Dropping Dark Matter from the Pisa Tower: A New Test of the Equivalence Principle with the Distortion of Time in Galaxy Clusters

A sketch of the Pisa tower on top of the Perseus cluster of galaxies observed by the Euclid satellite. Background image: ESA/Euclid/Euclid Consortium/NASA, image processing by J.-C. Cuillandre (CEA Paris-Saclay), G. Anselmi.

All types of ordinary matter fall in a gravitational potential in the same way, while dark matter may experience deviations. The depth of the gravitational potential can be measured through the impact of the time distortion on light, which changes frequency and hence colour in escaping the potential to reach an observer. © MPA

Predicted precision on deviations from the weak equivalence principle as a function of the total number of galaxies in galaxy clusters used to perform the test. The vertical line indicates the number used in the first detection of the distortion of time. The different colours correspond to more or less optimistic assumptions on the other free parameters involved in the test, ranging from assuming perfect knowledge of their values (black) to no knowledge (green).© MPA

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Does the mysterious dark matter experience gravity in the same way as ordinary matter? A team of scientists from MPA and the University of Geneva (Switzerland) has developed a new method to answer this question by measuring the time dilation in galaxy clusters. With future datasets, this method could detect violations of the equivalence principle at the level of a few percent.

In the 16th century, the Italian scientist Galileo Galilei is said to have dropped objects with different masses from the Leaning Tower of Pisa. With this experiment – possibly only imagined – he demonstrated that the acceleration of different bodies does not depend on their composition or mass. Since then, this seemingly counter-intuitive fact has become a fundamental pillar in our understanding of gravity, known as the weak equivalence principle. This principle states that any particle, regardless of its nature, experiences gravity in the same way.

Several experiments have confirmed with very high precision that the weak equivalence principle holds for all particles making up the ordinary matter around us. However, astrophysical and cosmological observations indicate that around 85 % of the matter in the Universe consists of unknown dark matter, which does not emit light and can only be probed through its gravitational impact on visible matter. If Galileo could have thrown a small amount of dark matter from the Pisa tower, would it have experienced the same acceleration as the other bodies? This remains a crucial open question, which could help shedding light on the nature of this mysterious component.

A team of researchers from MPA and the University of Geneva (Switzerland) – Sveva Castello, Enea Di Dio and Camille Bonvin – is determined to answer this question. Since dark matter has never been detected directly nor produced in a laboratory experiment, it is not possible to simply drop it from the Pisa tower. However, the team has designed a new method to perform an analogous experiment to Galileo’s in galaxy clusters. These are the largest gravitationally bound objects in the Universe and therefore provide the ideal environment to study the behaviour of dark matter under gravity. The new test consists in comparing the observed motion of the galaxies inside the clusters with the distortion of time generated by the clusters themselves.

Understanding the idea behind this test requires a small detour to the realm of Einstein’s theory of general relativity, providing our modern understanding of gravity. According to general relativity, the Universe can be described as a four-dimensional spacetime that gets distorted like a tablecloth in the presence of any object with a mass, such as galaxy clusters. This generates gravitational potential wells, which determine the motion of any particle under gravity. These distortions affect not only space but also time, so that a clock located at the bottom of a potential well ticks more slowly than one outside of it. This effect, known as time dilation or distortion of time, provides a direct measure of the depth of the gravitational potential well generated by a massive object.

If dark matter violates the weak equivalence principle, for example due to some unknown interactions, its motion under gravity will be different from the one predicted by general relativity. Since galaxies are mostly composed of dark matter, such a violation will impact their observed velocities inside a cluster. They will then move too fast or too slowly compared to the gravitational potential well of the cluster inferred from the distortion of time, clearly indicating an anomaly. Therefore, comparing galaxy velocities and the distortion of time in a galaxy cluster provides a powerful test of the weak equivalence principle.

Since we cannot send clocks across cosmological distances, how can we measure the distortion of time in galaxy clusters located billions of light-years away? This can be achieved by considering the impact of the distortion of time on light. Due to this effect, the wavelength of light emitted by galaxies in a cluster gets stretched and experiences a frequency shift, which is translated into a change of its observed colour. This leads to an observable gravitational redshift, which can be disentangled from other effects that change the colour of the light thanks to its symmetry properties when considering pairs of galaxies. This technique led to a first detection of this effect in 2011 by Radosław Wojtak, Steen H. Hansen and Jens Hjorth, who used a catalogue of around 100’000 galaxies in clusters by the Sloan Digital Sky Survey.

In this new study, the MPA-Geneva team predicted that existing measurements of the distortion of time can detect deviations from the weak equivalence principle at the level of 7-14 %. Ongoing galaxy surveys, such as the Euclid satellite and the Dark Energy Spectroscopic Instrument (DESI), will give access to larger samples of galaxy clusters and thus lead to an increased precision. In a realistic scenario, future datasets will be sensitive to violations of the equivalence principle at the level of a few percent.

As a next step, the team plans to apply the test to data. This will enable them to repeat Galileo’s experiment on astrophysical scales, providing crucial information on the properties of the mysterious dark matter in galaxy clusters. The discovery of a violation of the weak equivalence principle would have profound implications for cosmology, astrophysics and particle physics, and may also affect our fundamental understanding of gravity.

Source: Max Planck Institute for Astrophysics/Research Highlights

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Author:

Dr. Sveva Castello
Postdoc
Tel: 2007
Email: svevacas@mpa-garching.mpg.de

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Radiative Transfer Shapes Hydrogen Lines in Little Red Dots

Schematic illustration of resonance scattering in a hydrogen atom. Interactions with electrons in the ground state (1s–2p) are called Lyman-α (green), whereas excited electrons on n=2 contribute to the Balmer series (Hα and Hβ, red and blue). The next higher excitation level is then called the Paschen series (yellow). © MPA

Due to distinctive features in the spectra of the 'Little Red Dots', a new class of objects spotted by the James Webb Space Telescope, it was thought that these were distant galaxies with massive black holes at their centres. However, new research suggests that the light from these galaxies is shaped not only by the motion of gas near the central black hole, but also by the effects of radiation. MPA scientists have modelled three key processes – resonance, Raman, and Thomson scattering – and found that these, acting together, can explain the formation of hydrogen emission lines in the Little Red Dots.

Little Red Dots (LRDs) are among the most surprising discoveries of the James Webb Space Telescope. These compact, reddish sources appear in the early universe, within the first billion years of cosmic history, and exhibit unusual hydrogen spectra. Their light shows broad hydrogen emission lines, Balmer absorption features, and a pronounced break between ultraviolet and optical wavelengths. At first glance, these properties seem to point to active galactic nuclei, where broad hydrogen lines are typically interpreted as signatures of rapidly moving gas surrounding a supermassive black hole.

Yet this interpretation creates a major puzzle. If the widths of these hydrogen lines are directly interpreted as tracers of gas motion around a black hole, many Little Red Dots appear to host black holes that are unexpectedly massive compared to their young host galaxies. Such enormous black holes would challenge current ideas of how quickly black holes and galaxies could have formed and grown in the early universe. This tension raises an important question: do these spectral features truly provide a direct measure of black hole mass, or are they significantly shaped by the dense environments through which the radiation propagates?

