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 Hα and Hβ lines whose width and luminosity indicate a low black hole (BH) mass of 1.6 × 10⁶ M _⊙. We present a detailed multiwavelength study of the nuclear and host galaxy properties of UM 625. Analysis of Chandra and XMM-Newton observations suggests that this system contains a heavily absorbed and intrinsically X-ray weak (α_ox = -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 broadband spectral energy distribution constrains the bolometric luminosity to L _bol ≈ (0.5-3) × 10⁴³ erg s^-1 and L _bol/L _Edd ≈ 0.02-0.15. 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 (Sérsic 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 M _⊙ yr^-1. The mass of the central BH roughly agrees with the value predicted from its bulge velocity dispersion but is significantly lower than that expected from its bulge luminosity.