Stellar Population Synthesis

In this paper we review the study of stellar population synthesis.So far there exist three methods in the study of the integrated light of stellar population-trail-and error, automated, and evolutionary population synthesis (EPS).We have discussed advantages and disadvantages for these methods. Among the three methods the EPS is the most direct approach to model galaxies.In this scheme,the model builder starts with knowledge of stellar evolution and attempts to build a model galaxy with physical input parameters such as star formation rate (SFR)and the initial mass function (IMF)slope.Therefore we have discussed emphatically the EPS method.First we have described and given the often used grids of several key ingredients in the EPS studies:(1)the library of evolutionary tracks used to calculate isochrones in the color-magnitude diagram (CMD),(2)the libraries of spectra adopted, which include empirical and theoretical stellar spectral libraries,star cluster library,active galactic nuclear (AGN)library and galaxy library, to derive the integrated spectral energy distributions (ISED) or magnitudes and colors in the suitable passbands,(3)the IMF used to evaluate the relative proportions of stars in the various evolutionary phases, and (4)the assumption for the underlying star formation rate (SFR)and chemical enrichment.Then we have listed several population synthesis criterions, i.e.broadband color indices, the integrated spectral energy distribution (ISED)and narrow band color indices, given the basic method of calculating broadband colors and flux-distribution for a simple stellar population (SSP).At last we have discussed simply the existed limitations, which are caused by some uncertainties in its two principal building blocks:stellar evolution models and spectral libraries in the studies of the EPS. Stellar evolution models are often subject to limitations in the following areas:the atomic data (radiative opacities, heavy element mixture, helium content, reaction rates), the convection theory (mixing length, overshooting, semiconvection), the mixing in radiative regions (rotational mixing,thermal diffusion, gravitational settling), and the mass loss. As for stellar spectral libraries, most of them, including the empirical and the theoretical, are available only in some effective temperature ranges, surface gravity and/or metallicity. Thus inmost EPS studies several stellar spectral libraries are often incorporated to convert stellar evolutionary parameters into colors. There also exist some mismatches in temperature to color conversion between synthetic/empirical estimates and the shape of the flux curve, especially that of type K and M stars