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UM 625, previously identified as a narrow-line active galactic nucleus (AGN),actually exhibits broad \ha\ and \hb\ lines whose width and luminosity indicatea low black hole mass of $1.6 \times 10^6$ \msun. We present a detailedmultiwavelength study of the nuclear and host galaxy properties of UM 625.Analysis of \chandra\ and \xmm\ observations suggests that this system containsa 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 aminority of low-mass AGNs detected in the radio. The broad-band spectral energydistribution constrains the bolometric luminosity to$\lbol\approx(0.5-3)\times10^{43}$ \lum\ and $\lratio\approx0.02-0.15$. Acomprehensive analysis of Sloan Digital Sky Survey and {\it Hubble SpaceTelescope}\ images shows that UM 625 is a nearly face-on S0 galaxy with aprominent, relatively blue pseudobulge (\sersic\ index $n = 1.60$) thataccounts for $\sim$60% of the total light in the $R$ band. The extended disk isfeatureless, but the central $\sim150-400$ pc contains a conspicuous semi-ringof bright, blue star-forming knots, whose integrated ultraviolet luminositysuggests a star formation rate of $\sim$0.3 \msun yr$^{-1}$. The mass of thecentral black hole roughly agrees with the value predicted from its bulgevelocity dispersion but is significantly lower than that expected from itsbulge luminosity.

UM 625 Revisited: Multiwavelength Study of A Seyfert 1 Galaxy with a Low-mass Black Hole