Morphological evolution of the hosts of far-infrared/submillimeter galaxies
Authors:   , Ling Chenxiaoji 
Published: September 2022

Luminous and Ultra-luminous InfraRed Galaxies (LIRGs, LIR = 10^11-12L; ULIRGs, LIR [greater than] 10^12L) were discovered as the brightest objects in the infrared sky. While they are rare objects in the local universe, their number density is gradually higher with increasing redshift up to z = 3. In fact, they seem to dominate the total infrared luminosity density of the universe at z = 2 - 3, which makes them a key population in our understanding of galaxy evolution across cosmic time. Their extreme IR emission is mainly due to their extremely high star formation rates (LIRGs, SFRs [greater than] 10 M yr^-1; ULIRGs, SFRs [greater than] 100 M yr^-1 ). The mechanism of such high SFRs is still under debate, especially for high redshift ULIRGs. Most local ULIRGs are found to be in merger systems, and therefore it is widely believed that ULIRGs are merger-driven. However, the situation beyond the local universe is highly uncertain. Some observations and numerical simulations over the past two decades suggest that in situ, secular gas accretion through disk instability within a normal disk galaxy could also sustain such a high SFR. The most direct approach to understanding the triggering mechanism of ULIRGs is through morphological study. This direct approach, however, has to overcome two main problems. First, most ULIRGs were discovered in wide-field surveys done by low-resolution single dish telescopes in the farinfrared/ submillimeter wavelengths, which greatly hinder the optical/near- IR counterpart identification because there could be tens of candidates within a single resolution element. To solve this problem, interferometry data in the submillimeter and radio wavelength can be used as the bridge to find the correct counterparts. Second, galaxies at the high redshift have very small apparent sizes, and this leaves the data from the Hubble Space Telescope the only suitable choice for morphological study at z [greater than] 0:5. To solve this problem, high-resolution images in the near-IR wavelength are needed, and near-infrared(near-IR) images that were taken by Hubble Space Telescope (HST) are the only choice before the launch of the James Webb Space Telescope(JWST). In this thesis, I present the morphological study of 1,266 far-infrared galaxies (FIRGs) and sub-millimeter galaxies (SMGs) in the Cosmic Evolution Survey field (COSMOS) using the F160W and F814W images obtained by the HST. This is the largest sample of its kind. SMGs can be regarded as the population at the high-redshift tail of FIRGs. The FIRGs and the SMGs are selected from the Herschel Multi-tiered Extragalactic Survey and the SCUBA-2 Cosmology Legacy Survey. Their precise locations are based on the interferometry data from the Atacama Large Millimeter/submillimeter Array and the Very Large Array. The morphological study was based on their near-IR counterparts inside the COSMOS-DASH mosaic. Their redshifts of the FIRGs/SMGs were retrieved from the achieved data. The vast majority of them have either spectroscopic redshifts (623 in total) or highly reliable photometric redshifts (612 in total), and they span over. The morphological classification was done in three different ways, namely visual classification, Sersic profile fitting, and non-parametric statistics Most of our FIRGs/SMGs have total infrared luminosity (LIR) in the regimes of (U)LIRGs. We find that their morphological types are predominantly disk galaxies (type "D") and irregular/interacting systems (type "Irr/Int"), and that their relative importance changes with cosmic time. There is a morphological transition at z = 1:25, above which the Irr/Int galaxies dominate and below which the D and the Irr/Int galaxies have nearly the same contributions. This suggests a shift in the relative importance of galaxy mergers/ interactions versus secular gas accretions in "normal" disk galaxies as the major triggering mechanism of ULIRGs. As local ULIRG are predominantly merger driven, our results imply that there must be yet another transition between z = 0 and z = 1:25 where secular gas accretion in disk galaxies began to lose its importance among ULIRGs. The verification of this will have to wait for a future space-based, wide-field survey.