A galaxy with almost no stars? New Cloud-9 observations strengthen the case

A galaxy with almost no stars was found by astronomers using deep imaging of Cloud-9, a candidate "dark galaxy." The study, published on the arXiv preprint server, observed the region using a high-speed camera called HiPERCAM on the Gran Telescopio Canarias, achieving a limiting surface brightness of 31.4 magnitudes per square arcsecond in the g-band. No starlight was detected across the object's central region, placing an upper limit of just 16,000 solar masses of stars within a 4,200 light-year region. This non-detection supports the idea that Cloud-9 is an excellent candidate for a starless galaxy.

The team used five-filter imaging to maximize sensitivity to faint stellar signals, shifting the camera pointing between exposures and masking the region before subtracting the background. This method ensured that any real stellar emission was not accidentally removed. The observations reached a limiting surface brightness of 31.4 magnitudes per square arcsecond in the g-band, deep enough to detect extremely faint stellar populations invisible to previous surveys.

The result sits between two prior estimates: an earlier survey-based limit that was "not designed to preserve the low surface brightness signal," and a deeper HST star-counting estimate that heavily relied on a comparison galaxy that may be too compact. The new observations independently strengthen earlier evidence that Cloud-9 could be a rare starless galaxy, a system dominated by gas and dark matter that has somehow failed to form stars.

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