* Astronomy

Title: UM 625 Revisited: Multiwavelength Study of A Seyfert 1 Galaxy with a Low-mass Black Hole
Authors: Ning Jiang, Luis C. Ho, Xiao-bo Dong, Huan Yang, Junxian Wang

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 x 10^6 solar masses. 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) x 10^{43} \lum\ and \lratio\approx0.02-0.15. A comprehensive analysis of Sloan Digital Sky Survey and 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 ~60% of the total light in the R band. The extended disk is featureless, but the central ~150-400 pc contains a conspicuous semi-ring of bright, blue star-forming knots, whose integrated ultraviolet luminosity suggests a star formation rate of ~0.3 solar masses 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.

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