Publication Date:
2014-12-12
Description:
We present an analysis of nine S0–Sb galaxies which have (photometric) bulges consisting of two distinct components. The outer component is a flattened, kinematically cool, disc-like structure: a ‘discy pseudo-bulge’. Embedded inside is a rounder, kinematically hot spheroidal structure: a ‘classical bulge’. This indicates that pseudo-bulges and classical bulges are not mutually exclusive phenomena: some galaxies have both. The discy pseudo-bulges almost always consist of an exponential disc (scalelengths = 125–870 pc, mean size ~440 pc) with one or more disc-related subcomponents: nuclear rings, nuclear bars, and/or spiral arms. They constitute 11–59 per cent of the galaxy stellar mass (mean PB / T = 0.33), with stellar masses ~7 10 9 –9 10 10 M . The classical-bulge components have Sérsic indices of 0.9–2.2, effective radii of 25–430 pc and stellar masses of 5 10 8 –3 10 10 M ; they are usually 〈10 per cent of the galaxy's stellar mass (mean B / T = 0.06). The classical bulges do show rotation, but are clearly kinematically hotter than the discy pseudo-bulges. Dynamical modelling of three systems indicates that velocity dispersions are isotropic in the classical bulges and equatorially biased in the discy pseudo-bulges. In the mass–radius and mass–stellar mass density planes, classical-bulge components follow sequences defined by ellipticals and (larger) classical bulges. Discy pseudo-bulges also fall on this sequence; they are more compact than large-scale discs of similar mass. Although some classical bulges are quite compact, they are as a class clearly distinct from nuclear star clusters in both size and mass; in at least two galaxies they coexist with nuclear clusters. Since almost all the galaxies in this study are barred, they probably also host boxy/peanut-shaped bulges (vertically thickened inner parts of bars). NGC 3368 shows isophotal evidence for such a zone just outside its discy pseudo-bulge, making it a clear case of a galaxy with all three types of ‘bulge’.
Print ISSN:
0035-8711
Electronic ISSN:
1365-2966
Topics:
Physics
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