An Investigation into the Prominence of Spiral Galaxy Bulges

From a diameter-limited sample of 86 low-inclination (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 that 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. In the B band, only eight galaxies have a bulge shape parameter n consistent with the exponential value 1, and only five galaxies do in the K band. Use of the exponential 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, unlike exponential bulge models, results in a larger mean re/h ratio for the early-type spiral galaxies than for 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 σ) for the early-type spirals than for 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 level at which the bulge and disk contribute equally. The other test uses the radius at which the contribution from the disk and bulge light are equal, normalized for the effect of intrinsically different galaxy sizes. Both of these parameters reveal that the early-type spiral galaxies "appear" to have significantly (more than 2 σ 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 stellar density while maintaining the same effective bulge-to-disk ratio. The B/D luminosity ratio and the concentration index C31, in agreement with past studies, are positively correlated and decrease as one moves along the spiral Hubble sequence toward later spiral 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 rank-order correlation coefficient, 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 elliptical galaxies appear to be the faint extension to the brighter elliptical galaxies, the bulges of spiral galaxies do not; for a given luminosity they have a noticeably smaller shape parameter and hence a more dramatic decline of stellar density at large radii.