Highly-ionized gas in lensed z = 6.027 Little Red Dot seen through [OIII] 88$μ$m with ALMA

Kirsten K. Knudsen, Johan Richard, Mathilde Jauzac, Tom J. L. C. Bakx, Thiago S. Goncalves, Eiichi Egami, Kiana Kade, Rahul Rana, Laura Sommovigo, Flora Stanley, Daniel P. Stark

First listed 2025-12-04 | Last updated 2025-12-04

Abstract

Determining the physical properties of galaxies during the first billion years after the big bang is key to understanding both early galaxy evolution and how galaxies contributed to the epoch of reionization. We present deep ALMA observations of the redshifted [OIII] 88um line for the gravitationally lensed ($μ= 11.4\pm1.9$) galaxy A383-5.1 (z=6.027) that has previously been detected in [CII] 158um. Recent James Webb Space Telescope (JWST) imaging identified this sub-L* galaxy as a ''Little Red Dot'' (LRD). With a line luminosity of $L_{\rm [OIII]} = (1.29\pm0.24)\times10^8$ L$_\odot$ (corrected for lensing magnification) A383-5.1 is one of the faintest galaxies with combined [CII] and [OIII] detections. The ALMA data reveal no dust continuum emission, consistent with previous observations. The high line luminosity ratio of [OIII]/[CII] $\sim 14\pm5$ is consistent with A383-5.1 being low-metallicity and dust-poor. The non-detection of dust continuum in bands 6 and 8 is consistent with the high [OIII]/[CII] ratio and suggests a presence of a strong ultraviolet radiation field, which would be less affect by dust attenuation, implying that galaxies of this type could contribute significantly to the ionization of the intergalactic medium. The presence of strong ionizing field could provide an important piece of information for understanding the nature of LRDs and their role in cosmic reionization.

Short digest

Deep ALMA Band-8 detects the [OIII] 88 μm line from the lensed (μ=11.4±1.9) Little Red Dot A383-5.1 at z=6.027, yielding a demagnified L_[OIII]=(1.29±0.24)×10^8 L⊙ and no dust continuum in Bands 6/8. Together with the prior [CII] 158 μm detection, the high [OIII]/[CII]≈14±5 indicates a low-metallicity, dust-poor ISM. The continuum non-detections support a strong, hard UV radiation field with little attenuation. This makes A383-5.1 one of the faintest systems with both [CII] and [OIII] detected and a useful benchmark for what powers LRDs and their potential contribution to reionization.

Key figures to inspect

• Figure 1: Check the [OIII] moment-0 map over −100 to +100 km/s against JWST F200W and [CII] contours to see if [OIII] is co-spatial with [CII] or offset, and inspect the single-Gaussian fit/residuals for kinematic simplicity.
• Figure 2: Use the Band-6 and Band-8 continuum upper limits (with and without magnification correction) to visualize how the far-IR SED constrains L_IR and reinforces the lack of dust emission.
• Figure 3: Read off the lensing-corrected dust-mass upper limits versus assumed dust temperature; note the >×8 swing across plausible T_d, and how normalizing to Band-6 vs Band-8 changes Mdust and allowable T_d.
• Figure 4: Locate A383-5.1 on L_[OIII]–SFR relative to local SF and low-Z dwarf relations; compare the two plotted SFR estimates (Richard+2011 vs Golubchik+2025 limit) to gauge how [OIII] brightness per SFR signals low metallicity/strong ionization.