Signs of the stellar lifecycle Signs of the stellar lifecycle

NGC 1637

A spiral galaxy filling the view. Its disc is filled with bright red spots where stars are forming, dark reddish threads of dust that obscure light, and bluish glowing areas where older stars are concentrated. It has a large, glowing yellow oval area at the centre, from which two spiral arms wind through the galaxy’s disc. The bottom side of the disc is rounded while the top side is somewhat squared-off. Credit: ESA/Hubble & NASA, D. Thilker

The subject of this NASA/ESA Hubble Space Telescope Picture of the Week is NGC 1637, a spiral galaxy located 38 million light-years from Earth in the constellation Eridanus.

This image comes from an observing programme dedicated to studying star formation in nearby galaxies. Stars form in cold, dusty gas clouds that collapse under their own gravity. As young stars grow, they heat their nurseries through starlight, winds, and powerful outflows. Together, these factors play a role in controlling the rate at which future generations of stars form.

Evidence of star formation is scattered all around NGC 1637, if you know where to look. The galaxy’s spiral arms are dotted with what appear to be pink clouds, many of which are accompanied by bright blue stars. The pinkish colour comes from hydrogen atoms that have been excited by ultraviolet light from young, massive stars. This contrasts with the warm yellow glow of the galaxy’s centre, which is home to a densely packed collection of older, redder stars.

The stars that set their birthplaces aglow are comparatively short-lived, and many of these stars will explode as supernovae just a few million years after they’re born. In 1999, NGC 1637 played host to a supernova, pithily named SN 1999emsupe, that was lauded as the brightest supernova seen that year. When a massive star expires as a supernova, the explosion outshines its entire home galaxy for a short time. While a supernova marks the end of a star’s life, it can also jump start the formation of new stars by compressing nearby clouds of gas, beginning the stellar lifecycle anew.

Source: ESA/Hubble/potw.

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Field-Level Inference: Unlocking the Full Potential of Galaxy Maps to Explore New Physics210

Fig 1: Summary statistics like the two- and three-point correlation functions compress the galaxy field into spatial correlations between pairs and triplets of galaxies (left panel). Field-level statistics bypass the compression step to access the entire information in the galaxy field. © MPA

Fig 2: The comparison between FLI and 2+3-point inference adopts the same forward model, LEFTfield, for both inference schemes. The key difference is FLI analyzes the entire galaxy field while 2+3-point inference analyzes only the 2+3-point summaries of the (same) galaxy field. © MPA

Fig. 3: Constraints on the amplitude of growth of structure σ8 are improved by up to a factor of 5 when analyzing the whole galaxy field compared to just the 2- and 3-point correlation functions. © MPA

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Galaxies are not islands in the cosmos. While globally the universe expands – driven by the mysterious ‘dark energy’ – locally, galaxies cluster through gravitational interactions, forming the cosmic web held together by dark matter’s gravity. For cosmologists, galaxies are test particles to study gravity, dark matter and dark energy. For the first time, MPA researchers and alumni have now used a novel method that fully exploits all information in galaxy maps and applied it to simulated but realistic datasets. Their study demonstrates that this new method will provide a much more stringent test of the cosmological standard model, and has the potential to shed new light on gravity and the dark universe.

From tiny fluctuations in the primordial Universe, the vast cosmic web emerged: galaxies and galaxy clusters form at the peaks of (over)dense regions, connected by cosmic filaments with empty voids in between. Today, millions of galaxies sit across the cosmic web. Large galaxy surveys map those galaxies to trace the underlying spatial matter distribution and track their growth or temporal evolution.

Observing and analyzing millions of galaxies turns out to be a daunting task. Hence, standard analyses first compress the three-dimensional galaxy distribution into measurements of the spatial correlation between pairs and triplets of galaxies, technically known as the two- and three-point correlation functions (Figure 1).

These restricted statistics, however, potentially leave out a lot of information in galaxy maps, especially information encoded on smaller spatial scales. In addition, they do not tell us where in the maps to look further, should some surprising result turn up in these statistics. How much more information can be extracted? A recent study in Physical Review Letters by MPA researchers and alumni, led by Dr. Minh Nguyen, provides compelling evidence for significant information beyond the reach of two- and three-point functions.

For the study, the team have developed and validated a rigorous probabilistic framework, LEFTfield, to model the clustering of galaxies. How the LEFTfield framework leverages the Effective Field Theory of Large-Scale Structure (EFTofLSS) to produce robust and accurate predictions of the observed galaxy field with high efficiency was the topic of another MPA research highlight. LEFTfield foward-models the evolution of primordial fluctuations into large-scale structure and galaxy clustering, preserving the entire information in the three-dimensional distribution of galaxies. Further, the LEFTfield forward model is differentiable, allowing for field-level inference (FLI) of both parameters in the cosmological model and the primordial fluctuations from which all structure in the Universe emerged.

In the study, the team set up an apples-to-apples comparison between FLI and the standard two-point plus three-point (‘2+3-pt’) inference. Both inference pipelines adopt the same LEFTfield forward model, and use the observed maps on strictly the same scales, as illustrated by Figure 2.

