Astronomy & Astrophysics (A&A) is an international journal which publishes papers on all aspects of astronomy and astrophysics

Here we describe the first results of the ALHAMBRA survey which provides cosmic tomography of the evolution of the contents of the Universe over most of Cosmic history. Our novel approach employs 20 contiguous, equal-width, medium-band filters covering from 3500 ûA to 9700 ûA, plus the standard JHKs near-infrared bands, to observe a total area of 4 square degrees on the sky. The

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 ∼300-Å 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 ∼700 h of on-target science images. Here we present multicolour point-spread function (PSF) corrected photometry and photometric redshifts for ∼438 000 galaxies, detected in synthetic F814W images. The catalogues are complete down to a magnitude I ∼ 24.5 AB and cover an effective area of 2.79 deg². 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 δ_z/(1 + z_s) = 1 per cent for I < 22.5 and δ_z/(1 + z_s) = 1.4 per cent for 22.5 < I < 24.5. The global n(z) distribution shows a mean redshift 〈z〉 = 0.56 for I < 22.5 AB and 〈z〉 = 0.86 for I < 24.5 AB. Given its depth and small cosmic variance, ALHAMBRA is a unique data set for galaxy evolution studies.

We present the catalogue resulting from the ESO Nearby Abell Cluster Survey (the ENACS), which contains redshifts and magnitudes for 5634 galaxies in the directions of 107 rich, nearby southern Abell cluster candidates. We describe the contents of the catalogue and discuss the results of a comparison between the ENACS catalogue and the COSMOS Galaxy Catalogue. When cross-correlating the two catalogues we find that, at least in the areas of the ENACS clusters, the completeness of the COSMOS catalogue is somewhat lower than was estimated previously for the carefully analyzed and well-calibrated part of the COSMOS catalogue known as the Edinburgh-Durham Southern Galaxy Survey (EDSGC). The galaxy positions in the COSMOS and ENACS catalogues are found to be on the same system to within about one arcsecond. For the clusters for which the photometry in the ENACS and COSMOS catalogues is based on the same survey plates, the two magnitude scales agree very well. We confirm that the photometric calibration in the EDSGC subset of the COSMOS catalogue is of higher quality than in the EDSGC complement. The ENACS galaxy samples are unbiased subsets of the COSMOS catalogue as far as the projected galaxy distribution is concerned, except in only a few cases. We summarize how the ENACS galaxy samples are subsets of the COSMOS catalogues in the ENACS apertures, with respect to magnitude. For the ENACS catalogue as a whole, we describe the apparent incompleteness at faint magnitudes and towards higher redshifts. Finally, we provide some detailed information about the ENACS catalogue that is essential for its proper statistical use and we summarize some facts that must be remembered when selecting subsets of galaxies from it.

In the coming years, several cosmological surveys will rely on imaging data to estimate the redshift of galaxies, using traditional filter systems with 4-5 optical broad bands; narrower filters improve the spectral resolution, but strongly reduce the total system throughput. We explore how photometric redshift performance depends on the number of filters n_f , characterizing the survey depth by the fraction of galaxies with unambiguous redshift estimates. For a combination of total exposure time and telescope imaging area of 270 hr m², 4-5 filter systems perform significantly worse, both in completeness depth and precision, than systems with n_f 8 filters. Our results suggest that for low n_f the color-redshift degeneracies overwhelm the improvements in photometric depth, and that even at higher n_f the effective photometric redshift depth decreases much more slowly with filter width than naively expected from the reduction in the signal-to-noise ratio. Adding near-IR observations improves the performance of low-n_f systems, but still the system which maximizes the photometric redshift completeness is formed by nine filters with logarithmically increasing bandwidth (constant resolution) and half-band overlap, reaching ~0.7 mag deeper, with 10% better redshift precision, than 4-5 filter systems. A system with 20 constant-width, nonoverlapping filters reaches only ~0.1 mag shallower than 4-5 filter systems, but has a precision almost three times better, δz = 0.014(1 + z) versus δz = 0.042(1 + z). We briefly discuss a practical implementation of such a photometric system: the ALHAMBRA Survey.

A study of CO and FIR emission in a distance limited (z<0.03) complete sample of Hickson Compact Group (HCG) galaxies was conducted in order to examine the effects of their unique environment on the ISM and star formation activity. Ubiquitous tidal interactions in these dense groups would predict enhanced activities among the HCG galaxies compared to isolated galaxies. Instead their CO and FIR properties are surprisingly similar to isolated spirals. The CO data for 80 HCG galaxies presented here indicate that the spirals globally show the same H2 content as the isolated comparison sample, although 20% are deficient in CO emission. The CO deficiency appears linked with the group evolution, and gas exhaustion through past star formation and removal of external gas reserve by tidal stripping of the outer HI disk offer a possible explanation. The IRAS data for the entire sample of 161 HCG galaxies were re-analyzed using ADDSCAN/SCANPI, improving the sensitivity by a factor of 3-5 over the existing Point Source Catalog (PSC) and better resolving the contribution from individual galaxies. We find that FIR emission in HCG galaxies is similar to isolated, Virgo cluster, and weakly interacting galaxies. A factor two enhancement in the 25 micr/100 micr flux ratio among the HCG spirals is found, which suggests intense, localized nuclear starburst activity similar to HII galaxies. A number of early-type galaxies in HCGs are detected in CO and FIR, lending further support to the idea that tidal interactions and tidally induced evolution of the groups and member galaxies are important in our sample.Comment: 34 pages (Latex), 8 tables, 11 figures. To be published in Ap