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Published March 1992 | Published
Journal Article Open

Planetary Camera observations of the M87 stellar cusp


Analysis of V and I band HST Planetary Camera images of the giant elliptical galaxy M87 show that its central starlight distribution is consistent with the black hole M_• = 2.6 X 10^9 M_☉ cusp model proposed for M87 by Young et al. [ApJ, 221, 721 (1978)]. A combined approach of image deconvolution and modeling is used to investigate the starlight distribution into limiting radii of ≈0".04 (3 pc at 16 Mpc). The central structure of M87 can be described by three components: a power-law starlight profile of the form µ(r)∝a:r^(-1/4) for r<3", a central nonthermal point source, and optical counterparts of the jet knots N1 and M identified by VLBI observations. M87 lacks a constant surface brightness core, and its central starlight luminosity density exceeds 10^3L_☉ pc^(-3) (I band) for r< 10 pc. The profile strongly resembles a stellar cusp associated with a massive black hole. A review of existing velocity dispersion observations suggests that the Young et al. black hole mass can be accommodated to the observations with minor adjustments of dynamical models. The central luminosity spike itself remains unresolved at HST resolution, with r_c < 1 pc. The spike has optical spectral index ɑ= -0.46 ± 0.20 and is at least as blue if not bluer than the rest of the M87 jet. The total nonthermal flux in the inner 1" of M87 agrees well with the central radio flux and the radio-optical spectral index of the rest of the jet. It is also consistent with the spectral-line dilution seen by Dressler & Richstone [ApJ, 348, 120 ( 1990)]; we thus argue that the spike is completely nonthermal.

Additional Information

© 1992 American Astronomical Society. Provided by the NASA Astrophysics Data System. Received 2 October 1991; revised 12 November 1991. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA Contract No. NAS5-26555. The National Optical Astronomy Observatories are operated by the Association of Universities for Research in Astronomy, Inc. (AURA) under cooperative agreement with the National Science Foundation. This research was conducted by the WFPC Investigation Definition Team, supported in part by NASA Grant No. NAS5-1661.

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