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

Abstract

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 $\sim 150-400$ 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.

Accepted for publication by ApJ; 13 pages, 2 tables and 7 figures

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