Context : 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 .
The scenario is clear evidence of the difficulties in estimating the redshift of BL-Lac objects , whose optical spectra are typically featureless .
Neither of the estimated redshifts for PKS 0447-439 are confirmed as yet .

Aims : 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 .

Methods : 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 .

Results : 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 \gtrsim 97 \% .
Therefore , we propose to adopt z = 0.343 \pm 0.002 as the most likely redshift for PKS 0447-439 .

Conclusions :