An investigation into the prominence of spiral galaxy bulges

Abstract

From a diameter-limited sample of 86 `face-on&#39; spiral galaxies, the bulge-to-disk size and luminosity ratios, and other quantitative measurements for the prominence of the bulge are derived. The bulge and disk parameters have been estimated using a seeing convolved Sersic r^(1/n) bulge and a seeing convolved exponential disk. In general, early-type spiral galaxy bulges have Sersic values of n>1, and late-type spiral galaxy bulges have values of n<1. In the B-band, only 8 galaxies have a bulge shape parameter n consistent with the exponential value of 1, and only 5 galaxies do in the K-band. Application of the r^(1/n) bulge models results in a larger mean r_e/h ratio for the early-type spiral galaxies than the late-type spiral galaxies. Although, this result is shown not to be statistically significant. The mean B/D luminosity ratio is, however, significantly larger (> 3-sigma) for the early-type spirals than the late-type spirals. This apparent contradiction with the r_e/h values can be explained with an iceberg-like scenario, in which the bulges in late-type spiral galaxies are relatively submerged in their disk. This can be achieved by varying the relative bulge/disk stellar density while maintaining the same effective bulge-to-disk size ratio. The absolute bulge magnitude - log(n) diagram is used as a diagnostic tool for comparative studies with dwarf elliptical and ordinary elliptical galaxies. At least in the B-band, these objects occupy distinctly different regions of this parameter space. While the dwarf ellipticals appear to be the faint extension to the brighter elliptical galaxies, the bulges of spiral galaxies are not.