Hubble Space Telescope/faint object spectrograph observations of M87: Ly alpha emission from the active galactic nucleus

The Faint Object Spectrograph on the Hubble Space Telescope was used to obt ain spectra of the central region of M87. These spectra cover the wavelengt h range 1140-1606 Angstrom and have a resolution of similar to 1 Angstrom. The nuclear continuum is clearly visible in the spectra. The only strong li ne that is observed is Ly alpha, which has a velocity width of about 3000 k m s(-1) (higher than the width of any line previously observed in the M87 n ucleus). There is also a marginal detection of C IV lambda 1549. The ratio of Ly alpha to C IV in the nuclear spectrum is at least a factor of 2 highe r than in a spectrum taken at a position on the disk similar to 0." 6 away from the nucleus by Dopita et al. This enhancement of Ly alpha at the nucle us could point to significant differences in the properties of the emitting gas and/or the excitation mechanism between the outer and inner disk regio ns. The strength of the observed Ly alpha places limits on the properties o f the absorbing gas present within M87. For instance, if the hydrogen colum n at the systemic velocity of M87 is greater than about 10(18) cm(-2), then it can cover only a small fraction of the line-emitting region. Spectra se parated by 5 days show a 60% difference in the Ly alpha flux, but the same continuum level. This could either be due to a displacement between the ape rture positions for the two sets of observations, or it could be due to int rinsic 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.