A multi-wavelength study of the stellar content of nearby galaxies

Abstract

We present an investigation of the stellar content of nearby galaxies, focusing on: (a) a census of the stellar mass in star-forming galaxies in the Local Universe, and its relation with their star-forming activity; (b) a case study of the effects of galaxy interaction on the globular cluster (GC) population of an elliptical galaxy. First, we present the study of the stellar content and morphological parameters of a sample of FIR-selected galaxies representative of the star-formation activity in the Local Universe: the Star Formation Reference Survey (SFRS; Ashby et al., 2011). We perform a detailed two-dimensional fit of the galaxian surface brightness by using a combination of Sersic, exponential disk, and PSF models, intended to account for the different bulge, disk, and AGN components. We use the light profiles of the models to obtain integrated stellar luminosities and masses for the galaxies, as well as disentangle their bulge and disk sub-components. Based on these results we derive the luminosity and mass functions of nearby star-forming galaxies, and we explore the relations between star-formation activity and galaxian stellar mass, morphology, and AGN content. From the morphological analysis, we find that: (1) the sample consists of ∼ 62% diskdominated galaxies, and ∼ 38% bulge-dominated galaxies; (2) 25% of the non-AGN SFRS galaxies require a PSF component in their morphological decomposition, which could represent either a weak or unresolved bulge, or a yet-unknown AGN; (3) there is no clear correlation between the specific star-formation rate (sSFR) and morphological parameters as represented by the the bulge-to-total luminosity ratio and/or the Sersic index. Our stellar mass function (MF), which covers the whole range of galaxy masses from dwarf galaxies to large spirals, is in general agreement with the previously derived MF of FIR-selected galaxies, while its faint end appears to be flatter than that of MFs of optically-selected samples. We also introduce a “specific SFR function” (the volume density of galaxies at a given specific SFR), which spans 4 orders of magnitude, and is characterized by a sharp peak at sSFR ∼ 5 . 10−10 M_yr−1/M_. As a case study of a post-merger galaxy in the Local Universe, we present an analysis of the GC population of the elliptical galaxy NGC 4261 based on HST WFPC2 data in the B, V and I bands. We study the spatial distribution of the GCs in order to probe the anisotropy in the azimuthal distribution of the discrete X-ray sources in the galaxy revealed by Chandra images (Zezas et al., 2003). The luminosity function of our GC sample (complete at the 90% level for mV = 23.8 mag) peaks at mV = 25.1+1.0 −0.6 mag, which corresponds to a distance consistent with previous measurements. The colour distribution can be interpreted as being the superposition of a blue and red GC component with average colours V −I = 1.01+0.06 −0.06 mag and 1.27+0.06 −0.08 mag, respectively. This is consistent with a bimodal colour distribution typical of elliptical galaxies. The red GC’s radial profile is steeper than that of the galaxy surface brightness, while the profile of the blue sub-population looks more consistent with it. The most striking finding is the significant asymmetry in the azimuthal distribution of the GC population about a NE-SW direction. The lack of any obvious feature in the morphology of the galaxy suggests that the asymmetry could be the result of a past interaction or a dry merger.