The ALHAMBRA (Advance Large Homogeneous Area Medium Band Redshift Astronomical) survey has observed 8 different regions of the sky, including sections of the COSMOS, DEEP2, ELAIS, GOODS-N, SDSS and Groth fields using a new photometric system with 20 optical, contiguous ∼300Å 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 3.5m telescope using the wide field optical camera LAICA and the NIR instrument Omega-2000, represent a total of ∼700hrs of on-target science images. Here we present multicolor PSF-corrected photometry and photometric redshifts for ∼438,000 galaxies, detected in synthetic F 814W images. The catalogs are complete down to a magnitude I∼24.5AB and cover an effective area of 2.79 deg 2 . Photometric zeropoints 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 BPZ2.0 code, obtaining a precision of δ z /(1+z s )=1% for I<22.5 and δ z /(1+z s )=1.4% for 22.5=0.56 for I<22.5 AB and =0.86 for I<24.5 AB. Given its depth and small cosmic variance, ALHAMBRA is a unique dataset for galaxy evolution studies.

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 3500Å to 9700Å, plus the standard JHK s near-infrared bands, to observe a total area of 4 square degrees on the sky. The -2optical photometric system has been designed to maximize the number of objects with accurate classification by Spectral Energy Distribution type and redshift, and to be sensitive to relatively faint emission features in the spectrum. The observations are being carried out with the Calar Alto 3.5m telescope using the wide field cameras in the optical, LAICA, and in the NIR, Omega-2000. The first data confirm that we are reaching the expected magnitude limits (for a total of 100 ksec integration time per pointing) of AB ≤ 25 mag (for an unresolved object, S/N = 5) in the optical filters from the blue to 8300Å, and from AB = 24.7 to 23.4 for the redder ones. The limit in the NIR, for a total of 15 ks exposure time per pointing, is (in the Vega system) K s ≈ 20 mag, H ≈ 21 mag, J≈ 22 mag. Some preliminary results are presented here to illustrate the capabilities of the ongoing survey. We expect to obtain accurate redshift values, ∆z/(1 + z) ≤ 0.03 for about 5 ×10 5 galaxies with I≤ 25 (60% completeness level), and z med = 0.74. This accuracy, together with the homogeneity of the selection function, will allow for the study of the redshift evolution of the large scale structure, the galaxy population and its evolution with redshift, the identification of clusters of galaxies, and many other studies, without the need for any further followup. It will also provide targets for detailed studies with 10m-class telescopes. Given its area, spectral coverage and its depth, apart from those main goals, the ALHAMBRA-Survey will also produce valuable data for galactic studies.

Baryon Acoustic Oscillations (BAO) provide a "standard ruler" of known physical length, making it one of the most promising probes of the nature of dark energy. The detection of BAO as an excess of power in the galaxy distribution at a certain scale requires measuring galaxy positions and redshifts. "Transversal" (or "angular") BAO measure the angular size of this scale projected in the sky and provide information about the angular distance. "Line-of-sight" (or "radial") BAO require very precise redshifts, but provide a direct measurement of the Hubble parameter at different redshifts, a more sensitive probe of dark energy. The main goal of this paper is to show that it is possible to obtain photometric redshifts with enough precision (σ z ) to measure BAO along the line of sight. There is a fundamental limitation as to how much one can improve the BAO -2measurement by reducing σ z . We show that σ z ∼ 0.003(1 + z) is sufficient: a much better precision will produce an oversampling of the BAO peak without a significant improvement on its detection, while a much worse precision will result in the effective loss of the radial information. This precision in redshift can be achieved for bright, red galaxies, featuring a prominent 4000Å break, by using a filter system comprising about 40 filters, each with a width close to 100Å, covering the wavelength range from ∼ 4000Å to ∼ 8000Å, supplemented by two broad-band filters similar to the SDSS u and z bands. We describe the practical implementation of this idea, a new galaxy survey project, PAU * , to be carried out with a telescope/camera combination with an etendue about 20 m 2 deg 2 , equivalent to a 2 m telescope equipped with a 6 deg 2 -FoV camera, and covering 8000 sq. deg. in the sky in four years. We expect to measure positions and redshifts for over 14 million red, early-type galaxies with L > L ⋆ and i AB 22.5 in the redshift interval 0.1 < z < 0.9, with a precision σ z < 0.003(1 + z). This population has a number density n 10 −3 Mpc −3 h 3 galaxies within the 9 (Gpc/h) 3 volume to be sampled by our survey, ensuring that the error in the determination of the BAO scale is not limited by shot-noise. By itself, such a survey will deliver precisions of order 5% in the dark-energy equation of state parameter w, if assumed constant, and can determine its time derivative when combined with future CMB measurements. In addition, PAU will yield high-quality redshift and low-resolution spectroscopy for hundreds of millions of other galaxies, including a very significant high-redshift population. The data set produced by this survey will have a unique legacy value, allowing a wide range of astrophysical studies.Subject headings: large-scale structure of universe -cosmological parameters * Physics of the Accelerating Universe (PAU): http://www.ice.cat/

