Broad Ly alpha Emission from Three Nearby BL Lacertae Objects
  We present far-UV HST/COS spectra of four nearby BL Lac Objects. BL Lac
spectra are dominated by a smooth, power-law continuum which arises in a
relativistic jet. However, the spectra are not necessarily featureless; weak,
broad- and/or narrow-line emission is sometimes seen in high-quality optical
spectra. We present detections of Lya emission in HST/COS spectra of Mrk421
(z=0.030) and PKS2005-489 (z=0.071) as well as an archival HST/GHRS observation
of Mrk501 (z=0.0337). Archival HST/STIS observations of PKS2155-304 (z=0.116)
show no Lya emission to a very low upper limit. Using the assumption that the
broad-line region (BLR) clouds are asymmetrically placed around the AGN, we use
these measured Lya emission features to constrain either the relativistic Gamma
values for the ionizing continuum produced by the jet (in the
ionization-bounded case) or the mass of warm gas (in the density-bounded case).
While realistic Gamma values can be obtained for all four cases, the values for
Mrk421 and PKS2155-304 are high enough to suggest that covering factors of
broad-line-region clouds of ~1-2% might be required to provide consistency with
earlier values of Doppler boosting and viewing angles suggested for this class
of BL Lacs. This discrepancy also exists in the case of M87, where the amount
of Doppler boosting in our direction is expected to be minimal, again
suggestive of a small covering factor of broad-line-region clouds. If, as these
small covering factors might suggest, the assumptions of a density-bounded
model could be more correct, then the observed Lya luminosities require that
BL\,Lac/FRI nuclei possess very little warm gas (10^-4 to 10^-5 Msun) as
suggested by Guilbert, Fabian & McCray (1983). If these clouds are in pressure
balance with a hotter (~10^6 K) gas, the BLR contains too little mass to power
the AGN by accretion alone.
