What is Cloud-9, the mysterious hydrogen cloud near Messier 94, and how did ultra-deep HiPERCAM observations by Ignacio Trujillo’s team—reaching surface-brightness limits about ten times deeper than previous imaging—find no stellar emission across its roughly 4,200-light-year cen

Cloud-9 is a compact cloud of neutral atomic hydrogen (H I) near the spiral galaxy Messier 94 that may be a genuinely starless “dark galaxy”: gas retained inside a dark-matter halo, with little or no stellar component. It is the strongest current candidate for a reionization-limited H I cloud (RELHIC), although that classification remains an interpretation rather than a final proof. 23

• Trujillo and colleagues used HiPERCAM on the Gran Telescopio Canarias to reach surface-brightness limits of 31.4 mag/arcsec² in g and 31.0 in r—about ten times fainter than earlier deep images. They detected no diffuse stellar emission in the central 1′ × 1′, about 1.3 × 1.3 kpc (roughly 4,200 light-years) at Cloud-9’s distance. 1

• If any stars there are an old, metal-poor population, the non-detection limits their surface density to about 0.01 solar masses per square parsec and their total mass to less than 16,000 solar masses. This is an integrated-light constraint independent of—and consistent with—the prior Hubble resolved-star search. 12

• Radio observations show Cloud-9 is compact, dynamically cold, apparently non-rotating, and contains roughly 1 million solar masses of H I. 2 Models used to interpret it infer a halo mass of order 5 billion solar masses; compared specifically with its H I mass, that gives the commonly quoted ∼5,000:1 halo-to-neutral-hydrogen ratio. That is not the usual total dark-matter-to-all-baryons ratio, since helium and any ionized gas also count as ordinary matter.

• In ΛCDM simulations, reionization heats intergalactic gas and prevents it from settling into the smallest halos. Halos somewhat larger than Cloud-9’s proposed regime should cool gas efficiently enough to form stars; substantially smaller ones should lose or fail to retain their gas. Cloud-9 therefore sits near the predicted narrow “sweet spot” where a halo could retain H I but remain effectively starless. 23

• The result is important because it offers a potential observational test of ΛCDM’s predicted population of low-mass dark-matter halos. But “starless” means no stars detectable at exceptionally stringent present limits—not an absolute demonstration that zero stars exist—and the halo mass is model-dependent. 12

• DESI’s Imaging Legacy Survey was already important to the story: the initial work found no optical counterpart down to about 29.15 mag/arcsec². Future wide-area H I and deep optical surveys can seek more compact, isolated, extremely metal-poor or starless candidates to determine whether Cloud-9 is a rare survivor or the first member of a real population. 3