Bulge Evolution in Face-On Spiral and Low Surface Brightness Galaxies

It is an observational fact that bulges of spiral galaxies contain a highfraction of old and metal-rich stars. Following this observational fact, wehave investigated colors of 21 bulges hosted by a selected sample of highsurface brightness spirals and low surface brightness galaxies observed in Band R optical bands and in J and Ks near-IR bands. Using structural parametersderived from these observations we obtain evidence that bulges could be formedby pure disk evolution (secular evolution), in agreement with the suggestion bysome authors. The color profiles, especially the near-IR ones show null oralmost null color gradients, supporting the hypothesis that the disk stellarpopulations are similar to those present in the bulge, and/or some bulges canbe understood as disks with enhanced stellar density (or pseudobulges). In theoptical, half of the galaxies present an inverse color gradient, givingadditional evidence in favor of secular evolution for the sample investigated.The comparison of the observed colors with those obtained fromspectrophotometric models of galaxy evolution suggests that bulges of theselected sample have solar and subsolar metallicity, and are independent of thecurrent stellar formation rate. Also, we obtain evidence suggesting thatgalaxies hosting small bulges tend to be systematically metal poor compared tothose with larger bulges. These results are being checked more carefully withhigh S/N spectroscopy.