This work explores a new possibility. Rather than assuming that hydrogen line widths primarily trace gas dynamics near a black hole, it investigates how radiative transfer through dense surrounding gas can fundamentally alter the observed spectrum. The presence of Balmer absorption and strong spectral breaks already hints that light in these systems may undergo substantial scattering and reprocessing. If so, some of the broad and complex hydrogen features in Little Red Dots may arise not only from fast-moving gas, but also from the way photons interact with thick, hydrogen-rich environments before escaping.

Understanding how radiative transfer shapes these spectral signatures therefore offers more than an alternative explanation for broad lines: it provides a new tool for probing the physical conditions, structure, and nature of Little Red Dots themselves, revealing how gas, radiation, and black hole growth interact in some of the earliest galaxies. Our focus is on three key processes:

1. Resonance scattering, where photons interact with hydrogen atoms in the excited n=2 state.
2. Raman scattering, where ultraviolet photons are converted into optical emission through inelastic scattering by atomic hydrogen.
3. Thomson scattering, where photons scatter off free electrons. Each process contributes differently to the observed spectral features.

Resonance scattering: shaping line profiles and ratios

Resonance scattering plays a crucial role when hydrogen atoms populate the n=2 or Balmer state, as indicated by Balmer absorption features and strong Balmer breaks. In this regime, Balmer photons can undergo multiple scatterings before escaping, which significantly modifies the emerging line profiles. These repeated interactions can produce asymmetric line shapes, particularly in the presence of gas motions such as outflows.

Notably, the radiative transfer of Hα and Hβ differs due to the atomic structure of hydrogen. While Hα photons predominantly remain in the same transition, Hβ photons can be converted into other lines, such as Paschen-α and Hα, through cascades involving the n=3 state. Consequently, Hβ photons are efficiently depleted in optically thick gas, while more Hα photons are produced. This leads to enhanced Hα emission and naturally increases the Hα/Hβ flux ratio beyond its intrinsic value.

Left: schematic illustration of Raman scattering of far-ultraviolet photons and the energy levels involved in neutral hydrogen. An UV photon excites the atom near the n=3 or n=4 state (green). If the electron drops down again to the ground state, it emits a Rayleigh photon (blue). If it drops down to an intermediate energy level, it emits a Raman photon (yellow or red). Right: The width of the emission line around Hα and Hβ depends on the column density (coloured lines), with the Hα wings being approximately three times broader than the Hβ wings for the same column density.© MPA

Raman scattering: generating broad wings

Raman scattering introduces a distinct spectral signature. Ultraviolet (UV) photons near the hydrogen Lyman series can be inelastically scattered by neutral hydrogen into optical wavelengths, producing broad wings around emission lines and showing systematic differences between certain hydrogen transitions. In particular, Raman scattering predicts that the wings of Hα should be significantly broader than those of Hβ.

Although broad emission lines are a defining feature of the Little Red Dots, such strong differences between lines are not always observed. This suggests that, although Raman scattering may contribute to the observed spectra, it is unlikely to be the dominant origin of the broad emission features. than those of Hβ.

Thomson-scattered line profiles for different electron temperatures. The line width increases with electron temperature.
© MPA

Thomson scattering: similar broad wings in hydrogen emission lines

Among the processes considered, Thomson scattering by free electrons provides a particularly compelling explanation for the broad components observed. Since electrons move thermally, the scattering introduces a symmetric broadening that depends on the electron temperature rather than on the motion of the bulk gas. Under typical conditions, this naturally produces line widths of around 1000 km/s, which is consistent with observations of the Little Red Dots. than those of Hβ.

The resulting profiles often exhibit exponential wings — a distinctive feature of electron scattering that has also been identified in other astrophysical environments. Importantly, this mechanism affects all emission lines in a similar way, which is consistent with the observed spectra. than those of Hβ.

Simulated spectra of the Hα, Hβ and Paα lines (red, blue and yellow) in a model combining an inner ionised region producing Thomson scattering (green) and an outer neutral region producing resonance scattering (grey). The resulting profiles illustrate how multiple scattering processes shape the observed line features together. © MPA

Implications for interpreting the Little Red Dots

The combined effects of resonance, Raman and Thomson scattering demonstrate that the Little Red Dots' diverse spectral features can naturally arise from radiative transfer in dense gas. Broad wings, absorption features and differences between hydrogen lines do not necessarily require extreme gas velocities or a classical broad-line region.

This has important consequences. If line widths are interpreted purely as indicators of gas motion, the mass of black holes may be significantly overestimated. Instead, the spectra of Little Red Dots encode the physical properties of their surrounding gas, such as density, temperature and ionisation state, through radiative processes.

These results provide a new framework for interpreting the spectra of Little Red Dots and similar systems in the early universe, offering a new perspective on early galaxy evolution. Rather than being straightforward indicators of black hole dynamics, hydrogen emission lines can reflect the complex interplay between radiation and dense gas.

Understanding this interplay is essential for correctly inferring the physical properties of galaxies and black holes at high redshifts and for developing a consistent model of their co-evolution during the first billion years of cosmic history. Current work focuses on analysing observed line profiles and using these models to decode the physical conditions imprinted in their shapes.

Source:

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Contact:

Dr. Seok-Jun Chang
Chang, Seok-Jun
Postdoc
2245
sjchang@mpa-garching.mpg.de

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Original Publication

Chang, Seok-Jun; Gronke, Max; Matthee, Jorryt; Mason, Charlotte
Impact of resonance, Raman, and Thomson scattering on hydrogen line formation in Little Red Dots
MNRAS, 545, 4, id.staf2131, 21 pp

Source | DOI

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X-ray panorama of the “Manatee Nebula” by SRG/eROSITA

Figure 1 shows a composite X-ray image of the radio nebula W50 taken with the eROSITA telescope. The surface brightness of the X-ray emission is colour-coded in the 0.5–1 keV (red), 1–2 keV (green), and 2–4 keV (blue) energy bands. The white arrows depict the projection of the SS433 jets' precession cone extrapolated to distances of more than 100 pc. The hard and soft X-ray diffuse emission can be convincingly split into two components: softer filamentary emission (red-yellow) and harder (green-blue) emission from EXJs. Additionally, there are numerous nearby compact sources, such as active stars and accreting white dwarfs, as well as distant compact sources, mostly AGN. For distant sources, absorption by Milky Way gas suppresses emission below 1 or 2 keV, giving them a blue colour. © MPA; eROSITA/SRG

Figure 2 shows a schematic summary of the W50 nebula superimposed on a composite X-ray (red and green) and radio (VLA at 1.4 GHz, blue) image. Radio emission most likely arises at the outer, shell-like boundary of the nebula, while soft X-ray emission (0.3–0.9 keV) traces shock-heated interstellar medium (ISM) gas behind it, filling almost the entire interior of the nebula. The harder X-ray emission (0.9–2.7 keV in this case) is of a non-thermal (synchrotron) nature and may be produced by ultrarelativistic electrons that are accelerated at the shocks in the axial outflows from the system. The central part of the nebula, within 25 pc of SS433 (dashed circle), is likely to be of very low density and could be a wind-blown cavity created by an almost spherically symmetric outflow with a kinetic luminosity close to the Eddington limit. © MPA; eROSITA/SRG

