  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 colors of 21 bulges hosted by a selected sample of high
surface brightness spirals and low surface brightness galaxies observed in B
and R optical bands and in J and Ks 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 the suggestion 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 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 metallicity, 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 S/N spectroscopy.
