Three roads diverge in a galaxy’s future, and the chosen one is almost entirely random!

Title:The progenitor galaxies of stellar halos as “failed” Milky Ways

Authors: Sovnak Bose and Alis J. Deason

Institution of first author: Institute for Computational Cosmology, Department of Physics, Durham University, Durham DH1 3LE, UK

Status: available as arXiv preprint; submitted to Monthly Notices of the Royal Astronomical Society (MNRAS)

Much like halos around the heads of religious and mythological figures, many galaxies also contain a large halo of stars surrounding their central regions – aptly called the stellar halo! They often extend well beyond the brightest and densest regions of a galaxy; Galaxies like our own Milky Way have those that extend to hundreds of kiloparsecs (for comparison, the radius of our galaxy’s disk is about 15 kiloparsecs). According to our current models of galaxy formation, massive galaxies form from the merger of lower-mass galaxies, and stellar halos are the remnants of destroyed lower-mass galaxies that have merged with the more massive one, with stars being ejected from the merging systems into giants. random orbits – creating a diffuse “halo” structure.

One galaxy, multiple futures

Observations have shown that the dominant merger events in the past that led to the formation of our own galaxy’s stellar halo involved galaxies such as the Large Magellanic Cloud (LMC) (with stellar masses of about one billion solar masses, as opposed to the Milky Way’s about 10 solar masses). billion solar masses) fall into the proto-Milky Way, the most significant of which is the Gaia-Enceladus sausage. Because previous studies have shown that LMC-mass galaxies are the primary building blocks of our galaxy’s stellar halo, the authors of today’s paper use simulations to predict the evolution of LMC-mass galaxies from redshift z=2 (approx 10 billion years) to investigate today. In particular, they conduct a careful analysis of three possible fates of these galaxies:

(i) those that are destroyed and end up in the stellar halos of Milky Way mass galaxies

(ii) those that are the progenitors of galaxies like the Milky Way today, and

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(iii) those that survive to this day and end up forming galaxies like the LMC

Specifically, they ask: Are there identifiable properties of LMC-mass (~10^9 Msun) galaxies at z=2 that determine which of the three evolutionary paths they will follow? Their study is also motivated by the related question: Could multi-wavelength observations of such galaxies at z=2 allow us to distinguish which ones will eventually be destroyed and end up in stellar halos, which will form Milky Way-like galaxies, and which will form LMC-like galaxies?

I SAM what I SAM

To examine a statistical sample of galaxies and their evolutionary paths, the authors turn to the theoretical astrophysicist’s favorite tool: simulations! Cosmological simulations rely on the use of supercomputers to model the growth of large-scale structures in the universe and statistical properties of large samples of galaxies (check out some fun videos of the EAGLE simulations here as an example!). In particular, they only rely on dark matter Copernicus Complex Low resolution or COLOR simulations (phew, talk about astronomers’ penchant for convoluted acronyms). These simulations allow them to create an accurate model of how the dark matter halos that inhabit galaxies merge and grow over time, dictating the merger and evolution of the galaxies themselves. Note that dark matter halos are entirely different from stellar halos, and the important idea to keep in mind is that most, if not all, galaxies live in gigantic clumps of dark matter. More information on dark halos can be found here and here.

But wait, there’s more! Since pure dark matter simulations would provide no information about stellar astrophysics Formation of stellar halosthe authors combine the simulations with a semi-analytical model (SAM) called GALFORM. SAMs are a technique used to model the stellar and gas physics of galaxies analytically (rather than numerically as in simulations) and are often calibrated against observations. SAMs are much less computationally intensive than full simulations, and since stellar and gas physics are much more complex than dark matter physics, they allow for much faster modeling than relying on full simulations. Since dark matter obeys only the laws of gravity and is computationally cheap to model, the authors rely on the combined tool of the dark-matter-only COLOR simulations with the semi-analytical GALFORM model to model a large sample of about 17,000 LMC-mass galaxies at z = 2 and their evolutionary future!

