UV slope of z$\sim$3 bright ($L>L^{\ast }$) Lyman-break galaxies in the COSMOS field

Abstract

We analyse a unique sample of 517 bright ($L>L^{\ast }$) LBGs at redshift z$\sim$3 in order to characterise the distribution of their UV slopes ${\hat{I}}^2$ and infer their dust extinction under standard assumptions. We exploited multi-band observations over 750 arcmin${}^2$ of the COSMOS field that were acquired with three different ground-based facilities: the Large Binocular Camera (LBC) on the Large Binocular Telescope (LBT), the Suprime-Cam on the SUBARU telescope, and the VIRCAM on the VISTA telescope (ULTRAVISTA DR2). Our multi-band photometric catalogue is based on a new method that is designed to maximise the signal-to-noise ratio in the estimate of accurate galaxy colours from images with different point spread functions (PSF). We adopted an improved selection criterion based on deep Y-band data to isolate a sample of galaxies at $z\sim 3$ to minimise selection biases. We measured the UV slopes (${\hat{I}}^2$) of the objects in our sample and then recovered the intrinsic probability density function of ${\hat{I}}^2$ values (PDF(${\hat{I}}^2$)), taking into account the effect of observational uncertainties through detailed simulations. The galaxies in our sample are characterised by mildly red UV slopes with $<{\hat{I}}^2>\simeq -1.70$ throughout the enitre luminosity range that is probed by our data ($-24\lesssim M_{1600}\lesssim -21$). The resulting dust-corrected star formation rate density (SFRD) is $log(SFRD)\simeq -1.6M_{\odot }/yr/Mp{c}^3$, corresponding to a contribution of about 25% to the total SFRD at z$\sim$3 under standard assumptions. Ultra-bright LBGs at $z\sim 3$ match the known trends, with UV slopes being redder at decreasing redshifts, and brighter galaxies being more highly dust extinct and more frequently star-forming than fainter galaxies. [abridged]

Matched to journal version. 11 pages, 13 figures, Astronomy & Astrophysics in press