Science with the Hubble Space Telescope -- II
Book Editors: P. Benvenuti, F. D. Macchetto, and E. J. Schreier
Electronic Editor: H. Payne

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HST FOS/COSTAR Small Aperture Spectroscopy of the Disk of Ionized Gas in M87

H. Ford, Z. Tsvetanov, G. Hartig, G. Kriss, R. Harms and L. Dressel

Department of Physics and Astronomy, Johns Hopkins University, 3701 San Martin Drive, Baltimore, MD 21218 USA

Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 USA

RJH Scientific, Inc., 5904 Richmond Highway, Suite 401, Alexandria, VA 22303 USA

Abstract:

HST COSTAR plus small aperture () FOS observations across M87's nucleus in the direction perpendicular to jet and aperture observations across the nucleus parallel to the jet show large non-circular velocities superposed on the motion of a rapidly rotating disk. The non-circular velocities in the direction of the jet suggest gas is being entrained by the jet and ejected from the nucleus. The best fit of the organized motion to a Keplerian disk gives a central mass of M. The mass to light ratio is , strengthening the conclusion that M87 hosts a massive black hole.

Keywords: M87, Gaseous Disks, Black Holes

Introduction

Ford et al. 1994 (hereafter F94) used HST WFPC2 F658N H[NII] on-band images and F547M off-band images of M87 to find a 100 pc scale disk of ionized gas with a major axis approximately perpendicular to the jet. Harms et al. (1994; hereafter H94) used COSTAR and the FOS to measure the velocity at five positions in the disk found by F94, and concluded that there was Keplerian rotation around a mass of M. The large values of the mass-to-light ratio, and , led them to conclude there is a massive black hole in the center of M87. If this is indeed true, the velocities should rise parabolically toward the center. To further investigate the dynamics in the disk and the mass of the black hole, we re-observed M87 with the apertures and positions shown in Figure 1a. In addition to the small aperture positions along the apparent major axis, we used the aperture to measure the velocities at two positions along the apparent minor axis of the disk, the direction of the jet.

Representative spectra at these new positions are shown in Figure 1b. We interpret the multiple components measured in Position 7 as major outflows along the axis defined by the jet. The gas everywhere is very turbulent with a typical FWHM kms.

The conclusion that we are observing Keplerian motion around a MBH is supported and strengthened by the new observations. Figure 2 shows a preliminary fit of the H94 [OIII] velocities and the new [OIII] velocities to the projection of a Keplerian disk onto the line of sight (the equation for in H94). The slope of the fit gives the Keplerian mass. The plot would be a straight line for a perfect fit, and a scatter diagram in the absence of a central mass. Because at least three parameters can be varied (central mass, inclination of the disk, and position angle of the line of nodes), the fit is not unique. However, all combinations of the parameters which minimize the rms residuals require a central mass M. The best fit kinematical inclination is close to the geometrical inclination of the disk (F94), and the best fit kinematical line of nodes agrees acceptably well with determined geometrically. The kinematical line of nodes is only from the direction perpendicular to the jet, again suggesting a causal relationship between the angular momentum in the disk and the direction of the jet.

 
Figure: The left panel shows a schematic of the FOS aperture positions relative to the nucleus and jet in M87. The small square aperture observations were made at two different epochs at each position, accounting for the small differences in centering. The right panel shows representative COSTAR plus FOS spectra at four positions in the center of M87. The ordinate is observed flux in counts per second.

 
Figure: A preliminary fit of the H94 [OIII] M87 velocities and the new [OIII] velocities to the projection of a Keplerian disk onto the line of sight. The slope of the fit gives the Keplerian mass in units of M.

The dashed lines in Figure 2 are the extremes which bound the data, and show that the fit requires a mass between 1 and M. Several factors suggest that the dynamics in the disk are more complicated than simple Keplerian motion. The first is the turbulence in the disk, which shows that the motion cannot be perfectly Keplerian. The apparent spiral structure noted by F94 suggests shocks, which can accelerate gas. Finally, the disk in NGC 4261 is eccentric relative to the nucleus, and shows signs of mild warps. There may be similar effects in the M87 disk. In view of these, it is remarkable that the Keplerian model works as well as it does. Though we likely could improve the fit to the data by adding more parameters such as eccentricity, we do not think this is justified at this time.

The mean radius of the four points closest to the nucleus is pc. Using the Lauer et al. (1992) WFPC1 observations of M87 to estimate the luminosity (excluding light from the non-thermal point source), we provisionally get and ! In view of the good fit of the data to the model, and the fact that M/L continues to rise and reaches very high values as we approach the center, we conclude there is a MBH with a mass M in the center of M87.

The mass of the black hole and M/L are likely underestimated because we have not accounted for the kinetic energy in the turbulent motion. Two important unanswered questions are, what drives the turbulence in the disk, and how do we kinematically and physically account for the outflow (large non-circular velocities)?

Acknowledgments:

This research was supported by NASA grants NAS5-29293 and NAG5-1630.

References:

Ford, H.C., Harms, R.J., Tsvetanov, Z.I., Hartig, G.F., Dressel, L.L., Kriss, G.A., Davidsen, A.F., Bohlin, R.A., & Margon, B. 1994, ApJ 435, L27

Harms, R.J., Ford, H.C., Tsvetanov, Z.I., Hartig, G.F., Dressel, L.L., Kriss, G.A., Bohlin, R.A., Davidsen, A.F., Margon, B., & Kochhar, A.K. 1994, ApJ, 435, L35

Lauer, T.R., Faber, S.M., Lynds, C.R., Baum, W.A., Ewald, S.P., Groth, E.J., Hester, J.J., Holtzman, J.A., Kristian, J., & Light, R.M. 1992, AJ, 103, 703

H. Ford, Z. Tsvetanov, G. Hartig, G. Kriss, R. Harms, and L. DresselFord et al.HST FOS/COSTAR Small Aperture Spectroscopy of Ionized Disk Gas in M87