The ALHAMBRA Survey: Bayesian photometric redshifts with 23 bands for 3 deg2

The Advance Large Homogeneous Area Medium-Band Redshift Astronomical (ALHAMBRA) survey has observed eight different regions of the sky, including sections of the Cosmic Evolution Survey (COSMOS), DEEP2, European Large-Area Infrared Space Observatory Survey (ELAIS), Great Observatories Origins Deep Survey North (GOODS-N), Sloan Digital Sky Survey (SDSS) and Groth fields using a new photometric system with 20 optical, contiguous similar to 300-angstrom filters plus the JHKs bands. The filter system is designed to optimize the effective photometric redshift depth of the survey, while having enough wavelength resolution for the identification of faint emission lines. The observations, carried out with the Calar Alto 3.5-m telescope using the wide-field optical camera Large Area Imager for Calar Alto (LAICA) and the near-infrared (NIR) instrument Omega-2000, represent a total of similar to 700 h of on-target science images. Here we present multicolour point-spread function (PSF) corrected photometry and photometric redshifts for similar to 438 000 galaxies, detected in synthetic F814W images. The catalogues are complete down to a magnitude I similar to 24.5 AB and cover an effective area of 2.79 deg(2). Photometric zero-points were calibrated using stellar transformation equations and refined internally, using a new technique based on the highly robust photometric redshifts measured for emission-line galaxies. We calculate Bayesian photometric redshifts with the Bayesian Photometric Redshift (bpz)2.0 code, obtaining a precision of delta(z)/(1 + z(s)) = 1 per cent for I < 22.5 and delta(z)/(1 + z(s)) = 1.4 per cent for 22.5 < I < 24.5. The global n(z) distribution shows a mean redshift aEurozaEuro parts per thousand = 0.56 for I < 22.5 AB and aEurozaEuro parts per thousand = 0.86 for I < 24.5 AB. Given its depth and small cosmic variance, ALHAMBRA is a unique data set for galaxy evolution studies.