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Rare or unusual astrophysical objects are used to test the limits of theoretical models because of their extreme properties. The bright X-ray source SS433 in our galaxy undoubtedly belongs to this category. Initially identified as an Hα emitter, it was later recognised as a black hole in a binary system. Since then, SS433, which emits strongly in the radio and X-ray bands, has been targeted by almost every space- and ground-based observatory, leading to a flurry of discoveries.  In contrast, the surrounding huge W50 nebula, spanning more than two degrees, is much fainter and difficult to study. The complete radio image earned W50 the nickname 'Manatee Nebula', while X-ray maps were mostly patches from different observatories or lacked spatial or energy resolution. This shortcoming has finally been overcome by the recently published SRG/eROSITA map of W50 in multiple X-ray colours, which reveals a beautiful blend of thermal and non-thermal processes within an elongated cocoon.

At the core of the W50 nebula lies a compact source (most likely a stellar-mass black hole) that accretes matter from a companion star at an astonishingly high rate — thousands of times greater than the amount the black hole can digest. This limiting rate (known as the Eddington accretion rate) arises due to the pressure exerted by the radiation produced by the infalling gas. This configuration has an immediate impact on the observational appearance of the compact source and its large-scale environment. The key prediction of the accretion theory is that most of the gas supplied to the black hole will be expelled from the system, depositing a large amount of energy into the ambient medium in the process (see Highlight September 2024).

The W50 nebula is well known in radio astronomy for its croissant-like shape. Mapping this large nebula in X-rays used to be problematic due to the limited field of view of space telescopes. Additionally, strong and inhomogeneous absorption by gas and dust occurs in the direction of W50, which is located just two degrees away from the Galactic Plane. These problems can be resolved by using a telescope with a large field of view and high sensitivity to diffuse emission — the very characteristics of the eROSITA telescope on board the SRG observatory.

The full-size X-ray map of the W50 nebula is shown in Fig. 1. The central bright spot is the black hole that powers the entire nebula. It appears extended because it is much brighter than the nebula emission, causing the central part of the image to become saturated.

The 'X-ray colours' in this figure serve the same role as red, green, and blue colours in visible light. Specifically, red corresponds to X-ray photons with a longer wavelength, while green and blue correspond to progressively shorter wavelengths. Remarkably, this simple approach immediately reveals the nature of the X-ray emission: red and yellow colours dominate where thermal plasma with a temperature of 2–10 million degrees is present. Conversely, in the bluer regions, non-thermal emission from relativistic particles dominates.

The nebula is clearly asymmetric, most likely due to a gradient in the ambient gas density surrounding it. The most remarkable feature is the so-called 'Extended X-ray Jets' (EXJs), which have sharp inner edges located around 25 parsecs from the central black hole SS433. Their spectra do not have the emission lines characteristic of thermal plasma. Rather, they must be due to the emission of relativistic particles accelerated by shocks powered by SS433’s outflows. These structures have recently been detected at TeV energies; each TeV photon carries a billion times more energy than a soft X-ray photon at keV energies.

These new X-ray data support the idea that the energy flow from SS433 evolves through three distinct stages:

1) an invisible 'dark' flow of energy between the black hole and the EXJs, presumably carried by a cold wind from the binary system;

2) a 'non-thermal' flow of energy over some 30 pc in the form of EXJs; and

3) a thermal flow (i.e., shock-heated interstellar medium (ISM)) that envelops the EXJs.

The thermal part of the W50 X-ray emission can be reasonably well described by a shock-heated plasma that has not yet reached temperature and ionisation equilibrium. Such emission is typical of middle-aged or old supernova remnants (SNRs). The outer radio boundary of the nebula also resembles SNR shocks (see Fig. 2).

In contrast, the 'extended X-ray jets' are the most remarkable features of this system on tens-of-pc scales. Their sharp inner edges plausibly correspond to extreme shocks that accelerate particles and power the X-ray (synchrotron) and TeV emission, which is 9–10 orders of magnitude more energetic. The W50/SS433 system clearly illustrates the important role that hyper-Eddington accretors might play in the energetics of the interstellar medium in galaxies at different redshifts, as well as in the production of ultra-high-energy particles.

Source: Max Planck Institute for Astrophysics/Research Highlights

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Authors:

Rashid Sunyaev
Emeritus Director
Tel: 2244
Email: rsunyaev@mpa-garching.mpg.de

Eugene Churazov
Scientific Staff
Tel: 2219
Email: echurazov@mpa-garching.mpg.de

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Original publication

Sunyaev R., Khabibullin I., Churazov E., Gilfanov M., Medvedev P., Sazonov S.
X-ray panorama of the SS433/W50 complex by SRG/eROSITA
A&A, in press

DOI

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A Sea of Light: HETDEX Astronomers Reveal Hidden Structures in the Young Universe

Section of the Line Intensity Map created by charting the distribution and concentration of excited hydrogen (via the Lyman alpha wavelength) in the universe ten billion years ago. The stars mark where HETDEX has found galaxies. The inset simulates the structure present in this map once it is zoomed in on and background noise is removed from the data. Credit: Maja Lujan Niemeyer/Max Planck Institute for Astrophysics/HETDEX, Chris Byrohl/Stanford University/HETDEX

Example of a spectrum created by statistically combining the spectra of 50,000 Lyman alpha emitters from the first Public HETDEX Source Catalog. The wavelength associated with Lyman alpha appears as a dramatic peak, making it a particularly useful tool for identifying the location of bright galaxies in the early universe. Credit: HETDEX

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An international team of astronomers has created the most detailed 3D map yet of Lyman alpha light emitted by hydrogen in the early universe. Using Line Intensity Mapping on data by the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX), they identified faint galaxies and gas that were previously difficult to observe. This can now be compared to simulations of the structures in the early universe. The team processed half a petabyte of data to refine their map, revealing unseen objects and enhancing our understanding of galaxy evolution.

Astronomers with the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX), have used data from the project to make the largest, most accurate 3D map yet of the light emitted by excited hydrogen in the early universe, 9 billion to 11 billion years ago. This specific form of light, called Lyman alpha, is emitted in large quantities when hydrogen atoms are exposed to a star’s energy. That makes it a great tool for finding bright galaxies in this far-off time, which experienced a rash of star creation. However, the locations of fainter galaxies and gas, which also emit Lyman alpha, have remained largely unknown.

“Observing the early universe gives us an idea of how galaxies evolved into their current form, and what role intergalactic gas played in this process,” said Maja Lujan Niemeyer, a HETDEX scientist and recent graduate from the Max Planck Institute for Astrophysics who led the development of the map. “But because they are far away, many objects in this time are faint and difficult to observe.”

Using a technique called Line Intensity Mapping, the new map pulls these objects into view, adding shape and nuance to this formative era in our universe. Results were published on March 3 in The Astrophysical Journal.

