2010A&A...509L...5H

We aim to place new, strengthened constraints on the luminosity function (LF) of H-alpha (Hα) emitting galaxies at redshift z≃2.2, and to further constrain the instantaneous star-formation rate density of the universe ({dot}(ρ)_*). We have used the new HAWK-I instrument at ESO-VLT to obtain extremely deep narrow-band (line; NB2090) and broad-band (continuum; Ks) imaging observations. The target field is in the GOODS-South, providing us with a rich multi-wavelength auxiliary data set, which we utilise for redshift confirmation and to estimate dust content. We use this new data to measure the faint-end slope (α) of LF(Hα) with unprecedented precision. The data are well fit by a Schechter function and also a single power-law, yielding α=(-1.72±0.20) and (-1.77±0.21), respectively. Thus we are able to confirm the steepening of α from low- to high-z predicted by a number of authors and observed at other wavelengths. We combine our LF data-points with those from a much shallower but wider survey at z∼2.2 (Geach et al. 2008), constructing a LF spanning a factor of 50 in luminosity. Re-fitting the Schechter parameters, we obtain logL_*=(43.07±0.22)erg/s; logφ_*=(-3.45±0.52)Mpc^–3; α=(-1.60±0.15). We integrate over LF(Hα) and apply a correction for dust attenuation to determine the instantaneous cosmic star-formation rate density at z∼2 without assuming α or extrapolating it from lower-z. Our measurement of {dot}(ρ)_* is (0.215±0.090)M_☉/yr/Mpc³, integrated over a range of 37≤log(L_Hα/erg/s)≤47.