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We aim to illustrate the potentiality of the Advanced Large, HomogeneousArea, Medium-Band Redshift Astronomical (ALHAMBRA) survey to investigate thehigh redshift universe through the detection of quasi stellar objects (QSOs) atredshifts larger than 5. The search for z>5 QSOs candidates was done by fittingan extensive library of spectral energy distributions --including active andnon-active galaxy templates as well as stars-- to the photometric database ofthe ALHAMBRA survey (composed of 20 optical medium-band plus the 3 broad-bandJHKs filters). Our selection over ~1 square degree of ALHAMBRA data (~1/4 ofthe total area covered by the survey), combined with GTC/OSIRIS spectroscopy,has yielded the identification of an optically faint QSO at very high redshift(z = 5.41). The QSO has an absolute magnitude of ~-24 at the 1450{\AA}continuum, a bolometric luminosity of ~2x10^46 erg/s and an estimated blackhole mass of ~10^8 Msolar. This QSO adds itself to a reduced number of known UVfaint sources at these redshifts. The preliminary derived space density iscompatible with the most recent determinations of the high-z QSO luminosityfunctions (QLF). This new detection shows how ALHAMBRA, as well as forthcomingwell designed photometric surveys, can provide a wealth of information on theorigin and early evolution of this kind of objects.

The ALHAMBRA survey: Discovery of a faint QSO at z = 5.41