Analyzing the same catalogs of dark-matter halos from the same set of N-body simulations, the team found that FLI improves constraints on the amplitude of structure growth by a factor of 3-5, even with conservative scale cuts in both analyses. The improvement implies that even without agressively pushing down to very small scales – where we expect EFTofLSS or even N-body simulations to fail – much more information can still be extracted from galaxy clustering simply by opening up another dimension: getting rid of the compression of the input data.

Figure 3 compares the constraints on the amplitude of structure growth from the FLI and ‘2+3-pt’ analyses. The parameter σ8 quantifies the typical amplitude of structure in the initial (“linear”) density field on a certain scale. Essentially, galaxy clustering constraints on σ8 probe the growth of structure from the early universe (where we have precise measurements thanks to the cosmic microwave background) to late times. For this reason, this is a parameter that is generally modified in non-standard cosmological models, for example if gravity is not correctly described by General Relativity, or if dark matter is not cold.

A factor of 5 improvement in parameter constraints effectively ‘increases’ the survey volume by more than an order of magnitude, which is a huge improvement given the time-consuming and expensive process of mapping out the galaxy distribution over a large volume. Moreover, FLI in principle guarantees optimal extraction of cosmological information: there is no data compression, hence no information loss.

While this study used dark matter halos in simulations, the conclusions also hold for significantly more realistic simulated galaxies, which were the subject of a parallel study by the Beyond-2pt Collaboration that includes two researchers from the MPA team, the FLI approach based on the LEFTfield framework again returns unbiased and improved constraint on growth of structure.

Beyond improved parameter constraints, FLI also offers numerous ways to find out where evidence for physics beyond the standard model of cosmology might come from, should such evidence appear. Since we have samples of Universes that are compatible with the data, we can look for those regions most strongly deviant from the standard model, and investigate what is unusual about them. We can also employ independent datasets, for examply by correlating the inferred matter density with gravitational lensing maps, which are an entirely different probe of structure.

The team now set their eyes on applying the novel FLI approach and LEFTfield framework to real data from galaxy surveys. To connect FLI to observations, a better understanding, hence more studies, of how observational systematics impact the model predictions at the field level will be required. A flexible-yet-efficient foward-modelling framework like LEFTfield will be the key for such studies, and for unlocking the full potential of FLI from galaxy maps.

Source: Galaxy Clusters,Max Planck Institute for Astrophysics/News

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Authors:

Fabian Schmidt
Scientific Staff
Member of the works council, Representative of the Scientific Coworkers
tel: 2274
fschmidt@mpa-garching.mpg.de

Beatriz Tucci Schiewaldt, Beatriz
PhD student
tel: 2358
tucci@mpa-garching.mpg.de

Dr. Minh Nguyenh
Personal Homepage
Kavli IPMU, University of Tokyo

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Original publications

1. Nguyen, Nhat-Minh ; Schmidt, Fabian ; Tucci, Beatriz ; Reinecke, Martin ; Kostić, Andrija
How much information can be extracted from galaxy clustering at the field level?
Phys. Rev. Lett. 133, 221006

Source | DOI

2. E. Krause et al (Beyond-2pt Collaboration)
A Parameter-Masked Mock Data Challenge for Beyond-Two-Point Galaxy Clustering Statistics
submitted

Source

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An improved quantification of the intergalactic medium and cosmic filaments

Smoothed 0.3–1.2 keV eROSITA count rate map of the analysis footprint. The missing pixels show source-masked regions. Credit: Astronomy & Astrophysics (2024). DOI: 10.1051/0004-6361/202450933

Much of the mass in the universe lies not in stars or galaxies, but in the space between them, known as the intergalactic medium. It is warm and even hot, and is called the "warm-hot intergalactic medium," or WHIM. It holds about 50% of the normal mass (viz. baryonic, not including dark matter) of the universe but with a density of hydrogen ions less than 100 per cubic meter.

At temperatures between 100,000 and 10 million Kelvin, it is a web of "cosmic filaments" that are regions of hot, diffuse gas stretching between galaxies. These cosmic filaments, also called "galactic filaments," are the largest structures known in the universe, commonly 150 to 250 megaparsecs long (500 to 800 million light-years), the latter 8,000 times the width of the Milky Way galaxy.

Together they form the cosmic web, and they form the boundaries between cosmic voids, enormous regions of empty space containing almost no galaxies.

"The properties of the warm-hot intergalactic medium in cosmic filaments are among the least quantified units in modern astrophysics," writes a team of scientists from Europe, mostly Germany.

Using an instrument on a satellite that started surveying the universe in late 2019, they examined the X-ray emissions from almost 8,000 cosmic filaments and used a model to determine the temperature and baryon density contrast of the detected WHIM. Their work was published in the journal Astronomy & Astrophysics.

Cosmic filaments span almost the entire universe. Between them are voids with atom densities around one per cubic meter. (That is an extremely intense vacuum—by comparison, the density in interstellar space inside our own galaxy is a million to a trillion atoms per cubic meter, and the best vacuums that can be created on Earth is on the order of 1016 atoms per cubic meter.)

