Dwarf Galaxies with Optical Signatures of Active Massive Black Holes

TL;DR

The paper presents the largest systematic search for optical signatures of accreting massive black holes in dwarf galaxies (M★ ≤ 3×10^9 M⊙) using SDSS DR8 spectra and NSA-derived properties. By modeling continuum, measuring emission lines, and applying BPT diagnostics, the authors identify 136 dwarfs with narrow-line signatures of BH activity (35 AGN, 101 composites) and 15 additional broad-Hα detections in star-forming hosts, with 25 robust broad-line AGN candidates. Broad-line BH masses inferred from virial methods lie in the ~10^5–10^6 M⊙ range (median ~2×10^5 M⊙), contingent on assumptions about BLR geometry and radius–luminosity scaling; the broad-line sample is limited by detectability to higher accretion luminosities. The results imply that even very small galaxies can host massive BHs, informing seed formation scenarios and BH feedback in low-mass systems, and underscoring the need for complementary multiwavelength follow-up to complete the census.

Abstract

We present a sample of 151 dwarf galaxies (10^8.5 < M_stellar < 10^9.5 Msun) that exhibit optical spectroscopic signatures of accreting massive black holes (BHs), increasing the number of known active galaxies in this stellar mass range by more than an order of magnitude. Utilizing data from the Sloan Digital Sky Survey Data Release 8 and stellar masses from the NASA-Sloan Atlas, we have systematically searched for active BHs in ~25,000 emission-line galaxies with stellar masses comparable to the Magellanic Clouds and redshifts z<0.055. Using the narrow-line [OIII]/H-beta versus [NII]/H-alpha diagnostic diagram, we find photoionization signatures of BH accretion in 136 galaxies, a small fraction of which also exhibit broad H-alpha emission. For these broad-line AGN candidates, we estimate BH masses using standard virial techniques and find a range of 10^5 < M_BH < 10^6 Msun and a median of M_BH ~ 2 x 10^5 Msun. We also detect broad H-alpha in 15 galaxies that have narrow-line ratios consistent with star-forming galaxies. Follow-up observations are required to determine if these are true type 1 AGN or if the broad H-alpha is from stellar processes. The median absolute magnitude of the host galaxies in our active sample is Mg = -18.1 mag, which is ~1-2 magnitudes fainter than previous samples of AGN hosts with low-mass BHs. This work constrains the smallest galaxies that can form a massive BH, with implications for BH feedback in low-mass galaxies and the origin of the first supermassive BH seeds.

Figures (15)

• Figure 1: Left: BPT [O III]/H$\beta$ versus [N II]/H$\alpha$ narrow-line diagnostic diagram for all $\sim 25,000$ dwarf emission-line galaxies analyzed in this work. Galaxies with spectra dominated by an AGN are plotted as red points and galaxies with spectra dominated by star formation are plotted as blue points. Composite galaxies with significant contributions from both an AGN and star formation are plotted as purple points. The typical error is shown in the lower right corner (individual flux errors are given in Tables \ref{['tab:bptlines']} and \ref{['tab:sflines']}). The dashed line is an empirical separation of pure star-forming galaxies and those with some contribution from an AGN from kauffmannetal2003agn. The solid line is from kewleyetal2001, indicating the 'maximum starburst line' given by pure stellar photoionization models. We note that the red point falling to the far left of the diagram and just above the dividing line is unusual in a number of ways and rather suspect (see footnote in text). Right: BPT diagram for broad-line AGN candidates only (§\ref{['sec:broad']}). We consider the sources falling in the AGN and composite regions of the diagram the most secure broad-line candidates. Significant contamination from luminous Type II supernovae is likely in the star-forming region of the diagram.
• Figure 2: BPT narrow-line diagnostic diagrams for our sample of active galaxies. Regions are delineated according to the classification scheme outlined in kewleyetal2006. Top: [O III]/H$\beta$ versus [N II]/H$\alpha$ diagram. There are 35 galaxies in the AGN part of the diagram (6 with broad H$\alpha$ emission) and 101 galaxies in the composite region of the diagram (4 with broad H$\alpha$ emission). An additional 15 galaxies have broad H$\alpha$ emission, yet have HII-region-like narrow-line ratios. Middle: [O III]/H$\beta$ versus [S II]/H$\alpha$ diagram. Colors indicate classification based on the OIII/H$\beta$ versus [N II]/H$\alpha$ diagram. Bottom: [O III]/H$\beta$ versus [O I]/H$\alpha$ diagram for galaxies in which we detect the [O I] emission line (Tables \ref{['tab:bptlines']} and \ref{['tab:sflines']}). Colors indicate classification based on the OIII/H$\beta$ versus [N II]/H$\alpha$ diagram.
• Figure 3: SDSS redshift-corrected spectra of galaxies falling in the AGN region of the [O III]/H$\beta$ versus [N II]/H$\alpha$ diagram. Continuum and absorption-line fits are shown in blue (see Section \ref{['sec:contsub']}). An identification number (Table \ref{['tab:bptgals']}) is given in the upper left corner of each plot.
• Figure 4: Same as Figure \ref{['fig:nlagn_spec1']}.
• Figure 5: Example of a broad-line AGN candidate (ID 119). The narrow-line ratios of this source place it in the composite section of the [O III]/H$\beta$ versus [N II]/H$\alpha$ diagram. Top: The redshift-corrected spectrum with the continuum and absorption-line fit plotted in blue. Bottom: Chunks of the emission-line spectrum (after continuum and absorption-line subtraction). Best-fitting models are plotted in red and the individual narrow Gaussian components are plotted in yellow. The broad H$\alpha$ and H$\beta$ components are plotted in dark blue. Residuals are plotted in gray with a vertical offset for clarity. Reduced $\chi^2$ values are shown in the upper left-hand corner of the H$\alpha$ + [N II] and H$\beta$ chunks. For comparison, the reduced $\chi^2$ values from the fits not including a broad component are shown in parenthesis. Spectra of the other 24 broad-line AGN candidates are shown in the Appendix.