All light can be broken apart into its various wavelengths. The result is called a spectrum. Astronomers examine spectra (the plural of “spectrum”) for peaks and valleys which correspond to the presence of different elements. Line Intensity Mapping charts the distribution and concentration of specific elements across an entire region, rather than observing objects one-by-one.”

“Imagine you're in a plane looking down. The ‘traditional’ way to do galaxy surveys is like mapping the brightest cities only: you learn where the big population centers are, but you miss everyone thatlives in the suburbs and small towns,” explained Julian Muñoz, a HETDEX scientist, assistant professor at The University of Texas at Austin, and co-author on the paper. “Intensity mapping is like viewing the same scene through a smudged plane window: you get a blurrier picture, but you capture all the light and not just the brightest spots.

”Although Line Intensity Mapping isn’t a new technique, this is the first time it’s been used to chart Lyman alpha emissions in such a large set of data and with such high precision. Using the Hobby-Eberly Telescope at McDonald Observatory, HETDEX is charting the position of over one million bright galaxies in its quest to understand dark energy. The project is unique in gathering so much data – over 600 million spectra – for such a large swath of sky, measuring over 2,000 full Moons.

“However, we only use a small fraction of all the data we collect, around 5%,” explained Karl Gebhardt, HETDEX principal investigator, chair of UT Austin’s astronomy department, and co-author on the paper. “There’s huge potential in using that remaining data for additional research.”

“HETDEX observes everything in a patch of sky, but only a tiny amount of that data is related to the galaxies that are bright enough for the project to use,” added Lujan Niemeyer. “But those galaxies are only the tip of the iceberg. There’s a whole sea of light in the seemingly empty patches in between.”

To create its map, the team wrote custom programming and used supercomputers at the Texas Advanced Computing Center to sift through roughly half a petabyte of HETDEX data. It then used the location of bright galaxies already identified by HETDEX to calculate the location of fainter galaxies and gas glowing nearby. Thanks to gravity’s propensity for making matter clump together, where there is one bright galaxy, other objects are sure to be close.”

“So, we can use the location of known galaxies as a signpost to identify the distance of the fainter objects,” said Eiichiro Komatsu, a HETDEX scientist, scientific director at the Max Planck Institute for Astrophysics, and co-author on the paper. The resulting map brings the regions around bright galaxies into greater focus and adds detail to the stretches in between.

“We have computer simulations of this period,” continued Komatsu. “But those are just simulations, not the real universe. Now we have a foundation which can let us know if some of the astrophysics underpinning those simulations is correct.”

Moving forward, the team hopes to compare their map with others that overlap the same region of the universe and focus on different elements. For example, a Line Intensity Map of carbon monoxide - which is associated with the dense, cold clouds where stars form - could add insight to the conditions surrounding the young stars emitting Lyman alpha wavelengths.

“This study is a first detection, which is exciting on its own, and it opens the door to a new era of intensity-mapping the universe,” said Muñoz. “The Hobby-Eberly is a pioneering telescope. And with new, complementary instruments coming online, we're entering a golden age for mapping the cosmos.”

Source: Max Planck Institute for Astrophysics/Press Releases

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Contacts:

Lujan Niemeyer
Postdoc
Tel: 2357
maja@mpa-garching.mpg.de

Eiichiro Komatsu
Director
Tel: 2208
komatsu@mpa-garching.mpg.de

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Original publication

Maja Lujan Niemeyer, Eiichiro Komatsu, José Luis Bernal et al.
Lyα Intensity Mapping in HETDEX: Galaxy-Lyα Intensity Cross-Power Spectrum
published on March 3 in The Astrophysical Journal.

Source

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Proto-stellar disks in their natural habitat

Figure 1: Young disks observed with ALMA at a wavelength of 3 mm. These disks clearly display substructure and the early presence of companion stars. from: Maureira et al., 2025, A&A, 705, A96

Figure 2: Zoom simulations of proto-stellar disk formation inside a molecular cloud (large image). The colour scale indicates the the gas column densities. The insets show multiple zoom regions, in which several dense cores have formed. Six cores (labels and orange borders) were studied in more detail. © MPA

Figure 3: High resolution views of the resulting proto-stellar disks for various simulations of Core 1: the ‘control’ case without magnetic fields called ‘hydro’ (left), the absence of a disk with ‘ideal’ MHD (middle), and the disk forming in the most realistic ‘non-ideal’ MHD simulation (right), including ambipolar diffusion. © MPA

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Sun-like stars form within turbulent molecular clouds, encircled by disks of gas and dust - the birthplaces of planets. While the earliest phases of the disk assembly process are obscured by the surrounding dense gas, ALMA can observe proto-stellar disks shortly after their formation. In a project supported by the Excellence Cluster ORIGINS, researchers from MPA, MPE, Harvard, and the University of Cologne performed high-resolution non-ideal magneto-hydrodynamical simulations that self-consistently follow proto-stellar disk formation from their parental turbulent molecular clouds down to stellar scales, spanning over 10 orders of magnitude. The study uncovers the complex paths by which disks assemble and demonstrates that magnetic fields play a central role in their formation and early evolution.

The interstellar medium (ISM), the site of star formation in galaxies, is a very complex environment. Diffuse hot regions (with temperatures of several million Kelvin) often exist in close proximity to cold, dense molecular clouds (with temperatures below a few hundred Kelvin). ‘Stellar feedback’, e.g. the explosion of massive stars as supernovae, creates the hot gas and drives turbulent gas motion in the ISM. This turbulence also causes cooling and can lead to gravitational collapse in certain regions, which form molecular clouds. Stars and their proto-stellar disks form in these molecular clouds from dense cores.

This process covers a large range of spatial scales: using the distance between earth and sun, an ‘astronomical unit’ or AU, as a ruler, the scales range from several 10 million AU for the size of molecular clouds, to a million AU large ‘bubbles’ created by supernovae, to regions smaller than a per cent of an AU for a newly forming proto-star. Specific numerical techniques are required to simulate such a system, as using equally high resolution everywhere would overwhelm even supercomputers. Most previous studies of disk formation simplify the problem and focus on the final disk formation phase after the collapse of dense cloud cores with uniform densities and turbulent velocities imposed by hand. This, however, misses the self-consistent formation of the cloud core structure, kinematics, and magnetic fields from its the large-scale environment.

How important are magnetic fields in this picture? It is well established observationally that clouds cores are strongly magnetized, which impacts their evolution. ‘Ideal’ magneto-hydrodynamical (MHD) models assume that magnetic fields are carried along with the gas. They back-react on the gas through the Lorentz force and provide support against gravitational collapse. The Lorentz force also works against the rotational twisting of magnetic field lines - a situation encountered where a rotating disk surrounds a young star. This resistance slows down gas so much that it falls onto the star, while fast-rotating material leaves the system in a proto-stellar wind - leaving no disks behind. However, extended proto-stellar disks are regularly observed around young stars (Figure 1) – inconsistent with ‘ideal’ MHD models.

