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

TL;DR

This study investigates molecular gas presence in the Leo ring's extreme environment, revealing low molecular gas surface density and suggesting rapid gas depletion, challenging existing star formation scaling relations.

Contribution

It provides the first sensitive CO line search in the Leo ring, showing molecular gas is scarce and may be depleted quickly, thus testing the limits of star formation models.

Findings

Detected marginal CO emission in one region, consistent with star formation activity.

Molecular gas surface density is lower than predicted by galaxy disk relations.

Indicates rapid molecular gas depletion in the Leo ring environment.

Abstract

We carry out sensitive searches for the CO 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. We use the IRAM-30m telescope to observe eleven regions close to HI gas peaks or where sparse young massive stars have been found. For all pointed observations we reached a spectral noise between 1 and 5~mK for at least one observed frequencies at 2~km/s spectral resolution. We marginally detect two CO J=1-0 lines in the star forming region Clump~1 of the Leo ring, whose radial velocities are consistent with those of Halpha lines but 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 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.}