Explaining the reportedly overmassive black holes in early-type galaxies with intermediate-scale discs

Abstract

The classification "early-type" galaxy includes both elliptically-and lenticular-shaped galaxies. Theoretically, the spheroid-to-disc flux ratio of an early-type galaxy can assume any positive value, but in practice studies often consider only spheroid/disc decompositions in which the disc neatly dominates over the spheroid at large galaxy radii, creating an inner "bulge" as observed in most spiral galaxies. Here we show that decompositions in which the disc remains embedded within the spheroid, labelled by some as "unphysical", correctly reproduce both the photometric and kinematic properties of early-type galaxies with intermediate-scale discs. Intermediate-scale discs have often been confused with large-scale discs and incorrectly modelled as such; when this happens, the spheroid luminosity is considerably underestimated. This has recently led to some surprising conclusions, such as the claim that a number of galaxies with intermediate-scale discs (Mrk 1216, NGC 1277, NGC 1271, and NGC 1332) host a central black hole whose mass is abnormally large compared to expectations from the (underestimated) spheroid luminosity. We show that when these galaxies are correctly modelled, they no longer appear as extreme outliers in the (black hole mass)-(spheroid mass) diagram. This not only nullifies the need for invoking different evolutionary scenarios for these galaxies but it strengthens the significance of the observed (black hole mass)-(spheroid mass) correlation and confirms its importance as a fundamental ingredient for theoretical and semi-analytic models used to describe the coevolution of spheroids and their central supermassive black holes.

Implications for the Origin of Early-type Dwarf Galaxies: A Detailed Look at the Isolated Rotating Early-type Dwarf Galaxy LEDA 2108986 (CG 611), Ramifications for the Fundamental Plane’s Kinematic Scaling, and the Spin–Ellipticity Diagram

“…As noted in Section 2.2.2, CG 611 possibly consists of an intermediate-scale stellar disk embedded within a much larger spheroid (see Savorgnan & Graham 2016b for other examples of such ES galaxies). Ann, Seo & Ha (2015) reclassified CG 611 from E/S0 to SA0/a; most likely because they detected the faint bar and the weak spiral structure from their visual inspection.…”

“…Specific recent examples include NGC 3412 (Erwin et al 2005, their Figure 3a), NGC 3115 (Arnold et al 2011) and NGC 3377 (Arnold et al 2014). Other such galaxies of interest that have intermediatescale disks, include: NGC 1271 ; Mrk 1216 and NGC 1332 (Savorgnan & Graham 2016b); NGC 821, NGC 3377, and NGC 4697 (Savorgnan & Graham 2016a); and NGC 4473 (Foster et al 2013;Alabi et al 2015). Galaxies like NGC 1277 and NGC 3115 (Savorgnan & Graham 2016b) also have disks fully embedded in their spheroids, but they extend quite far out and thus globular clusters or planetary nebula, rather than long-slit or integral field unit (IFU) spectroscopy will likely be required to sample the dynamics beyond the disk (e.g.…”

Disky Elliptical Galaxies and the Allegedly Over-Massive Black Hole in the Compact Massive “Es” Galaxy NGC 1271

“…However, as done here with NGC 1271, and in Graham et al (2016) with NGC 1277, the spheroid masses first need to be reliably derived. This has now been done for NGC 1332, NGC 3115 and Mrk 1216 (Savorgnan & Graham 2016b) 12 and NGC 821, NGC 3377 and NGC 4697 (Savorgnan & Graham 2016a). …”

“…NGC 1277 was one such ES galaxy noted in GDS15 -after van den Bosch et al 2012 identified the galaxy as compact and massive 3 -as was NGC 1332 (Savorgnan & Graham 2016b), NGC 5493 (Krajnović et al 2013) and NGC 5845 (Jiang et al 2012). NGC 1271 is another nearby, compact early-type galaxy (Brunzendorf & Meusinger 1999) that contains a nearly edge-on, intermediate-scale disk.…”

A Normal Supermassive Black Hole in Ngc 1277

“…Beyond R 10 , the contribution from the disk light starts declining more rapidly than the spheroid light (e.g., Liller 1966). This suggests a somewhat embedded, intermediate-size stellar disk for NGC1277 (see Savorgnan & Graham 2016b for other examples). This explanation is also in accord with the results from the model fitting seen in Figure 3.…”