2.1 Kinematics of Optical Emission Lines
High-resolution optical images taken with ground-based telescopes and especially with the Hubble Space Telescope (HST) show that many giant elliptical galaxies contain nuclear disks of dust and ionized gas. The most famous case is M 87 (Figure 2a). The disk measures ~ 150 pc across, and its rotation axis is closely aligned with the optical and radio jet. This is in accord with the BH accretion picture. The disk is in Keplerian rotation (Figure 2b) around an object of mass MBH 3 x 109 M. Furthermore, this object is dark: the measured mass-to-light ratio exceeds 100 in solar units, and this is much larger than that of any known population of stars. Moreover, the dark mass must be very compact: the velocity field limits its radial extent to be less than 5 pc. Therefore its density exceeds 107 M pc-3. Another illustration of this technique is given in Figure 3. M 84, also a denizen of the Virgo cluster of galaxies, is a twin of M 87 in size, and it, too, harbors an inclined nuclear gas disk (diameter ~ 80 pc), whose rotation about the center betrays an invisible mass of MBH 2 x 109 M. Other cases are reported (NGC 4261, NGC 6251, NGC 7052), and searches for more are in progress.
Figure 2a. HST image of the ionized gas disk near the center of the giant elliptical galaxy M 87. The data were taken with the Second Wide Field/Planetary Camera through a filter that isolates the optical emission lines H and [N II] 6548, 6583. The left inset is an expanded viewof the gas disk; for an adopted distance of 16.8 Mpc, the region shown is 5" x 5" or 410 x 410 pc. The disk has a major axis diameter of ~ 150 pc, and it is oriented perpendicular to the optical jet. [Image courtesy of NASA/Space Telescope Science Institute, based on data originally published by Ford, H. C., et al. Astrophys. J. 435, L27 (1994).]
Figure 2b. Optical emission-line rotation curve for the nuclear disk in M 87. The data were taken with the Faint Object Camera on HST. The curves in the upper panel correspond to two different Keplerian thin disk models, and the bottom panel shows the residuals for the best-fitting model. [Figure adapted from Macchetto, F., et al. Astrophys. J. 489, 579 (1997).]
Figure 3. (Left) HST image of the central region of the giant elliptical galaxy M 84; the box measures 22" x 19" or 1.8 kpc x 1.6 kpc for an adopted distance of 16.8 Mpc. The data were taken with the Second Wide Field/Planetary Camera through a filter that isolates the optical emission lines H and [N II] 6548, 6583. The slit of the Space Telescope Imaging Spectrograph was placed along the major axis of the nuclear gas disk (blue rectangle). (Right) Resulting spectrum of the central 3" (240 pc). The abscissa is velocity and the ordinate is distance along the major axis. The spectrum shows the characteristic kinematic signature of a rotating disk. The velocity scale is coded such that blue and red correspond to blue and red shifts, respectively; the total velocity range is 1445 km s-1. [Image courtesy of NASA/Space Telescope Science Institute, based on data originally published by Bower, G. A., et al. Astrophys. J. Lett. 483, L33 (1997) and Bower, G. A., et al. Astrophys. J. Lett. 492, L111 (1998).]