Mass-to-light Ratio of Ly-alpha Emitters: Implications of Ly-alpha Surveys at Redshifts z=5.7, 6.5, 7, and 8.8

TL;DR

This study interprets the luminosity functions of Ly-alpha emitters from redshifts 5.7 to 8.8 to constrain their mass-to-light ratios and assess implications for reionization and galaxy evolution.

Contribution

It introduces a simple method to estimate mass-to-light ratios of Ly-alpha emitters across high redshifts considering metallicity and stellar spectra variations.

Findings

Ly-alpha emitters are consistent with starburst or normal galaxies depending on photon survival fraction.

No evidence found for the end of reionization up to z=7 from luminosity functions.

Non-detection at z=8.8 does not significantly constrain high-redshift galaxy properties.

Abstract

Using a simple method to interpret the luminosity function of Ly-alpha emitters, we explore properties of Ly-alpha emitters from 5.7 < z < 8.8 with various assumptions about metallicity and stellar mass spectra. We constrain a mass-to-'observed' light ratio, M_h/L_band. For narrow-band surveys, L_band is simply related to the intrinsic Ly-alpha luminosity with a survival fraction of Ly-alpha photons, alpha_esc. The mass-to-'bolometric light', M_h/L_bol, can also be deduced, once the metallicity and stellar mass spectrum are given. The inferred M_h/L_bol is more sensitive to metallicity than to the mass spectrum. We find the following constraints on a mass-to-light ratio of Ly-alpha emitters from 5.7 < z < 7: (M_h/L_bol)(alpha_{esc}epsilon^{1/gamma})^{-1}=21-38, 14-26, and 9-17 for Z=0, 1/50, and 1 Z_sun, respectively, where epsilon is the 'duty cycle' of Ly-alpha emitters, and gamma ~ 2 is a local slope of the cumulative luminosity function. Only weak lower limits are obtained for z=8.8. Therefore, Ly-alpha emitters are consistent with either starburst galaxies M_h/L_bol ~ 0.1-1 with a smaller Ly-alpha survival fraction, alpha_{esc}epsilon^{1/gamma} ~0.01-0.05, or normal populations (M_h/L_bol ~ 10) if a good fraction of Ly-alpha photons survived, alpha_{esc}epsilon^{1/gamma} ~ 0.5-1. We find no evidence for the end of reionization in the luminosity functions of Ly-alpha emitters discovered in the current Ly-alpha surveys, including recent discovery of one Ly-alpha emitter at z=7. The data are consistent with no evolution of intrinsic properties of Ly-alpha emitters or neutral fraction in the intergalactic medium up to z=7. No detection of sources at z=8.8 does not yield a significant constraint yet. We also show that the lack of detection at z=8.8 does not rule out the high-z galaxies being the origin of the excess NIRB.