On the Redshift of the Very High Energy Blazar 3C 66A
  As a bright gamma-ray source, 3C 66A is of great interest to the high-energy
astrophysics community, having a potential for placing cosmological constraints
on models for the extragalactic background light (EBL) and the processes which
contribute to this photon field. No firm spectroscopic redshift measurement has
been possible for this blazar due to a lack of intrinsic emission and
absorption features in optical spectra. We present new far-ultraviolet spectra
from the Hubble Space Telescope/Cosmic Origins Spectrograph (HST/COS) of the BL
Lac object 3C 66A covering the wavelength range 1132 to 1800 Angstroms . The
data show a smooth continuum with intergalactic medium absorption features
which can be used to place a firm lower limit on the blazar redshift of
z>0.3347. An upper limit is set by statistically treating the non-detection of
additional absorbers beyond z=0.3347, indicating a redshift of less than 0.41
at 99% confidence and ruling out z>0.444 at 99.9% confidence. We conclude by
showing how the redshift limits derived from the COS spectra remove the
potential for this gamma-ray emitting blazar to place an upper limit on the
flux of the EBL using high energy data from a flare in October of 2009.
