2008MNRAS.386..608L

Motivated by the recent observational and theoretical evidence that long gamma-ray bursts (GRBs) are likely associated with low metallicity, rapidly rotating massive stars, we examine the cosmological star formation rate (SFR) below a critical metallicity Z_crit∼ 1/10-1/5Z_solar, to estimate the event rate of high redshift long GRB progenitors. To this purpose, we exploit a galaxy formation scenario already successfully tested on a wealth of observational data on (proto)spheroids, Lyman break galaxies, Lyman α emitters, submm galaxies, quasars and local early-type galaxies. We find that the predicted rate of long GRBs amounts to about 300 events/yr/sr, of which about 30 per cent occur at z ≳ 6. Correspondingly, the GRB number counts well agree with the bright SWIFT data, without the need for an intrinsic luminosity evolution. Moreover, the above framework enables us to predict the properties of the GRB host galaxies. Most GRBs are associated with low-mass galaxy haloes M_H≲ 10¹¹M_solar, and effectively trace the formation of small galaxies in such haloes. The hosts are young, with age smaller than 5x10⁷ yr, gas rich, but poorly extincted (A_V≲ 0.1) because of their chemical immaturity; this also implies high specific SFR and quite extreme α-enhancement. Only the minority of hosts residing in large haloes with M_H≳ 10¹²M_solarhas larger extinction (A_V∼ 0.7 - 1), SFRs exceeding 100M_solar/yr and can be detected at submm wavelengths. Most of the hosts have ultraviolet magnitudes in the range -20 ≲ M₁₃₅₀≲ -16, and Lyman α luminosity in the range 2x10⁴⁰ ≲ L_Lymanα≲ 2x10⁴² erg/s. GRB hosts are thus tracing the faint end of the luminosity function of Lyman break galaxies and Lyman α emitters. Finally, our results imply that the population of `dark' GRBs occur mostly in faint hosts at high redshift, rather than in dusty hosts at low redshift.