Cosmological Simulations of Galactic Disc Assembly

Abstract

We address the issue of kinematic heating in disc
galaxies by analysing a suite of cosmological Milky Way-type disc simulations
run with different particle-and grid-based hydrodynamical codes and different resolution, and
compare them with observations of the Milky Way. By studying the kinematics of disc stars in
these simulations, we seek to determine whether or not the existence of a fragile thin disc
is possible within a cosmological framework, where multiple mergers and interactions are the
essence of galaxy formation.

We study the velocity dispersion-age relation for disc stars at $z=0$ and find that four
of the simulations, the stellar disc appears to undergo continual/secular heating. Two other
simulations suggest a “saturation” in the heating profile for young stars in the disc. None of the simulations have thin discs as old as that of the Milky Way.

We also analyse the kinematics of disc stars at the time of their birth, and find that in some simulations old stars are born cold within the disc and are subsequently heated, while other simulations possess old stellar populations, which are born relatively hot. The models which are in better agreement with
observations of the Milky Way's stellar disc undergo significantly lower minor-merger/assembly activity after the last major merger.

By running a set of isolated Milky Way-type simulations with different resolution and different density thresholds for star formation we conclude that, on top of the effects of mergers, there exists a ``floor'' in the dispersion that is related to the underlying treatment
of the heating and cooling of the interstellar medium, and the low density threshold which such codes use for star formation.

A persistent issue in simulations of disc galaxies is the formation of large spheroidal components,
and disc galaxies with larger bulge to disc ratios than is observed. This problem is alleviated by supernova feedback. We found that by increasing the feedback in the simulations, we decrease the amount of stars that are accreted onto the main galaxy. The star formation is quenched more efficiently in low mass satellites when stronger feedback is implemented as well as in the main halo. These effects result in a disc galaxy, which has formed less stars overall, but more importantly, contains less accreted stars. As the strong stellar feedback quenches the star formation in the small building blocks, the metallicity of the accreted stars is lower than in the case where less feedback was used.

In the context of hierarchical formation, mass assembly is expected to be scale free. Yet the properties of galaxies depend strongly on their mass. We examine how baryonic physics has different effects at different mass scales by analysing three cosmological simulations using the same initial conditions that are scaled to three different masses. Despite their identical dark matter merger history, we show that the simulated galaxies have significantly different stellar accretion histories. As we go down in mass, the lowest mass progenitors are unable to form stars, resulting in a low mass galaxy with less accreted stars. The overall chemical properties are also distinct at the different mass scales, as one might expect from the mass-metallicity relation of observed galaxies. We examine gradients of chemical abundances with radius and with height above the disc, and look for properties that are retained at different mass scales and properties which change, often dramatically.

We analyse the kinematic and chemical properties of their accreted and in-situ populations. Again,
trends can be found that persist at all mass scales, providing signatures of hierarchical structure formation. We find that accreted populations in the high mass simulation did not resemble any of the populations in the lower mass galaxies, showing that the chemical properties of proto-galaxies, which merge at high redshift to form massive galaxies, differ from the properties of low mass galaxies that survive at z=0.

We probe further the signatures of hierarchical structure formation at smaller scales, in dwarf galaxies. We analysed the morphologies, kinematics and chemical properties of two simulated dwarf galaxies with different merger histories. We again analyse the accreted and in-situ populations. Observations of dwarf galaxies have found that they are comprised of multiple components. Our simulated dwarfs indicate that such populations may indeed be a manifestation of the hierarchical formation process in action in these lower mass galaxies.

In one simulated dwarf, the in-situ stellar component forms a thin disc and a thick disc. We show that the thick disc in this simulation forms from in-situ stars that are born kinematically hot in the disc from
early gas-rich mergers. The thin disc is formed quiescently from the later infall of gas. The accreted stars in the simulation were found to form an extended stellar halo. Chemical signatures of the three
populations are also explored.

The second dwarf we analysed has different galactic components, a result found to be due to the different
merger history of this galaxy. The last major merger in this simulation occurs early on in the formation process
between two proto-galaxies of similar mass. The result is a dwarf galaxy comprised of a disc formed of in-situ stars and a flattened rotating stellar halo formed of accreted stars. The angular momentum of the accreted and old in-insitu stars is obtained from the last major merger. We discuss the resemblance of this flattened rotating stellar halo to fast rotating flattened elliptical galaxies, and propose that such structures may explain some of the observed extra-galactic thick discs.

These studies show that galactic properties emerge through the complex inter-play between hierarchical
structure formation, star formation, and feedback from supernovae. Different modelling of these processes
will alter the simulated galaxy's properties, and detailed comparisons with observations can then be made to determine the dominant processes responsible for different galactic properties. We remain optimistic that
further improvement in modelling will allow deeper insights into the processes of galaxy formation and evolution.