Breathless BEARS: [O$_{\rm \,III}$] 88$μ$m Emission of Dusty Star-Forming Galaxies at $z = 3-4$

TL;DR

This study targets the ionized ISM in four dusty star-forming galaxies at $z = 3-4$ by observing [O \textsc{iii}] 88 μm with the ACA, complemented by ancillary [C II] data for two sources. All targets show >3σ detections, revealing $L_{\mathrm{[OIII]}}/L_{\mathrm{IR}}$ in the range $10^{-4.2}$ to $10^{-3}$, akin to local spiral galaxies and an order of magnitude below local dwarfs and high-redshift LBGs. The measured $L_{\mathrm{[OIII]}}/L_{\mathrm{CII]}}$ ratios of $0.3$–$0.9$ imply that the ionized gas fraction is smaller in these DSFGs, consistent with metal-rich, dust-dominated ISM and substantial neutral gas reservoirs. Collectively, the results suggest that DSFGs at cosmic noon are evolved, metal-rich systems with large dust masses and longer depletion times, occupying a transitional phase between gas-rich disks and more evolved, gas-poor galaxies; expanding samples with multi-line diagnostics will be crucial to situate this phase within galaxy evolution.

Abstract

We present [O$_{\rm \,III}$] 88$μ$m observations towards four ${\it Herschel}$-selected dusty star-forming galaxies (DSFGs; log$_{10}$ $μ$L$_{\rm IR}$/L$_{\odot}$ = 13.5 - 14 at $z = 2.9 - 4$) using the Atacama Compact Array (ACA) in Bands 9 and 10. We detect [O$_{\rm \,III}$] emission in all four targets at >3$σ$, finding line luminosity ratios ($L_{\rm [O_{\rm \,III}]}$ / L$_{\rm IR}$ = 10$^{-4.2}$ to 10$^{-3}$) similar to local spiral galaxies, and an order of magnitude lower when compared with local dwarf galaxies as well as high-redshift Lyman-break galaxies. Using the short-wavelength capabilities of the ACA, these observations bridge the populations of galaxies with [O$_{\rm \,III}$] emission at low redshift from space missions and at high redshift from ground-based studies. The difference in [O$_{\rm \,III}$] emission between these DSFGs and other high-redshift galaxies reflects their more evolved stellar populations (> 10 Myr), larger dust reservoirs (M$_{\rm dust}$ $\sim$ 10$^{9 - 11}$ M$_{\odot}$), metal-rich interstellar medium ($Z \sim 0.5 - 2$ Z$_{\odot}$), and likely weaker ionization radiation fields. Ancillary [C$_{\rm \,II}$] emission on two targets provide $L_{[{\rm O}_{\rm \,III}]} / L_{[{\rm C}_{\rm \,II}]}$ ratios at 0.3 - 0.9, suggesting that ionized gas represents a smaller fraction of the total gas reservoir in DSFGs, consistent with theoretical models of DSFGs as transitional systems between gas-rich, turbulent disks and more evolved, gas-poor galaxies. Expanding samples of DSFGs with [O$_{\rm \,III}$] emission will be key to place this heterogeneous, poorly-understood galactic phase in its astrophysical context.

Figures (5)

