Inconstant, Fine Structures

Title: Probing the merger history of red early-type galaxies with their faint stellar substructures
Authors: B. Mancillas, P.-A. Duc, F. Combes, F. Bournaud, E. Emsellem, M. Martig, L. Michel-Dansac
First author’s institution: Sorbonne Université, Observatoire de Paris, Paris, France
Status: Accepted to Astronomy & Astrophysics, open access on arXiv

Galaxies tend to interact with one another in a dramatic and destructive fashion. They exert enormous gravitational force over large scales and when they are drawn together, their shapes can be distorted to the point of being unrecognizable. Once galaxies are close enough that they can no longer stay separate, they end up merging together over millions or even billions of years.

Fine structural traces of mergers are clues to these encounters: tails of gas, streams of stars, and peculiar shell-like structures that appear out of place, all classified under an umbrella term of “stellar substructure” (see Figure 1 for an example). Is it possible to learn about a galaxy’s history of mergers by searching for these substructures in images?

The Tadpole Galaxy, so-named for a tidal tail extending outward many times its own diameter.
Figure 1: The Tadpole Galaxy, showing a fine structural trace in the form of a long tail. Tidal tails are one of the substructures studied in today’s paper. Image credit: NASA.

The bad news is that our short lifespans prevent us from observing mergers in “real-time”: the timescales involved are simply too great. The good news is that the authors of today’s paper have circumnavigated this obstacle by using simulations to do the “observing” instead! By examining the different kinds of substructures in simulations, they have determined how such features are created and how long they survive.

Recipe for simulating galaxy mergers

After running a sequence of simulations that begins with a cosmological N-body and ends with a zoomed-in hydrodynamical one (i.e. one complete with atomic matter), the authors select a galaxy comparable to the Milky Way that undergoes a major merger with another galaxy (a “satellite”) and eventually settles into an elliptical shape. The “major” identifier here indicates that the galaxy merged with a satellite whose mass was comparable to its own, with a mass ratio of M(satellite)/M(galaxy) of greater than 1/3 (in this case, of 1/1.5). For reference, mergers where there is a larger discrepancy in mass are called intermediate (mass ratio between roughly 1/3 and 1/7) or minor (mass ratio less than 1/7).

To construct a chronological record of the galaxy’s evolution with time, “snapshots” are taken spaced over about 11 billion years, showing its growth from a small, trifling blob only slightly larger than the Large Magellanic Cloud to a Milky Way-sized elliptical galaxy. Using these snapshots, the authors hunt for stellar substructures.

Figure 2: Posterchildren of the three stellar substructures examined in today’s paper, with real galaxy examples on the left and simulated examples on the right. Figure 5 from the paper.

There are three classes of structures, all observed in numerous real galaxies, that the authors are interested in: tidal tails, stellar streams, and shells (Figure 2). A tidal tail is defined as a broad, elongated, and radial structure. In the real universe, these often appear when there are still two visually distinct galaxies interacting with one another, before a final merger has occurred. In contrast, a stellar stream is a thin, elongated structure, and isn’t necessarily emanating radially from a galaxy. Streams are believed to arise when one galaxy strips some stars off another, but the stars remain in the same approximate orbit as the now-disrupted satellite. One example close to home is the Sagittarius Stream orbiting the Milky Way, whose stars may have originated in the Sagittarius Dwarf Spheroidal Galaxy. A shell is an arc-like shape resembling a concentric circle around the galaxy. They aren’t very luminous and are probably pressure waves gradually propagating outwards in response to interaction with a satellite.

To identify the substructures, a team of five people examine the snapshots and count the number of each type they see. The numbers are averaged between the five people to arrive at the final incidence (N in the lower panel of Figure 3). By examining the evolution in the number of features with time, the authors attempt to relate them to merger events in the galaxy’s history and learn how long such features might persist after a merger has occurred.

Figure 3: The top panels shows the simulated galaxy’s merger history showing increases in stellar mass. The arrows indicate all merging events, while the vertical dash-dot lines indicate the most significant merger events involving the greatest increase in stellar mass. The bottom panels shows the maximum incidence of all three types of substructure selected from three possible viewing angles. Figure 10 from the paper.

Add satellites to taste, and let rest for several billion years

The evolutionary history of this simulated galaxy can be split into three phases. The first is fairly active as the galaxy is involved in several minor and intermediate mergers, and lasts roughly from ages 3 to 8 billion years (Figure 3). The second is deemed a “quiet” phase, where it experiences nothing in the way of significant merger activity, and lasts from ages 8 to 11.5 billion years. The final phase, like the first, is active and kicks off with a major merger event where it accretes a significant amount of its final stellar mass (Figure 4).

Tidal tails appear to be generated by any type of merger event (major, intermediate or minor), and their abundance decreases a bit during the simulated galaxy’s quiet phase. They are rare in number, and are relatively short-lived with an average survival time of about 2 billion years.

Figure 4: The major merger event in the simulation. The galaxy considered in this study is located in the center of each panel, while the merging satellite enters from the top right. Excerpt from Figure A.3 in the paper.

Streams are a bit more mysterious, because while they also appear in response to the first phase of mergers, their numbers continue to gradually increase during the quiet phase before the major merger event. Their abundance peaks at an age of about 11 billion years, and then falls off as the satellite responsible for the major merger begins to disrupt the environment. Why are they increasing during this window? It could simply be that they are persistent features that will only dissipate if disturbed by a new gravitational influence, or perhaps they are somehow being produced by accreting gas that is unaffiliated with any outside galaxy. Typically, they are longer lived than tidal tails, with lifespans of about 3 billion years.

Shells appear to be formed by both intermediate and major mergers (see peaks in the bottom panel of Figure 3 around 7 and 11.5 billion years). In the simulation, they seem to appear when the merging satellite finally falls into the central potential well of the main galaxy, creating waves that flow outward to form the circular, arc-like shape. They persist for about 4 billion years after their creation, taking a long time to propagate away from the central galaxy. Consequently, they are the longest lived of the examined structures, followed by stellar streams, and then tidal tails.

By examining observations of galaxies, today’s authors have shown that the presence of substructures can tell us if a galaxy underwent a recent merger, the mass of the offending satellite, and the approximate time that the event occurred, adding to the astronomer’s toolbox for understanding galaxy evolution.

About Caitlin Doughty

Caitlin Doughty is a fifth year graduate student at New Mexico State University. They use cosmological simulations to study galaxy evolution during the epoch of reionization, with a focus on metal absorption in the intergalactic medium.

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