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The stellar halos of Milky Way type galaxies in the EAGLE simulations

Walton, SRJ (2017) The stellar halos of Milky Way type galaxies in the EAGLE simulations. Masters thesis, Liverpool John Moores University.

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Abstract

The stellar halo component of a galaxy contains a wealth of valuable information on the host galaxy's formation history. As the diffuse outskirts of galaxies which have been built up through hierarchical assembly, their content and structure is directly tied to the material that was accreted onto them over the course of the galaxy's life. This thesis presents the results of a study into the stellar halos of Milky Way (MW) type galaxies in the EAGLE simulations. Analysis was primarily carried out on two data sets from the EAGLE simulations; one set of 352 galaxies (from the `Reference' model) and 9 higher resolution galaxies (from a `recalibrated' smaller box size model). Galaxies were selected with criteria of approximately the MW halo mass and a prominent disc component. We find that EAGLE is able to reproduce a number of observational properties of stellar halos, including the surface brightness and density profiles. This lends weight to the suitability of cosmological simulations to accurately model stellar halos, and in particular shows that EAGLE is a good basis for deeper analysis of MW-mass disc galaxy stellar halos. Negative metallicity gradients are ubiquitous in the galaxy samples analysed, with gradients in the range -0.002 to -0.02 dex/kpc, consistent with recent observations of M31. The gradient was found to be primarily driven by the transition from high metallicity in situ stars to low metallicity accreted stars with increasing radius, and sensitive to the simulation feedback efficiency employed in the simulation. The in situ and accreted components individually, however, do both have negative metallicity gradients. Observations of some galaxies, including the MW, find flat metallicity profiles, with one proposed explanation being the tendency to observe along only the minor axis. Measuring along the minor or major axes in this simulated galaxy sample does not explain the lack of gradients found in observational data as we fail to reproduce any flat metallicity profiles. Altering the feedback efficiency does, however, have an effect on the metallicity gradients, with weak feedback resulting in shallower gradients. The results are not strongly sensitive to the AGN subgrid parameters adopted in the EAGLE simulations. When tested against runs with varied stellar feedback efficiency, we find that the metallicity is affected by up to a few tenths of a dex, with the Reference model having higher metallicity out to around 40-50kpc. The density profiles were not altered significantly in the regions analysed here, with only the central - bulge dominated - region differing between model runs. No correlation was found between the mass-metallicity relation of a galaxy's z=0 satellite population and its stellar metallicity gradient, and testing against two extreme EAGLE models showed that feedback strength had no effect on the gradient of the z=0 mass-metallicity relation. While strong and weak stellar feedback changed the absolute metallicity of galaxy satellites, the overall mass-metallicity gradient remained the same. The outer stellar halo fractions of galaxies in a wider mass range were measured for comparison with recent observational stellar halo surveys and found to be generally higher, with no galaxies having an undetectable stellar halo unlike for certain galaxies recently observed. The stellar halo mass fraction of EAGLE galaxies was found to be correlated with galaxy stellar mass, with the mean fraction rising from 1% at M* ~ 5x10^9Msun to 18% at M* ~ 5x10^11Msun. The sample has an overall mean halo mass fraction of 3%, ranging from 0.1% to 30%.

Item Type: Thesis (Masters)
Uncontrolled Keywords: astrophysics stellar halos eagle
Subjects: Q Science > QB Astronomy
Q Science > QC Physics
Divisions: Astrophysics Research Institute
Date Deposited: 12 Jan 2018 10:16
Last Modified: 12 Jan 2018 10:16
Supervisors: Font, A
URI: http://researchonline.ljmu.ac.uk/id/eprint/7791

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