Author: Alister W. Graham
Abstract: From a diameter-limited sample of 86 `face-on' 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 Sérsic r1/n bulge and a seeing convolved exponential disk which were fitted to the optical (B, R, and I) and near-infrared (K) galaxy light profiles. In general, early-type spiral galaxy bulges have Sérsic values of n>1, and late-type spiral galaxy bulges have values of n<1. Use of the exponential (n=1) bulge model is shown to restrict the range of re/h and B/D values by more than a factor of 2. Application of the r1/n bulge models results in a larger mean re/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 (>3σ) for the early-type spirals than the late-type spirals.
Two new parameters are introduced to measure the prominence of the bulge. The first is the difference between the central surface brightness of the galaxy and the surface brightness where the bulge and disk contribute equally. The other test uses the radius where the contribution from the disk and bulge light is equal — normalised for the effect of intrinsically different galaxy sizes. Both of these parameters reveale that the early-type spiral galaxies `appear' to have significantly (>2σ in all passbands) bigger and brighter bulges than late-type spiral galaxies. This apparent contradiction with the re/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 B/D luminosity ratio and the concentration index C31 are, in agreement with past studies, positively correlated and decrease as one moves along the spiral Hubble sequence towards later galaxy types. Although for galaxies with large extended bulges, the concentration index no longer traces the B/D luminosity ratio in a one-to-one fashion. A strong (Spearman's rs=0.80) and highly significant positive correlation exists between the shape, n, of the bulge light profile and the bulge-to-disk luminosity 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; for a given luminosity they have a noticeably smaller shape parameter and hence a more dramatically declining stellar density profile at larger radii.