The ionizing photon production efficiency of bright z ∼ 2 − 5 galaxies

Aims. We investigate the production e fficiency of ionizing photons (xi(*)(ion)) of 1174 galaxies with secure redshift at z = 2-5 from the VANDELS survey to determine the relation between ionizing emission and physical properties of bright and massive sources. Methods. We constrained xi(*)(ion) ion and galaxy physical parameters by means of spectrophotometric fits performed with the BEAGLE code. The analysis exploits the multi-band photometry in the VANDELS fields and the measurement of UV rest-frame emission lines (CIII] lambda 1909, HeII lambda 1640, and OIII] lambda 1666) from deep VIMOS spectra.Results. We find no clear evolution of xi*(ion) with redshift within the probed range. The ionizing e fficiency slightly increases at fainter MUV and bluer UV slopes, but these trends are less evident when the analysis is restricted to a complete subsample at log(M-star /M-circle dot) > 9 :5. We find a significant trend of increasing xi*(ion) with increasing EW(Ly(alpha)), with an average log(xi*(ion) /Hz erg(-1)) > 25 at EW > 50 angstrom and a higher ionizing e fficiency for high-EW CIII] lambda 1909 and OIII] lambda 1666 emitters. The most significant correlations are found with respect to stellar mass, specific star formation rate (sSFR), and SFR surface density ( Sigma(SFR)). The relation between xi*(ion) and sSFR increases monotonically from log(xi*(ion) /Hz erg(-1)) similar to 24 :5 at log(sSFR) similar to -9 .5 yr(-1) to similar to 25.5 at log(sSFR) similar to -7.5 yr(-1). This relation has a low scatter and only a weak dependence on mass. The objects above the main sequence of star formation consistently have higher than average xi*(ion). A clear increase in xi*(ion) with Sigma(SFR) is also found, with log(xi*(ion)/Hz erg(-1)) > 25 for objects at Sigma(SFR) > 10 M-circle dot yr(-1) kpc(-2).Conclusions. Bright (M-UV <= 20) and massive (log( M-star/M-circle dot) >= 9.5) galaxies at z = 2-5 have a moderate ionizing e fficiency. However, the correlation between xi*(ion) and sSFR, together with the known increase in the average sSFR with redshift at fixed stellar mass, suggests that similar galaxies in the epoch of reionization can be e fficient sources of ionizing photons. The availability of sSFR and Sigma(SFR) as proxies for xi*(ion) can be fundamentally important in determining the role of galaxy populations at z >= 10 that were recently discovered by the James Webb Space Telescope in the onset of reionization.