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

TL;DR

The study uses deep ALMA observations of the [OIII] 88 μm line to characterize the ISM conditions and ionizing radiation in a highly magnified, sub-L$^$ galaxy at $z=6.027$, A383-5.1, which JWST identifies as a Little Red Dot. The authors detect [OIII] with $L_{ m [OIII]}=(1.29\pm0.24)\times10^{8}\,L__ $ and find a high [OIII]/[CII] ratio of ~14, while the non-detection of dust continuum implies a dust mass $M_d<0.8\times10^{6}\,M_$ and a dust-poor ISM. The emission is resolved over ~0.3 kpc$^2$ and likely arises in a two-component system, raising questions about the powering source (star formation vs AGN/BH*) and indicating potentially strong LyC leakage. Together with JWST imaging, these results suggest that sub-L$^$ galaxies with extreme ionization may significantly contribute to reionization, highlighting the importance of high-frequency ALMA observations for faint EoR populations.

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.

Figures (4)

• Figure 1: The [O iii] 88 $\mu$m detection of A383-5.1. Top: Moment-0 map obtained through collapsing the data cube over the velocity range -100 to 100 km/s. The contours show $3, 5, 7, 9\sigma$, and dashed show $-3\sigma$ overlaid on the JWST NIRCam/F200W image. Additionally, the magenta contours show the moment-0 of [C ii] using the combined data from knudsen16 and additional ALMA archive data; [C ii] contours show $3, 4, 5\sigma$. Bottom: ALMA spectra extracted at the position of A383-5.1 and centered at the frequency of the redshifted [O iii] line. The red dashed line shows the best-fit Gaussian. The vertical lines indicate the velocity range over which the moment-0 map has been extracted. The lower panel shows the residuals after subtracting the best-fit Gaussian profile from the spectrum.
• Figure 2: The far-IR SED of A383-5.1 with the two continuum upper limits from bands 6 and 8. The grey squares show the $3\sigma$ observed limits, while the black squares show the same corrected for gravitational lensing magnification. The modified blackbody functions are normalised to the band-6 data point.
• Figure 4: [O iii] line luminosity, $L_{\rm [OIII]}$, vs. star formation rate. The filled black square shows the detection in A383-5.1 using the SFR of richard11, and the open black square shows the same but with the SFR limit of golubchik2025, while the coloured symbols show recent results inoue16carniani17laporte17marrone18hashimoto18tamura19hashimoto19harikane20binggeli20akins22algera23tadaki22witstok22bakx2024zavala24schouws25carniani25knudsen2025. The $L_{\rm [OIII]}$ - SFR relation, where The green region shows the $L_{\rm [OIII]}$ - SFR relation for local star-forming galaxies, and the light-blue one shows that for low-metallicity dwarf galaxies delooze14.
• Figure 5: The [O iii] / [C ii] line luminosity ratio vs. the Ly$\alpha$ equivalent width. Black square shows the detection in A383-5.1, and coloured symbols show results for other $z>6$ galaxies vanzella11shibuya12furusawa16inoue16carniani17laporte17laporte19hashimoto18tamura19hashimoto19harikane20.