Intense CIII] 1907,1909 emission from a strong Lyman continuum emitting galaxy

TL;DR

This study presents the first complete UV spectrum of a low-redshift Lyman continuum emitter, revealing strong emission lines and high ionizing efficiency, which enhances understanding of galaxy properties related to ionizing photon escape.

Contribution

It provides the first UV spectrum of a low-redshift LyC emitter and links UV emission line strength with ionizing photon production efficiency, supported by photoionization models.

Findings

Strong LyC emitters can have high equivalent width UV emission lines.

The galaxy J1154+2443 has a high ionizing photon production efficiency.

A trend of increasing ionizing efficiency with CIII] 1909 equivalent width was identified.

Abstract

We have obtained the first complete ultraviolet (UV) spectrum of a strong Lyman continuum(LyC) emitter at low redshift -- the compact, low-metallicity, star-forming galaxy J1154+2443 -- with a Lyman continuum escape fraction of 46% discovered recently. The Space Telescope Imaging Spectrograph spectrum shows strong Lya and CIII] 1909 emission, as well as OIII] 1666. Our observations show that strong LyC emitters can have UV emission lines with a high equivalent width (e.g. EW(CIII])$=11.7 \pm 2.9 \AA$ rest-frame), although their equivalent widths should be reduced due to the loss of ionizing photons. The intrinsic ionizing photon production efficiency of J1154+2443 is high, $\log(\xi_{\rm ion}^0)=25.56$ erg$^{-1}$ Hz, comparable to that of other recently discovered $z \sim 0.3-0.4$ LyC emitters. Combining our measurements and earlier determinations from the literature, we find a trend of increasing $\xi_{\rm ion}^0$ with increasing CIII] 1909 equivalent width, which can be understood by a combination of decreasing stellar population age and metallicity. Simple ionization and density-bounded photoionization models can explain the main observational features including the UV spectrum of J1154+2443.