Quasi-stellar objects in the ALHAMBRA survey. I. Photometric redshift
  accuracy based on a 23 optical-NIR filter photometry
  We characterize the ability of the ALHAMBRA survey to assign accurate
photo-z's to BLAGN and QSOs based on their ALHAMBRA very-low-resolution
optical-NIR spectroscopy. A sample of 170 spectroscopically identified BLAGN
and QSOs have been used together with a library of templates (including SEDs
from AGN, normal, starburst galaxies and stars) in order to fit the 23
photometric data points provided by ALHAMBRA in the optical and NIR (20
medium-band optical filters plus the standard JHKs). We find that the ALHAMBRA
photometry is able to provide an accurate photo-z and spectral classification
for ~88% of the spectroscopic sources over 2.5 deg^2 in different areas of the
survey, all of them brighter than m678=23.5 (equivalent to r(SLOAN)~24.0). The
derived photo-z accuracy is better than 1% and comparable to the most recent
results in other cosmological fields. The fraction of outliers (~12%) is mainly
caused by the larger photometric errors for the faintest sources and the
intrinsic variability of the BLAGN/QSO population. A small fraction of outliers
may have an incorrectly assigned spectroscopic redshift. The definition of the
ALHAMBRA survey in terms of the number of filters, filter properties, area
coverage and depth is able to provide photometric redshifts for BLAGN/QSOs with
a precision similar to any previous survey that makes use of medium-band
optical photometry. In agreement with previous literature results, our analysis
also reveals that, in the 0<z<4 redshift interval, very accurate photo-z can be
obtained without the use of near-IR broadband photometry at the expense of a
slight increase of outliers. The NIR importance is expected to increase at
higher redshifts (z>4). These results are relevant for the design of future
optical follow-ups of surveys with a large fraction of BLAGN, as it is the case
for X-rays or radio surveys.
