Bibcode
Matute, I.; Márquez, I.; Masegosa, J.; Husillos, C.; del Olmo, A.; Perea, J.; Alfaro, E. J.; Fernández-Soto, A.; Moles, M.; Aguerri, J. A. L.; Aparicio-Villegas, T.; Benítez, N.; Broadhurst, T.; Cabrera-Cano, J.; Castander, F. J.; Cepa, J.; Cerviño, M.; Cristóbal-Hornillos, D.; Infante, L.; González Delgado, R. M.; Martínez, V. J.; Molino, A.; Prada, F.; Quintana, J. M.
Referencia bibliográfica
Astronomy and Astrophysics, Volume 542, id.A20
Fecha de publicación:
6
2012
Revista
Número de citas
22
Número de citas referidas
20
Descripción
Context. Even the spectroscopic capabilities of today's ground and
space-based observatories can not keep up with the enormous flow of
detections (>105 deg-2) unveiled in modern
cosmological surveys as: i) would be required enormous telescope time to
perform the spectroscopic follow-ups and ii) spectra remain unattainable
for the fainter detected population. In the past decade, the typical
accuracy of photometric redshift (photo-z) determination has drastically
improved. Nowdays, it has become a perfect complement to spectroscopy,
closing the gap between photometric surveys and their spectroscopic
follow-ups. The photo-z precision for active galactic nuclei (AGN) has
always lagged behind that for the galaxy population owing to the lack of
proper templates and their intrinsic variability. Aims: Our goal
is to characterize the ability of the Advanced Large, Homogeneous Area
Medium-Band Redshift Astronomical (ALHAMBRA) survey in assigning
accurate photo-z's to broad-line AGN (BLAGN) and quasi-stellar objects
(QSOs) based on their ALHAMBRA very-low-resolution optical-near-infrared
(NIR) spectroscopy. This will serve as a benchmark for any future
compilation of ALHAMBRA selected QSOs and the basis for the statistical
analysis required to derive luminosity functions up to z ~ 5.
Methods: We selected a sample of spectroscopically identified BLAGN and
QSOs and used a library of templates (including the SEDs of AGN and both
normal and starburst galaxies, as well as stars) to fit the 23
photometric data points provided by ALHAMBRA in the optical and NIR (20
medium-band optical filters plus the standard JHKs). Results: We
find that the ALHAMBRA photometry is able to provide an accurate photo-z
and spectral classification for ~88% of the 170 spectroscopically
identified BLAGN/QSOs over 2.5 deg2 in different areas of the
survey and brighter than m678 = 23.5 (equivalent to
rSLOAN ~ 24.0). The derived photo-z accuracy is below 1% and
is comparable to the most recent results in other cosmological fields
that use photometric information over a wider wavelength range. 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. Conclusions: The
definition of the ALHAMBRA survey in terms of the number of filters,
filter properties, areal 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 NIR broadband photometry at the
expense of a slight increase in the outliers. The importance of NIR data
is expected to increase at higher z (z > 4). These results are
relevant for the design of future optical follow-ups of surveys
containing a large fraction of BLAGN, such as many X-ray or radio
surveys.
Based on observations collected at the German-Spanish Astronomical
center, Calar Alto (Almeria, Spain), jointly operated by the
Max-Planck-Institut für Astronomie at Heidelberg and the Instituto
de Astrofísica de Andalucía (CSIC).
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