UM 625 Revisited: Multiwavelength Study of A Seyfert 1 Galaxy with a
  Low-mass Black Hole
  UM 625, previously identified as a narrow-line active galactic nucleus (AGN),
actually exhibits broad \ha\ and \hb\ lines whose width and luminosity indicate
a low black hole mass of $1.6 \times 10^6$ \msun. We present a detailed
multiwavelength study of the nuclear and host galaxy properties of UM 625.
Analysis of \chandra\ and \xmm\ observations suggests that this system contains
a heavily absorbed and intrinsically X-ray weak ($\aox=-1.72$) nucleus.
Although not strong enough to qualify as radio-loud, UM 625 does belong to a
minority of low-mass AGNs detected in the radio. The broad-band spectral energy
distribution constrains the bolometric luminosity to
$\lbol\approx(0.5-3)\times10^{43}$ \lum\ and $\lratio\approx0.02-0.15$. A
comprehensive analysis of Sloan Digital Sky Survey and {\it Hubble Space
Telescope}\ images shows that UM 625 is a nearly face-on S0 galaxy with a
prominent, relatively blue pseudobulge (\sersic\ index $n = 1.60$) that
accounts for $\sim$60% of the total light in the $R$ band. The extended disk is
featureless, but the central $\sim150-400$ pc contains a conspicuous semi-ring
of bright, blue star-forming knots, whose integrated ultraviolet luminosity
suggests a star formation rate of $\sim$0.3 \msun yr$^{-1}$. The mass of the
central black hole roughly agrees with the value predicted from its bulge
velocity dispersion but is significantly lower than that expected from its
bulge luminosity.
