Exploring the Low-Mass End of The M-Sigma Relation with Active Galaxies

We present new measurements of stellar velocity dispersions, using spectra obtained with the Keck Echellette Spectrograph and Imager (ESI) and the Magellan Echellette (MagE), for 76 Seyfert 1 galaxies from the recent catalogue of Greene & Ho. These objects were selected from the Sloan Digital Sky Survey (SDSS) to have estimated black hole (BH) masses below 2\times10^6 M\odot. Combining our results with previous ESI observations of similar objects, we obtain an expanded sample of 93 galaxies and examine the relation between BH mass and velocity dispersion (the M-Sigma relation) for active galaxies with low BH masses. The low-mass active galaxies tend to follow the extrapolation of the M-Sigma relation of inactive galaxies. Including results for active galaxies of higher BH mass from the literature, we find a zero point{\alpha}= 7.68\pm0.08 and slope of {\beta}= 3.32\pm0.22 for the M-Sigma relation [in the form log Mbh={\alpha}+{\beta}log({\sigma}*/200 km s-1)], with intrinsic scatter of 0.46\pm0.03 dex. This result is consistent, within the uncertainties, with the slope of the M-Sigma relation for reverberation-mapped active galaxies with BH masses from 10^6 to 10^9 M\odot. For the subset of our sample having morphological information from Hubble Space Telescope images, we examine the slope of the M-Sigma relation separately for subsamples of barred and unbarred host galaxies, and find no significant evidence for a difference in slope. We do find a mild offset between low-inclination and high-inclination disk galaxies, such that more highly inclined galaxies tend to have larger {\sigma}* at a given value of BH mass, presumably due to the contribution of disk rotation within the spectroscopic aperture. We also find that the velocity dispersion of the ionized gas trace the stellar velocity dispersion well for this large sample of low-mass Seyfert 1 galaxies.