Authors: Philip Taylor, Chiaki Kobayashi, & Christoph Federrath
First Author’s Institutions: Research School of Astronomy and Astrophysics, Australian National University, Australia; ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), Australia
Status: Accepted to MNRAS, closed access
Cataclysms in slow motion
Space is a battleground for giants. When galaxies drift too close to one another, their immense gravitational influences can bend, deform, or even tear apart the structure of each other. In the most extreme cases, the galaxies will collide and merge into single structures. Unfortunately, mergers happen over hundreds of millions of years and cannot be observed in their entirety. Luckily, the Universe provides so many galaxies that we can combine snapshots (Figure 1) of many different mergers with simulations to understand the process. Extracting the history of individual galaxies is much more difficult. Mergers between two large galaxies of similar size (major mergers) can result in distinct elliptical galaxies, but a small galaxy being subsumed into a much larger one (a minor merger) leaves a significantly smaller mark. Today’s paper studies simulations of collided galaxies to try and find observational signatures of galaxies that are still reeling from minor mergers in the past.
The predecessor of today’s paper is a different paper by the same authors from a year ago. There, they discuss results from a cosmology simulation that allows for the formation and subsequent collisions of galaxies. Dark matter filaments, thin strands of dark matter that connect distant galaxies, are also included in the simulation. These filaments are of particular importance because as a galaxy passes through a dark matter filament, gas along the filament will accrete into the galaxy.
When observing galaxies, distinguishing the individual stars is usually a hopeless endeavor. Even the brightest nearby galaxies are often an order of magnitude smaller in angular size than the Moon. A simulation, however, allows us insight into the properties of each individual particle. A significant challenge in this work is translating that information into realistic observations of the hypothetical galaxies. In order to mimic the effects of integral field spectroscopy (IFS), the authors calculate the weighted average of various quantities for hypothetical pixels. Typically, observations are weighted by luminosity; the brightest objects will have the largest impact on what we see. For this work, the authors were able to assign weights based on luminosity or mass but note that stellar population models could allow conversion between the different weighting systems in observations.
A tale of two interactions
In their previous paper, the authors identified five simulated “kinematically atypical” galaxies. The rotational direction of these galaxies varied with radius. According to their interpretation, galaxies could pass through dark matter filaments at a particular angle such that the accreted gas would rotate in the opposite direction of the galaxy, forming a counter-rotating gas disc (CRGD) along the outside. If such a galaxy then had a minor merger with a smaller galaxy on a retrograde orbit, it would possess counter-rotating stars from the absorbed smaller galaxy. The result is a kinematically distinct core (KDC). Of the five studied galaxies, three had CRGDs and two were KDC galaxies. All five galaxies had relatively small effective radii for their masses which removes the possibility of major mergers which would expand the galaxy.
Given the distinct importance of the gas and stars, the authors simulated observations of both (Figure 2). Two important quantities that can be determined from the observed spectra of galaxies are metallicities (how much of the material consists of elements heavier than hydrogen and helium) and chemical abundances (what is the makeup of those elements). Iron is created in the violent extreme heat of supernovae while elements like oxygen and carbon are created over the lifetime of stars. Metallicity and chemical abundances can therefore help determine the age of parts of a galaxy.
A number of interesting features exist in the maps of the stars and the gas. One particularly striking characteristic is the sudden drop in gas metallicity for the CRGD galaxies which the authors attribute to the recently accreted gas from the dark matter filament. Gas that has always been in the galaxy will have been enriched by nearby stars producing heavier elements; the much more isolated dark matter filaments will be relatively gas-poor. KDC galaxies, on the other hand, will have brought in metal-enriched gas from their minor mergers.
The authors also draw attention to the relatively shallow rise in chemical abundances for KDC galaxies. Lower mass galaxies exert weaker gravitational forces on their star forming material. As a result, stars are much slower to form and will have higher concentrations of iron from other stars in the Universe going supernova first. Since the stars around the edge of the KDC galaxies will have come from the smaller galaxies, the authors blame the shallower chemical abundance rise on the existence of iron-rich stars acquired during the minor merger. For similar reasons, the authors find a slight excess of young stars on the edges of KDC galaxies.
A sharp decline in metallicity with radius does not guarantee that a galaxy has a CRGD. The authors note that any galaxy that passes through a dark matter filament will acquire such a feature. Similarly, a small gradient in chemical abundances could occur in non KDC galaxies. Confirmation of either requires intimate knowledge of the individual stars, which is only possible for simulated data. But the presence of either feature is a good first hint in uncovering the past of distant galaxies.