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

TL;DR

This study provides a comprehensive multiwavelength analysis of UM 625, a low-mass black hole Seyfert 1 galaxy, revealing its nuclear properties, host galaxy structure, and star formation activity, with implications for black hole-galaxy relations.

Contribution

It offers the first detailed multiwavelength characterization of UM 625, highlighting its low-mass black hole, nuclear absorption, and host galaxy features, expanding understanding of low-mass AGNs.

Findings

UM 625 hosts a low-mass black hole (~1.6 million solar masses).

The galaxy is a nearly face-on S0 with a blue pseudobulge.

Star formation rate in the central region is about 0.3 solar masses per year.

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 $\sim150-400$ 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.