The Host Galaxies of Low-Mass Black Holes

Abstract

Using Hubble Space Telescope observations of 147 host galaxies of low-mass black holes (BHs), we systematically study the structures and scaling relations of these active galaxies. Our sample is selected to have central BHs with virial masses of ∼10 5 -10 6 M . The host galaxies have total I-band magnitudes of −23.2 < M I < −18.8 mag and bulge magnitudes of −22.9 < M I < −16.1 mag. Detailed bulge-disk-bar decompositions with GALFIT show that 93% of the galaxies have extended disks, 39% have bars, and 5% have no bulges at all at the limits of our observations. Based on the Sérsic index and bulge-to-total ratio, we conclude that the majority of the galaxies with disks are likely to contain pseudobulges and very few of these low-mass BHs live in classical bulges. The fundamental plane of our sample is offset from classical bulges and ellipticals in a way that is consistent with the scaling relations of pseudobulges. The sample has smaller velocity dispersion at fixed luminosity in the Faber-Jackson plane compared with classical bulges and elliptical galaxies. The galaxies without disks are structurally more similar to spheroidals than to classical bulges according to their positions in the fundamental plane, especially the Faber-Jackson projection. Overall, we suggest that BHs with mass 10 6 M live in galaxies that have evolved secularly over the majority of their history. A classical bulge is not a prerequisite to host a BH.

Observational evidence for intermediate-mass black holes

“…10). However, this is not the case for the M BH −M bulge relation: the BH mass of IMBHs/low-mass AGN tends to be lower at a given bulge mass than expected from an extrapolation of the relation found for classical bulges (e.g., Greene et al 2008;Jiang et al 2011;Graham & Scott 2013Baldassare et al 2015Baldassare et al , 2017Nguyen et al 2017). SMBHs in inactive galaxies with pseudobulges (Hu 2008;Kormendy & Ho 2013) and spiral galaxies with megamaser BH mass measurements (Greene et al 2010;Läsker et al 2016) are also found to fall below the M BH − M bulge relation of early-type galaxies (Kormendy & Ho 2013), which suggests that most IMBHs/low-mass AGN are hosted by late-type galaxies (Greene & Ho 2007a;Ho & Kim 2016).…”

Galaxy Bulges and Their Massive Black Holes: A Review

“…If one was to separate the galaxies in Figure 2 at M bh = 2×10 6 M ⊙ , one would (understandably but inappropriately) conclude that the lower mass spheroids do not follow an M bh -M sph, * relation (Jiang et al 2011). Busch et al 2014) are denoted by the cross hairs.…”

Low-Mass Active Galactic Nuclei With Rapid X-Ray Variability

“…We estimate virial BH masses using the formalism for singleepoch spectra, recently recalibrated for the Hβ line by Ho & Kim (2015), which takes into consideration the dependence of the virial coefficient (so-called f -factor) on bulge type (Ho & Kim 2014), as well as the latest updates to the M BH − σ * relation of inactive galaxies (Kormendy & Ho 2013), on which the calibration of the f -factor is based. As our sample specifically targets low-mass BHs, we expect, from experience with other similar samples Jiang et al 2011b), that the host galaxies contain pseudobulges. The BH mass estimator of Ho & Kim (2015) is based on the FWHM of broad Hβ and the AGN continuum luminosity at 5100 Å, whereas the primary measurements in this study come from the FWHM and luminosity of broad Hα.…”