An investigation into the prominence of spiral galaxy bulges

From a diameter-limited sample of 86 low-inclination (face-on) spiral galax ies, the bulge-to-disk size and luminosity ratios and other quantitative me asurements 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 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 values of n > 1, and late-type Sersic spiral galaxy bulges have values of n < 1. In the B band, only eight galaxies hav e a bulge shape parameter n consistent with the exponential value 1, and on ly five galaxies do in the K band. Use of the exponential bulge model is sh own to restrict the range of r(e)/h and B/D values by more than a factor of 2. Application of the r(1/n) bulge models, unlike exponential bulge models , results in a larger mean r(e)/h ratio for the early-type spiral galaxies than for the late-type spiral galaxies, although this result is shown not t o be statistically significant. The mean B/D luminosity ratio is, however, significantly larger (>3 sigma) for the early-type spirals than for the lat e-type spirals. Two new parameters are introduced to measure the prominence of the bulge. The first is the difference between the central surface brig htness of the galaxy and the surface brightness level at which the bulge an d disk contribute equally. The other test uses the radius at which the cont ribution from the disk and bulge light are equal, normalized for the effect of intrinsically different galaxy sizes. Both of these parameters reveal t hat the early-type spiral galaxies "appear" to have significantly (more tha n 2 sigma in all passbands) bigger and brighter bulges than late-type spira l galaxies. This apparent contradiction with the r(e)/h values can be expla ined 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 v arying the relative stellar density while maintaining the same effective bu lge-to-disk ratio. The B/D luminosity ratio and the concentration index in agreement with past studies, are positively correlated and decrease as one moves along the spiral Hubble sequence toward later spiral galaxy types, al though for galaxies with large extended bulges the concentration index no l onger traces the B/D luminosity ratio in a one-to-one fashion. A strong (Sp earman's rank-order correlation coefficient, and highly significant positiv e correlation exists between the shape, n, of the bulge light r(s) = 0.80) profile and the bulge-to-disk luminosity ratio. The absolute bulge magnitud e-log n diagram is used as a diagnostic tool for comparative studies with d warf elliptical and ordinary elliptical galaxies. At least in the B band th ese objects occupy distinctly different regions of this parameter space. Wh ile the dwarf elliptical galaxies appear to be the faint extension to the b righter elliptical galaxies, the bulges of spiral galaxies do not; for a gi ven luminosity they have a noticeably smaller shape parameter and hence a m ore dramatic decline of stellar density at large radii.