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

It is an observational fact that bulges of spiral galaxies contain a high fraction of old and metal-rich stars. Following this observational fact, we have investigated the colors of 21 bulges hosted by a selected sample of high surface brightness spiral galaxies and low surface brightness galaxies observed in the B and R optical bands and the J and K_s near-IR bands. Using structural parameters derived from these observations, we obtain evidence that bulges could be formed by pure disk evolution (secular evolution), in agreement with suggestions by some authors. The color profiles, especially the near-IR ones, show null or almost null color gradients, supporting the hypothesis that the disk stellar populations are similar to those present in the bulge and/or that some bulges can be understood as disks with enhanced stellar density (or pseudobulges). In the optical, half of the galaxies present an inverse color gradient, giving additional evidence in favor of secular evolution for the sample investigated. The comparison of the observed colors with those obtained from spectrophotometric models of galaxy evolution suggests that bulges of the selected sample have solar and subsolar metallicities and are independent of the current stellar formation rate. Also, we obtain evidence suggesting that galaxies hosting small bulges tend to be systematically metal-poor compared to those with larger bulges. These results are being checked more carefully with high signal-to-noise ratio spectroscopy.