The void closest to us is the "Local Void." The cosmic filaments connect galaxies in a vast web; they are mostly full of gas, dust, stars, and a lot of dark matter. They are very hot, in a plasma state, but not as hot or as dense as the sun, consisting of ionized hydrogen atoms (a proton), and are detected by the absorption of light given off by quasars.

To study these structures, the group used data from eROSITA, an X-ray instrument that was part of the Russian-German Spectrum Roentgen Gamma space observatory. (Launched in July 2019, eROSITA was to image the entire sky for seven years, but the instrument stopped collecting data in February 2022, two days after Russia invaded Ukraine and institutional relations broke down.)

Distribution of the selected filaments in the redshift and physical length space. Credit: Astronomy & Astrophysics (2024); DOI: 10.1051/0004-6361/202450933

"Stacked" scans—the same images taken multiple times, a common way to deal with weak single scan intensities—were collected between December 12 to 19, 2021 in the X-ray spectrum of about 1 kilo-electronvolt (wavelengths of about 1 nm), utilizing four stacks. They then used a catalog of optical filaments, compiled in 2011 from the Sloan Digital Sky Survey, which contains over 63,000 filaments.

Assuming the standard cosmological parameters for the canonical ΛCDM model—the Hubble constant, the matter density, the baryon density and dark matter energy density, they calculated the physical length of the filaments.

Lengthy data analysis followed. First, they obtained the surface brightness profile of all filaments at discrete distances along each, carefully accounting for a host of effects such as projection effects, overlapped filaments and subtracting out the local background near each filament.

Next, they estimated the fraction of each signal due to unmasked galactic sources such as X-ray detected point sources, galaxy clusters and groups and other complicating factors. Finally, detailed astrophysical models (some from established libraries), corrections for instrument bias and statistical reasoning gave the best-fit temperature and density profiles of the gas in the weak hot intergalactic medium (WHIM).

Their best-fit temperature was 106.84 Kelvin, which is about 7 million K. For the baryon density contrast—the difference between the density of baryons and the average density of baryons—they found 101.88, which is 76. The density of ordinary matter, which is mostly baryons, in the WHIM was 76 times greater than the background baryon density of space.

Their average density contrast agrees with numerical simulations, but the relatively simple temperature they calculated was near the upper boundary of the X-ray emitting WHIM. This was not unexpected, they write, as the simple temperature was expected to be "biased to the high end of the temperature distribution when fitting a spectrum with a multi-temperature nature."

Understanding the X-ray emitting cosmic filaments and WHIM through studies such as this is expected to significantly improve in the coming decade, as improved filament finders are completed and a better understanding develops of the X-ray properties of galaxy groups, active galaxy nuclei and fast radio bursts allows better subtraction from the total WHIM signal.

X-ray missions such as the Hot Universe Baryon Surveyor and Line Emission Mapper "will be able to explore a wider parameter space of the WHIM properties," throwing more metaphorical light on the mysterious intergalactic medium.

by David Appell , Phys.org

Source: Phys.org/Astronomy and Space

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More information: X. Zhang et al, The SRG/eROSITA all-sky survey, Astronomy & Astrophysics (2024). DOI: 10.1051/0004-6361/202450933

Journal information: Astronomy & Astrophysics

© 2024 Science X Network

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Explore further

• A technique for estimating the mass of gigantic filaments of dark matter

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Clusters

The images above show the galaxy cluster Abell 2034 in the optical, X-ray and radio. These show that the cluster contains 328 individual galaxies (including two massive brightest cluster galaxies) a disturbed ICM and several distinct sites of particle acceleration.

As the Universe evolves, gravity brings together hundreds, sometimes thousands of galaxies together to form galaxy clusters. The galaxies within these clusters usually account for about 1% of the total mass. They are encompassed by a hot low density gas (a million to 10 million Kelvin) known as the intra-cluster medium (ICM) which contains about 9% of the cluster mass. The other, approximately 90% of the mass is in the surrounding dark matter halo.

Observations at different frequencies

Observations at different frequencies help us to form a comprehensive picture of the structure and evolution of galaxy clusters. Optical telescopes, for example, can detect individual galaxies allowing us to determine the dynamics of the galaxies and infer the distribution of dark matter. X-rays observatories are used to measure thermal emission from the ICM. Radio telescopes offer a completely different view. They detect non-thermal emission, which reveals the cluster’s magnetic field and the sites of extreme particle acceleration in the ICM.

ASTRON interests

Our group at ASTRON is interested in studying the particle acceleration processes and magnetic fields within the tenuous ICM. We wish to understand the formation of radio halos which are characterised by cluster-wide radio emission and are thought to be caused by turbulence throughout the cluster. We also aim to understand the conditions that lead to the formation of radio relics. These objects are characterised by their peripheral location and are thought to be generated by large shock waves. Finally, we are studying other unusual structures showing intense particle acceleration in the ICM and the interaction between the ICM and discrete radio sources such as tailed radio galaxies.

Research staff: Tim Shimwell

Source: ASTRON-Netherlands Institute for Radio Astronomy/Astronomy

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