Study On The Morphology, Structure And Star Formation Activities Of High-redshift Galaxies

Morphology,structure,and star formation activity of galaxies are the very impor-tant parts in study of galaxy evolution.The most direct feature in observation,morphol-ogy,is constructed by the two-dimension projection of a bulk of stars with different mo-tions,colors and brightnesses.It is reported that galaxy morphology and structure are related to the other physical properties,like stellar mass,environment,star formation rate,color and the epoch.Several studies focus on the synergy between morphology transformation and star formation quenching and search for the evidences of how in-ternal and external factors influence the galaxy evolution.In the paper,we utilize the multiwavelength data from all five CANDELS fields(Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey)to explore the physical properties of,red sequence","green valley","blue cloud",and compact galaxy populations,including morphologies,structures,active galactic nucleus(AGN)fractions,star formation rates and local envi-ronments.In addition,we construct the map of environmental density over the redshift ranging from 2.5 to 0.5 and set out to discuss the mass and environmental influences on morphologies,structures,and star formation activities during the galaxy evolution.In Chapter 2,we investigate the dust content,morphologies,structures,active galactic nucleus(AGN)fractions of red,green,and blue galaxy populations.The mor-phological distinction between blue and red galaxies is found at a fixed mass bin,sug-gesting that the quenching processes should be accompanied with the transformations of galaxy structures and morphologies.Green valley galaxies are found to have inter-mediate dust attenuation and morphological parameters and reside in the middle of the regions occupied by quiescent and star-forming galaxies.The above findings seem to fa-vor the scenario that green galaxies are at a transitional phase when star-forming galax-ies are being quenched into quiescent status.We further discuss the mass dependence on galaxy evolution.The fractions of three populations as functions of mass support a"downsizing" quenching picture that the bulk of star formation in more massive galax-ies is completed earlier than that of lower-mass galaxies.The structural parameters,including effective radius(re),galaxy compactness(?1.5),and second-order moment of 20%brightest pixels(M2O),are found to be correlated with stellar mass.Sersic index(n),concentration(C),and Gini coefficient(G)seem to be insensitive to stellar mass.In Chapter 3,we present a study on structures and physical properties of massive(M->1010M(?))compact galaxies at 1<z<2 in five 3D-HST/CANDELS fields.Ac-cording to the two quantities ?1.5 and sSFR,the sample is divided into the four subpopu-lations,namely extended star-forming galaxies(eSFGs),compact star-forming galaxies(cSFGs),compact quiescent galaxies(cQGs),and extended quiescent galaxies(eQGs).Compared with eSFGs,cSFGs are found to have the lower level of star formation,and mainly distribute in the quiescent region of the UVJ diagram.Our analysis of paramet-ric and nonparametric morphologies shows that cQGs(eQGs)are more concentrated and have less substructures than cSFGs(eSFGs).It support the early evolutionary sce-nario that the cSFGs at 1<z<2 are the intermediate population after compaction but before a quick quenching.The cSFGs at 1.5<z<2(1<z<1.5)prefer to be in higher(lower)density environment,similar as cQGs(eSFGs).It suggests that merger or strong interaction might be the main driving mechanism of compaction at higher red-shifts,whereas the disk instability of individual galaxies might play a more important role on the formation of cSFGs at lower redshifts.In Chapter 4,We present a robust Bayesian method,reconstruct the map of en-vironmental overdensities,and explore the mass and environmental dependence of the structural parameters and star formation activities at 0.5<z<2.5.By using the known clusters/groups in UDS,we confirm that Bayesian method is a good tracker of environ-mental density.And it performs better than the 10th-nearest-neighbor method.It is found that quiescent fraction of massive galaxies(M*>1010.3M?)is positively corre?lated with the environmental overdensity at all redshifts(0.5<z<2.5).The similarity of the quiescent fraction and Sersic index distributions is consistent with the picture in which quenching processes are correlated with morphology,such as bulge buildup.It also demonstrates strong evidence of environmental quenching for low mass galaxies(M*<1010.3M?)at z<1,suggesting that environmental quenching dominates the low-mass galaxies at z<1.