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R. Sankrit, K.R. Sembach (JHU), C.R. Canizares (MIT)
The Faint Object Spectrograph on the Hubble Space Telescope was used to obtain spectra of the central region of M87. These spectra cover the wavelength range 1140~Å\ -- 1606~Å\ and have a resolution of ~ 1~Å. The nuclear continuum is clearly visible in the spectra. The only strong line that is observed is Ly\alpha, which has a velocity width of about 3000 km~s-1 (higher than the width of any line previously observed in the M87 nucleus). There is also a marginal detection of C~IV \lambda1549. The ratio of Ly\alpha to C~IV in the nuclear spectrum is at least a factor of 2 higher than in a spectrum taken at a position on the disk ~~0\farcs6 away from the nucleus by Dopita et al. This enhancement of Ly\alpha at the nucleus could point to significant differences in the properties of the emitting gas and/or the excitation mechanism between the outer and inner disk regions. The strength of the observed Ly\alpha places limits on the properties of the absorbing gas present within M87. For instance, if the hydrogen column at the systemic velocity of M87 is greater than about 1018 cm-2 then it can cover only a small fraction of the line emitting region. Spectra separated by 5 days show a 60% difference in the Ly\alpha flux, but the same continuum level. This could be due to either a displacement between the aperture positions for the two sets of observations, or it could be due to intrinsic variability of the source. The current observations do not strongly favor either of these alternatives. The observations do show, however, that the Ly\alpha line is a useful tracer of kinematics in the M87 nucleus. The work was funded in part by NASA.
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