The optical identification of a primeval galaxy atz 4.4

Abstract

We have obtained with the SUSI CCD camera on the ESO 3.5m NTT deep images in the BVRI bands of the field centered on the QSO BRI 1202-0725 ($z_{\it em}=4.694$). In the final combined frames the stellar images have FWHM of 1,1,0.6 and 0.65 arcsec respectively. The R and I images show clearly a galaxy $2.2''$ from the QSO, corresponding to $13h^{-1}_{50}$ kpc at $z\sim 4.5$. Possible identification with the metal absorption systems seen in the line of sight to the QSO, including the highest redshift damped system known to date at $z=4.383$, are discussed. We conclude that its colours can be reconciled only with the spectrum of a primeval galaxy at z >~ 4.4, making it the most distant galaxy detected so far. From its magnitudes and models of young galaxy evolution we deduce that it is forming stars at a rate $\sim 30 M_{\odot}$ yr$^{-1}$ and has an estimated age of the order of $10^8$ yr or less, implying that the bulk of the stellar population formed at $z < 6$.