The ALHAMBRA survey: Discovery of a faint QSO at z = 5.41
  We aim to illustrate the potentiality of the Advanced Large, Homogeneous
Area, Medium-Band Redshift Astronomical (ALHAMBRA) survey to investigate the
high redshift universe through the detection of quasi stellar objects (QSOs) at
redshifts larger than 5. The search for z>5 QSOs candidates was done by fitting
an extensive library of spectral energy distributions --including active and
non-active galaxy templates as well as stars-- to the photometric database of
the ALHAMBRA survey (composed of 20 optical medium-band plus the 3 broad-band
JHKs filters). Our selection over ~1 square degree of ALHAMBRA data (~1/4 of
the 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 black
hole mass of ~10^8 Msolar. This QSO adds itself to a reduced number of known UV
faint sources at these redshifts. The preliminary derived space density is
compatible with the most recent determinations of the high-z QSO luminosity
functions (QLF). This new detection shows how ALHAMBRA, as well as forthcoming
well designed photometric surveys, can provide a wealth of information on the
origin and early evolution of this kind of objects.