This problem can be solved with more realistic ‘non-ideal’ MHD models where neutral and ionized particles move differently (ambipolar diffusion).

With this process, magnetic fields in collapsing cores are reduced and proto-stellar disks are able to form. Numerical simulations of this process are expensive but essential to understand proto-stellar disk formation.

In a project supported by the DFG Excellence Cluster ‘ORIGINS’ researchers from the Max Planck Institute for Astrophysics (MPA,), the Max Planck Institute for Extraterrestrial Physics (MPE,), Harvard, and the University of Cologne performed high-resolution non-ideal MHD ‘zoom’ simulations to self-consistently follow proto-stellar disk formation from their parent turbulent, multi-phase molecular clouds down to stellar sub-AU scales. The unprecedented ‘non-ideal’ MHD simulations span over 10 orders of magnitude in spatial scales.

In this setup with a realistic large-scale turbulent environment (Figure 2), no extended proto-stellar disks can form with ‘ideal’ MHD, while ‘non-ideal’ MHD allows for the early formation of a disk, similar to what is seen in the ‘hydro’ model without any magnetic field (Figure 3). However, the substructures of the disks formed in these different models are clearly distinct from each other. The study indicates that magnetic fields, along with non-ideal MHD effects, and the large-scale, multi-phase and turbulent environment play a central role for proto-stellar disk formation.

Ongoing work building on this study will focus on the evolution of these disks formed in realistic environments over a longer time-span. This will also allow the researchers to study how early stellar companions form.

Source: Max Planck Institute for Astrophysics/Tesearch Highlights

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Authors:

Alexander Mayer
PhD student
Tel: 2042
amayer@mpa-garching.mpg.de

Thorsten Naab
Scientific Staff
tnaab@mpa-garching.mpg.de

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Original publication

Mayer, Alexander C.; Naab, Thorsten; Caselli, Paola; et al.

Protostellar discs in their natural habitat ─ the formation of protostars and their accretion discs in the turbulent and magnetized interstellar medium

Monthly Notices of the Royal Astronomical Society, Volume 543, Issue 4, pp. 3321-3344, 24 pp.

DOI

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Measuring the expansion of the universe with cosmic fireworks

High-resolution image taken with the Large Binocular Telescope on Mount Graham in Arizona, USA, displaying the two lens galaxies in a warm tone, and the five lensed copies of SN Winny in blue. © Credit: SN Winny Research Group

Munich astronomers image and model extremely rare gravitationally lensed supernova

That the universe is expanding has been known for almost a hundred years now, but how fast? The exact rate of that expansion remains hotly debated, even challenging the standard model of cosmology. A research team at the Technical University of Munich (TUM), the Ludwig Maximilians University (LMU) as well as the Max Planck Institutes for Astrophysics (MPA) and Extraterrestrial Physics (MPE) has now imaged and modelled an exceptionally rare supernova that could provide a new, independent way to measure how fast the universe is expanding.

• An image that could solve a long lasting cosmic mystery


• Unprecedented chance to measure the growth of the universe


• Collaboration between TUM, LMU and Max Planck Institutes

The supernova is a rare superluminous stellar explosion, 10 billion lightyears away, and far brighter than typical supernovae. It is also special in another way: the single supernova appears five times in the night sky, like cosmic fireworks, due to a phenomenon known as gravitational lensing. Two foreground galaxies bend the supernova’s light as it travels toward Earth, forcing it to take different paths. Because these paths have slightly different lengths, the light arrives at different times. By measuring the time delays between the multiple copies of the supernova, researchers can determine the universe’s present-day expansion rate, known as the Hubble constant.

Sherry Suyu, Associate Professor of Observational Cosmology at TUM and Fellow at the Max Planck Institute for Astrophysics, explains: “We nicknamed this supernova SN Winny, inspired by its official designation SN 2025wny. It is an extremely rare event that could play a key role in improving our understanding of the cosmos. The chance of finding a superluminous supernova perfectly aligned with a suitable gravitational lens is lower than one in a million. We spent six years searching for such an event by compiling a list of promising gravitational lenses, and in August 2025, SN Winny matched exactly with one of them.”

Large Binocular Telescope auf dem Mount Graham in Arizona, USA
© Credit: Dr. Christoph Saulder / MPE

High-resolution color image of unique supernova

Because gravitationally lensed supernovae are so rare, only a handful of such measurements have been attempted to date. Their accuracy depends strongly on how well one can determine the masses of the galaxies acting as a lens, because these masses control how strongly the supernova’s light is bent. To measure those masses, the team obtained images with the Large Binocular Telescope in Arizona, USA, using its two 8.4-meter diameter mirrors and an adaptive optics system that corrects for atmospheric blurring. The result is the first high-resolution color image of this system published to date.

The observations reveal the two foreground lens galaxies in the center and five bluish copies of the supernova - reminiscent of a firework exploding. This comes as a surprise, since galaxy-scale lens systems normally produce only two or four copies. Using the positions of all five copies, Allan Schweinfurth and Leon Ecker, junior researchers in the team, built the first model of the lens mass distribution.

“Until now, most lensed supernovae were magnified by massive galaxy clusters, whose mass distributions are complex and hard to model,“ says Allan Schweinfurth. “SN Winny, however, is lensed by just two individual galaxies. We find overall smooth and regular light and mass distributions for these galaxies, suggesting that they have not yet collided in the past despite their close apparent proximity. The overall simplicity of the system offers an exciting opportunity to measure the universe’s expansion rate with high accuracy.”

Members of the SN Winny Research Group at Research Campus Garching (from left): Stefan Taubenberger, Allan Schweinfurth, Alejandra Melo, Elias Mamuzic, Sherry Suyu, Christoph Saulder, Roberto Saglia, Leon Ecker, Limeng Deng. © Credit: Dr. Robert Reich / TUM

Two methods, two very different results

So far, scientists have mostly relied on two methods to measure the Hubble constant, but these methods yield conflicting results. This puzzle is known as the Hubble tension.

The first is the local method, which measures distances to galaxies one step at a time, much like climbing a ladder, where each step depends on the previous one; hence, it is referred to as the cosmic distance ladder. It uses objects with well-known brightness to estimate distances and then compares those distances with how fast galaxies are moving away. Because this method involves many calibration steps, even small errors can accumulate and affect the final result.

The second method looks much farther back in time. It studies the cosmic microwave background, the faint afterglow of the Big Bang, and uses models of the early universe to calculate today’s expansion rate. This approach is highly precise, but it relies heavily on assumptions about how the universe evolved, and these assumptions are still subject to debate.

SN Winny
Credit: Elias Mamuzic / MPA / TUM

A new, one-step approach

Animation (available in several languages) showing the gravitational lensing effect of the pair of foreground galaxies on the host galaxy of SN Winny. The host galaxy is lensed into multiple images, which are distorted and stretched out to form a bluish ring around the lens. The explosion of SN Winny itself and the time-delayed arrival of its multiple lensed copies on Earth are also simulated. Ultimately, the animation fades to a real observation of SN Winny, captured at the Large Binocular Telescope in Arizona.

