Enriched haloes at redshift z=2 with no star formation: implications for accretion and wind scenarios

In order to understand which process (e.g. galactic winds, cold accretion) is responsible for the cool (T similar to 10(4) K) halo gas around galaxies, we embarked on a programme to study the star formation properties of galaxies selected by their Mg II absorption signature in quasar spectra. Specifically, we searched for the H alpha line emission from galaxies near very strong z similar or equal to 2 Mg II absorbers (with rest-frame equivalent width W-r(lambda 2796) greater than or similar to 2 angstrom) because these could be the signposts of outflows or inflows. Surprisingly, we detect H alpha from only four hosts out of 20 sightlines (and two out of the 19 H I-selected sightlines), despite reaching a star formation rate (SFR) sensitivity limit of 2.9 M-circle dot yr(-1) (5 sigma) for a Chabrier initial mass function. This low success rate (4/20) is in contrast with our z similar or equal to 1 survey where we detected 66 per cent (14/21) of the Mg II hosts (down to 0.6 M-circle dot yr(-1); 5 sigma. Taking into account the difference in sensitivity between the two surveys, we should have been able to detect >= 11.4 (>= 7.6) of the 20 z similar or equal to 2 hosts - assuming that SFR evolves as proportional to(1 + z)(gamma) with gamma = 2.5 (or gamma = 0) respectively - whereas we found only four galaxies. Interestingly, all the z = 2 detected hosts have observed SFRs greater than or similar to 9 M-circle dot yr(-1), well above our sensitivity limit, while at z = 1 they all have SFR greater than or similar to 9 M-circle dot yr(-1), an evolution that is in good agreement with the evolution of the SFR main sequence, i.e. with gamma = 2.5. Moreover, we show that the z = 2 undetected hosts are not hidden under the quasar continuum after stacking our data. They also cannot be outside our surveyed area as this latter option runs against our sample selection criteria (W-r(lambda 2796) > angstrom) and the known W-r(lambda 2796)-impact parameter relation for low-ionization ions. Hence, strong Mg II absorbers could trace star-formation-driven winds in low-mass haloes (M-h <= 10(10.6) M-circle dot), provided that the winds do not extend beyond 20 kpc in order not to violate the evolution of the absorber number density dN/dz (Mg II). Alternatively, our results imply that z = 2 galaxies traced by strong Mg II absorbers do not form stars at a rate expected (3-10 M-circle dot yr(-1)) for their (halo or stellar) masses, supporting the existence of a transition in accretion efficiency at M-h similar or equal to 10(11) M-circle dot. This scenario can explain both the detections and the non-detections.