Galaxy formation in separate universes

Summary sketch of the Separate Universe Formalism: structure formation in special regions of our Universe is equivalent to structure formation in appropriately modified/separate Universes. The blue line is a mass perturbation: regions above the dashed line have more mass than the average. © MPA, Background image: Tess Cholson, Hi-res image

Rather than trying to study special regions in large-volume simulations, scientists at MPA have used the IllustrisTNG model to create whole separate universes with a modified cosmology. Their study of these separate universes shows that when the baryon density (the density of ordinary matter) changes, the number of galaxies can increase or decrease depending on how this number is measured. Also, the large-scale distribution of matter is affected by the effects of baryons, with various measures reacting differently.

Imagine we are travelling across the Universe and want to measure some property such as the number of galaxies around us. This number is not going to be the same everywhere during our journey because various regions of the Universe are not equal. For example, in some regions, there was a slight excess of mass and energy at the beginning of the Universe, the Big Bang, which means there was more material to form galaxies, and so we would count more galaxies there. Astrophysicists need to take this variability into account when analysing observational data. In particular, it could be the case that the observed part of the Universe is special and not representative of the whole Universe. Such an analysis can be performed with the aid of so-called Response Functions, which tell us how a given statistical measurement of the Universe changes when the properties of the underlying region change.

Researchers at MPA have been interested in studying response functions and their applications for some time now. This can be done with the “Separate Universe Formalism”, which establishes that structures forming in our Universe in a special (e.g. over- or under-dense) region are the same as the structures that would form in a normal region of a different/separate Universe (see Fig. 1). Studying responses is easier in this formalism, because numerical simulations can easily be used to study structure formation in other Universes -- this is much easier than to simulate structure formation in special regions of our Universe. In the past, numerical studies of response functions were done with simulations that took into account only the effect of gravity. A team of researchers at MPA has recently gone beyond this limitation by running separate universe simulations with the IllustrisTNG galaxy formation model, which, for the first time, allowed them to study response functions including also important baryonic effects such as hydro- dynamical forces, gas cooling, star and black hole formation.

Galaxy formation with an excess of baryons

Sketch of a compensated isocurvature perturbation (CIP): the total matter stays the same, but in some regions, more baryons are compensated by less dark matter. © MPA

Matter in the Universe can broadly be divided into two types: (i) dark matter, which does not interact with light and comprises the majority of the mass (80 %), and (ii) all the rest. This rest is made up of the particles detected in particle physics experiments, which are called baryons. While dark matter is the dominant source of gravitational energy that drives structure formation, stars and galaxies are made up of baryons. Therefore the number of galaxies should depend on the amount of baryons available inside some observed region. In other words, the number of galaxies responds to the baryonic density.

A few theoretical models of the very early Universe (also known as the period of Inflation) predict that there should be regions in the Universe with an excess of baryons that is exactly compensated by a suppression in the number of dark matter particles; these are called compensated isocurvature perturbations (CIP), see Fig. 2. Researchers at MPA have studied how the number of galaxies responds to these perturbations using the separate universe formalism by simulating galaxy formation in Universes with different total amounts of baryons and dark matter.

The results of this study showed that, indeed, the number of galaxies depends strongly on the amount of baryonic matter. More interestingly, however, the sign of this dependency depends also on the quantity used to classify the galaxies. If the number of galaxies is measured as a function of total mass (dark matter + baryons), the response to CIP perturbations is negative, i.e., there are fewer galaxies with a given total mass. However, if the number of galaxies is measured as a function of the mass in stars (not the total mass), the response now displays the opposite trend, i.e. there are more galaxies with a given stellar mass. The MPA researchers traced back the origin of this change of sign to the modifications that the CIP perturbations induce on the relation between total mass and stellar mass in the galaxies.

This study provided the first ever prediction of the impact of CIP perturbations on the observed number of galaxies, which can now be incorporated in theoretical models of the distribution of galaxies in the Universe. This in turn will allow astronomers to use the statistics of galaxies to look for important signatures from the early Universe.

Using responses to predict weak gravitational lensing

The top image shows a simulated weak-lensing map of the sky, as well as 2-, 3- and 4-point functions drawn on top. The lower panel shows the percentage impact of the baryonic effects on the N-point functions. The quantity on the x-axis is inversely proportional to distance: large scales on the left, smaller scales on the right. The vertical dashed lines correspond to those scales, where the effect of baryons exceed 1%. All N-point functions are affected by baryonic effects on small scales, but they all respond differently. © MPA

The separate universe formalism can also be applied to large-scale maps of the total matter distribution. The light emitted by distant galaxies travels towards Earth along trajectories that are perturbed by the gravitational effect of the intervening matter. This effect, known as weak gravitational lensing, distorts the observed images of the distant galaxies and can be used to construct sky maps of the total mass between Earth and the galaxies, see Fig. 3. These maps contain information about the physics of our Universe and a popular way to organize this information is in N-point correlation functions: how does the matter density correlate between N points.

For quite some time, cosmologists have considered only the effect of gravity to obtain theoretical predictions for these statistics, but recently the community became aware of the critical importance of baryonic effects. For example, the heating and ejection of gas by black holes at the centre of massive galaxies can significantly alter the total distribution of the mass that weak lensing observations are sensitive to. The impact of baryonic effects on higher-order functions has remained largely unexplored, but researchers at MPA have recently made progress on this front. Specifically, separate universe simulations of galaxy formation were used to measure the impact of baryonic effects on the response of the 2-point function, which was in turn used in theoretical models to predict the impact of baryons on 3- and 4-point functions.

