Publication Date

5-8-2013

Advisor(s)

Edward C. Moran

Department

Astronomy

Language

English

Abstract

As part of a search for intermediate-mass black hole candidates in the
local universe, we have assembled a new sample of nearby (d < 80 Mpc)
active galactic nuclei (AGN) in dwarf galaxies with stellar masses less than
1010 M⊙. 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 Chandra 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.
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