Research On Star Formation History Of Galaxies And Evolution Of Their Properties

Galaxy formation and evolution is an very important but complicated field in mod-ern astronomy. Using archive and our own data, we explored the star formation history, stellar mass-metallicity relation and its redshift evolution, and galaxy structural evolu-tion.The first part of the dissertation talked about an important process in the star for-mation history of galaxies, the quenching process and its time scale occuring rate among galaxies. It is well known that the star formation density of the universe has been declin-ing since z-2. Part of the driver for this trend is the secular evolution of star forming galaxies, while another important reason is the quenching process that shutdown the star formation in galaxies. The properties of the quenching process is not clear yet. For example, what is the time scale of quenching? How many galaxies will experience this process in a certain time? Since the NUV emission is dominated by massive stars, it is a good probe for the recent star formation activity and therefore sensitive to the rapid quenching process. Thus, to answer these questions, we combine the UV and optical imaging survey dataset and construct stellar popolation models to analyze galaxy num-ber density profile in NUV-u colour space and their distribution in NUV—u versus u — i colour-colour diagram. Our results show that the model with quenching time scale of 0.5 Gyr best fitting the data. Meanwhile, statistically speaking, most galaxies have a quenching time scale between 0.2-1 Gyr. In addition, by decomposing galax-ies’number density profile, we found the quenching rate is between 19%Gyr-33%Gyr. More massive galaxies show a higher quenching rate.The second part of this dissertation is about the mass-metallicity (MZ) relation. Using SDSS and other dataset, many previous works found the massive galaxies at z～0.1 show a strong correlation between their stellar mass and metallicity, i.e. MZ relation. There are several basic problems in this field, such as how does the MZ rela-tion evolve along with redshift? Does low mass galaxies show the same MZ relation with massive galaxies? What is the physical driver for the scatter in MZ relation? Due to the difficulties suffered by spectroscopic observation for high redshift galaxies, most of the previous works concerning the MZ relation at high redshift utilise a small sam-ple concentrating on high luminosity (or massive) galaxies and their results show large uncertainties. To overcome these difficulties, we selected a local sample of 700 Lyman Break Analogues (LBAs) using Ha luminosity and surface brightness and studied their MZ relation. We found that the MZ relation of LBAs is consistent with that of galaxies at z= 1.4—1.7. The MZ relation at high redshift has been extended to lower mass range by our result. A interesting finding is that massive galaxies at different redshifts show similar metallicity while low mass galaxies tend to be less metal-enriched at higher red-shift. Given the same stellar mass, the trend of lower metallicity in galaxies at higher redshift support the downsizing evolution effect. Using this LBA sample, we also ex-plored the origins of the scatter in MZ relation. Our results indicate that star formation rate, dust extinction, and gas mass fraction does not seem to contribute to scatter in the MZ relation while the stellar age, Dn(4000), plays a important role.To investigate the MZ relation evolution at low mass range, we selected a blue compact dwarf (BCD) galaxy sample using COSMOS dataset. By applying for MMT telescope observation time, we obtained high quality spectra for a sample of 74 BCDs at z～[0.2,0.5] using Hectospec on MMT. Meanwhile, we also selected a local BCD sample from SDSS dataset. By comparing the two BCD samples at different redshifts, we found these low mass galaxies show higher R23 but similar 032 line ratio at inter-mediate redshift. Furthermore, the metallicities of these low mass galaxies show weak redshift evolution.In the third part of the dissertation, we investigated the structural evolution of dwarf galaxies. As the most metal deficient and youngest galaxies in the universe, whether BCDs have consistent structure with other dwarf galaxies provide an important inspect to the evolution sequence between different types of dwarf galaxies. Since the near infrared (NIR) emission is dominated by old stellar population, deep NIR imaging data is neccessary to study the structure of old population of dwarfs. Benefit from the deep NIR imagings of CANDELS survey, we analyzed the structure of underlying population for a sample of 34 BCDs. We extracted surface brightness profile in optical from HST F435W and in NIR from F160W and fitted with Sersic function. The surface brightness in F160W reaches～26 mag arcsec-2, the deepest NIR image data for BCDs so far. Our results show that the BCDs have structure in consistent with the early-type dwarfs. All types of dwarfs seems to follow a similar luminosity-radius relationship which suggests a unified structural evolution for dwarf galaxies and no significant change of structure needed given the evolutionary relation between different dwarf galaxies.