16258

High-redshift luminous QSOs (zvirgul6) already host billion solar masses black holes in their centers comma and are predicted to live in high-density peaks of the dark matter distribution comma surrounded by overdensities of galaxies. Previous searches for Lyman Alpha Emitters (LAEs) with narrow-band (NB) imaging around radio-quiet high-z QSOs did not recover such overdensities. These QSOs redshifts were determined only via broad comma high- ionization emission lines in the rest-frame UV comma often strongly shifted with respect to the systemic redshifts. Alternatively comma radio-loud sources have been commonly found in rich environments comma pinpointing the first protoclusters in the universe. Here comma we propose HST imaging observations to search for LAEs in the field of PSO172+18 comma the most distant radio source and the radio loudest QSO at z>6 comma using a suite of narrow and broad band filters with WFC3 and ACS-WFC. The systemic redshift of PSO172+18 comma z=6.823+-0.002 comma is accurately measured from observations of the narrow MgII emission line comma and precisely locate the Lyman Alpha emission line in the response of the narrow band filter F953N comma uniquely available on board HST. Additionally comma we will detect (or derive tight constraints on) the stellar emission of the QSO host galaxy comma and search for any potential close (-kpc scale) companion.