There are three paths branching out into the galactic future, and the path chosen is almost completely random!

Title:Ancestor galaxies of stellar halos are ‘failed’ Milky Way ways

Authors: Sownak Bose and Alis J. Deason

First Author Foundation: Institute of Computational Cosmology, Department of Physics, Durham University, Durham DH1 3LE, UK

condition: Available as an arXiv primer; Submitted to the Monthly Notices of the Royal Astronomical Society (MNRAS)

Much like the halos of light around the heads of religious and mythical figures, many galaxies also have a large halo of stars surrounding their central regions – which is aptly named star aura! They often extend far beyond the brightest and densest regions of the galaxy; Galaxies like our Milky Way have galaxies that span hundreds of kiloparsecs (for comparison, the radius of our galaxy disc about 15 kilo leagues). Under our current models of galaxy formation, massive galaxies are formed by mergers Of those with less mass, stellar halos are remnants destroyed Low-mass galaxies that have merged into the most massive galaxies, with stars from merging systems thrown into giant, random orbits – creating a pervasive “halo” structure.

One Galaxy, Multiple Futures

Observations showed that the dominant past merger events that led to the formation of our galaxy’s stellar halo included galaxies such as Large Magellanic Cloud (LMC) (with stellar masses of about 1 billion solar masses, as opposed to the Milky Way at about 10 billion solar masses) located in the primary Milky Way, with more accurate Who are these being? Gaia-Enceladus sausage. Since previous studies showed that LMC galaxies are the building blocks of our galaxy’s stellar halo, the authors of today’s paper are using simulations to study the evolution of LMC galaxies from redshift z = 2 (nearly 10 billion years ago) to the present day. In particular, they carefully analyze three possible fates of those galaxies:

(1) that destroyed and ends in the stellar halos of the galaxies of the Milky Way

(ii) the predecessors of galaxies such as the Milky Way at present, and

(3) Those that survive to this day and end up forming galaxies like the LMC

In particular, they ask: Are there any identifiable properties of galaxies with an LMC mass (~10^9 Msun) at z = 2 that determine which of the three evolutionary paths they will follow? Their study also prompts the related question: Can multi-wavelength observations of such galaxies at z = 2 allow us to distinguish which galaxies will end in destruction and in stellar halos, which ones will form Milky Way-like galaxies, and which will form LMC-like galaxies?

I Sam what I believed in

To examine a statistical sample of galaxies and their evolutionary paths, the authors turn to the theoretical astrophysicist’s favorite tool: simulation! Cosmic simulations rely on the use of supercomputers to model the growth of a large-scale structure in the universe, and the statistical properties of large samples of galaxies (for example, check out some fun EAGLE simulation videos). over here!). In particular, they rely on dark matter only Copernicus Low Accuracy Complex or the color Simulations (phew, talk about astronomers’ penchant for complex acronyms). This simulation allows them to build an accurate model of how a file works Dark matter halos That galaxies inhabit, merge, and grow over time, thus dictating the merger and evolution of galaxies themselves. Note that dark matter halos are very different from stellar halos, and an important idea to remember is that most, if not all, galaxies live inside giant clumps of dark matter. For more information on dark circles, see previous astrocytes over here And the over here.

But wait there is more! Since only dark matter simulations will not provide any information about stellar astrophysics and The formation of star halosthe authors combine simulations with a Semi-Analytic Model (SAM) It’s called Galform. SAM systems are a technique used to analyze (rather than numerically, as in simulation) a model of stellar and gaseous physics of galaxies, often calibrated against observations. SAM systems are computationally less expensive than full simulations, and since the physics of stars and gases are more complex than that of dark matter, they allow for much faster modeling rather than relying on large-scale simulations. Because dark matter obeys only the laws of gravity and is computationally cheap, the authors rely on the combined tool of dark matter simulations only with the semi-analytic GALFORM model, to build a large sample of about 17,000 LMC-mass galaxies at z = 2 and their evolutionary future!

Figure 1 shows the mass growth history of the galaxies in their sample against redshift (or cosmic time). In this figure, the masses of galaxies that survive to this day to form their LMC analogues are shown, compared to the masses of galaxies that end up being the main progenitors of the Milky Way-like systems at present. Red lines show stellar masses, while black lines show dark matter masses (shown M200) of galaxies, over time from right to left.

The plot shows that although in early cosmic times (right side), the primordial Milky Way and proto-LMCs had similar stellar and halo masses, their evolutionary paths differed enough that nowadays the galaxies that end up forming LMCs have what Approximately blocks of an order of magnitude smaller than those that end up forming the Milky Way! You may ask, what causes this difference? The answer is the third group of galaxies – those that are destroyed and eventually fall into the stellar halos of the Milky Way, which is why the Milky Way galaxies end up with higher total masses.

Figure 1: Mass growth history of galaxies in their sample – broken down into galaxies remaining to this day to form LMC-like systems (faint red and faint black stripes), and those that survived to form Milky Way-like galaxies (dark red and dark black stripes). Shaded areas show scattering in both curves. The black lines show the overall growth dark matter mass For the two samples, while the red lines show the growth of their stellar mass. Time flows from right to left along the x axis, and the left side is present today. (Source: Figure 2 in today’s article).

For galaxies as with real estate – location, location, location!

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

Figure 2 shows exactly this. Advance each plate probability distribution of the different properties of a galaxy at z = 2: swelling size at top left, disc size at top right, chemical composition At the bottom left, the distance from a larger neighbor is at the bottom right. In each panel, the galaxies are divided into three samples: those that have survived to the present day as LMCs (“survivalists”), those that make up the Milky Way at present (“MW ancestors”), and those that have been destroyed and ended Command in the stellar halos of the Milky Way (“The Destroyer”). The final panel does not show the predecessors of the Milky Way because these are the reference galaxies from which the distances of the other two samples are calculated. As the upper and lower left panels show, the bulge sizes, disc sizes, and chemical compositions of the ‘survival’ and ‘destroyed’ samples are nearly identical. The final panel shows that the “destroyed” population is on average much closer to the ancestors of the Milky Way than the “surviving” population, and this appears to be the only significant criterion determining the overall fate of a galaxy at z = 2.

A 4-panel diagram showing the probability distributions of different parameters in their sample of galaxies.
Figure 2: Probability distributions of different properties of a sample of galaxies at z = 2. The upper left panel is shown swelling Size, top right is shown disc Size, bottom left appears chemical composition, and the bottom right shows the distance from a larger neighbor. The blue curves show the distributions of galaxies that end up being destroyed in the stellar halos of Milky Way-like systems, the black curve shows those that have survived to this day and form small and medium-sized galaxies, and the red curve shows those that eventually end up forming Milky Way-like galaxies. (Source: Figure 5 in today’s paper).

The authors ended by suggesting that if we performed an observational survey of such galaxies at z = 2, we might find examples of all three samples in the same regions, with their intervals giving us insight into which ones will survive in the future and which ones will be destroyed, feeding the stellar halos of others.

Edited by Maryam Said

Featured image credit: Getty Images / iStock

About Pratik Gandhi

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