Star formation at z=4.7 in the environment of the quasar BR 1202-07

We present the spectrum of the galaxy companion to the z = 4.7 quasar BR 1202-0725 in the optical range 6000-9000 Angstrom, corresponding to 1050-1580 Angstrom in the rest frame. We detect a strong Ly alpha emission line at z = 4.702, with an integrated flux of 2 x 10(-16) ergs cm(-2) s(-1), and a UV continuum longward of the Ly alpha emission at a flux level of similar or equal to 3 x 10(-19) ergs cm(-2) s(-1) Angstrom(-1). We fail to detect any C IV lambda 1550 emission with a 3 sigma upper limit of 3 x 10(-17) ergs cm(-2) s(-1). We show that the ratio between Ly alpha and continuum intensity and the absence of strong C IV emission imply that the UV continuum radiation is the result of intense star formation activity rather than of reprocessing of the QSO flux. The total estimated star formation rate of this z = 4.7 star-forming region is similar to 15-54 M. yr(-1), depending on the initial mass function and the metallicity. The present data suggest that the Ly alpha emission has a velocity and spatial structure, with possible velocity differences of similar or equal to 500 km s(-1) on scales of a few kilo-parsecs. These velocity patterns may be a signature of collapsing or merging phenomena in the QSO and its environment, as expected from current models of galaxy formation at high z.