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Black holes in galactic nuclei

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Massive BHs are thought to be responsible for the energy production in active galaxies, and are believed to be common in quiescent galaxies as well. Dynamical evidence for this can be sought by studying the kinematics of stars and gas in galactic nuclei.

In the 1970's M87 was the first galaxy for which stellar kinematical data hinted at a BH, but not conclusively. In [R9] I analyzed the kinematics of the nuclear gas, showed that it rotates and has a central FWHM exceeding 1000 km/s (Figure 1). I argued for gravitational motion of the gas, and showed that the seeing convolved predictions of a model with an approximately 3 x 10^9 solar mass BH provide an excellent fit to the data. This was subsequently confirmed by higher resolution observations with the HST (Ford, Harms et al. 1994), which also showed the gas to be in a disk perpendicular to the jet.

Kinematical observations of nuclear gas disks have since become a useful tool for BH mass determinations. In [R13] and [R22] we studied the nuclear disk of dust and gas in NGC~7052. Figure 2 shows HST/WFPC2 images of the disk. HST Faint Object Spectrograph (FOS) spectra reveal a steeper rotation curve and broader emission lines than ground-based data. Detailed modeling implies the presence of 3 x 10^8 solar mass BH. An HST study is also underway for the (active) galaxy IC 1459 [P2]. The stars in the central 10 arcsec counter-rotate with respect to the gas, and with respect to the stars at larger radii, suggesting that a strong accretion event has occurred in the history of this galaxy. To ensure that the small FOS apertures in my projects would be properly positioned, I wrote software to simulate target acquisitions which I made available to the HST community [C5]. The FOS spectra that I obtained also allowed me to perform a detailed study of the accuracy of the FOS pipeline wavelength calibration [C7].

In quiescent galaxies only stellar kinematics are generally available to study the presence of a BH. The galaxy M32 (see Figure 3) has been a major focus of my research, although I have also been involved in studies of other galaxies [R20]. The main problems for stellar kinematical studies have long been insufficient spatial resolution, and lack of knowledge on the stellar velocity dispersion anisotropy (an overabundance of stars on radial orbits can cause a central increase in the velocity dispersion, mimicking the presence of a BH). In [R6] I presented sub-arcsec resolution ground-based kinematical and velocity profile data for M32 along several position angles. Axisymmetric models with a central BH and a distribution function of the form f=f(E,L_z) were constructed in [R8] and [R12], and were found to fit the data well. N-body simulations on a Cray T3D supercomputer were performed to demonstrate the stability of these models [R18]. More recently I obtained spectra with the HST Faint Object Spectrograph (FOS). As shown in Figure 4, the observations show a steeper rotation curve and higher central velocity dispersion than the ground-based data; the nuclear dispersion measured through an 0.08 arcsec aperture is 156 +/- 10 km/s [R19], while the nuclear dispersion measured from the ground is only 85-95 km/s. To interpret these data we developed a new method to construct axisymmetric models with fully general distribution functions ([R21],[P1]), based on Schwarzschild's orbit superposition approach. The results show convincingly that M32 must have a nuclear dark mass of (3.4 +/- 0.7) x 10^6 solar masses. The size of the dark mass must be less than 0.3 pc, implying a density exceeding 10^8 solar masses per cubic pc. This rules out clusters of dark objects such as stellar remnants, brown dwarfs or planets, leaving a massive BH as the only plausible interpretation [R16].

In [R10] I showed that the central velocity profile of a galaxy with a BH has broader wings than a Gaussian, due to the stars that orbit close to the hole at high velocities. I argued that for some galaxies this additional BH signature should be observable with the HST. Just recently it has indeed been detected, by Kormendy et al. (1996) in the galaxy NGC 3115.

The presence of BHs in galaxies can also be studied through surface brightness profiles. I recently showed in [R23] that the surface brightness cusps observed with HST can be very well explained as the result of adiabatic BH growth, as first envisaged by Young. This scenario yields a roughly linear correlation between BH mass and galaxy luminosity, which is seen also in the combined results of kinematical studies. This relation agrees with predictions based on quasar statistics; hence, the `dead quasars' in the local universe have now been convincingly detected.

The unambiguous detection of BHs in several galaxies has been an important step. After decades of study it is now possible to study the demographics of BHs in galactic nuclei, rather than their mere existence. Some questions are: What fraction of galaxies has BHs? What is the BH mass function? Is the BH mass related to the host galaxy properties? My future research will attempt to answer some of these questions. The unambiguous detection of BHs in several galaxies has been an important step: it is now possible to study the demographics of BHs in galactic nuclei, rather than their mere existence. Some questions are: What fraction of galaxies has BHs? What is the BH mass function? Is the BH mass always proportional to galaxy luminosity? Is the BH mass related to the host galaxy morphology? I am involved in several ongoing projects that will attempt to answer some of these questions. These include a study of the BH masses in a complete sample of the nearest radio-loud ellipticals (for which we already have data available in a variety of wavelength regions) to address the origin of the AGN phenomenon, and a study of the BH masses in a sample of nearby spiral galaxies, to determine whether spiral bulges and ellipticals of similar luminosity harbor similar BHs.



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Last modified December 8, 1998.
Roeland van der Marel, marel@stsci.edu.
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