The Javalambre Photometric Local Universe Survey (J-PLUS ) is an ongoing 12-band photometric optical survey, observing thousands of square degrees of the Northern Hemisphere from the dedicated JAST/T80 telescope at the Observatorio Astrofísico de Javalambre (OAJ). The T80Cam is a camera with a field of view of 2 deg 2 mounted on a telescope with a diameter of 83 cm, and is equipped with a unique system of filters spanning the entire optical range (3500-10 000 Å). This filter system is a combination of broad-, medium-, and narrow-band filters, optimally designed to extract the rest-frame spectral features (the 3700-4000 Å Balmer break region, Hδ, Ca H+K, the G band, and the Mg b and Ca triplets) that are key to characterizing stellar types and delivering a low-resolution photospectrum for each pixel of the observed sky. With a typical depth of AB ∼21.25 mag per band, this filter set thus allows for an unbiased and accurate characterization of the stellar population in our Galaxy, it provides an unprecedented 2D photospectral information for all resolved galaxies in the local Universe, as well as accurate photo-z estimates (at the δ z/(1 + z) ∼ 0.005-0.03 precision level) for moderately bright (up to r ∼ 20 mag) extragalactic sources. While some narrow-band filters are designed for the study of particular emission features ([O ii]/λ3727, Hα/λ6563) up to z < 0.017, they also provide well-defined windows for the analysis of other emission lines at higher redshifts. As a result, J-PLUS has the potential to contribute to a wide range of fields in Astrophysics, both in the nearby Universe (Milky Way structure, globular clusters, 2D IFU-like studies, stellar populations of nearby and moderate-redshift galaxies, clusters of galaxies) and at high redshifts (emission-line galaxies at z ≈ 0.77, 2.2, and 4.4, quasi-stellar objects, etc.). With this paper, we release the first ∼1000 deg 2 of J-PLUS data, containing about 4.3 million stars and 3.0 million galaxies at r < 21 mag. With a goal of 8500 deg 2 for the total J-PLUS footprint, these numbers are expected to rise to about 35 million stars and 24 million galaxies by the end of the survey.Article published by EDP Sciences A176, page 1 of 25

In the next 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 hrs m 2 , 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 S/N . 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 9 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, non-overlapping filters reaches only ∼ 0.1 mag shallower than 4 − 5 filter systems, but has a precision almost 3 times better, δz = 0.014(1 + z) vs δz = 0.042(1 + z). We briefly discuss a practical implementation of such a photometric system: the ALHAMBRA survey.

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We have used William Herschel Telescope/INGRID K s images on two high-latitude fields, the Coppi and Groth strips, to obtain galaxy number counts over $180 arcmin 2 , to a depth of K s $ 21:0. Detection efficiency corrections as a function of object size have been calculated on each pointing. We have used a signalto-noise threshold in two complementary half-exposure images to remove spurious detections. Our data cover the range from K s ¼ 14:5 to K s ¼ 21:0, so they are useful for investigating a previously reported change in the number count slope (d log N=dm) at K $ 17. We find a slope b ¼ 0:54 0:63 for K < 17:5 and a slope f ¼ 0:25 0:29 for K > 17:5. A total contribution from galaxies to the extragalactic background light (EBL) in the K band of I ¼ 10:5 nW m À2 sr À1 has been calculated. This K-band EBL coming from galaxies accounts for only $50% of the recent measurements of the diffuse EBL. Standard number count models fail to reproduce the observed slope change at K $ 17:5 unless elliptical and spiral formation is pushed to zd2.

The ALHAMBRA survey aims to cover 4 deg 2 using a system of 20 contiguous, equal width, medium-band filters spanning the range 3500 Å-9700 Å plus the standard JHKs filters. Here we analyze deep near-IR number counts of one of our fields (ALH08) for which we have a relatively large area (0.5 deg 2 ) and faint photometry (J = 22.4, H = 21.3, and K = 20.0 at the 50% of recovery efficiency for point-like sources). We find that the logarithmic gradient of the galaxy counts undergoes a distinct change to a flatter slope in each band: from 0. . These observations together with faint optical counts are used to constrain models that include density and luminosity evolution of the local type-dependent luminosity functions. Our models imply a decline in the space density of evolved early-type galaxies with increasing redshift, such that only 30%-50% of the bulk of the present day red ellipticals was already in place at z ∼ 1.