Black Holes at the Centers of Nearby Dwarf Galaxies

TL;DR

This study identifies 28 active galactic nuclei in nearby dwarf galaxies, suggesting the presence of intermediate-mass black holes and indicating that such active nuclei are more common than previously believed.

Contribution

First optical survey to detect low-luminosity AGNs in nearby dwarf galaxies, revealing a higher prevalence of active nuclei and estimating black hole masses in the 10^3 to 10^4 solar mass range.

Findings

Detected AGNs in 28 nearby dwarf galaxies.

Estimated black hole masses mainly between 10^3 and 10^4 solar masses.

Found active nuclei in a few percent of surveyed dwarf galaxies.

Abstract

Using a distance-limited portion of the Sloan Digital Sky Survey (SDSS) Data Release 7, we have identified 28 active galactic nuclei (AGNs) in nearby (d < 80 Mpc) low-mass, low-luminosity dwarf galaxies. The accreting objects at the galaxy centers are expected to be intermediate-mass black holes (IMBHs) with M_BH < 1e6 M_sun. The AGNs were selected using several optical emission-line diagnostics after careful modeling of the continuum present in the spectra. We have limited our survey to objects with spectral characteristics similar to those of Seyfert nuclei, excluding emission-line galaxies with ambiguous spectra that could be powered by stellar processes. The host galaxies in our sample are thus the least massive objects in the very local universe certain to contain central black holes. Given our focus on the nearest objects included in the SDSS, our survey is more sensitive to low-luminosity emission than previous optical searches for AGNs in low-mass galaxies. The [O III] lambda5007 luminosities of the Seyfert nuclei in our sample have a median value of L_5007 = 2e5 L_sun and extend down to 1e4 L_sun. Using published data for broad-line IMBH candidates, we have derived an [O III] bolometric correction of log (L_bol/L_5007) = 3.0 +/- 0.3, which is significantly lower than values obtained for high-luminosity AGNs. Applying this correction to our sample, we obtain minimum black-hole mass estimates that fall mainly in the 10^3 M_sun -- 10^4 M_sun range, which is roughly where the predicted mass functions for different black-hole seed formation scenarios overlap the most. In the stellar mass range that includes the bulk of the AGN host galaxies in our sample, we derive a lower limit on the AGN fraction of a few percent, indicating that active nuclei in dwarf galaxies are not as rare as previously thought.