Possible detection of cold molecular gas in REBELS-25, 700 million years after the Big Bang

Astronomers have reported what they say is the most distant detection yet of low-J carbon monoxide, a key tracer of the cold molecular gas that fuels star formation, in a galaxy seen just 700 million years after the Big Bang. If the result holds up, it would offer rare direct evidence that the galaxy, known as REBELS-25, already contained an enormous reservoir of star-forming fuel at a redshift of 7.31.

The study, “Direct detection of cool molecular gas in a star-forming galaxy at z = 7.31,” was posted to arXiv on June 11 and has been accepted for publication in Monthly Notices of the Royal Astronomical Society. Led by Karin Cescon, the work comes with an important caveat: The two reported carbon monoxide line detections are modest, at about 3.5 sigma each, so the findings should be treated cautiously.

REBELS-25 is a massive star-forming galaxy in the REBELS survey. Earlier observations with the Atacama Large Millimeter/submillimeter Array, or ALMA, had already pinned down its distance using ionized carbon emission, measuring a redshift of 7.3065. In the new study, the team used the Karl G. Jansky Very Large Array’s Q-band and ALMA Band 3 to search for molecular gas more directly.

The authors report detecting two carbon monoxide lines, CO(3-2) and CO(7-6), and they also set an upper limit on a related atomic carbon line, C I. Of the two CO signals, the lower-energy CO(3-2) line is especially important because low-J CO transitions are widely used to trace the bulk reservoir of cold molecular gas in galaxies — the raw material from which new stars form.

Using the CO(3-2) measurement after correcting for the effects of the cosmic microwave background, the study estimates that REBELS-25 contains about 100 billion times the mass of the sun in molecular gas, with a substantial uncertainty. A separate model-based analysis puts the total closer to 180 billion solar masses. Either way, the paper says the galaxy appears overwhelmingly gas-rich, with a gas fraction of about 0.95, meaning most of its baryonic mass seems to be in gas rather than stars.

That matters because many observations of extremely distant galaxies rely on other tracers, such as high-J CO lines or ionized carbon, which can emphasize warmer, denser or otherwise different parts of a galaxy’s interstellar medium. Low-J CO is valued because it tracks the colder, more extended gas supply that can sustain future star formation.

Measuring that gas so early in cosmic history is especially difficult because the cosmic microwave background was much warmer then than it is today. At a redshift of 7.31, the background temperature was about 22 kelvins, high enough to reduce the contrast between cold gas emission and the sky behind it, making faint signals harder to detect.

If upheld, the result would push the frontier for low-J CO observations deeper into the early universe. The paper says the CO(3-2) signal from REBELS-25 is the highest-redshift detection of a low-J CO transition reported so far; prior published detections had reached only about redshift 6.9.

“Our results show galaxies just 700 million years after the Big Bang already contained large reservoirs of cold gas available for star formation,” Cescon said in a Hiroshima University news item adapted from National Radio Astronomy Observatory press material.