An Investigation into the Prominence of Spiral Galaxy Bulges

From a diameter-limited sample of 86 `face-on' spiral galaxies, thebulge-to-disk size and luminosity ratios, and other quantitative measurementsfor the prominence of the bulge are derived. The bulge and disk parameters havebeen estimated using a seeing convolved Sersic r^(1/n) bulge and a seeingconvolved exponential disk. In general, early-type spiral galaxy bulges haveSersic values of n>1, and late-type spiral galaxy bulges have values of n<1. Inthe B-band, only 8 galaxies have a bulge shape parameter n consistent with theexponential value of 1, and only 5 galaxies do in the K-band. Application ofthe r^(1/n) bulge models results in a larger mean r_e/h ratio for theearly-type spiral galaxies than the late-type spiral galaxies. Although, thisresult is shown not to be statistically significant. The mean B/D luminosityratio is, however, significantly larger (> 3-sigma) for the early-type spiralsthan the late-type spirals. This apparent contradiction with the r_e/h valuescan be explained with an iceberg-like scenario, in which the bulges inlate-type spiral galaxies are relatively submerged in their disk. This can beachieved by varying the relative bulge/disk stellar density while maintainingthe same effective bulge-to-disk size ratio. The absolute bulge magnitude - log(n) diagram is used as a diagnostic toolfor comparative studies with dwarf elliptical and ordinary elliptical galaxies.At least in the B-band, these objects occupy distinctly different regions ofthis parameter space. While the dwarf ellipticals appear to be the faintextension to the brighter elliptical galaxies, the bulges of spiral galaxiesare not.Comment: 33 pages (includes 27 figures, 4 tables). To be published in AJ (tentatively scheduled for Feb 2001