The Effects Of Galaxy Mergers And Large-scale Environments In Regulating Galaxy Evolution

Previous studies of galaxy mergers were mostly contributed to:(1)The effects of mergers on star formation and AGN activity;(2)The evolution of galaxy merger rate in relation to the formation of galaxies of different types;(3)Characterization of merging processes and links between parent galaxies,merger parameters and merger remnants through simulations.However,characterizing merger parameters of merging systems in the Universe is still lacking.In our first work,we examine the properties of long tidal tails using a sample of 461 merging galaxies with log(M*/M☉)≥ 9.5 within 0.2 ≤ z≤1 from the COSMOS survey.Our results show that long tidal tails can be simply divided into three shape types:straight(41%),curved(47%)and plume(12%),which are generated at different merger stages.By comparing with the simulations of merging galaxies,,we find that the long tidal tails are mainly formed in major mergers after the first pericenter.We identify 165 tidal dwarf galaxies,yielding a tidal dwarf galaxy production rate of 0.36 per merger.With given galaxy merger fraction and survival rate of TDGs,We estimate that 5 per cent of local dwarf galaxies are of tidal origin,suggesting the tidal formation is not an important formation channel for the dwarf galaxies.More than half of TDGs are located at the tip of their host tails.These TDGs have stellar masses in the range of 7.5≤log(M*/M☉)≤9.5 and appear compact with half-light radii following the M*-Rerelation of low-mass elliptical galaxies.However,their surface brightness profiles are generally flatter than those of local disc galaxies.The morphological classification of galaxies is key to studying the formation and evolution of different types of galaxies.It is still challenging to separate merging galaxies from irregular galaxies with the existing methods of morphology identification,leaving the detection of the merging galaxies inaccurate and highly incomplete.On the one hand,the morphological features used by different classification methods are different.On the other hand,some of merging galaxies have disturbed morphologies similar to those of irregular galaxies.In the second work,we select a sample of 28667 galaxies with 0.2≤z≤1.0 and stellar mass log(M*/M☉)≥9.5 from the COSMOS field and calculate their morphological parameters,including outer asymmetry(AO),the secondorder moment(M20)and asymmetry(A).We analyze the correlations between diferent parameters and the galaxy morphology.We find that AO-M20 diagram can be used to distinguish merger galaxies from other types of galaxies,and can also roughly probe minor and major mergers at different merger stages.When we compare this method with other non-parametric methods,we find that it is able to recognize mergers over a wide merging stage and insensitive to star formation activity.Finally,we study star formation and AGN activity for subsamples of galaxies across the AO-M20-A parameter space.Gas supply from the cosmic web is the key to sustain star formation in galaxies.The large-scale structures of the Universe have fundamental influence on the evolution of galaxies.So far efforts are still demanded to,the influence of large-scale environment on galaxy star formation.We think that to study the large-scale environment,we need to distinguish it from the local environment and other properties of galaxies.We aim to address the effects of the large-scale environment and the local environment on the properties of galaxies.In the third work,we examine galaxy star formation rate(SFR)as a function of local density in a LSS at z=0.735 in the Extended Chandra Deep Field South.The structure is mapped by galaxies with R<24 mag and 0.72≤z≤0.75 collected from the literature and our spectroscopic observations with Magellan/IMACS(414 spec-z and 318 phot-z),consisting of five galaxy clusters/groups and surrounding filaments over an area of 23.9 × 22.7 cMpc2.The spread of spec-z corresponds a velocity dispersion of 494 km s-1,indicating the z=0.735 LSS likely to be a thin sheet with a galaxy density 3.9 times that of the general field.These clusters/groups in this LSS mostly exhibit elongated morphologies and multiple components connected with surrounding filaments,suggesting that these substructure are still undergoing the virialization process.Strikingly,we find that star-forming galaxies(SFGs)in the z=0.735 LSS keep star formation at the same intensity level as field SFGs,and show no dependence on galaxy local density but stellar mass.Meanwhile,an increasing fraction of quiescent galaxies is detected at increasing local density in both the structure and the field,consistent with the expectation that galaxy mass and local dense environment hold the key to quench star formation.Combined together,we conclude that the cosmic environment of the z=0.735 LSS overtakes the local environment in remaining galaxy star formation to the level of the field.