KINEMATICS OF THE NUCLEAR IONIZED-GAS IN THE RADIO GALAXY M84 (NGC-4374)

We present optical long-slit spectroscopy of the nucleus of the nearby radio galaxy M84 (NGC 4374 = 3C 272.1) obtained with the Space Telesc ope Imaging Spectrograph aboard the Hubble Space Telescope. Our spectr a reveal that the nuclear gas disk seen in the Wide Field Planetary Ca mera 2 imaging by Bower et al. is rotating rapidly. The velocity curve has an S-shape with a peak amplitude of 400 km s(-1) at 0''.1 = 8 pc from the nucleus. To model the observed gas kinematics, we construct a thin Keplerian disk model that fits the data well if the rotation axi s of the gas disk is aligned with the radio jet axis. These models ind icate that the gasdynamics are driven by a nuclear compact mass of 1.5 x 10(9) M. with an uncertainty range of (0.9-2.6) x 10(9) M., and tha t the inclination of the disk with respect to the plane of the sky is 75 degrees-85 degrees. Of this nuclear mass, only less than or equal t o 2 x 10(7) M. can possibly be attributed to luminous mass. Thus, we c onclude that a dark compact mass (most likely a supermassive black hol e) resides in the nucleus of M84.