Relations Between Central Black Hole Mass and Total Galaxy Stellar Mass in the Local Universe

TL;DR

The paper quantifies how central black hole mass scales with the total stellar mass of the host in the local universe, using a broad sample of broad-line AGN, reverberation-mapped AGN, and galaxies with dynamical BH masses. It finds a robust, near-linear correlation for AGN hosts but with a normalization far below the canonical BH–bulge relations, implying that BH growth relative to total stellar mass is smaller in these systems. The authors show two distinct BH–host mass relations: one for AGN hosts with low normalization and another for ellipticals/classical bulges with higher normalization, underscoring the danger of applying bulge-based scaling to AGN hosts or to total stellar mass without bulge decomposition. These results have important consequences for interpreting high-redshift BH populations and for cosmological simulations that lack resolved bulges, highlighting the role of bulge-to-total mass and morphology in BH growth.

Abstract

Scaling relations between central black hole (BH) mass and host galaxy properties are of fundamental importance to studies of BH and galaxy evolution throughout cosmic time. Here we investigate the relationship between BH mass and host galaxy total stellar mass using a sample of 262 broad-line active galactic nuclei (AGN) in the nearby Universe (z < 0.055), as well as 79 galaxies with dynamical BH masses. The vast majority of our AGN sample is constructed using Sloan Digital Sky Survey spectroscopy and searching for Seyfert-like narrow-line ratios and broad H-alpha emission. BH masses are estimated using standard virial techniques. We also include a small number of dwarf galaxies with total stellar masses M_stellar < 10^9.5 Msun and a sub-sample of the reverberation-mapped AGNs. Total stellar masses of all 341 galaxies are calculated in the most consistent manner feasible using color-dependent mass-to-light ratios. We find a clear correlation between BH mass and total stellar mass for the AGN host galaxies, with M_BH proportional to M_stellar, similar to that of early-type galaxies with dynamically-detected BHs. However, the relation defined by the AGNs has a normalization that is lower by more than an order of magnitude, with a BH-to-total stellar mass fraction of M_BH/M_stellar ~ 0.025% across the stellar mass range 10^8 < M_stellar/Msun < 10^12. This result has significant implications for studies at high redshift and cosmological simulations in which stellar bulges cannot be resolved.

Figures (12)

• Figure 1: Spectrum of a broad-line AGN illustrating our fitting method. Top: the redshift-corrected spectrum is shown in black and the continuum plus absorption-line model is plotted in blue. Bottom: chunks of the emission-line spectrum (after subtracting the continuum and absorption-line model). The best-fit models for the emission line regions are shown in red. The individual narrow-line Gaussian components are plotted in yellow. Broad H$\alpha$ and H$\beta$ Gaussian components are plotted in dark blue. The residuals are shown in gray with a vertical offset for clarity. In the upper left-hand corner of the H$\alpha$ and H$\beta$ chunks we show the reduced $\chi^2$ values. The reduced $\chi^2$ values from the fits not including a broad component are shown below in parenthesis for comparison.
• Figure 2: Narrow-line diagnostic diagrams for sources with detectable broad H$\alpha$ emission. We use the classification scheme outlined in kewleyetal2006. Our sample of broad-line AGNs is restricted to objects with narrow line ratios placing them in both the AGN region of the [O III]/H$\beta$ vs. [N II]/H$\alpha$ diagram and the Seyfert region of the [O III]/H$\beta$ vs. [S II]/H$\alpha$ diagram (red points). The typical error for the red points is shown in the lower right corner of each plot.
• Figure 3: Distribution of the FWHM (left panel) and luminosity (middle panel) of broad H$\alpha$ emission for our sample of nearby broad-line AGN. The distribution of virial BH masses calculated from equation 1 is shown in the right panel.
• Figure 4: Top: Distributions of the ratio of AGN flux density to total (Host+AGN) flux density in the SDSS $g$ and $i$ bands for our sample of broad-line AGNs. Middle: Distributions of absolute $g$-band magnitudes for our sample of broad-line AGN. The blue histogram shows AGN-only magnitudes, the orange histogram shows host galaxy-only magnitudes and the black histogram shows the total (Host+AGN) magnitudes. Bottom: Total stellar mass without correcting for AGN contamination versus total stellar mass corrected for AGN contamination. See § \ref{['sec:mstar']} for details. The line shows the one-to-one relation.
• Figure 5: The distribution of host galaxy total stellar masses for our sample of broad-line AGN (corrected for AGN contamination) is shown in red. Our parent sample of emission-line galaxies is shown in blue, normalized to the number of galaxies in the red histogram (244 objects). We also show the mass distribution for the full NSA catalog (no emission-line cuts), again normalized by the number of galaxies in the red histogram. All masses were derived using $g$ and $i$-band data in the NSA with the color-dependent mass-to-light ratio given in equation \ref{['eqn:mlratio']}.