A Low-Mass Black Hole in the Nearby Seyfert Galaxy UGC 06728

We present the results of a recent reverberation mapping campaign for UGC 06728, a nearby low-luminosity Seyfert 1 in a late-type galaxy. Nightly monitoring in the spring of 2015 allowed us to determine an H$\beta$ time delay of $\tau = 1.4 \pm 0.8$ days. Combined with the width of the variable H$\beta$ line profile, we determine a black hole mass of $M_{\rm BH} = (7.1 \pm 4.0) \times 10^5$ M$_{\odot}$. We also constrain the bulge stellar velocity dispersion from higher-resolution long slit spectroscopy along the galaxy minor axis and find $\sigma_{\star} = 51.6 \pm 4.9$ km s$^{-1}$. The measurements presented here are in good agreement with both the $R_{\rm BLR} - L$ relationship and the $M_{\rm BH}-\sigma_{\star}$ relationship for AGNs. Combined with a previously published spin measurement, our mass determination for UGC 06728 makes it the lowest-mass black hole that has been fully characterized, and thus an important object to help anchor the low-mass end of black hole evolutionary models.