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

Aims. We investigate the production efficiency of ionizing photons ( ξ 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 ξ 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] λ 1909, HeII λ 1640, and OIII] λ 1666) from deep VIMOS spectra. Results. We find no clear evolution of ξ ion ∗ with redshift within the probed range. The ionizing efficiency slightly increases at fainter M UV and bluer UV slopes, but these trends are less evident when the analysis is restricted to a complete subsample at log( M star / M ⊙ ) > 9.5. We find a significant trend of increasing ξ ion ∗ with increasing EW(Ly α ), with an average log( ξ ion ∗ /Hz erg −1 ) > 25 at EW > 50 Å and a higher ionizing efficiency for high-EW CIII] λ 1909 and OIII] λ 1666 emitters. The most significant correlations are found with respect to stellar mass, specific star formation rate (sSFR), and SFR surface density (Σ SFR ). The relation between ξ ion ∗ and sSFR increases monotonically from log( ξ ion ∗ /Hz erg −1 )∼24.5 at log(sSFR) ∼ −9.5 yr −1 to ∼25.5 at log(sSFR) ∼ −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 ξ ion ∗ . A clear increase in ξ ion ∗ with Σ SFR is also found, with log( ξ ion ∗ /Hz erg −1 ) > 25 for objects at Σ SFR > 10 M ⊙ yr −1 kpc −2 . Conclusions. Bright ( M UV ≲ 20) and massive (log( M star / M ⊙ )≳9.5) galaxies at z = 2 − 5 have a moderate ionizing efficiency. However, the correlation between ξ 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 efficient sources of ionizing photons. The availability of sSFR and Σ SFR as proxies for ξ 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.