Wide and deep near-UV (360 nm) galaxy counts and the extragalactic background light with the Large Binocular Camera

¹ INAF – Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio, Italy e-mail: grazian@oa-roma.inaf.it
² INAF – Osservatorio Astronomico di Trieste, via G. B. Tiepolo 11, 34131 Trieste, Italy
³ MPIA Max-Planck-Institute für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁴ INAF – Osservatorio Astronomico di Padova, vicolo dell'Osservatorio 5, 35122 Padova, Italy
⁵ ESA, Space Science Department, 2200 AG Noordwijk, The Netherlands
⁶ INAF – Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁷ Large Binocular Telescope Observatory, University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721-0065, USA
⁸ INAF – Osservatorio Astronomico di Capodimonte, via Moiariello 16, 80131 Napoli, Italy

Received: 19 March 2009
Accepted: 15 June 2009

Abstract

Context. Deep multicolour surveys are the main tool for exploring the formation and evolution of the very faint galaxies that are beyond the spectroscopic limit of present technology. The photometric properties of these faint galaxies are usually compared with current renditions of semianalytical models to provide constraints on the detailed treatment of the fundamental physical processes involved in galaxy formation and evolution, namely the mass assembly and the star formation.

Aims. Galaxy counts over large sky areas in the 360 nm near-UV band are particularly difficult to obtain given the low efficiency of near-UV instrumentation, even at 8 m class telescopes. Observing in the near-UV bands can provide a first assessment of the distribution of star formation activity in distant (up to z ~ 3) galaxies. A relatively large instrumental field of view helps to minimize the biases caused by cosmic variance.

Methods. We obtained deep images in the 360 nm U band provided by the blue channel of the Large Binocular Camera at the prime focus of the Large Binocular Telescope. Over an area of 0.4 sq. deg., we derived the galaxy number counts down to in the Vega system (corresponding to in the AB system) at a completeness level of 30% reaching the faintest current limit for this wavelength and sky area.

Results. The shape of the galaxy number counts in the U band can be described by a double power-law, the bright side being consistent with the shape of shallower surveys of comparable or greater areas. The slope bends over significantly at ensuring the convergence of the contribution by star-forming galaxies to the extragalactic background light in the near-UV band to a value that is more than 70% of the most recent upper limits derived for this band. We jointly compared our near-UV and K band counts collected from the literature with a few selected hierarchical CDM models, concentrating on specific critical issues in the physical description of the galaxy formation and evolution.