The BL-Lac gamma-ray blazar PKS 0447-439 as a probable member of a group
  of galaxies at z=0.343
  The BL-Lac blazar PKS 0447-439 is one of the brightest HE gamma-ray sources
that were first detected by Fermi-LAT. It was also detected by H.E.S.S. at VHE
gamma-rays, which allowed constraining the redshift of PKS 0447-439 by
considering the attenuation caused by gamma-ray interactions with ambient
photons in the extragalactic background light (EBL). This constraint agreed
with color-magnitude and spectroscopic redshift constraints (0.179 < z < 0.56),
Recently, however, a much higher redshift was proposed for this blazar (z >
1.2). This value was debated because if true, it would imply either that the
relevant absorption processes of gamma-rays are not well understood or that the
EBL is dramatically different from what is believed today. This high redshift
was not confirmed by three independent new spectroscopic observations at high
signal-to-noise ratios.
  Given that BL-Lac are typically hosted by elliptical galaxies, which in turn
are associated with groups, we aim to find the host group of galaxies of PKS
0447-439. The ultimate goal is to estimate a redshift for this blazar.
  Spectra of twenty-one objects in the field of view of PKS 0447-439 were
obtained with the Gemini Multi-Object Spectrograph. Based on the redshifts and
coordinates of these galaxies, we searched for groups of galaxies. Using a deep
catalog of groups, we studied the probability of finding by chance a group of
galaxies in the line of sight of PKS 0447-439.
  We identified a group of galaxies that was not previously cataloged at z =
0.343 with seven members, a virial radius of 0.42 Mpc, and a velocity
dispersion of 622 km s^-1. We found that the probability of the host galaxy of
PKS 0447-439 to be a member of the new group is >= 97%. Therefore, we propose
to adopt z = 0.343 +- 0.002 as the most likely redshift for PKS 0447-439.