The fractional impact of the baryonic effects on 2-, 3- and 4-point correlation functions is shown in Fig. 3.All statistics display a suppression of their amplitude of approximately 5%-20% on the smallest scales (right part). This is as expected from the impact of black hole activity, which makes the density field smoother and the correlation of perturbations weaker. A key aspect revealed by this study was the fact that, quantitatively, the various N-point functions are affected differently by the same black hole activity. This work by the MPA researchers opens up a new window to study important effects on galaxy formation (like black hole activity) using combined analysis of different weak-lensing N-point functions.

Source: Max Planck Institute for Astrophysics

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Author

Alexandre Barreira

Postdoc

Tel.: 2241

barreira@mpa-garching.mpg.de

for the team: Alexandre Barreira, Giovanni Cabass, Dylan Nelson, Rüdiger Pakmor, Fabian Schmidt and Volker Springel

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Original publications

1. A. Barreira, G. Cabass, D. Nelson, F. Schmidt

Baryon-CDM isocurvature galaxy bias with IllustrisTNG

Submitted to JCAP

Source

2. A. Barreira, D. Nelson, A. Pillepich, V. Springel, F. Schmidt, R. Pakmor, L. Hernquist, M. Vogelsberger Separate Universe Simulations with IllustrisTNG: baryonic effects on power spectrum responses and higher-order statistics MNRAS, Volume 488, Issue 2, September 2019, Pages 2079–2092,  

Source - DOI

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Unpredicted Stellar Black Hole Discovered

Figure LB-1: Accretion of gas onto a stellar black hole from its blue companion star, through a truncated accretion disk (artist impression). Credit: YuJjingchuan, Beijing Planetarium, 2019.

Maunakea, Hawaii – Our Milky Way Galaxy is estimated to contain 100 million stellar black holes – cosmic bodies formed by the collapse of massive stars and so dense even light can’t escape.

Until now, scientists had estimated the mass of an individual stellar black hole in our Galaxy at no more than 20 times that of the Sun. But the discovery of a huge black hole by a Chinese-led team of international scientists has toppled that assumption.

The team, headed by Prof. LIU Jifeng of the National Astronomical Observatory of China of the Chinese Academy of Sciences (NAOC), spotted a stellar black hole with a mass 70 times greater than the Sun.

The monster black hole is located 15 thousand light-years from Earth and has been named LB-1 by the researchers. The discovery is reported in today’s issue of Nature.

The discovery came as a big surprise.

“Black holes of such mass should not even exist in our Galaxy, according to most of the current models of stellar evolution,” said Prof. LIU. “We thought that very massive stars with the chemical composition typical of our Galaxy must shed most of their gas in powerful stellar winds, as they approach the end of their life. Therefore, they should not leave behind such a massive remnant. LB-1 is twice as massive as what we thought possible. Now theorists will have to take up the challenge of explaining its formation.”

Until just a few years ago, stellar black holes could only be discovered when they gobbled up gas from a companion star. This process creates powerful X-ray emissions, detectable from Earth, that reveal the presence of the collapsed object.

The vast majority of stellar black holes in our Galaxy are not engaged in a cosmic banquet, though, and thus don’t emit revealing X-rays. As a result, only about two dozen Galactic stellar black holes have been well identified and measured.

To counter this limitation, Prof. LIU and collaborators surveyed the sky with China’s Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST), looking for stars that orbit an invisible object, pulled by its gravity.

This observational technique was first proposed by the visionary English scientist John Michell in 1783, but it has only become feasible with recent technological improvements in telescopes and detectors.

Still, such a search is like looking for the proverbial needle in a haystack: only one star in a thousand may be circling a black hole.

After the initial discovery, the world’s largest optical telescopes – Spain’s 10.4-m Gran Telescopio Canarias and W. M. Keck Observatory’s 10-m Keck I telescope on Maunakea, Hawaii – were used to determine the system’s physical parameters. The results were nothing short of fantastic: a star eight times heavier than the Sun was seen orbiting a 70-solar-mass black hole, every 79 days.

The discovery of LB-1 fits nicely with another breakthrough in astrophysics. Recently, the Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo gravitational wave detectors have begun to catch ripples in spacetime caused by collisions of black holes in distant galaxies. Intriguingly, the black holes involved in such collisions are also much bigger than what was previously considered typical.

The direct sighting of LB-1 proves that this population of over-massive stellar black holes exists even in our own backyard. “This discovery forces us to re-examine our models of how stellar-mass black holes form,” said LIGO Director Prof. David Reitze from the University of Florida in the U.S.

“This remarkable result along with the LIGO-Virgo detections of binary black hole collisions during the past four years really points towards a renaissance in our understanding of black hole astrophysics,” said Reitze.

This work was made possible by LAMOST (Xinglong, China), the Gran Telescopio Canarias (Canary Islands, Spain), the W. M. Keck Observatory (Hawaii, United States), and the Chandra X-ray Observatory (United States). The research team comprised scientists from China, the United States, Spain, Australia, Italy, Poland and the Netherlands.

By: Chinese Academy of Sciences Headquarters

Source:  W. M. Keck Observatory/Science News

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Media Contact:

XU Ang, annxu@nao.cas.cn

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About  W. M. Keck Observatory

The W. M. Keck Observatory telescopes are among the most scientifically productive on Earth. The two, 10-meter optical/infrared telescopes on the summit of Maunakea on the Island of Hawaii feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectrometers, and world-leading laser guide star adaptive optics systems. 

Some of the data presented herein were obtained at Keck Observatory, which is a private 501(c) 3 non-profit organization operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.

The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the Native Hawaiian community.  We are most fortunate to have the opportunity to conduct observations from this mountain.

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