Evidence for strong evolution of the cosmic star formation density at high redshifts

¹ INAF - Istituto di Radioastronomia, Largo E. Fermi 5, 50125 Firenze, Italy e-mail: filippo@arcetri.astro.it
² INAF - Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
³ INAF - Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monte Porzio Catone, Italy
⁴ INAF - Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy

Received: 7 July 2006
Accepted: 4 September 2006

Abstract

Deep HST/ACS and VLT/ISAAC data of the GOODS-South field were used to look for high-redshift galaxies in the rest-frame UV wavelength range and to study the evolution of the cosmic star-formation density at 7. The GOODS-South area was surveyed down to a limiting magnitude of about  = 25.5, looking for drop-out objects in the  ACS filter. The large sampled area would allow for the detection of galaxies that are 20 times less numerous and 1–2 mag brighter than similar studies using HST/NICMOS near-IR data. Two objects had initially been selected as promising candidates for galaxies at 7, but were subsequently dismissed and identified as Galactic brown dwarfs through detailed analysis of their morphology and Spitzer colors, as well as through spectroscopic information. As a consequence, we conclude that there are no galaxies at 7 down to our limiting magnitude in the field we investigated. Our non detection of galaxies at 7 provides clear evidence for a strong evolution of the luminosity function between and , i.e. over a time interval of only ~170 Myr. Our constraints also provide evidence of a significant decline in the total star formation rate at , which must be less than 40% of that at and 40–80% of that at . We also derive an upper limit to the ionizing flux at , which is only marginally consistent with what is required to completely ionize the Universe.