Colors and mass-to-light ratios of bulges and disks of nearby spiral galaxies

Abstract

We investigated the colors and mass-to-light (M/L) ratios of bulges and disks for 28 nearby spiral galaxies with various morphological types of Sab-Scd, using images in optical and near-infrared (V, I, and J) bands as well as published rotation curves. It is shown that the observed colors and M/ L values generally agree with the galaxy-formation model with an exponentially declining star-formation rate and a shallow slope initial mass function (e.g., Scalo IMF) for both the bulges and the disks. We find that the bulge M/ L is generally higher than the disk M/ L, and that the galaxies with a larger bulge-to-total luminosity (B/ T) ratio tend to have a smaller bulge M/L. This fact indicates that the luminosity-weighted average age of bulges for early-type spirals is younger than that for later-type spirals. These results support a formation scenario that the bulges of middle-type and early-type spirals produce young stars. © 2008. Astronomical Society of Japan.