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Figure 1 shows the mass growth history of the galaxies in their sample compared to redshift (or cosmic time). In this figure, they show the masses of galaxies that survive to this day to form LMC analogues compared to the masses of galaxies that are the main progenitors of Milky Way-like systems today. The red lines show the stellar masses and the black lines the dark matter masses (shown as M200) of the galaxies, with time moving from right to left.

The plot shows that although the proto-Milky Ways and proto-LMCs had similar stellar and halo masses in early cosmic times (right panel), their evolutionary paths are so different that the galaxies that eventually form LMCs today have almost one masses an order of magnitude smaller than those that eventually form Milky Ways! You might ask what causes this difference? The answer is the third group of galaxies — those that get destroyed and end up falling into the stellar halos of the Milky Ways, which is why the Milky Ways end up having a higher total mass.

Graph showing logarithmic mass on the y-axis and logarithmic redshift on the x-axis, with redshift increasing from left to right.  The faint and dark red lines, and the faint and dark black lines, all increase from lower right to upper left, with the black lines being at least an order of magnitude greater than the red.
Figure 1: Mass growth history of the galaxies in their sample – divided into those surviving to date to form LMC-like systems (faint red and faint black lines) and those surviving to form Milky Way-like galaxies (dark red and dark black lines). The shaded areas show the scatter in each of the curves. Black lines show the growth in total dark matter mass for the two samples, while red lines show the growth in their stellar mass. Time flows from right to left along the x-axis, with the left side being the current day. (Source: Figure 2 in today’s newspaper.)

With galaxies as with real estate – it’s location, location, location!

The question remains: if all galaxies at z = 2 are similar to each other (all about 10^9 solar masses), what exactly determines their overall final state: formation of a Milky Way, formation of an LMC, or destruction in a Milky Way’s stellar halo? The authors examine numerous properties of the galaxies at z = 2 to see if any of them influence the result, and their results may be surprising: the only property that seems to matter is the location of the galaxy! That is, if the galaxy is near a slightly more massive galaxy, it will be destroyed by that more massive neighbor and end up in its stellar halo. The more massive neighbor will end up forming a Milky Way-like system! However, if a galaxy is far from more massive ones, it will survive to this day and end up forming an LMC-like galaxy.

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Figure 2 shows just that. Each panel shows the probability distribution of various galaxy properties at z=2: bulge size at upper left, disk size at upper right, chemical composition at lower left, and distance from a more massive neighbor at lower right. In each panel, the galaxies are divided into the three samples: those that survive to this day as LMCs (“surviving”), those that form Milky Ways today (“MW progenitors”), and those that are destroyed and end up in stellar ones Halos from Milky Ways (“Destroyed”). The last panel shows no progenitors of the Milky Way, as these are the reference galaxies from which the distances of the other two samples are calculated. As the top two and bottom left figures show, their bulge sizes, disk sizes, and chemical compositions of the “surviving” and “destroyed” samples are nearly identical. The final panel shows that the ‘destroyed’ population is on average much closer to the progenitors of the Milky Way than the ‘surviving’ population, and this appears to be the only important parameter determining the overall fate of a galaxy at z=2.

4-panel plot showing probability distributions for various parameters in their sample of galaxies.
Figure 2: Probability distributions of various properties for the sample of galaxies at z = 2. The upper left panel shows the bulge size, the upper right the disk size, the lower left the chemical composition and the lower right the distance from a more massive neighbor. The blue curves show the distributions of galaxies that are destroyed in the stellar halos of Milky Way-like systems, the black curve shows those that survive to this day and form LMCs, and the red curve shows those that eventually form Milky Way-like galaxies. (Source: Figure 5 in today’s newspaper.)

The authors conclude by suggesting that in an observational survey of such galaxies at z=2, we would likely find examples of all three samples in the same regions, with their separations telling us which would survive in the future and which would be destroyed and feed the stellar halos of the others.

Astrobit edited by Maryum Sayeed

Credit: Getty Images/iStock

About Pratik Gandhi

I am a 3rd year Astrophysics PhD student at UC Davis, originally from Mumbai, India. I study the formation and evolution of galaxies and am really excited about the use of simulations and observations in the study of galaxies. I am interested in science communication, teaching and social issues in science. I’m also a huge fan of Star Trek, with Deep Space Nine and The Next Generation being my favorites!

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