Updated measurements of [O III] 88 $\mu$m, [C II] 158 $\mu$m, and Dust Continuum Emission from a z=7.2 Galaxy

TL;DR

This study provides updated measurements of key far-infrared emission lines and dust from a high-redshift galaxy, revealing its clumpy structure, low dust content, and a significant [C II] deficit, with implications for galaxy detection methods at early cosmic times.

Contribution

It offers the first detailed analysis combining archival and new ALMA data to refine emission line measurements and dust constraints for a z=7.2 galaxy, highlighting the limitations of [C II] surveys.

Findings

Detected [O III] emission with high significance and clumpy structure.

Non-detection of dust continuum implies very low dust mass.

Identified a strong [C II] deficit relative to SFR at high redshift.

Abstract

We present updated measurements of the [O III] 88 $\mu$m, [C II] 158 $\mu$m, and dust continuum emission from a star-forming galaxy at $z=7.212$, SXDF-NB1006-2, by utilizing Atacama Large Millimeter/submillimeter Array (ALMA) archival data sets analysed in previous studies and data sets that have not been analysed before. The follow-up ALMA observations with higher angular resolution and sensitivity reveal a clumpy structure of the [O III] emission on a scale of $0.32-0.85\,\rm{kpc}$. We also combined all the ALMA [O III] ([C II]) data sets and updated the [O III] ([C II]) detection to $5.9\sigma$ ($3.6\sigma-4.5\sigma$). The non-detection of [C II] with data from the REBELS large program implies the incompleteness of spectral-scan surveys using [C II] to detect galaxies with high star formation rates (SFRs) but marginal [C II] emission at high-$z$. The dust continuum at 90 $\mu$m and 160 $\mu$m remains undetected, indicating little dust content of $<3.9\times10^{6}\,M_\odot\,(3\sigma)$, and we obtained a more stringent constraint on the total infrared luminosity. We updated the [O III]/[C II] luminosity ratios to $10.2\pm4.7~(6.1\pm3.5$) and $20\pm12~(9.6\pm6.1$) for $4.5\sigma$ and $3.6\sigma$ [C II] detections, respectively, where the ratios in the parentheses are corrected for the surface brightness dimming effect on the extended [C II] emission. We also found a strong [C II] deficit ($0.6-1.3$ dex) between SXDF-NB1006-2 and the mean $L_{\rm{[CII]}}-\rm{SFR}$ relation of galaxies at $0<z<9$.