Black Holes at the Centers of Nearby Dwarf Galaxies: The X-ray Perspective

<p>As part of a search for intermediate-mass black hole candidates in the local universe, we have assembled a new sample of nearby (<em>d</em> < 80 Mpc) active galactic nuclei (AGN) in dwarf galaxies with stellar masses less than 1010 <em>M</em>‚äô. Collectively, these 28 objects are the least massive galaxies known to contain central black holes. Surprisingly, only two of them show clear evidence of broad emission lines in their optical spectra, indicating a much higher incidence of narrow-line (type 2) AGN in our low-mass sample than in samples of classical Seyfert galaxies. There are two possible explanations for this. First, our objects may have the same basic structure as luminous Seyfert galaxies but a higher probability that their broad-line regions are obscured along the line of sight. Alternatively, theoretical work suggests that the broad-line region becomes increasingly difficult to detect (or may even cease to exist) as luminosity or black-hole mass decreases. To investigate which scenario is more plausible, we have observed a subset of 6 galaxies from our sample with the <em>Chandra</em> X-ray Telescope. We find that the ratios of their observed X-ray luminosities and their [O III] emission-line luminosities are very low compared to the intrinsic ratios measured for luminous AGNs, suggesting that our objects are heavily absorbed. Consistent with this conclusion, spectral fitting of one well-detected object (NGC 4117) provides further evidence of heavy absorption. 1</p>