Browsing by Subject "galaxy evolution"
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(2022)Results from cosmological zoom-in simulations focusing on the formation and evolution of massive early-type galaxies starting from redshift z=50 until the present time are presented in this thesis. In addition, the dynamics of supermassive black hole (SMBH) binaries found in a subset of the galaxies as a result of galaxy mergers are studied. The first 12 zoom-in simulations were run with GADGET-3, an N-body hydrodynamical tree code on the Puhti supercomputer at the Finnish IT Centre for Science (CSC). The zoom-in regions were chosen from a low resolution large-volume simulation, run with the University of Helsinki computing cluster Kale. The SMBH binary dynamics were simulated with the regularized integrator code KETJU. In total three simulations using KETJU were run on the CSC supercomputer Mahti. The GADGET-3 simulations included both dark matter only runs and runs containing also baryons. The simulations were run at two different resolutions. No significant differences between the two resolutions were found for the dark matter only runs, whereas the runs including a baryonic component showed large differences. The medium resolution runs had too low particle numbers and too large particle masses to correctly resolve star formation and feedback, leading to rotation curves that missed their central peaks. The results from the high resolution simulations agreed with earlier published results. The rotation curves peak in the central regions of the galaxies with the curves becoming almost constant at large radii. The star formation rates peak in the redshift range z~2-3 and at smaller redshifts star formation can momentarily increase due to the occuring of galaxy mergers. At the present day, all of the studied galaxies include mainly old stellar populations, resulting in red colours for all the galaxies. Still, the galaxy formation efficiency parameter of each galaxy is somewhat higher than what is seen in the observations. Finally, all three SMBH binaries for which the dynamics were studied using KETJU led to a coalescence of the two black holes. The orbital decay of SMBH binaries occur in three phases: dynamical friction, three-body interactions and gravitational wave emission. The merger times strongly depend on the eccentricity of the binaries with the semimajor axis of the binary with the highest eccentricity decreasing the fastest. This is expected from theory, with the evolution from subparsec scales to coalescence agreeing very well with the theoretical evolution taking into account the post-Newtonian correction term 2.5PN, which is the lowest order post-Newtonian term responsible for gravitational wave emission.
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(2024)This thesis studies gas-rich galaxy mergers at redshifts of z ∼ 1-2 using numerical simulations, with a particular focus on the effect of feedback from active galactic nuclei (AGNs). In total, 16 galaxy mergers at redshifts z = 1 and z = 2 were modeled using the simulation codes KETJU and GADGET-3. The simulations were performed on the supercomputer Mahti located at the Finnish IT Centre for Science (CSC). AGN feedback can be described as the radiative and mechanical energy released through accretion, which act to heat and disperse the remaining gaseous material surrounding the central supermassive black hole (SMBH). The feedback mechanisms include, for example, photoionization heating due to high-energy photons and winds and jets driven by the AGN. Numerically, AGN feedback was implemented using two models in this thesis: thermal and kinetic AGN feedback, in which the gas particles are either heated or ‘kicked’, respectively. In addition to AGN feedback, the simulations included metal-dependent gas cooling, stochastic star formation, and stellar feedback. The simulated progenitor galaxies were gas-rich spirals consistent with observed galaxies at redshifts z = 1 and z = 2. The virial masses of the progenitors were set to correspond to typical massive galaxies at their redshifts using the Press-Schechter mass function, while the initial masses for the central SMBHs were set using observed MBH-M⋆ and MBH-σ⋆ relations. The gas fractions and metal abundances of the progenitors were calibrated using observational data at their respective redshifts. The KETJU and GADGET-3 simulations produced very similar results for the overall evolution of a given merger configuration. Consistent with earlier studies, the kinetic feedback was observed to be significantly more effective at removing gas from the galaxies than the thermal feedback. The combined effect of AGN and stellar feedback was observed to strongly suppress star formation, with the star formation of one merger being almost completely shut down. The thermal and kinetic feedback models caused noticeable differences in the orbital evolution of the SMBH binaries. Merger timescales were significantly longer for the SMBHs in the KETJU simulations with kinetic feedback. In general, the merger timescales increased with decreasing initial eccentricity for the SMBH binary. The merger remnants were compared to observed MBH-σ⋆, R1/2-M⋆, fgas-M⋆, and mass-metallicity relations. Overall, the remnants were reasonably consistent with the observed relations. Hence, we can conclude that AGN feedback plays a crucial role in galaxy evolution and that both the thermal and kinetic feedback models are able to produce realistic high-redshift galaxies.
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