ELDAR, a new method to identify AGN in multi-filter surveys: the
  ALHAMBRA test-case
  We present ELDAR, a new method that exploits the potential of medium- and
narrow-band filter surveys to securely identify active galactic nuclei (AGN)
and determine their redshifts. Our methodology improves on traditional
approaches by looking for AGN emission lines expected to be identified against
the continuum, thanks to the width of the filters. To assess its performance,
we apply ELDAR to the data of the ALHAMBRA survey, which covered an effective
area of $2.38\,{\rm deg}^2$ with 20 contiguous medium-band optical filters down
to F814W$\simeq 24.5$. Using two different configurations of ELDAR in which we
require the detection of at least 2 and 3 emission lines, respectively, we
extract two catalogues of type-I AGN. The first is composed of 585 sources
($79\,\%$ of them spectroscopically-unknown) down to F814W$=22.5$ at $z_{\rm
phot}>1$, which corresponds to a surface density of $209\,{\rm deg}^{-2}$. In
the second, the 494 selected sources ($83\,\%$ of them
spectroscopically-unknown) reach F814W$=23$ at $z_{\rm phot}>1.5$, for a
corresponding number density of $176\,{\rm deg}^{-2}$. Then, using samples of
spectroscopically-known AGN in the ALHAMBRA fields, for the two catalogues we
estimate a completeness of $73\,\%$ and $67\,\%$, and a redshift precision of
$1.01\,\%$ and $0.86\,\%$ (with outliers fractions of $8.1\,\%$ and $5.8\,\%$).
At $z>2$, where our selection performs best, we reach $85\,\%$ and $77\,\%$
completeness and we find no contamination from galaxies.
