Bayesian Inference Of Galaxy Spectra

The galaxy spectra contain a wealth of physical information.To analyse the observed galaxy spectra and to infer those important galaxy properties remain hot issues in galaxy formation and evolution studies.We develop a suit of full spectra fitting code,which is called Bayesian Inference of Galaxy Spectra(BIGS),to infer the various galaxy properties from spectra of unresolved stellar populations.This code uses the state-of-the-art stellar population synthesise models to model the star formation and evolution,as well as the dust attenuation in galaxies.The Bayesian inferences method used in BIGS allows one to rigorously explore the posterior distributions of model parameters as well as potential degeneracy between them,and to distinguish different models with Bayesian evidence.Using this code,we analyse the stellar initial mass functions(IMF)of a large sample of early type galaxies(ETGs)provided by Ma NGA.The large number of IFU spectra of individual galaxies provide high signal-to-noise composite spectra that are essential for constraining IMF and to investigate possible radial gradients of the IMF within individual galaxies.We find that the IMF slope depends systematically on galaxy velocity dispersion,in that galaxies of higher velocity dispersion prefer a more bottom-heavy IMF,but the dependence is almost entirely due to the change of metallicity,Z,with velocity dispersion.The IMF near the centers of massive ETGs appears more bottom-heavy than that in the outer parts,while a weak opposite trend is seen for low-mass ETGs.We also apply this Bayesian spectra fitting methods to study the star formation history(SFH)in Ma NGA galaxies,especially the least massive ones.Resolved observations have revealed the existences of a old stellar population in those low mass,blue galaxies,which gives a strong constrain on galaxy formation and evolution models.We find that old stellar populations in unresolved low mass galaxies can be detected as long as SNR is high.With the help of UKIDSS nir-infered photometry,the mass fractions of those old population and be further constrained.By comparing the SFH of stacked spectra in different mass bins and radial bins,we find that massive ETGs formed earlier then less massive,late-type ones,and outskirts of LTGs are systematically younger.