Applications Of Evolutionary Population Synthesis

The formation and evolution of galaxies has been the biggest challenge in astronomy and astrophysics. It can help us not only to understand the histories of galaxies but also to join the studies of stars and cosmology. Because most of the observable mass in galaxies is contained in stars, the evolution of stars determine many integrated peculiarities of galaxies, e.g., luminosity, colours, and spectra. Therefore, one can investigate the formation and evolution of galaxies via studying the stellar contents of galaxies. In fact, the determination of stellar contents of galaxies has been a very important part in astronomy and astrophysics studies. Evolutionary stellar population synthesis is just a powerful technique for such works. We discuss some key problems on the applications of evolutionary population synthesis technique and show some new results in the thesis .First, we investigate the ability of colours for determining the stellar age and metallicity of stellar populations. The motivation is that it is difficult to measure the above parameters of distant (redshift > 0.3) galaxies and star clusters via spectra-like methods as these is no available spectra for them, but colours are actually available for such galaxies and star clusters. Our results show that colours can be used to determine the stellar-population parameters of populations. The best colour pairs for measuring the stellar age and metallicity are shown, with their possible result uncertainties. The results can help us to get the information about the stellar populations of distant galaxies, and then to give further studies on the formation and evolution of galaxies.Second, because observations showed that most galaxies including early-type ones have recent (within about 1 Gyr) star formations, and young populations can obviously affect the determination of stellar-population parameters, we investigate how young populations can affect the determination of stellar ages and metallicities of populations. A statistical method to correct the effects of young populations is brought forward by the work. Using this technique, we will be able to obtain the distributions and averages of the ages and metallicities of stellar populations. We also analyze the effects of young populations on the fundamental plane and Kormendy relation of galaxies. It shows that the scatter in the fundamental relations may be smaller when taking the effects of young populations into account, and we will measure less distances for galaxies after correcting the effects of young populations.Third, to solve the problem that it is difficult to calculate the isochrones of binary stellar populations via stellar evolution code directly, we build an isochrone database for conveniently and quickly modeling populations. Then we calculate the spectral energy distributions, Lick indices, and colours of both single stellar populations and binary stellar populations. This makes it possible to take binary interactions into account in stellar population studies, and galaxy formation and evolution studies more conveniently.Forth, we investigate the effects of binary interactions on the results of stellar population synthesis and the results of stellar population studies. The results show that binary interactions make the colours of populations bluer, Hβindex larger, and metal line indices smaller. When we use a Lick index method to determine the age and metallicity, we will obtain obviously less ages compared to the real values of populations. When we take a colour method to measure the stellar-population parameters, we will get obviously less stellar metallicities. We also find that the real ages of populations are related to the values fitted via single stellar populations. Thus we can calculate the stellar ages fitted via binary populations from the stellar ages and metallicities fitted via single populations. It is also shown that the relative results obtained via both single populations and binary populations are similar.Fifth, we determine the stellar ages and metallicities of about 80 elliptical galaxies and test a new model for the formation of elliptical galaxies. Our results show supports to the model, while show a few opposition. This is actually useful for galaxy formation investigations.As a whole, this work solved some key problems on the applications of evolutionary populations synthesis, and make the population synthesis technique more useful and powerful in astrophysics studies.