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.

Popular Abstract

How galaxies interact with each other, and what effect(s) this has on star formation, is a key ingredient for understanding how galaxies form and evolve. Interactions between galaxies can often times lead to so-called merger events, where two galaxies collide and merge into one. In these merger events, unique morphological tidal features can be produced, and be left behind to remain visible long after the merger event itself. Tidal features are therefore a powerful tool to help reconstruct assembly histories of galaxies. An example of where this has been put in action, is the galaxy NGC 474 and its outer shell, which is a tidal feature that can be described as a wide concentric arc of stars. Besides constraints on its formation history, observations have also shown that NGC 474’s outer shell contains several massive star clusters, two of which that are much younger than the overall population of stars in the shell itself. Instead, they match a population of stars close to the nucleus of NGC 474. This shows evidence of a rather complex star formation history, and raises questions about where star formation takes place, the conditions for it, and how it evolves in these types of mergers and galaxies with shells: shell galaxies.

In this project, we perform two sets of numerical simulations. The first involves a parameter survey, where we explore different sets of parameters between two colliding galaxies, to see what configuration is the most favourable for shell formation. In order to get a shell galaxy with distinct shells, the results suggest a merger between two disk galaxies on a head-on trajectory towards each other, with a mass ratio between 1:10 to 1:2. With this information, we perform our second set of simulations, which is a single high-resolution simulation of a merger and its formation into a shell galaxy.

The results from our high-resolution simulation show evidence of three epochs of star formation. Pericentre passages are when the two galaxies are as closest to each other. Before the first pericentre passage, the rate at which stars form is relatively stable, however, as the two galaxies approach the first passage, the star formation activity increases. This is due to interactions between the galaxies’ gas reservoirs, which generates an excess of dense gas that can form stars. During the merger event, gravitational interactions puffs up the disk of stars, such that the gravitational force on the gas becomes weaker. Because of this, the gas can not get dense enough to form stars, which causes the star formation activity to go into its third and last epoch, where the rate at which stars form is close to zero.

On a local scale, our high-resolution simulation shows indications of different behaviours between regions with high and low fraction of molecular H2 gas. Regions with a high fraction of H2 correlates better to a classical star formation law, where the rate at which stars form is proportional to the gas density, than regions with a low fraction of H2. The central areas of the two galaxies, and later the shell galaxy, show evidence of large fractions of H2. This is also where many of the star-forming regions are located, however, we also see star formation in spiral arms and in the outskirts early in the merger. As the two galaxies evolve into a shell galaxy, only the innermost area close to the nucleus of the galaxy contains star-forming molecular H2 gas. Since shells form at large distances from the nucleus, and contain very little star-forming gas, our simulation suggests that there is no star formation in shells.

In conclusion, assuming that our high-resolution simulation forms a ‘typical’ shell galaxy, and therefore is representative for shell galaxies as a whole, our results suggest that the two young massive star clusters in NGC 474’s outer shell, have an external origin. Our simulation instead advocates for a formation scenario where the star clusters were formed during a merger event with enhanced star formation activity, when the material that later formed the shell was close to the associated material of the nucleus.

• author: Petersson, Jonathan
• supervisor: Florent Renaud
• organization: Lund Observatory - Undergoing reorganization; Department of Physics
• course: ASTM32 20221
• year: 2022
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• keywords: shell galaxies, simulations, starbursts, quenching, mergers
• publication/series: Lund Observatory Examensarbeten
• report number: 2022-EXA194
• language: English
• id: 9094760
• date added to LUP: 2022-06-30 13:58:02
• date last changed: 2023-08-30 15:14:57