| The formation and evolution of galaxies is one of the most important frontiers in modern astrophysics.Clusters of galaxies are the largest gravitational bound system in the universe,where the stellar density is much higher than that of the field environment.The evolution of member galaxies in clusters is much more complicated than that of the isolated galaxies in field due to the dense environment of galaxy clusters.By advancing on the physical properties of member galaxies in clusters,we can not only improve the overall study of galaxy evolution,but also reveal the assembling history of galaxy clusters.High energy X-ray emission is an important method for studying the physical properties of galaxies.X-ray emission is prevalent in universe on different physical scales,from close binary systems to galaxies to galaxy clusters.Thanks to the high resolution and high sensitivity observations accumulated by Chandra Observatory in last 20 years,we can conduct systematic surveys regarding to the stellar population and gas content of member galaxies reside in nearby clusters,as well as the active galactic nuclei in the central region of interacting galaxies,to shed light on the evolution of galaxies in dense environment and the co-evolution between central supermassive black holes and host galaxies.In Chapter 2,we present a survey of intracluster X-ray point sources around member galaxies in the nearest and dynamically young galaxy cluster,Virgo,using archival Chandra observations.We detect a total of ?1200 in the sample of 80 early-type member galaxies in Virgo down to a limiting 0.5-8 keV luminosity of ?2×1038erg s-1.Based on the cumulative spatial and flux distributions of these sources,we statistically identify ?120 excess sources that are not associated with the main stellar content of the individual galaxies,nor with the cosmic X-ray background.This excess is significant at a 3.5 level.On the other hand,no significant excess sources are found at the outskirt of a control sample of field galaxies,suggesting that at least some fraction of the excess sources around the Virgo galaxies are truly intracluster X-ray sources.Assisted with ground-based and HST optical imaging of Virgo,we discuss the origins of these intracluster X-ray sources,in terms of supernova-kicked low-mass X-ray binaries(LMXBs),globular clusters,LMXBs associated with the diffuse intra-cluster light,stripped nucleated dwarf galaxies and free-floating massive black holes.The identification of such intracluster X-ray source population open up a new window to study the structure evolution of member galaxies in cluster environment.In Chapter 3,we present a systematic study of the diffuse hot gas around early-type galaxies(ETGs)residing in the Virgo cluster,based on archival Chandra observa-tions.Our representative sample consists of 79 galaxies with low-to-intermediate stellar masses(*?109-11M?),a mass range that has not been extensively explored with X-ray observations thus far.We detect diffuse X-ray emission in eight galaxies,and in only five cases a substantial fraction of the detected signals can be unambigu-ously attributed to truly diffuse hot gas,based on their spatial distribution and spectral property.For the remaining galaxies,especially the low mass early-type galaxies,there is no significant detectable diffuse X-ray emission.We constrain their average X-ray emission by performing a stacking analysis,finding that the specific X-ray luminosity is consistent with the expected contribution from unresolved stellar populations.The apparent paucity of truly diffuse hot gas in these low-and intermediate-mass ETGs may be the result of efficient ram pressure stripping by the hot intracluster medium.However,we also find no significant diffuse hot gas in a comparison sample of 57 field ETGs of similar stellar masses,using archival Chandra observations with similar sensi-tivity.This leaves us with the possibility that a galactic wind driven by nuclear activities and/or Type Ia supernovae is the primary mechanism of evacuating hot gas from the inner region of low-and intermediate-mass ETGs,regardless of the galactic environ-ment.Nevertheless,we do find strong evidence for on-going ram pressure stripping in two galaxies(NGC 4417 and NGC 4459),thus substantially increasing the inventory of hot gas stripping from small galaxies in Virgo.A better understanding of the roles of ram pressure stripping and galactic winds in regulating the hot gas content of ETGs,in particular ones with moderate masses,invites more sensitive X-ray observations for a large galaxy sample.In Chapter 4,we present a Chandra archival X-ray survey of optically selected nearby AGN pairs.Out of 1286 AGN pairs(with projected separations<100 kpc and velocity offsets?<600 km s-1)optically identified from the Sloan Digital Sky Survey Seventh Data Release,we find 67 systems with archival Chandra observations,which represent the largest sample of optically selected AGN pairs studied in the X-ray.Among the 67 AGN pairs,21 systems have both nuclei detected in the X-rays,36have one nucleus detected in the X-rays,whereas 10 have no X-ray detection.The X-ray detection rate,78/134=58%(±7%1 Poisson errors),is significantly higher than that(23/134=17%±4%)of a comparison sample of star-forming galaxy pairs,lending support to the optical AGN classification.In the conservative case where X-ray contamination from star formation is removed,the X-ray detection rate becomes27%±4%,consistent with predictions from latest galaxy merger simulations.The2–10 keV X-ray luminosity 2-10 keVincreases with decreasing projected separation in AGN pairs for 15 kpc,suggesting an enhancement of black hole accretion even in early-stage mergers.On the other hand,2-10 keVappears to decrease with decreasing projected separation at 15 kpc,which is contradictory to predictions from merger simulations.The apparent decrease in 2-10 keVof AGN pairs at 15 kpc may be caused by:(i)enhanced absorbing columns from merger-induced gas inflows,(ii)feedback effects from early-stage mergers.Future X-ray studies with larger samples are needed to put our results on firmer statistical ground.In Chapter 5,we summarize the full paper and propose the next step research plan in the future.We propose that the on-going eROSITA all-sky survey will afford a much larger galaxy sample with sensitive X-ray observations,which can help to further investigate the aforementioned studies in the future. |
PDF Full Text Request