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Equal-mass Galaxy Mergers : Initial Conditions and Subresolution Astrophysics

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Title: Equal-mass Galaxy Mergers : Initial Conditions and Subresolution Astrophysics
Author(s): Lahén, Natalia
Contributor: University of Helsinki, Faculty of Science, Department of Physics
Discipline: Astronomy
Language: English
Acceptance year: 2015
Interactions and mergers play important roles in the evolution of galaxies. In this Master's thesis I have studied some basic properties of merging galaxies using numerical simulations. I have created initial conditions for equal-mass disk galaxy mergers and run the simulations using the numerical simulation codes GADGET-3 and its recently updated version SPHGal. In this Thesis I have also reviewed the basic physics modeled by GADGET-3 and the differences between the astrophysical subresolution models in the standard GADGET-3 and SPHGal versions of the code. The aim of the Thesis was to see how different initial conditions affect the mergers of disk galaxies, and how the updated astrophysics affect the properties of the mergers. The simulations were run on the supercomputer Sisu and the supercluster Taito at CSC, the Finnish IT Center for Science. From the results of the simulations I have studied the effect of having a dark matter halo with either an NFW density profile or a Hernquist density profile. Four observed mergers were succesfully reproduced with NFW profiles using position and velocity maps, and then compared to their Hernquist profile analogues. By comparing the results of the mergers between galaxies with the two types of haloes, I found that the steeper gravitational potentials of the NFW haloes produced more prominent tidal tails than the Hernquist haloes. The more compact morphology of the disks within the Hernquist haloes before the final coalescence enabled also faster coalescence times in the Hernquist halo mergers. As expected, the NFW halo mergers experienced slower orbital decay, were more violent, experienced higher star formation rates and produced thus 2-17% more new stars than the Hernquist halo mergers. In the Thesis I have also studied the effect of the updated subresolution models on the star formation process in the merging galaxies. The star formation in the standard GADGET-3 is self-regulated, whereas the star formation in SPHGal is regulated by detailed feedback processes. The star formation rates vary both spatially and on much shorter time scales in the SPHGal compared to the analogous simulations with the standard GADGET-3. I analyzed also the metal yields in the merger remnants of the Antennae galaxies, and found a metallicity gradient within the inner 2 kpc from the center of mass of the remnant. A gradient was also present in the oxygen abundance of the remnants, following the fact that the metallicity was dominated by the abundance of oxygen. Gradients in the inner regions of the merger remnants are to be expected, since star formation is most active in the central regions of the remnant.

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