A third, independent method now enters the picture: using a gravitationally lensed supernova. Stefan Taubenberger, a leading member of Professor Suyu’s team and first author of the supernova-identification study, explains that by measuring the time delays between the multiple copies of the supernova and knowing the mass distribution of the lensing galaxy, scientists can directly calculate the Hubble constant: “Unlike the cosmic distance ladder, this is a one-step method, with fewer and completely different sources of systematic uncertainties.”

Astronomers worldwide are currently observing SN Winny in detail using both ground-based and space-based telescopes. Their results will provide crucial new insights and help clarify the long-standing Hubble tension.

Source: Max Planck Institute for Astrophysics/Press Releases

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Contacts:

Prof. Dr. Sherry Suyu
Scientific Staff
Tel: 2015

Stefan Taubenberger
Tel: 2019
tauben@mpa-garching.mpg.de

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Original publication

1. Taubenberger et al.
HOLISMOKES XIX: SN 2025wny at z = 2, the first strongly lensed superluminous supernova
accepted by Astronomy & Astrophysics (A&A), December 2025

Source

2. Ecker, Schweinfurth et al.
HOLISMOKES XX. Lens models of binary lens galaxies with five images of Supernova Winny
submitted to Astronomy & Astrophysics (A&A)

Source

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An old puzzle solved: astronomers discover the world is flat

The average distribution of dark matter for a large number of computer simulations, each of which was required to form a Milky Way and an Andromeda Nebula (the two bright blobs at the centre) with the observed position and velocity, and also to match the observed velocity at the position of 31 nearby galaxies (cyan dots). The box size is 20 times the Milky Way-Andromeda separation with a depth of one-half this separation. Colour represents the amount of dark matter at each point, while arrows show its velocity relative to a uniformly expanding universe. The left image is looking down onto the Local Mass Sheet, while the right one views it from the side. Notice that velocities relative to a uniform Hubble flow are small in both panels in the region occupied by the cyan dots, implying that these galaxies appear to match Hubble’s Law almost perfectly in the simulated universes. © MPA

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A pan-European group of astronomers has used newly developed computer technology to solve a 100 year-old puzzle. While most galaxies in our neighborhood move away from us almost as expected for an unperturbed cosmic expansion, our nearest giant neighbour is approaching at high speed. Systematic numerical experimentation demonstrates this rapid approach is due to massive dark matter haloes surrounding both Andromeda and our own Milky Way, but this mass does not slow down somewhat more distant galaxies because its effects are counteracted by more distant dark matter which lies in a vast flattened sheet out to distances well beyond the neighboring galaxies considered.

Why is the Andromeda Nebula heading straight for us, while other nearby galaxies are receding?

It is nearly a century since the American astronomer Edwin Hubble discovered the expansion of the Universe. Distant galaxies similar to our own Milky Way move away from us at speeds that increase in proportion to their distance, reflecting the origin of the Universe in a Big Bang, an enormous explosion 14 billion years ago. Hubble already knew, however, that this is not true for our nearest giant neighbour, the Andromeda Nebula, which is 2.5 million light-years away and coming towards us at 100 kilometers per second. In 1959, two European astronomers, Franz Kahn and Lodewijk Woltjer, calculated that in order for the gravity of the two galaxies to have reversed the initial expansion, their total mass must be more than 1000 billion times the mass of the Sun – much more than the mass of all their stars put together. This was the first detection of unseen Dark Matter around our Milky Way and its neighbour.

In the 1970s and 1980s, accurate distances began to be measured for somewhat more distant galaxies. It became clear that not only are they are mostly moving away from us but that their speeds are close to those predicted by the overall cosmic expansion – starting in a “Big Bang” 14 billion years ago. Studies of galaxies at distances from 1.5 to 4 times the Milky Way-Andromeda separation found the deviations to be actually quite small – the total amount of matter required to account for these deviations out to the most distant galaxy cannot be larger than that already needed to explain the approach speed of the Milky Way and Andromeda. However, there are several other large galaxies in this region, which should contribute additional mass. Why then does the cosmic expansion around us appear so weakly perturbed?

A pan-European group of astronomers has recently used newly developed computer technology to find the solution to this puzzle. They set the machine the following task: Find representative regions of the early Universe with small deviations from uniformity that are statistically similar to the Cosmic Microwave Background, but that evolve to produce galaxies similar to the Milky Way and Andromeda, with the appropriate positions and velocities. At the same time, other nearby galaxies should show motions and positions matching those of observed nearby galaxies.

Apparently, the puzzle was not hard for the computer: it was able to find hundreds of examples satisfying all the given conditions. The average mass distribution for a large number of these is shown in the figure. In the region containing the local galaxies, motions relative to a uniform expansion are indeed small – the Hubble flow is almost unperturbed – while at larger distances material is actually moving away from the Milky Way faster than the Hubble flow.

Max Planck Institute for Astrophysics How the computer solved the puzzle can be seen in the right image of the figure, which shows a view of the same box rotated by 90 degrees. The mass is concentrated to a flattened sheet extending well beyond the region occupied by the local galaxies considered. All the galaxies are inside the sheet and even at larger distances most known galaxies are still found in a flattened distribution known as the Local Supercluster. The computer has inferred this larger structure even though it was not told about its existence. The large low-density regions above and below the sheet are also seen in the galaxy distribution and are known as the Local Voids. However, the large velocities predicted there are not observable, because in the real universe there are no galaxies there to be measured.

Thus, there are two reasons why the local Hubble flow seems so weakly perturbed despite the large combined mass of the Milky Way and Andromeda. Mass at larger distances is counteracting the gravity of the central galaxies by pulling material outwards. In addition, there are no galaxies where the predicted infall effects are large, so inflow onto the Local Sheet is hidden.

The solution to the puzzle is that the total mass distribution in our environment is at least as sheet-like as the distribution of galaxies. The world around our Local Group of galaxies is indeed flat out to distances of tens of millions of light-years.

Source: Max Planck Institute for Astrophysics/Press Releaes

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Author:

Simon White
Emeritus Director
Tel: 2211
Tel: +49 170 248 1178
swhite@mpa-garching.mpg.de

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Original publication

E. Wempe et al. The mass distribution in and around the Local Group
Nature Astronomy, 27 January 2026

Source

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Weitere Informationen

L’anomalie d’Andromède résolue : une feuille cosmique explique son mouvement et l’expansion locale
CNRS Press Release
(in French)

Een ‘platte’ omgeving van de Melkweg verklaart de beweging van nabije sterrenstelsels
Dutch press release

Gammal gåta löst: astronomer upptäcker att vårt kosmiska närområde är platt
University Stockholm press release

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Astronomers confounded by a mysterious million solar-mass dark object whose inner structure defies explanation

Schematic view of the gravitational lens system.
image generated with AI

Representation of a possible scenario for the object, which includes a black hole that has a mass of about 300,000 times that of our Sun and an extended dark disk with even more mass, which can only be characterised through their combined gravitational lensing effect on the distant Universe. image generated with AI

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Million–Solar-Mass Enigma Shows Traits of a Tiny Galaxy—But Its Inner Structure Defies Explanation

An international team of astronomers have uncovered what may be a new type of unseen dark object in the distant Universe that does not seem to resemble anything observed before. The object, which has a mass of about one million times that of our Sun, was previously discovered through the subtle gravitational distortions it imprints on the images of a strongly lensed radio jet. From a new analysis, published today in the journal Nature Astronomy, the team have now tested various models for how the mass is structured within the enigmatic object in the hope of uncovering what it could be. To their surprise, the data rule out all conventional explanations, but instead point towards an extremely compact object, like a black hole or a dense stellar nucleus, that is embedded in an extended disk of matter that, so far, also does not seem to emit any detectable light.

