I: We first investigate the triggering mechanism and the structural properties of obscured luminous active galactic nuclei from a detailed study of the rest-frame B and I HST images of 29 nearby (z=0.04-0.4) optically selected type 2 quasars. Morphological classification reveals that only a minority (34%) of the hosts are mergers or interacting galaxies. More than half (55%) of the hosts contain regular disks, and a substantial fraction (38%), in fact, are disk-dominated (B/T≤ 0.2) late-type galaxies with low Sersic indices (n < 2), which is characteristic of pseudo bulges. Nuclear star formation seems to be ubiquitous in the central regions, leading to positive color gradients within the bulges and enhancements in the central surface brightness of most systems. We then carry out a statistic study on the host galaxies of a large sample of 887 type 2 quasars at z<0.83. We find that majority of host galaxies distribute in the blue cloud in the color-mass diagram, and on/above the main-sequence in the SFR-mass diagram. Our results also show that as host galaxies become more massive, they become redder evolving to the green valley and red sequence, and their SFRs decrease. AGN properties intrinsically correlate with properties of host galaxies: AGNs with higher [O III] luminosities and Eddington ratios have bluer hosts with higher SFRs. Additionally, we identify a subsample of 111 host galaxies in mergers at z≤0.25. It seems that mergers could moderately enhance [O II] star formation rates and star formation efficiency for host galaxies of type 2 quasars, but take a very minor role in affecting other properties.
II: The most advanced cosmological simulation IllustrisTNG has successfully produced galaxies with a reasonable visual morphology in a fully cosmological context and provides a great opportunity to study how important bars are in the evolution history of galaxies. We find that the bar fraction reaches 61% in galaxies of stellar mass logM*≥10.6, which is consistent with the NIR observations. However, the bar fraction decreases sharply in lower-mass galaxies. Only about 14% galaxies of logM*<10.6 have a bar. Distributions of bar strength and size are in general agreement with observations, except that IllustrisTNG is more likely to create small bars in massive galaxies. We also discuss the selection effect of disc galaxies on bar fractions.
