When Is a Bulge Not a Bulge? Inner Disks Masquerading as Bulges in NGC 2787 and NGC 3945

Abstract

We present a detailed morphological, photometric, and kinematic analysis of two barred S0 galaxies with large, luminous inner disks inside their bars. We show that these structures, in addition to being geometrically disk-like, have exponential profiles (scale lengths ∼ 300-500 pc) distinct from the central, non-exponential bulges. We also find them to be kinematically disk-like. The inner disk in NGC 2787 has a luminosity roughly twice that of the bulge; but in NGC 3945, the inner disk is almost ten times more luminous than the bulge, which itself is extremely small (half-light radius ≈ 100 pc, in a galaxy with an outer ring of radius ≈ 14 kpc) and only ∼ 5% of the total luminosity -a bulge/total ratio much more typical of an Sc galaxy. We estimate that at least 20% of (barred) S0 galaxies may have similar structures, which means that their bulge/disk ratios may be significantly overestimated. These inner disks dominate the central light of their galaxies; they are at least an order of magnitude larger than typical "nuclear disks" found in ellipticals and early-type spirals. Consequently, they must affect the dynamics of the bars in which they reside.

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

“…Its orientation is very similar to that of NGC 4371 (bar ∆PA = 88 • , inclination = 55 • ). Unlike NGC 4371, NGC 3945 also has a small nuclear bar (Erwin et al 2003), which our model does not.…”

“…In this Section we compare the simulation to three early-type disc galaxies where previous study has shown the existence of distinct, moderately large NDs and/or massive stellar nuclear rings (which could plausibly be the most visible signature of an extended ND) inside bars (Erwin & Sparke 1999;Erwin et al 2003;Erwin 2004;Nowak et al 2010;Comerón et al 2010). Further details concerning the NDs, including their relation to the central bulges, can be found in Erwin et al (2014).…”

“…As shown by Erwin & Sparke (1999), there is a stellar nuclear ring within this disc, which produces the slight shoulder in the ellipticity profile at a ∼ 600 pc. Further inside is a secondary bar oriented almost parallel to the primary bar and thus close to the galaxy minor axis; the projection of this produces the ellipticity minimum at a ∼ 250 pc; a small classical-bulge component dominates the isophotes at a 100 pc (Erwin et al 2003;Erwin et al 2014). Erwin et al (2014) estimate a ND mass of 2.7 × 10 10 M⊙ for NGC 3945, or 36 per cent of the total stellar mass, larger than the 29 per cent we estimate for the model at 10 Gyr using the same method.…”

SUPERMASSIVE BLACK HOLES AND THEIR HOST SPHEROIDS. II. THE RED AND BLUE SEQUENCE IN THE MBH–M*,SPH DIAGRAM

“…For example, mergers can create bulges that rotate (e.g., Bekki 2010; Keselman & Nusser 2012), and bars can spin-up classical bulges (e.g., Saha et al 2012;Saha 2015), thus rotation is not a definitive signature of a pseudo-bulge. Furthermore, many galaxies host both a classical and a pseudo-bulge (e.g., Erwin et al 2003Erwin et al , 2015Athanassoula 2005;Gadotti 2009;MacArthur et al 2009;Erwin 2010;dos Anjos & da Silva 2013;Seidel et al 2015). In the recent literature, pseudo-and classical bulges have frequently been divided at the Sérsic index n 2 sph = (e.g., Sani et al 2011;Beifiori et al 2012), although, from a selection of hundreds of disk galaxies imaged in the Kband, Graham & Worley (2008) observed no bimodality in the bulge Sérsic indices about n 2 sph = or any other value.…”

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.…”