Uncovering the invisible

Astronomers have long relied on a phenomenon known as strong gravitational lensing – the bending and magnifying of light by massive objects – to probe invisible structures in the Universe. In many cases, these distortions act as natural cosmic microscopes, revealing otherwise undetectable concentrations of dark matter, the elusive substance that is thought to make up most of the mass in our Universe. By analyzing the way background light is warped, scientists can map the mass of intervening objects with extraordinary precision, even when those objects emit no light of their own.

For their analysis, the team combined radio telescopes from around the world, including the Green Bank Telescope (GBT), the Very Long Baseline Array (VLBA) and the European Very Long Baseline Interferometric Network (EVN). The data from this international network of radio telescopes were correlated at the Joint Institute for VLBI ERIC (JIVE) in the Netherlands, forming an Earth-sized super-telescope that captured the precise details of the tiny gravitational lensing distortions caused by the thus far invisible low mass object.

Dr. Simona Vegetti (MPA), lead author of the paper published today in Nature Astronomy, explained how difficult the modeling analysis was, “Trying to separate all of the different mass components of such a distant and low mass object with gravitational lensing was both extraordinarily challenging and incredibly exciting. We’re working with high data quality and complex models, and just when I thought we’d pinned it down, its properties threw us another curveball. That combination of difficulty and mystery is exactly what makes this object so compelling.”

By combining the high fidelity of the lensing data with sophisticated analysis tools, and a massive amount of computing power, Dr. Vegetti and her team were able to characterise the structure of the low mass object over a range of radii with unprecedented detail. Vegetti continued, “The central inner part is strikingly compact, consistent with either a black hole or a dense stellar nucleus that surprisingly makes up about a quarter of the total mass of the object. As we go out from the centre, however, the density of the object flattens into a broad, disk-like component. This is a structure that we really haven’t seen before, and so, it may be a new class of dark object.”

A possible problem for dark matter theories

In terms of its overall size, structure and mass, the object could fall within the family of ultra-compact dwarf galaxies with some extended stellar halo of stars. These are rare systems that bridge the gap between massive star clusters and small galaxies, but the team have yet to detect the light from any stars embedded within the object. Even for this category of compact galaxies, the measured internal structure of the object remains highly unusual. If it were instead purely dominated by dark matter, its strange structure would be inconsistent with astronomers’ expectations for what such dark objects should look like.

Prof. Simon White, emeritus director at MPA and a co-author of the study said, “Our standard view of how cosmic structure forms predicts that there should be many starless dark matter lumps with mass a million times that of the Sun. But it predicts a structure for them which is very different, in particular much less centrally concentrated, than what we have found here.”

Although the observed properties of the object deviate dramatically from the predictions of the standard cold dark matter model, which underpins much of our understanding of the Universe, one speculative alternative is that the dark matter is self-interacting. In such a scenario, the object could be a dark-matter halo whose centre has collapsed to form a black hole. However, additional numerical modelling will be required to test whether such a theory can replicate the observed density profile of the object.

This is the third such object to be identified using the so-called gravitational imaging method, but it is by far the smallest in terms of mass and the first to be characterised to such a precise level. All three detections exhibit properties that sit uneasily within the standard dark matter framework. Identifying more examples will be crucial for determining whether these systems are rare outliers or are the first hints of physics beyond the current dark matter model.

Co-author Prof. John McKean added: “We’re hopeful that this detection is just the first of many to be studied in such amazing detail with high-resolution radio telescopes. With ongoing wide-sky surveys and the ever-improving power of high–angular-resolution follow-up observations, we should soon be able to uncover a whole population of these elusive low-mass systems, which will definitely teach us something new about our Universe.”

Source: Max Planck Institute for Astrophysics/Press Releases

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Additional information

Gravitational lensing: This is an astrophysical tool used by astronomers to measure the mass properties of structures in the Universe. It is a consequence of Einstein’s Theory of General Relativity, where mass in the Universe curves space. If the mass of the foreground lensing object (typically a galaxy or cluster of galaxies) is sufficiently dense, then the light from distant objects is distorted and multiple images are even seen.

Very Long Baseline Interferometry: The radio observations were taken using a combination of radio telescopes that are combined to form a so-called Very Long Baseline Interferometer. This observational method allows astronomers to improve the imaging sharpness of the data and reveal very small fluctuations in the brightness that otherwise could not be seen. The telescopes included in the observations used in this analysis were the Green Bank Telescope and the Very Long Baseline Array of the National Radio Astronomy Observatory in the United States, and the telescopes of the European Very Long Baseline Interferometric Network.

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Contacts:

Dr. Simona Vegetti
Scientific
Staff
tel:2285
svegetti@mpa-garching.mpg.de

Prof. Dr. Simon White
Emeritus Director
tel:2211
tel:+49 170 248 1178
swhite@mpa-garching.mpg.de

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Original publication

Vegetti et al.
Title
Nature Astronomy, 5 Jan 2026

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Related Press Releases

Astronomers ‘image’ a mysterious dark object in the distant Universe

October 09, 2025

An international team of astronomers has found a low mass dark object in the distant Universe, not by directly observing any emitted light, but by detecting its tiny gravitational distortion of the light from another distant galaxy. This mysterious object has a mass of about one million times that of our Sun, and its discovery seems consistent with the current best theory about how galaxies like our own Milky Way formed.

more

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Explosive endpoints of a life next to a black hole

Binary systems composed of a black hole and a massive stellar companion can maintain stable mass transfer if the orbit is not too tight. The plot shows the orbital separation at the end of mass transfer (in units of solar radii) as a function of the initial mass of the star (in units of solar masses). The blue-shaded region marks all systems that remain stable and survive the interaction. The newly identified separation limit is indicated by the dashed red line, corresponding to approximately ten solar radii. All binaries in which the orbit shrinks below this limit are found to be dynamically unstable: the black hole plunges into the star and destroys the system. In surviving systems, the star continues its evolution until it collapses to form a second black hole. Because of the critical separation limit and the exclusion of very tight orbits, the resulting binary black hole systems take a very long time – billions of years – to merge as gravitational-wave sources detectable by LIGO/Virgo/KAGRA. © MPA/Klencki

Sequence of events leading to the black hole destroying its companion star. Phase 1: A long-lasting (~1000 years) phase of stable mass transfer from the massive star onto the black hole. Most of the mass lost by the star is not accreted by the black hole but is instead ejected from the system at low velocities (~30 km/s). Over time, this outflow forms an extended dusty cocoon that enshrouds the central binary, reaching nearly one trillion kilometers in size. © MPA/Klencki/Metzger

