UC Irvine

This thesis demonstrates the potential of the cosmological hydrodynamical simulation Illus-trisTNG 300-1 to investigate and to provide fresh insights into the search for massive relic galaxies that are believed to skip the second phase of the two-phase formation process for massive ETGs. We present an analysis of the stellar mass and size of massive early-type galaxies (ETGs) in the TNG300-1 at redshifts z = 0 and z = 2, using simulation particle level data. We implemented a dust attenuation process via a semi-analytical approach for star light, and we derive the stellar mass-size relation. We find that the derived median mass-size relation is in good agreement with the observations (within 10% of the predictions from Sloan Digital Sky Server). We developed methods to define the relicness of massive compact galaxies physically by constraining dark matter and stellar mass after redshift 2, and examine the stellar and dark matter properties of the selected relics. We find that constraining dark matter mass within a proper length of 100 kpc is better suited than virial radii to quantify the passiveness of dark matter mass growth. We have also demonstrated that the relicness defined physically corrob- orates the definition based on compactness that has been used in observations. Additionally, we have demonstrated that the dark matter halo concentration of the selected relics shows a positive correlation with relicness (i.e., passiveness). We also explored various methods that xihave been used in observations, to define relicness using compactness of galaxies. We find that massive relics with low virial mass (M200 < 1013 M⊙ ) have a higher dark matter halo concentration (c200 ), i.e., above the ΛCDM c200 − M200 relationship. Motivated by the massive relic galaxy Mrk 1216 which has an extremely high dark matter halo concentration, we have presented our analysis of dark matter halo concentrations (c200 and c2500 ) in TNG300. We have verified the expected c200 − M200 relation for halos with mass 1012 M⊙ ≤ M200 ≤ 3 × 1013 M⊙ and for the first time we have presented the power law fit for c2500 −M2500 for simulated halos in this mass range. We verify that both concentrations follow a log-normal distribution with similar scatter for a given mass bin. Furthermore, we have been able to find a few halos with extremely high concentrations (i.e., positive outliers with above 5 σ from the ΛCDM c200 − M200 relationship). We investigate the gas and interactions with the local environment of these halos and find that all these halos do not have any gas and have shown many interactions (i.e., flybys) with other subhalos since redshift 2.