Resolving the Host Galaxy of a Distant Blazar with LBT/LUCI1 + ARGOS
  BL Lac objects emitting in the Very High Energy (VHE) regime are unique tools
to peer into the properties of the Extragalactic Background Light (EBL).
However, due to the typical absence of features in their spectra, the
determination of their redshifts has proven challenging. In this work we
exploit the superb spatial resolution delivered by the new Advanced Rayleigh
guided Ground layer adaptive Optics System (ARGOS) at the Large Binocular
Telescope to detect the host galaxy of HESS J1943+213, a VHE emitting BL Lac
shining through the Galaxy. Deep H-band imaging collected during the ARGOS
commissioning allowed us to separate the contribution of the nuclear emission
and to unveil the properties of the host galaxy with unprecedented detail. The
host galaxy is well fitted by a S\'ersic profile with index of n~2 and total
magnitude of H~16.15 mag. Under the assumption that BL Lac host galaxies are
standard candles, we infer a redshift of z~0.21. In the framework of the
current model for the EBL, this value is in agreement with the observed dimming
of the VHE spectrum due to the scatter of energetic photons on the EBL.
