Discovery of An Active Intermediate-Mass Black Hole Candidate in the
  Barred Bulgeless Galaxy NGC 3319
  We report the discovery of an active intermediate-mass black hole (IMBH)
candidate in the center of nearby barred bulgeless galaxy $\rm NGC~3319$. The
point X-ray source revealed by archival Chandra and XMM-Newton observations is
spatially coincident with the optical and UV galactic nuclei from Hubble Space
Telescope observations. The spectral energy distribution derived from the
unresolved X-ray and UV-optical flux is comparable with active galactic nuclei
(AGNs) rather than ultra-luminous X-ray sources, although its bolometric
luminosity is only $3.6\times10^{40}~\rm erg~s^{-1}$. Assuming an Eddington
ratio range between 0.001 and 1, the black hole mass ($M_\rm{BH}$) will be
located at $3\times10^2-3\times10^5~M_{\odot}$, placing it in the so-called
IMBH regime and could be the one of the lowest reported so far. Estimates from
other approaches (e.g., fundamental plane, X-ray variability) also suggest
$M_\rm{BH}\lesssim10^5~M_{\odot}$.Similar to other BHs in bulgeless galaxies,
the discovered IMBH resides in a nuclear star cluster with mass of
$\sim6\times10^6~M_{\odot}$. The detection of such a low-mass BH offers us an
ideal chance to study the formation and early growth of SMBH seeds, which may
result from the bar-driven inflow in late-type galaxies with a prominent bar
such as $\rm NGC~3319$.
