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Composite bulges: the coexistence of classical bulges and discy pseudo-bulges in S0 and spiral galaxies
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2015 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 446, no 4, 4039-4077 p.Article in journal (Refereed) Published
Abstract [en]

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 similar to 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 similar to 7 x 10(9)-9 x 10(10) M-circle dot. The classical-bulge components have Sersic indices of 0.9-2.2, effective radii of 25-430 pc and stellar masses of 5 x 10(8)-3 x 10(10) M-circle dot; 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'.

Place, publisher, year, edition, pages
2015. Vol. 446, no 4, 4039-4077 p.
Keyword [en]
galaxies: bulges, galaxies: elliptical and lenticular, cD, galaxies: evolution, galaxies: kinematics and dynamics, galaxies: spiral, galaxies: structure
National Category
Astronomy, Astrophysics and Cosmology
URN: urn:nbn:se:su:diva-115928DOI: 10.1093/mnras/stu2376ISI: 000350272400062OAI: diva2:805128


Available from: 2015-04-14 Created: 2015-04-08 Last updated: 2015-04-14Bibliographically approved

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Nowak, Nina
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Department of AstronomyThe Oskar Klein Centre for Cosmo Particle Physics (OKC)
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