Phase 2: Once the orbit tightens sufficiently and approaches the stability threshold (Figure 1), the interaction becomes unstable and the black hole plunges into the star. Over the course of several days, the black hole spirals inward until it reaches the compact helium core. Energy released during this phase causes hot gas from the inner layers of the star to expand at thousands of kilometers per second, creating a nearby hot bubble roughly one billion kilometers in size. © MPA/Klencki/Metzger

Phase 3: The black hole spirals into the stellar core and tidally rips it apart, forming a dense, thick accretion disk. Over just a few hours, the black hole accretes a fraction of a solar mass, releasing energy equivalent to ten million years of solar output. A powerful jet is launched near the black hole: a relativistic outflow of ionized matter and particles traveling at tens of percent of the speed of light (~100,000 km/s). The jet rapidly breaks out and shocks the nearby hot bubble (blue), powering a superluminous transient whose peak brightness is reached after several days, as observed in LFBOTs. Over the following months, the jet continues to propagate through and interact with the extended dusty cocoon (red), producing the strong radio emission observed in LFBOTs. © MPA/Klencki/Metzger

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More and more black holes are found orbiting a luminous massive stellar companion. The future of these systems holds a fundamental puzzle: once the companion star expands and begins to lose mass onto the black hole, will the interaction remain stable or will the black hole plunge into the star and destroy it from within? Using state-of-the-art computational models, a team led at MPA has identified a surprisingly simple rule: the interaction is stable as long as the distance between the black hole and the star remains larger than about ten times the radius of the Sun. The newly found separation threshold will play a key role in determining which systems survive to form gravitational-wave sources and will help interpret the growing population of LIGO/Virgo/Kagra detections. Binaries that fail to remain stable, however, are no less remarkable. Such black hole-star mergers could be the explanation for luminous fast blue optical transients, linking these rare and powerful explosions to the violent end states of binary evolution.

Black holes are invisible by nature, but some of them reveal their presence by orbiting a luminous companion star. Over the last few years, astronomers have discovered several black holes in binaries with a massive stellar companion – at least ten times heavier than the Sun – by carefully tracking the motion of the visible star. These systems are likely just the tip of the iceberg: population studies suggest that hundreds more may be hidden in our Milky Way.

Massive stars do not stay compact forever. Within a few million years, the stars we see today will expand by factors of tens to a hundred, until the black hole’s gravity pulled their outer layers away. This process, known as mass transfer, lights up the system as an X-ray binary, with hot gas spiralling into the black hole via an accretion disk. Crucially, this mass exchange does not only transform the star itself, but also reshapes the entire binary: depending on how mass and angular momentum are redistributed, the orbit can widen or tighten dramatically, in some cases by orders of magnitude.

A long-standing mystery is whether this interaction remains stable or ends catastrophically. In some cases, the black hole may accrete matter peacefully for millions of years, gradually stripping away the hydrogen envelope of its companion and revealing the helium core beneath. In others, the binary becomes dynamically unstable and the black hole plunges deep into the star, destroying it from the inside. In a recent study, a research team led by an MPA fellow used detailed computer simulations with the state-of-the-art stellar evolution code MESA to show that, despite the complex gas dynamics in systems with black hole accretors, the outcome is governed by a surprisingly simple rule: how close the binary orbit becomes.

The team found that stable mass transfer has a hard limit. If the orbit tightens below about ten solar radii – roughly one-twentieth of the Earth-Sun distance – the massive star reacts by rapidly expanding. The black hole then plunges into its stellar companion, spirals through it, and ultimately merges with the helium core, destroying the star and thus the binary. This separation limit is not set by the uncertain details of how mass is exchanged, but by how massive stars respond to mass loss when forced into very tight orbits. Different stars have different “comfort zones”: some trigger instability at slightly wider separations than others do. In every case, however, the threshold can be traced back to the star’s internal structure, in particular to deep layers near the core that are normally hidden from our view.

This orbital size limit has important consequences for gravitational-wave astronomy. Compact orbits are required to form pairs of black holes or neutron stars that later spiral together and merge, producing detectable gravitational waves. The newly identified separation threshold therefore shapes which binaries can become gravitational-wave sources and which cannot, helping to clarify the origins of the growing population of mergers observed across the Universe.

But systems that cross the stability threshold may give rise to something even more dramatic. In a follow-up study, researchers from MPA and Columbia University propose that these “failed” gravitational-wave sources power one of the most mysterious explosions in the Universe: luminous fast blue optical transients, or LFBOTs.

LFBOTs are among the most extreme stellar explosions known. They can shine as brightly as the most luminous supernovae (up to a hundred times brighter than typical stellar explosions) while rising and fading on timescales of just a few days. They launch powerful outflows at tens of percent of the speed of light and emit X-rays that can persist for years after the initial flash. Radio observations add another puzzling clue: these explosions occur inside an enormous cloud of dense gas, extending to distances nearly a hundred times larger than Pluto’s orbit. Such extreme environments have posed a major challenge for models attempting to explain LFBOTs. These events are also exceedingly rare, occurring roughly a thousand times less frequently than ordinary supernovae. Illustration of a black hole absorbing a stellar core, causing radiation in radio, IR, optical/UV, and X-rays.

The new model naturally brings all these pieces together. When a black hole plunges into the star following a dynamical instability, it spirals into the compact helium core, tidally rips it apart, and accretes a fraction of a solar mass in just a few hours. This rapid accretion releases an enormous amount of energy and drives powerful, asymmetric outflows that propagate through what remains of the star, producing the observed brightness, colors, and rapid evolution of LFBOTs..

Crucially, such a merger does not happen overnight. The study shows that before the orbit tightens below the critical separation and a delayed dynamical instability is triggered, the black hole will strip mass from its companion for thousands of years in a long-lived, stable phase. Only a small fraction of this material is accreted; most of it is expelled into space, naturally building the vast and dense circumstellar medium inferred from radio observations. When the final explosion occurs, it does so inside this cocoon – explaining one of the most puzzling features of LFBOTs..

Taken ; together, the new studies led at MPA draw a direct line from the quiet lives of black hole binaries to both gravitational-wave sources and some of the most powerful stellar explosions known. Get too close to a black hole, it seems, and the result is fireworks.

Source:  Max Planck Institute for Astrophysics/Research Highliughts

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Author:

Dr. Jakub Klencki
Postdoc
2282
jklencki@mpa-garching.mpg.de

Original publication

1. Klencki, Jakub; Podsiadlowski, Philipp; Langer, Norbert; Olejak, Aleksandra; Justham, Stephen; Vigna-Gómez, Alejandro; de Mink, Selma E.
A fundamental limit to how close binary systems can get via stable mass transfer shapes the properties of binary black hole mergers Accepted by A&A

Source

2. Klencki, Jakub; Metzger, Brian D.
Luminous Fast Blue Optical Transients as "Failed" Gravitational Wave Sources: Helium Core− Black Hole Mergers Following Delayed Dynamical Instability

Submitted to ApJ

Source

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