Colliding Galaxies in a (Nut)Shell

Abstract: Galaxy interactions and mergers are natural recurring events in the current cosmological paradigm, events that often coincide with starbursts and enhanced star formation activity. To understand why that is, one can use idealised (non-cosmological) merger simulations, which provide freedom to recreate specific tidally distorted galaxies, and analysis of the time evolution and underlying physics of the star formation activity at a high spatial resolution. One particular class of tidally distorted galaxies are shell galaxies. These galaxies are characterised by wide concentric shell(s), that extends out to large galactocentric distances with sharp outer edges. Shells, together with other unique morphological tidal features, have shown to be remnants of previous merger events, and can therefore serve as a powerful tool to help reconstruct assembly histories of galaxies. An example of where this have been put in action, is the shell galaxy NGC 474 and its outer shell. However, besides constraints on its formation history, observations have also found evidence of young massive star clusters in its outer shell, which asks questions about where and when star formation takes place in shell-forming mergers and shell galaxies. The purpose of this project is to study the merger-driven star formation activity in a shell galaxy, how it evolves with time and within the system, and the physical conditions for it. To fulfil this, we perform idealised merger simulations. We begin with a short parameter survey, in which we explore different sets of initial conditions, to find the most favourable orbital configuration for shell formation. The conclusion is a near-radial intermediate to major merger (1:10 to 1:2 mass ratio) between two disk galaxies. From this, we perform an idealised high-resolution N-body + hydrodynamical simulation of two merging galaxies and their formation into a shell galaxy, and analyse its star formation activity. In our analysis, we find that as the system approaches the first pericentre passage, it goes into a starburst phase with enhanced star formation activity, due to an excess of dense gas generated. For spatially resolved star formation, the Kennicutt-Schmidt relation varies during the merger, but with no general trend over time. A break is consistently shown in the relation at low gas surface densities, due to mixing of atomic HI and molecular H2 gas. By analysing HI and H2 individually, we find that the surface density of H2 shows a better correlation to the surface density of SFR than that for all the gas, while for HI, we see the opposite with no correlation at all. Star formation therefore mainly takes place in regions with large amounts of H2 gas, including the nucleus, spiral arms, and occasionally in the outskirts of the system early in the merger. Tidal interactions during the merger scatters stars into a stellar spheroid around the system, and as the system approaches coalescence, morphological quenching stops star formation (without the need of AGN feedback). Only the innermost ∼1.5 kpc is left with star-forming H2 gas. The first stellar shell does not appear until after coalescence, and due to its position at a large galactocentric distance and lack of gas, it shows an absence of in situ star formation, and so does forthcoming stellar shells as well. Our results suggest that shell-forming mergers can be part of the process in turning blue-late types galaxies into red and dead early-types ones in galaxy evolution, including blue nuggets into red ones at high redshift, due to similarities with the compaction model. However, further investigation of this calls for more simulations, both idealised and cosmological, which would provide valuable statistics on e.g what the effects the orbital properties, mass ratio and continues accretion of gas would have on the quenching phase.