The H-alpha luminosity function at redshift 2.2 A new determination using VLT/HAWK-I

We aim to place new, strengthened constraints on the luminosity function (LF) of H-alpha (H alpha) emitting galaxies at redshift z approximate to 2.2, and to further constrain the instantaneous star-formation rate density of the universe ((rho) over dot(star)). We have used the new HAWK-I instrument at ESO-VLT to obtain extremely deep narrow-band (line; NB2090) and broad-band (continuum; K-s) 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 (alpha) of LF(H alpha) with unprecedented precision. The data are well fit by a Schechter function and also a single power-law, yielding alpha = (-1.72 +/- 0.20) and (-1.77 +/- 0.21), respectively. Thus we are able to confirm the steepening of a 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 similar to 2.2 (Geach et al. 2008), constructing a LF spanning a factor of 50 in luminosity. Re-fitting the Schechter parameters, we obtain log L-star = (43.07 +/- 0.22) erg s(-1); log phi(star) = (-3.45 +/- 0.52) Mpc(-3); alpha = (-1.60 +/- 0.15). We integrate over LF(H alpha) and apply a correction for dust attenuation to determine the instantaneous cosmic star-formation rate density at z similar to 2 without assuming alpha or extrapolating it from lower-z. Our measurement of (rho) over dot(star) is (0.215 +/- 0.090) M-circle dot yr(-1) Mpc(-3), integrated over a range of 37 <= log(L-H alpha/erg s(-1)) <= 47.