The Efficiency of Stellar Reionization: Effects of Rotation, Metallicity, and Initial Mass Function

TL;DR

This paper calculates the photon production rates in stellar populations considering rotation, metallicity, and initial mass function, providing insights into their role in cosmic reionization at high redshift.

Contribution

It offers a revised calibration of ionizing photon production rates from stellar models, incorporating recent stellar evolutionary tracks and atmosphere models, crucial for understanding reionization.

Findings

Median LyC photon production is (6±2)×10^60 per Msun of star formation.

Efficiencies in helium continua are approximately 10^60 and 10^56 photons per Msun at solar metallicity.

The critical star formation rate at z=7 is estimated as 0.012 Msun/yr/Mpc^3, depending on IGM clumping and escape fraction.

Abstract

We compute the production rate of photons in the ionizing Lyman continua (LyC) of H I (lambda < 912 A), He I (lambda < 504 A), and He II (lambda < 228 A) using recent stellar evolutionary tracks coupled to a grid of non-LTE, line-blanketed (WM-basic) model atmospheres. The median LyC production efficiency is Q_LyC = (6+/-2)x10^60 LyC photons per Msun of star formation (range [3.1-9.4]x10^60) corresponding to a revised calibration of 10^{53.3+/-0.2} photons/s per Msun/yr. Efficiencies in the helium continua are Q_HeI ~ 10^60 photons/Msun and Q_HeII ~ 10^56 photons/Msun at solar metallicity and larger at low metallicity. The critical star formation rate needed to maintain reionization against recombinations at z = 7 is rho_SFR = (0.012 Msun/yr/Mpc^3) [(1+z)/8]^3 [(C_H /3) (0.2/ f_esc) for fiducial values of IGM clumping factor C_H = 3 and LyC escape fraction f_esc = 0.2. The boost in LyC production efficiency is an important ingredient, together with metallicity, C_H, and f_esc, in assessing whether IGM reionization was complete by z ~ 7. Monte-Carlo sampled spectra of coeval starbursts during the first 5 Myr have intrinsic flux ratios of F(1500)/F(900) = 0.4-0.5 and F(912^-)/F(912^+) = 0.4-0.7 in the far-UV (1500A), the LyC (900A), and at the Lyman edge (912A). These ratios can be used to calibrate the LyC escape fractions in starbursts.