The supermassive black hole in the Seyfert 2 galaxy NGC 5252

Abstract

We present results from HST/STIS long-slit spectroscopy of the gas motions in the nuclear region of the Seyfert 2 galaxy NGC 5252. The observed velocity field is consistent with gas in regular rotation with superposed localized patches of disturbed gas. The dynamics of the circumnuclear gas can be accurately reproduced by adding to the stellar mass component a compact dark mass of MBH = 0.95 (-0.45;+1.45) 10E9 M(sun), very likely a supermassive black hole. Contrarily to results obtained in similar studies rotational broadening is sufficient to reproduce also the behaviour of line widths. The MBH estimated for NGC 5252 is in good agreement with the correlation between MBH and bulge mass. The comparison with the MBH vs sigma relationship is less stringent (mostly due to the relatively large error in sigma); NGC 5252 is located above the best fit line by between 0.3 and 1.2 dex, i.e. 1 - 4 times the dispersion of the correlation. Both the galaxy's and MBH of NGC 5252 are substantially larger than those usually estimated for Seyfert galaxies but, on the other hand, they are typical of radio-quiet quasars. Combining the determined MBH with the hard X-ray luminosity, we estimate that NGC 5252 is emitting at a fraction ~ 0.005 of L(Edd). In this sense, this active nucleus appears to be a quasar relic, now probably accreting at a low rate, rather than a low black hole mass counterpart of a QSO.