The BL-Lac gamma-ray blazar PKS 1424+240 associated with a group of
  galaxies at z=0.6010
  PKS 1424+240 is a BL-Lac blazar with unknown redshift that was detected at
high-energy gamma rays by Fermi-LAT with a hard spectrum. At VHE, it was first
detected by VERITAS and later confirmed by MAGIC. Its spectral energy
distribution is highly attenuated at VHE gamma rays, which is coherent with
distant sources. Several estimations enabled the redshift to be constrained to
the range 0.6 < z < 1.3. These results place PKS 1424+240 in the very
interesting condition of being probably the most distant blazar that has been
detected at VHE. The ambiguity in the redshift is still large enough to prevent
precise studies of the EBL and the intrinsic blazar spectrum. Given the
difficulty of measuring spectroscopic redshifts for BL-Lac objects directly, we
aim to establish a reliable redshift value for this blazar by finding its host
group of galaxies. Elliptical galaxies are associated with groups, and BL-Lac
objects are typically hosted by them, so we decided to search for the host
group of the blazar. For this, we performed optical spectroscopic observations
of thirty objects in the field of view of PKS 1424+240 using the Gemini MOS.
After analysing the data for groups, we evaluated the probability of finding
groups of galaxies by chance around the position of PKS 1424+240, using a deep
catalogue of groups. We also used photometric data from the SDSS catalogue to
analyse the red sequence of the proposed blazar host group. We found a new
group of galaxies with eight members at z = 0.6010 +- 0.003, a virial radius of
R_vir = 1.53 Mpc, and a velocity dispersion of sigma_v = 813 +- 187 km/s. The
photometric study indicates that more members are probably populating this
previously uncatalogued group of galaxies. The probability of PKS 1424+240
being a member of this group was found to be 98%. The new group of galaxies
found at z = 0.6010 +- 0.003 is very likely hosting PKS 1424+240.
