ALMA Observations of [OI]145um and [NII]205um Emission lines from Star-Forming Galaxies at $z\sim 7$

Abstract

We present results of new observations of [OI]145um and [NII]205um emission lines from four star-forming galaxies at redshifts between $z=6.58$ and $7.68$ that have previous detections of \Ciium\ and dust continua. Using ALMA, we successfully detect [OI]145um emission from all targets at $>4\ddot{I}ƒ$ significance. However, [NII]205um emission is undetected in all galaxies (SNR $<3.5\ddot{I}ƒ$) except for a tentative detection from A1689-zD1. From the observed high [CII]/[NII] emission line ratios ($\gtrsim 20-80$), we find that most of the [CII]158um emission arise from neutral gas regions ($3\ddot{I}ƒ$ lower limits of $\gtrsim 74-96\%$). From [OI]145um, [CII]158um lines, and infrared luminosities, we estimate the neutral gas densities of $n_{\mathrm{H}}=10^{3.5}$ - $10^6\mathrm{c}{\mathrm{m}}^{-3}$ and the far-ultraviolet (FUV) radiation strengths of $G_0\sim 10^{2.5}$-$10^3$. While the neutral gas densities are similar to those of high-redshift starburst galaxies, the FUV strengths are lower compared to both local and high-redshift starbursts. Finally, we estimate atomic hydrogen masses using [OI]145um emission lines and the oxygen abundances measured from recent JWST observations. We find gas mass ratios of $f_{\mathrm{gas}}\sim 0.3$ - $0.8$, which are similar to earlier studies using [CII]158um. Starting from this pilot observation, future large [OI]145um emission line surveys will provide us with currently little-known neutral gas properties of star-forming galaxies in the early Universe.

18 pages, 6 figures, 3 tables, submitted to ApJ