Statistics Of Active Galatic Nuclei And Star-Forming Galaxies In Galaxy Groups

Active galactic nuclei(AGNs)refers to active galaxies or active galactic nuclei where exist violent activity or dramatic physical processes in their centers.It has been appreciated that AGNs have played an important role in star formation and evolution of galaxies.Many evidences show that most galaxies harbour supermassive black holes(SMBHs)in their centers.The AGNs are powered by gas accretion on to SMBHs,which promotes the rapid growth of black holes.Intense star formation in galaxies is accompanied by assembly of galactic structures such as bulge,bar,disk and spiral arms.The AGN and star formation activities are important driving forces for evolution of galaxies.There is a strong correlation between the growth of central black holes and star formation.It is well known that cold gas is raw material for star formation and the main source of fueling central black hole.Several physical processes have been supposed to affect the utilization and transport of cold gases,such as mergers,ram pressure strip-ping,evaporation of interstellar media,tidal effects due to the cluster potential,and gas starvation due to the absence of infall of new cold gas.Above these physical processes will directly influence the intensity of star formation and the turn-on and off of AGN in galaxies.Each galaxy is in a different gravitational environment such as galaxy clusters,galaxy groups and isolated fields.The incidences of AGN and star-forming galaxies(SFGs)in different environments provide valuable additional observational constraints on AGN fueling mechanisms and their feedback on star formation activities.Previous studies showed that the AGN fraction in groups is larger than that in clusters,suggesting that galaxies in groups/fields retain larger reservoirs of cold gas to fuel AGN activity than their counterparts in clusters.However,according to the results of previous studies,the AGN fraction in galaxy groups/clusters vary greatly with AGN selection criteria,redshift coverage and limiting flux.In addition,AGN and SFG show some differences in spatial distribution,morphological characteristics and star formation properties(such as stellar mass,star formation rate,stellar age,etc.)among galaxy groups/clusters with a wide coverage of richness.In this thesis,the above observational properties of AGNs and SFGs in the environment of galaxy groups/clusters are studied statistically.Firstly,on the basis of our previous study on the fraction of the X-ray selected AGNs with optical emission lines in COSMOS(Cosmological Evolution Survey)field,the normalized projection distance of AGN in the galaxy groups,R/R200 and its vari-ance with the X-ray lumonosity of AGN host groups are further studied,as well as the changes of the corrected overall AGN fraction with redshift and the group richness.For the X-ray groups at z>0.5,most member AGNs are X-ray bright,optically dull,which results in a lower AGN fraction at higher redshifts.The AGN fraction in isolated fields also exhibits a rising trend with redshift,and the slope is consistent with that in groups.The environment of galaxy groups seems to make no difference in detection probability of the X-ray selected AGNs with emission lines.For the groups in vari-ous redshift ranges,a larger AGN fractions are found in poorer groups.For those poor groups with less than 30 member galaxies,a larger fraction of group AGNs might still be in the high-accretion phase,whereas the AGN population in rich clusters is mostly in the low-accretion X-ray-dominanted phase.Secondly,we take use of a large sample of 254220 galaxies in 81089 groups,which are selected from the spectroscopic galaxy sample of the SDSS DR12.Regarding the group richness as a direct indicator of gravity environment on a scale of 0.1-1 Mpc,we investigate the projected distribution,morphologies,environmental densities,and star formation properties of the AGN host galaxies and SFGs in the groups at z<0.2.Partic-ularly,we compare the observational features between the AGNs and SFGs in different gravity environments such as galaxy pair,poor groups,rich groups,and poor clusters.All member galaxies in groups are spectroscopically classified into AGN host,star-forming,composite,and unclassified galaxies.It is found that AGN fraction slightly declines with richness for the groups/clusters.The SFG fraction is on average about 2 times larger than the AGN fraction,with a significant declining trend with richness.The group AGNs are preferentially reside in lenticular and buge-dominated disc galaxies,whereas the majority of SFGs are late-type discs.Compared with the SFGs,the AGNs in poor groups(5 ?Nrich ?10)are closer to group center.The AGN fraction does not change with the distance to the group center,whereas the SFG fraction tends to be higher in the outskirts.The AGNs in groups have a higher incidence than the SFGs for the massive(log(M*/M(?))>10.7)galaxies,and the mean SFG fraction is about 6 times as that of AGNs in the late-type galaxies with lower masses at larger radius.The distribution of environmental luminosity densities shows that the AGNs are likely to be reside in a denser environment relative to the SFGs.Compared with the SFGs in groups,the group AGNs are found to have a higher mean stellar mass,a lower mean star formation rate,and an older mean stellar age.The mean stellar age for AGN host galaxies seem to be independent on group richness,implying that their star formation histories are insensitive to the environment of groups.