Exploring extreme conditions for star formation: A deep search for molecular gas in the Leo ring

Abstract

Aims. We carried out sensitive searches for the 12CO J = 1–0 and J = 2–1 lines in the giant extragalactic HI ring in Leo to investigate the star formation process within environments where gas metallicities are close to solar, but physical conditions are different than those typical of bright galaxy disks. Our aim is to check the range of validity of known scaling relations. Methods. We used the IRAM-30 m telescope to observe 11 regions close to HI gas peaks or where sparse young massive stars have been found. For all pointed observations we reached spectral noise between 1 and 5 mK for at least one of the observed frequencies at 2 km s−1 spectral resolution. Results. We marginally detect two 12CO J = 1–0 lines in the star-forming region Clump 1 of the Leo ring, whose radial velocities are consistent with those of Hα lines, but whose line widths are much smaller than observed for virialized molecular clouds of similar mass in galaxies. The low signal-to-noise ratio, the small line widths, and the extremely low number densities inferred by virialized cloud models suggest that a more standard population of molecular clouds, still undetected, might be in place. Using upper limits to the CO lines, the most sensitive pointed observations show that the molecular gas mass surface density is lower than expected from the extrapolation of the molecular Kennicutt–Schmidt relation established in the disk of galaxies. The sparse stellar population in the ring, possibly forming ultra diffuse dwarf galaxies, might then be the result of a short molecular gas depletion time in this extreme environment.

BST1047+1156: A (Failing) Ultradiffuse Tidal Dwarf in the Leo I Group

“…The peak column density in BST1047 today is very low (∼1 M e pc −2 Mihos et al 2018a), and it is likely incapable of fueling any additional star formation. While the relatively large beam size of the H I data leaves open the possibility of pockets of high-density gas on small scales, recent CO observations of BST1047 have failed to detect molecular gas in the system (Corbelli et al 2023), although those studies did not survey BST1047ʼs full H I extent. Nonetheless, there is no evidence for any current, ongoing star formation in BST1047 today.…”