Bibcode
DOI
Graham, Alister W.
Referencia bibliográfica
The Astronomical Journal, Volume 121, Issue 2, pp. 820-840.
Advertised on:
2
2001
Número de citas
216
Número de citas referidas
189
Descripción
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-logn 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.