Physical properties and H-ionizing-photon production rates of extreme nearby star-forming regions

Measurements of the galaxy UV luminosity function at z6 suggest that young hosted in low-mass star-forming galaxies produced the bulk of-ionizing photons necessary to reionize the intergalactic medium (IGM) redshift z~6. Whether star-forming galaxies dominated cosmic reionization, , also depends on their stellar populations and interstellar medium, which set, among other things, the production rate of H-ionizing, \_^\, and the fraction of these escaping into the. Given the difficulty of constraining with existing observatories the properties of z6 galaxies, in this work we focus on a sample of ten objects showing UV spectral features comparable to those observed at6. We use the new-generation Beagle tool to model the UV-to-optical and UV/optical emission lines of these Local 'analogues' of-redshift galaxies, finding that our relatively simple, yet fully-consistent, physical model can successfully reproduce the different considered. Our galaxies span a broad range of metallicities and characterised by high ionization parameters, low dust attenuation, and very stellar populations. Through our analysis, we derive a novel diagnostic the production rate of H-ionizing photons per unit UV luminosity, \_^\, based on the equivalent width of the bright \ 4959, 5007 doublet, which does not require measurements H-recombination lines. This new diagnostic can be used to estimate\_^\ from future direct measurements of the \ 4959, 5007 line using JWST/NIRSpec (out to z~9. 5), and by exploiting contamination by \ + \ 4959, 5007 of observations of distant galaxies, for instance from existing/IRAC data and from future ones with JWST/NIRCam.