The Black Hole–bulge Mass Relation in Megamaser Host Galaxies*

Abstract

We present HST images for nine megamaser disk galaxies with the primary goal of studying photometric BH-galaxy scaling relations. The megamaser disks provide the highest-precision extragalactic BH mass measurements, while our high-resolution HST imaging affords us the opportunity to decompose the complex nuclei of their late-type hosts in detail. Based on the morphologies and shapes of the galaxy nuclei, we argue that most of these galaxies' central regions contain secularly evolving components (pseudo-bulges), and in many cases we photometrically identify co-existing "classical" bulge components as well. Using these decompositions, we draw the following conclusions: (1) The megamaser BH masses span two orders of magnitude (10 6 -10 8 M ) while the stellar mass of their spiral host galaxies are all ∼ 10 11 M within a factor of three; (2) the BH masses at a given bulge mass or total stellar mass in the megamaser host spiral galaxies tend to be lower than expected, when compared to an extrapolation of the BH-bulge relation based on early-type galaxies; (3) the observed large intrinsic scatter of BH masses in the megamaser host galaxies raises the question of whether scaling relations exist in spiral galaxies.

Megamaser Disks Reveal a Broad Distribution of Black Hole Mass in Spiral Galaxies

“…On the other hand, we see a similar trend in the M BH -M bul relations (Läsker et al 2016). Thus, we briefly consider the possible ramifications should this difference prove real.…”

“…We calculate the average offset ΔM BH between M BH and that is predicted by the * s M BH -relation fit to early-type galaxies. The offset is calculated as the weighted mean of the difference between the measured M BH and that calculated from the best-fit relation, with the weights including both y and x uncertainties combined in quadrature (Läsker et al 2016). We find an offset of −0.39 ± 0.12 for late-type galaxies and an offset of −0.45 ± 0.12 for pseudobulges.…”

“…The BHs in the megamaser disk galaxies are too large to be primordial seeds (e.g., Volonteri 2010), so they have certainly undergone some accretion episodes, but the lack of correlation with M * strongly suggests that BH growth does not remove gas from galactic disks (i.e., does not act beyond bulge scales). After removing the disk component, we still see considerable scatter between M BH and bulge properties in late-type spiral galaxies, even when we attempt to isolate the "classical" bulge component from the nuclear disks, rings, and bars that characterize "pseudobulges" (Läsker et al 2016;Saglia et al 2016). We show elliptical (red circles), S0 (green triangles), spiral (blue squares), and megamaser disk (blue circles) galaxies.…”

“…We reproduce our aperture measurements within 0.1 dex, with no systematic dependence on mass or n. Deprojection uncertainties may play a larger role for the pseudobulges, particularly for nearly edge-on inclinations. We explore the importance of the inclination using ellipse fitting to our Hubble Space Telescope (HST) data for the maser galaxies (Greene et al 2014;Läsker et al 2016) and find that the differences between circular and elliptical apertures are never larger than 0.2 mag, translating into <0.1 dex in stellar mass.…”

“…It is possible that M BH is built up along with the bulge mass. However, in late-type galaxies where there is clear evidence for ongoing bulge growth, M BH appear to be systematically lower than expected based on BH-bulge relations in early-type galaxies (e.g., Läsker et al 2016). Here, we use megamaser disks that reside well within the BH sphere of influence to probe the distribution of M BH in spiral galaxies.…”