The Star Formation And Quenching Mechanisms In Early Type Galaxies

Galaxies from z～3 to the present can be mainly classified into two types(i.e.,red sequence and blue cloud)in the color-magnitude diagrams.Red sequence consists of quiescent,bulge-dominated early-type galaxies(ETGs),while blue cloud mainly contains active,disk-dominated late-type galaxies(LTGs).The significant increase in the number density of red galaxies at low-z universe indicates that blue galaxies might suffer from some physical processes that quench their star formation.The processes involving stellar mass are referred to as "mass quenching",while the effects related to environment are referred to as "environmental quenching".Lenticular galaxies(S0s),as a type of ETGs,are thought to be the intermediate transition population between the elliptical and the spiral galaxies in the Hubble tuning fork diagram.The prominent features of S0 galaxies are the absence of spiral arms and noticeable star formation regions.We find an S0 galaxy PGC 26218,which presents the central star-formation and post-starburst outside in the disk,and study its physical properties.The main purpose of this thesis is to investigate the mechanisms for galaxy quench-ing at z<2.5,as well as the origin of star formation and starburst in PGC 26218.The research content of this thesis will give insight into the relative role played by stel-lar mass and environment in the galaxy quenching,as well as the diverse evolutionary paths in the Hubble sequence of the galaxy.The research mainly consists of three parts.(1)We use a sample of massive galaxies from Sloan Digital Sky Survey Data Release 7(SDSS DR7)at z<0.2 to investigate the variation of physical properties from the blue cloud to green valley to red sequence.We find that the physical parameters related to the central density show a continuous change from the blue,to the green valley and to the red galaxies.Besides,massive quiescent galaxies(M*>1010M⊙),on average,are found to have denser cores than both green valley and star-forming galaxies in the same stellar mass range.These results suggest that the quenching of local galaxies is accompanied by the growth of bulge,at least in massive galaxies.(2)We select a galaxy sample from Cosmic Assembly Near infrared Deep Extra-galactic Legacy Survey(CANDELS)at 0.5<z<2.5 and study the mechanisms of galaxy quenching.We confirm that massive quiescent galaxies at 0.5<z<2.5,on average,have larger stellar mass surface densities within the central 1 kpc(∑1)and smaller sizes than that of star-forming galaxies with the same stellar mass.The differ-ence in structures between quiescent and star-forming galaxies is smoothed with the decreasing redshift.At 0.5<z<1.5,both the quiescent fractions of centrals and satel-lites increase with the increasing ∑1/M*·The increasing trend is mild for low-mass centrals(M*<～1010M⊙,while the trend is steep for massive centrals.In addition,the quiescent fraction of low-mass satellites is significantly higher than that of low-mass centrals(20%vs.5%).Our results reveal that mass quenching dominates the quench-ing of massive galaxies since z～2.5,while the environmental quenching might play an important role in the quenching of low-mass galaxies at z≤1.5.(3)For the nearby S0 galaxy PGC 26218,we present 2D-spectroscopic observa-tions from Centro Astronomico Hispano Aleman(CAHA)3.5 m telescope and the mil-limetre observation from NOrthern Extended Millimeter Array(NOEMA).It is found that PGC 26218 follows the star forming main sequence relation and the Kennicutt-Schmidt law.Based on the kinematics of CO(J=1-0),stars and Ha,we find a large difference between the rotational axes of CO(J=1-0)and Ha and the difference reaches 45°.Furthermore,an inflow component(～46 km s-1)is indispensable when fitting the CO(J=1-0)emission line.Based on a pure disk model for the decomposition of the optical image,we find that PGC 26218 shows multiple nuclear structures.Our find-ings suggest that PGC 26218 may experience a gas-rich minor merger,which led to an inward migration of gas and triggered the starburst a few hundred Myr year ago and current star formation.In general,the stellar mass and the external environment are important factors that regulate the formation and evolution of the galaxies.At the same time,the minor mergers of galaxies also play an important role in trigging the star formation of nearby S0 galaxies.