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

Author:

M. Castellano, D. Belfiori, L. Pentericci, A. Calabrò, S. Mascia, L. Napolitano, F. Caro, S. Charlot, J. Chevallard, E. Curtis-Lake, M. Talia, A. Bongiorno, A. Fontana, J. P. U. Fynbo, B. Garilli, L. Guaita, R. J. McLure, E. Merlin, M. Mignoli, M. Moresco, E. Pompei, L. Pozzetti, A. Saldana Lopez, A. Saxena, P. Santini, D. Schaerer, C. Schreiber, A. E. Shapley, E. Vanzella, G. Zamorani

Keyword:

Astrophysics, Astrophysics of Galaxies, Astrophysics of Galaxies (astro-ph.GA)

journal:

date:

2023-05-21 16:00:00

Abstract

We investigate the production efficiency 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. We constrain $\xi_{ion}^*$ 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 $\xi_{ion}^*$ 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 $\xi_{ion}^*$ with increasing EW(Ly$\alpha$), with an average log($\xi_{ion}^*$/Hz erg$^{-1}$)$>$25 at EW$>$50\AA, and a higher ionizing efficiency for high-EW CIII]$\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 $\xi_{ion}^*$ and sSFR shows a monotonic increase from log($\xi_{ion}^*$/Hz erg$^{-1}$) $\sim$24.5 at log(sSFR)$\sim$-9.5$yr^{-1}$ to $\sim$25.5 at log(sSFR)$\sim$-7.5$yr^{-1}$, a low scatter and little dependence on mass. The objects above the main-sequence of star-formation consistently have higher-than-average $\xi_{ion}^*$. A clear increase of $\xi_{ion}^*$ with $\Sigma_{SFR}$ is also found, with log($\xi_{ion}^*$/Hz erg$^{-1}$)$>$25 for objects at $\Sigma_{SFR}>$10 M$_{\odot}/yr/kpc^2$.(Abridged)

PDF: The ionizing photon production efficiency of bright z$\sim$2-5 galaxies.pdf