First Spectroscopic Evidence for High Ionization State and Low Oxygen Abundance in Lya Emitters

TL;DR

This study provides the first spectroscopic evidence that high-redshift Lya emitters have high ionization states and low oxygen abundances, indicating they are in an early galaxy formation stage with potential implications for cosmic reionization.

Contribution

It presents the first measurements of ionization parameters and oxygen abundances in high-z LAEs, revealing their extreme properties compared to other galaxy populations.

Findings

LAEs have high ionization parameters (~10^8 cm/s)

LAEs exhibit low oxygen abundances (12+log(O/H) < 8.4)

LAEs are likely early-stage galaxies contributing to reionization

Abstract

We present results from Keck/NIRSPEC and Magellan/MMIRS follow-up spectroscopy of Lya emitters (LAEs) at z=2.2 identified in our Subaru narrowband survey. We successfully detect Ha emission from seven LAEs, and perform a detailed analysis of six LAEs free from AGN activity, two out of which, CDFS-3865 and COSMOS-30679, have [OII] and [OIII] line detections. They are the first [OII]-detected LAEs at high-z, and their [OIII]/[OII] ratios and R23-indices provide the first simultaneous determinations of ionization parameter and oxygen abundance for LAEs. CDFS-3865 has a very high ionization parameter (q_{ion}=2.5^{+1.7}_{-0.8}x10^8 cm s^{-1}) and a low oxygen abundance (12+log(O/H)=7.84^{+0.24}_{-0.25}) in contrast with moderate values of other high-z galaxies such as Lyman-break galaxies (LBGs). COSMOS-30679 also possesses a relatively high ionization parameter (q_{ion}=8^{+10}_{-4}x10^7 cm s^{-1}) and a low oxygen abundance (12+log(O/H)=8.18^{+0.28}_{-0.28}). Both LAEs appear to fall below the mass-metallicity relation of z~2 LBGs. Similarly, a low metallicity of 12+log(O/H)<8.4 is independently indicated for typical LAEs from a composite spectrum and the [NII]/Ha index. Such high ionization parameters and low oxygen abundances can be found in local star-forming galaxies, but this extreme local population occupies only ~0.06% of the SDSS spectroscopic galaxy sample with a number density ~100 times smaller than that of LAEs. With their high ionization parameters and low oxygen abundances, LAEs would represent an early stage of galaxy formation dominated by massive stars in compact star-forming regions. High-q_{ion} galaxies like LAEs would produce ionizing photons efficiently with a high escape fraction achieved by density-bounded HII regions, which would significantly contribute to cosmic reionization at z>6.