Galaxy Evolution
NASA, ESA, CSA, STScl, Judy Schmidt
In the early Universe, dark matter collected itself into structures called halos, guiding ordinary matter with their gravitational pull. Ordinary matter accumulates, forming galaxies embedded inside of dark matter halos. Massive dark matter halos gathers more ordinary matter than less massive ones, such that more massive galaxies tend to form in larger halos. More mass fundamentally changes the physics occurring in galaxies, playing a role in the panoply of galaxies we see today. Studying the link between galaxies and the dark matter halos that host them is the first step to understanding the physics responsible for shaping galactic evolution. Furthermore, the link also aides our ability to infer the distribution of matter, as galaxies trace the dark matter halos.
The video of a galaxy cluster from the
Millenium Simulation
is shown above as an example of how dark matter halos shape
galaxy evolution.
The bright ovals in the left panel are dark matter halos,
which often host galaxies. The galaxies are visible as the
blue/white spots in the top right panel. Note how the temperature
of these galaxies, shown in the bottom right panel, is strongly
dependent on the size of the dark matter halo the galaxy is hosted
in. The large halo at the center of the video contains very hot gas,
whereas the smaller halos possess much cooler gas comparatively.
The blue cold spots are even maintained while entering the main
central halo.
This is just one example of how dark matter halos alter the physics
taking place in galaxies.
(Credit: Volker Springel, Max-Planck-Institute for Astrophysics)
Star formation is a cornerstone in the grand narrative
of galaxy evolution.
Cold, massive gas clumps serve as the fuel for star formation,
and as these galaxies mature they exhaust their gas reservoirs.
In the video on the left, you can see this gas as the blue clumps.
When these clumps are abundant, the star-formation rate is relatively
large (SFR in the top). As the clumps dissipate towards the end of the
video, the SFR correspondingly falls.
Whether or not a galaxy is actively forming stars significantly affects
its luminous properties. Massive stars tend to be blue, but also
have extremely short lives. If the galaxy is
not actively forming stars, it cannot maintain a substantial population of
blue stars, shifting the entire galaxy's color to redder colors. This process
occurs relatively quickly
such that galaxies largely belong to two groups. The first group
is young, actively star-forming, blue galaxies, and the second is mature, passive,
red galaxies. The link between the state of star-formation and the properties of the
galaxy as a whole underscores the intricate relationship between star formation
and galaxy evolution.
However, while astronomers have studied this dichotomy in
galaxy evolution in detail, the many complicated mechanisms that influence the
the star-formation and gas abundance in a galaxy remains an active field of study.
As described in the first section, dark matter halos play a pivotal role in shaping the physical processes that dictate a galaxy's evolution. Massive halos, in particular, host a plethora of mechanisms that serve to dissipate the very gas necessary for nurturing new stars.
This is exemplified in the video on the left. Galaxies initially form in relative isolation, their star formation proceeding smoothly. However, a massive merger event and subsequent chaotic mixing disperses the gas in this region, hampering the star-formation of the galaxies within them. Extrapolating from this example, we would then expect massive halos to typically host many red galaxies. The correlation between the star-formation state of galaxies and their host halos is just one step to understanding galaxy evolution.
Simulations provide us a way to test our models and precisely understand the complicated physics responsible for galaxy evolution. I spearheaded a project with partners from UW, UMD and the Flatiron Institute to take advantage of a state-of-the-art cosmological simulation called IllustrisTNG.
We studied the spatial connection between blue and red galaxies and an ingredient for
star-formation, neutral hydrogen gas, using their distributions to examine the spatial extent of the influence of environment (via the host halo mass) on the gas abundance and star-formation state of galaxies.
We found that galaxies often have their gas content and star-formation altered beyond the virial radius of even the largest halos. In other words, galaxies are influenced by their environment even outside of massive halos, which is surprising since intuitively one would expect galaxies to start losing their gas inside of the massive halos.
Another surprising result came from studying the time evolution of the clustering of galaxies and hydrogen. If you were to leave galaxies to their own devices, you would expect them to approach each other due to gravity, increasing their clustering. However, We found that all three of red galaxies, blue galaxies and hydrogen actually cluster less with time, which is an entirely unexpected and novel. As it turns out, the changing galaxy demographics and gas reduction in the most clustered regions in the simulation are enough to reverse the expected evolution; for further explanation, please see the paper Osinga+23. In upcoming work, we hope to understand the implications this has on efforts use hydrogen and galaxies to map the large-scale structure of the Universe.