Near-Infrared Spectroscopy of Galaxies During Reionization: Measuring CIII] in a Galaxy at $\mathbf{z}=7.5$

Abstract

We present Keck/MOSFIRE $H$-band spectroscopy targeting CIII] $\hat{I}»$1907,1909 in a $z=7.5056$ galaxy previously identified via Ly$\hat{I}\pm$ emission. We detect strong line emission at $1.621\pm 0.002\hat{I}¼$m with a line flux of ($2.63\pm 0.52$)$\times 10^{-18}$ erg s${}^{-1}$ cm${}^{-2}$. We tentatively identify this line as [CIII] $\hat{I}»$1907, but we are unable to detect CIII] $\hat{I}»$1909 owing to sky emission at the expected location. This gives a galaxy systemic redshift, $z_{sys}=7.5032\pm 0.0003$, with a velocity offset to Ly$\hat{I}\pm$ of $\hat{I}”$v${}_{Ly\hat{I}\pm }$ = $88\pm 27$ km s${}^{-1}$. The ratio of combined CIII]/Ly$\hat{I}\pm$ is 0.30-0.45, one of the highest values measured for any $z>2$ galaxy. We do not detect SiIII] $\hat{I}»\hat{I}»$1883, 1892, and place an upper limit on SiIII]/CIII] $<$ 0.35 ($2\ddot{I}ƒ$). Comparing our results to photoionization models, the CIII] equivalent width (W${}_{\text{CIII]}}=16.23\pm 2.32$à ), low SiIII]/CIII] ratio, and high implied [OIII] equivalent width (from the $Spitzer$/IRAC [3.6]$-$[4.5]$\simeq$0.8 mag color) require sub-Solar metallicities ($Z\simeq 0.1-0.2Z_{\odot }$) and a high ionization parameter, logU $\gtrsim -1.5$. These results favor models that produce higher ionization, such as the BPASS models for the photospheres of high-mass stars, and that include both binary stellar populations and/or an IMF that extends to 300 M${}_{\odot }$. The combined CIII] equivalent width and [3.6]$-$[4.5] color are more consistent with ionization from young stars than AGN, however we cannot rule out ionization from a combination of an AGN and young stars. We make predictions for $JamesWebbSpaceTelescope$ spectroscopy using these different models, which will ultimately test the nature of the ionizing radiation in this source.

19 pages, 12 figures, accepted for publication in ApJ