Cataclysms in slow motion: When galaxies come too close to one another, their vast gravitational influences can bend, deform even tear apart the structure of each other.

In most extreme cases, the galaxies will collide and merge into single structures. Mergers happen over hundreds of millions of years and cannot be observed in their entirety. However, the Universe provides so many galaxies that we can combine snapshots of many different mergers with simulations to understand the process.

Extracting the history of individual galaxies is 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 (minor merger) leaves a significantly smaller mark. This 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 this paper is a different paper from twelve months ago. In it is discussion of 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 individual stars is usually a very difficult and at times impossible practice. Even the brightest nearby galaxies are often an order of magnitude smaller in angular size than the Moon.

A simulation, however, allows 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), scientists 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.

Scientists 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 story of two interactions: In a previous paper, scientists identified five simulated "kinematically atypical" galaxies. The rotational direction of the 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 five studied galaxies, three had CRGD’s 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, scientists simulated observations of both.

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 are present in the maps of the stars and gas.

One striking characteristic is the sudden drop in gas metallicity for the CRGD galaxies which scientists 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. Researchers 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, scientists blame the shallower chemical abundance rise on the existence of iron rich stars acquired during the minor merger.

For similar reasons, scientists 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. Researchers 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.

However, the presence of either feature is a good first hint in uncovering the past of distant galaxies.

References: Federrath, C. Kobayashi, C. Taylor, P. (Research School of Astronomy and Astrophysics, Australian National University, Australia; ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), Australia). Full article on facebook: @OxfordProf.MarkRSmith