Probing the faint end of the Galaxy luminosity function at z = 3w ith Ly emission ?

We present spectroscopic observations obtained with the ESO Very Large Telecope (VLT) of seven candidate Ly emitting galaxies in the eld of the radio quiet Q1205-30 at z =3 :04 previously detected with deep narrow band imaging. Based on equivalent widths and limits on line ratios we conrm that all seven objects are Ly emitting galaxies. Deep images also obtained with the VLT in the B and I bands show that ve of the seven galaxies have very faint continuum fluxes (I(AB) 26: 8a ndB(AB) 27:3). The star formation rates of these seven galaxies estimated from the rest-frame UV continuum around 2000 A, as probed by theI-band detections, as well as from the Ly luminosities, are 1{4 M yr 1 assuming a Hubble constant of 65 km s 1 Mpc 1 ,m =0 :3, and = 0:7. This is 1{3 orders of magnitude lower than for other known populations of star-forming galaxies at similar redshifts (the Lyman-Break galaxies and the sub-mm selected sources). The inferred density of the objects is high, 16 4 per arcmin 2 per unit redshift. This is consistent with the integrated luminosity function for Lyman-Break galaxies down to R = 27 if the fraction of Ly emitting galaxies is70% at the faint end of the luminosity function. However, if this fraction is 20% as reported for the bright end of the luminosity function then the space density in this eld is signicantly larger (by a factor of 3.5) than expected from the luminosity function for Lyman-Break galaxies in the HDF{North. This would be an indication that at least some radio quiet QSOs at high redshift reside in overdense environments or that the faint end slope of the high redshift luminosity function has been underestimated. We nd evidence that the faint Ly galaxies are essentially dust{free. These observations show that Ly emission is an ecient method by which to probe the faint end of the luminosity function at high redshifts.