Composite bulges: the coexistence of classical bulges and discy pseudo-bulges in S0 and spiral galaxies

Abstract

We study nine S0-Sb galaxies with (photometric) bulges consisting of two distinct components. The outer component is a flattened, kinematically cool, disklike structure: a "disky pseudobulge". Embedded inside is a rounder, kinematically hot spheroid: a "classical bulge". This indicates that pseudobulges and classical bulges are not mutually exclusive: some galaxies have both.The disky pseudobulges almost always have an exponential disk (scale lengths = 125-870 pc, mean ∼ 440 pc) with disk-related subcomponents: nuclear rings, bars, and/or spiral arms. They constitute 11-59% of the galaxy stellar mass (mean P B/T = 0.33), with stellar masses ∼ 7 × 10 9 -9 × 10 10 M . 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 (usually < 10% of the galaxy's stellar mass; mean B/T = 0.06). The classical bulges show rotation, but are kinematically hotter than the disky pseudobulges. Dynamical modeling of three systems indicates that velocity dispersions are isotropic in the classical bulges and equatorially biased in the disky pseudobulges.In the mass-radius and mass-stellar mass density planes, classical-bulge components follow sequences defined by ellipticals and (larger) classical bulges. Disky pseudobulges also fall on this sequence; they are more compact than similar-mass large-scale disks. Although some classical bulges are quite compact, they are distinct from nuclear star clusters in both size and mass, and coexist with nuclear clusters in at least two galaxies.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 evidence for such a zone outside its disky pseudobulge, making it a galaxy with all three types of "bulge".

The formation of stellar nuclear discs in bar-induced gas inflows

“…A similar offset can be seen in NGC 4371 (PA = 86 • for the outer disc and 92 • for the inner ellipticity peak associated with the nuclear ring), which suggests that the nuclear disc/ring in NGC 4371 is slightly elliptical as well. Erwin et al (2014) estimate a ND mass of 7.6 × 10 9 M⊙ for NGC 4371, or 17 per cent of its total stellar mass, comparable to, but smaller than, the mass fraction measured in the model at 10 Gyr. The size of the ND, as a fraction of the deprojected radius at which the bar ellipticity peaks, is 0.17.…”

“…Further details concerning the NDs, including their relation to the central bulges, can be found in Erwin et al (2014). For each galaxy, we orient the simulation to match the galaxy as best as possible, first reproducing the (deprojected) position angle of the bar relative to the line of nodes and then inclining the simulation by the same angle as the galaxy.…”

“…Near-infrared Hubble Space Telescope (HST) imaging has also provided indirect evidence for NDs as discy isophotes in the nuclei of early-type galaxies (Ravindranath et al 2001) or photometrically distinct exponential components in bulges (Balcells et al 2003). Erwin et al (2014) also found evidence for NDs which they call "discy pseudobulges".…”

On the dependence of galaxy morphologies on galaxy mergers

“…It is difficult to directly compare this sample with the Fisher & Drory (2011) catalogue, as the latter contains only 11 galaxies with M⋆ > 10 10.5 M⊙, while the former does not allow for a finer classification between pseudo and classical bulges. It is thus worth stressing that in the following plots fE formally represents an upper limit for the fraction of Elliptical galaxies, as it is not possible to discriminate the relative contribution of disk instabilities and merger processes in the formation of a spheroid, but only its main shape (but see Erwin et al 2015 for recent progress on these issues). Nonetheless, a more detailed classification should have a limited impact on our conclusions, since we do not expect the formation of B/T > 0.7 objects to be driven by disk instabilities alone, although they might contribute to increase the mass of a merger-induced spheroid.…”

Separation of stellar populations by an evolving bar: implications for the bulge of the Milky Way

“…Many external galaxies have been found to host compound bulges. For instance, Méndez-Abreu et al (2014) found that ∼ 30% of their sample of face-on barred galaxies showed signs of multiple bulge components, while Erwin et al (2015) presented a sample of nine galaxies with composite bulges (see also Erwin et al 2003). Thus, composite bulges are not rare, and it is conceivable that the Milky Way has one too.…”