Deep ALMA redshift search of a z~12 GLASS-JWST galaxy candidate

Abstract: The James Webb Space Telescope (JWST) has discovered a surprising abundance of bright galaxy candidates in the very early Universe ($< 500$ Myrs after the Big Bang), calling into question current galaxy formation models. Spectroscopy is needed to confirm the primeval nature of these candidates, as well as to understand how the first galaxies form stars and grow. Here we present deep spectroscopic and continuum ALMA observations towards GHZ2/GLASS-z12, one of the brightest and most robust candidates at $z > 10$ identified in the GLASS-JWST Early Release Science Program. We detect a $5.8 \sigma$ line, offset 0.5" from the JWST position of GHZ2/GLASS-z12 that, associating it with the [OIII] 88 micron transition, implies a spectroscopic redshift of $z = 12.117 \pm 0.001$. We verify the detection using extensive statistical tests. The oxygen line luminosity places GHZ2/GLASS-z12 above the [OIII]-SFR relation for metal-poor galaxies, implying an enhancement of [OIII] emission in this system while the JWST-observed emission is likely a lower-metallicity region. The lack of dust emission seen by these observations is consistent with the blue UV slope observed by JWST, which suggest little dust attenuation in galaxies at this early epoch. Further observations will unambiguously confirm the redshift and shed light on the origins of the wide and offset line and physical properties of this early galaxy. This work illustrates the synergy between JWST and ALMA and paves the way for future spectroscopic surveys of $z > 10$ galaxy candidates.

• The paper demonstrates a significant 5.8σ [OIII] detection that confirms a spectroscopic redshift of z=12.117 ± 0.001 for the galaxy candidate.
• It employs deep ALMA spectroscopic observations over four tunings to validate the JWST photometric redshift estimate with high precision.
• The study identifies a 0.5 arcsecond offset between ALMA and JWST positions, suggesting complex dynamical processes and early galactic evolution.

Overview of Deep ALMA Redshift Search of a $z\sim12$ GLASS-JWST Galaxy Candidate

The recent exploration of the early Universe has been significantly enhanced by both the James Webb Space Telescope (JWST) and the Atacama Large Millimeter/submillimeter Array (ALMA). A highly pertinent study in this domain is the examination of the high-redshift galaxy candidate GHZ2/GLASS-z12, which has been identified as one of the most promising galaxies at redshift $z>10$. This paper outlines the efforts to spectroscopically validate this candidate using deep ALMA observations, particularly targeting the [OIII] 88 µm emission.

Methodology and Observations

The study leverages the advanced capabilities of ALMA to conduct deep spectroscopic and continuum observations. By focusing on a frequency range that would capture the redshifted [OIII] emission for $z \approx 12$, the observations span four tunings covering from 233.42 GHz to 263.04 GHz. The observations aim to confirm the photometric redshift estimates derived from JWST data, which suggest GHZ2/GLASS-z12 could be at a redshift as high as 12.5.

Results

A significant detection, quantified at a $5.8 \sigma$ level, was achieved with this observational strategy. The line is distinctly identified at 258.7 GHz, indicating a spectroscopic redshift of $z=12.117 \pm 0.001$, which aligns closely with the projected JWST photometric estimate. Interestingly, the emission line appears spatially offset by approximately 0.5 arcseconds from the JWST position, corresponding to a physical distance of about 1.5 kpc, suggesting a possible outflow scenario or other dynamical phenomena in this early galaxy.

Discussion

This detection provides a rare insight into the conditions of the interstellar medium (ISM) in galaxies at such an early epoch. The tentative identification of the line with [OIII] emission implies the presence of a chemically evolved region, potentially indicative of significant star formation and metal enrichment processes having already occurred just 400 Myrs after the Big Bang. This poses challenges to current galaxy formation models which need to account for such a rapid pace of chemical enrichment and galaxy assembly.

The study also reports a lack of significant dust emission, down to an upper limit at 13.8 µJy, which suggests that despite potential metal enrichment, dust formation might be less efficient in these early galaxies or could be subject to sublimation by intense radiation fields or feedback processes.

Implications for Future Research

By confirming the redshift through ALMA, this study demonstrates a potent synergy between JWST and ALMA, enabling enhanced understanding of the early Universe. Future observations, potentially leveraging forthcoming upgrades in ALMA's bandwidth, could further clarify the characterization of early galaxies. JWST's NIRSpec instrument could be instrumental for diagnosing other emission lines and confirming additional high-redshift candidates. As a roadmap for future studies, these findings stress the importance of comprehensive spectroscopic campaigns in elucidating the nature and evolution of the earliest cosmic structures.

The spectroscopic confirmation of such high-redshift candidates also serves as a critical testbed for refining cosmological models, as it may inform or challenge the existing hierarchical framework of galaxy formation. Continued detailed studies at these epochs are vital for piecing together the timeline and processes of cosmic reionization and the subsequent evolution of the Universe.