Spectroscopy of CASSOWARY gravitationally-lensed galaxies in SDSS: characterisation of an extremely bright reionization-era analog at $z=1.42$

TL;DR

This study investigates the properties of gravitationally lensed galaxies at redshifts 1-3, focusing on a bright reionization-era analog at z=1.42, revealing insights into UV emission lines, stellar populations, and ionizing radiation escape.

Contribution

It provides detailed spectroscopic analysis of a bright high-redshift galaxy, CSWA-141, and compares UV line emission properties to understand factors influencing strong CIII] emission.

Findings

CSWA-141 exhibits strong UV emission lines similar to z>6 galaxies.

Galaxies with strong UV lines tend to have young stars and low-metallicity, highly ionized gas.

Gas traced by CIII] likely has higher densities than that traced by [OII] and [SII].

Abstract

We present new observations of sixteen bright ($r=19-21$) gravitationally lensed galaxies at $z\simeq 1-3$ selected from the CASSOWARY survey. Included in our sample is the $z=1.42$ galaxy CSWA-141, one of the brightest known reionization-era analogs at high redshift (g=20.5), with a large sSFR (31.2 Gyr$^{-1}$) and an [OIII]+H$\beta$ equivalent width (EW$_{\rm{[OIII]+H\beta}}$=730~\r{A}) that is nearly identical to the average value expected at $z\simeq 7-8$. In this paper, we investigate the rest-frame UV nebular line emission in our sample with the goal of understanding the factors that regulate strong CIII] emission. Whereas most of the sources in our sample show weak UV line emission, we find elevated CIII] in the spectrum of CSWA-141 (EW$_{\rm{CIII]}}$=4.6$\pm1.9$~\r{A}) together with detections of other prominent emission lines (OIII], Si III], Fe II$^\star$, Mg II). We compare the rest-optical line properties of high redshift galaxies with strong and weak CIII] emission, and find that systems with the strongest UV line emission tend to have young stellar populations and nebular gas that is moderately metal-poor and highly ionized, consistent with trends seen at low and high redshift. The brightness of CSWA-141 enables detailed investigation of the extreme emission line galaxies which become common at $z>6$. We find that gas traced by the CIII] doublet likely probes higher densities than that traced by [OII] and [SII]. Characterisation of the spectrally resolved Mg II emission line and several low ionization absorption lines suggests neutral gas around the young stars is likely optically thin, potentially facilitating the escape of ionizing radiation.