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L.L. Dressel (STScI), H.C. Ford (Johns Hopkins University), G.A. Kriss (STScI), Z.I. Tsvetanov (Johns Hopkins University)
A discrete rotating emission line disk is ideal for determining the mass of a supermassive black hole in the nucleus of a galaxy. One can use high resolution imaging in combination with a matrix of spatially resolved velocity measurements to confirm the simple geometry and kinematics of the disk. If the disk remains bright and orderly beyond the sphere of influence of the black hole, one can also measure the density of the surrounding matter in the nucleus. Accordingly, we have used HST to make one of the most direct and complete high resolution measurements of a supermassive black hole and the density of its surrounding nuclear matter ever made. We first imaged the nucleus of the radio-bright Liner galaxy NGC 3998 in H alpha+[NII] emission and in two adjacent continuum bands with the WFPC2 PC, and found a remarkably bright subarcsecond elliptical emission line region. We then observed this apparent disk with STIS in H alpha, [NII], and [SII], placing the 0.1 arcsec slit along the major axis and in 4 parallel positions to map the emission line kinematics. Because the disk is so bright, velocities could be measured at nearly 20 points along the major axis, at spacings of 6 pc. The major axis rotation curve shows a 108 solar mass black hole in its inner part, then rolls over at 25 parsecs to show a density of about 2000 solar masses per cubic parsec in its outer part. The minor axis velocities are approximately systemic, as expected for a simple rotating disk.
This work is supported by NASA grants GO-05924, GO-07354, and NAG5-8375.