• Figure 1: The spectra of the [O iii] emission line of HerBS-28, HerBS-90, HerBS-103 and HerBS-160. The poststamps shows the 12" by 12" moment-0 map of the [O iii] 88 $\mu{\rm m}$ emission line as the background and black solid (positive) and dashed (negative) contours at $2,3,5 \sigma$ starting at $\pm 2$, centered on the positions listed in Table \ref{['tab:sourcesAndLines']}. The white contour indicates the high-resolution Band 7 continuum data Bakx2024ANGELS, which is used as the basis for the aperture as shown in yellow contours. This aperture uses the $5 \sigma$ continuum flux contours, and is subsequently extended by a number of beams (N$_{\rm beam}$) to capture most of the line flux without diluting the signal with excess noise. The spectra show the emission line as a function of velocity (bottom $x$-axis) and frequency (top $x$-axis), where the filled yellow region indicates the range over which the moment-0 map is generated, and the red solid line indicates a single-Gaussian fit to the line profile.
• Figure 2: The [O iii]-to-IR luminosity ratio as a function of IR luminosity. The open circles (dwarf galaxies; Madden2013Cormier2015Cigan2016Ura2023) and squares (local spiral galaxies; Ferkinhoff2010HerreraCamus2018) represent low-redshift galaxies. High-redshift galaxies are shown in open Bakx2024GoldenRatio and filled pentagons for Lyman-Break Galaxies Inoue2016carniani:2017oiiiLaporte2017Hashimoto2018Hashimoto2019Zavala2024Bakx2023glassSchouws2024Carniani2025Algera2024Ren2023Akins2022Schouws2021Fujimoto2024, with AGN shown in filled circles Hashimoto2019QSODecarli2023 and distant DSFGs in open Zhang2018 and filled triangles Marrone2018Zavala2018DSFGTadaki2022.
• Figure 3: Oxygen-to-carbon line ratio ($L_\mathrm{[O\,\textsc{iii}]}/L_\mathrm{[C\,\textsc{ii}]}$) as a function of redshift from $z = 0$ to 15, showing the observational accessibility of these key diagnostic lines with ALMA. The upper panel displays the $L_\mathrm{[O\,\textsc{iii}]}/L_\mathrm{[C\,\textsc{ii}]}$ ratios measured in various galaxy populations across cosmic time, with our two DSFG sources with both [O iii] and [C ii] data (orange stars) showing low values compared to local dwarf galaxies Madden2013Cormier2015Cigan2016Ura2023, and in line with local spiral galaxies DiazSantos2017 and Herschel/FTS-observed galaxies Zhang2018. The $L_\mathrm{[O\,\textsc{iii}]}/L_\mathrm{[C\,\textsc{ii}]}$ ratios of high-redshift LBGs Carniani2025Schouws2024Schouws2025Zavala2024Zavala2025HalphaHashimoto2018Laporte2019Fujimoto2024Tamura2019Bakx2020Fudamoto2021NaturAlgera2024Algera2025OIIIratioRowland2024Inoue2016Ren2023Hashimoto2019Sugahara2022Akins2022Wong2022Carniani2017Witstok2022Harikane2019, companion galaxies Venemans2020Bakx2024GoldenRatioAlgera2025OIIIratio, high-$z$ DSFGs Marrone2018Zavala2018DSFGTadaki2022 and quasar host galaxies Hashimoto2019QSOWalter2018. The lower panel shows the atmospheric transmission for [O iii] 88 $\mu{\rm m}$ and [C ii] across ALMA’s Bands, with the combined transmission indicating optimal redshift windows for simultaneous observations. The systematic [O iii] deficit in DSFGs is observable across the full redshift range where both lines can be detected, suggesting this is a fundamental property of dusty star-forming environments rather than an observational bias.
• Figure 4: [C ii] spectra of HerBS-28 and HerBS-103, similar to Figure \ref{['fig:lineprofiles']}. The poststamps shows the 12" by 12" moment-0 map of the [C ii] 158 $\mu{\rm m}$ emission line as the background and black contours at $2,3,5 \sigma$ starting at $\pm 2$, centered on the positions listed in Table \ref{['tab:sourcesAndLines']}. The white contour indicates the high-resolution Band 7 continuum data Bakx2024ANGELS, which is used as the basis for the aperture as shown in yellow contours for the spectrum shown on the right-hand side.
• Figure 5: The dust attenuation affects line ratios differently for a varying transitional absorption wavelength, $\lambda_{\rm thick}$. The ratio varies between 1 (no effect of dust attenuation) to $(\lambda_1 / \lambda_2)^\beta$ in the optically-thick case. Assuming Milky Way dust properties, a spherical dust distribution and $\beta_{\rm dust} = 2$, the required dust mass to achieve a transition wavelength is shown in the top x-axis.