HEROES: Strong lensing of Lyman-alpha background emitters
Guenther Hasinger
European Space Agency

HEROES (the Hawaii EROsita Ecliptic pole Survey) is a ground-based optical/NIR large-area survey carried out on the Subaru and CFHT telescopes in Hawaii. We covered 40 Square degrees around the North-Ecliptic Pole (NEP) in seven broad band and two narrow-band filters. This area will be covered in a deep survey by the upcoming German/Russian eROSITA all-sky survey on the Russian Spectrum-Roentgen-Gamma mission. This survey will be about 30 times more sensitive than the ROSAT observations carried out almost 28 years ago. In preparation of eROSITA we observed a number of ROSAT-detected clusters of galaxies with the new KCWI imaging spectrograph on the Keck 2 telescope. Imaging spectrographs provide a spectrum in each pixel of the field of view without the need to preselect a spectroscopic target. They are the only way to detect faint background emission line galaxies in the presence of the foreground cluster emission. In several of our cluster observations we discovered evidence of strong lensing of background Ly-alpha emitters by the foreground cluster lenses. In one case we even found an Einstein Cross. This provides a new method of detecting these very rare geometric coincidences and puts interesting constraints on the space density of Ly-alpha emitters.