The ionizing photon production efficiency of bright z$\sim$2-5 galaxies

Abstract

We investigate the production efficiency of ionizing photons (${\xi }_{ion}^{\ast }$) 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. We constrain ${\xi }_{ion}^{\ast }$ and galaxy physical parameters by means of spectro-photometric 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$\lambda 1640$, OIII]$\lambda 1666$) from deep VIMOS spectra. We find no clear evolution of ${\xi }_{ion}^{\ast }$ 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 restricting the analysis to a complete subsample at log(M${}_{star}$/M${}_{\odot }$)$>$9.5. We find a significant trend of increasing ${\xi }_{ion}^{\ast }$ with increasing EW(Ly$\alpha$), with an average log(${\xi }_{ion}^{\ast }$/Hz erg${}^{-1}$)$>$25 at EW$>$50Å, and a higher ionizing efficiency for high-EW CIII]$\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 ${\xi }_{ion}^{\ast }$ and sSFR shows a monotonic increase from log(${\xi }_{ion}^{\ast }$/Hz erg${}^{-1}$) $\sim$24.5 at log(sSFR)$\sim$-9.5$y{r}^{-1}$ to $\sim$25.5 at log(sSFR)$\sim$-7.5$y{r}^{-1}$, a low scatter and little dependence on mass. The objects above the main-sequence of star-formation consistently have higher-than-average ${\xi }_{ion}^{\ast }$. A clear increase of ${\xi }_{ion}^{\ast }$ with ${\Sigma }_{SFR}$ is also found, with log(${\xi }_{ion}^{\ast }$/Hz erg${}^{-1}$)$>$25 for objects at ${\Sigma }_{SFR}>$10 M${}_{\odot }/yr/kp{c}^2$.(Abridged)

11 pages, 9 figures, accepted for publication in A&A

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