Stellar populations in bulges of spiral galaxies

Abstract

To better understand bulge formation, we have used long-slit spectroscopy and imaging to obtain luminosity-weighted (SSP) ages and abundances, line of sight velocity distributions, and bulge-to-disk decomposition for 38 spiral galaxies. We specifically included several blue bulges to see if these show evidence for secular evolution. Here we describe the stellar populations and how they relate to the kinematics and dynamics. We find that red bulges are similar to luminous ellipticals in their central line strengths, with large SSP ages and super-solar metallicities and α/Fe ratios. Blue bulges are similar to low-luminosity ellipticals in their central line strengths. They have solar α/Fe ratios and either small age or small metallicity. The colors, metallicities, and α/Fe ratios of bulges are correlated with the central velocity dispersion and maximum disk rotational velocity. Most bulges have metallicity gradients, but the metallicities of the bulge and disk are correlated. These observations suggest that the star-formation histories of ellipticals, bulges, and disks are sensitive primarily to the depth of the galactic potential well, but the scatter in the scaling relations leaves room for multiple formation mechanisms.