Hubble Space Telescope/Faint Object Spectrograph Observations of M87:Lyα Emission from the Active Galactic Nucleus

Abstract

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 1140-1606 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 Lyα, 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 λ1549. The ratio of Lyα 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 ~0.6" away from the nucleus by Dopita et al. This enhancement of Lyα 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 Lyα 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 Lyα flux, but the same continuum level. This could either be due to 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 Lyα line is a useful tracer of kinematics in the M87 nucleus.

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 5-26555.