2021A&A...651A..77C

Context. Chemical abundances in the Leo ring, the largest HI cloud in the local Universe, have recently been determined to be close to or above Solar values. This is a finding that is incompatible with prior claims that the ring was primordial in origin. The gas, pre-enriched in a galactic disk and tidally stripped, did not manage to efficiently form stars in the intergalactic space.
Aims. Using Hα emission and a multi-wavelengths analysis of its extremely faint optical counterpart, we investigate the process of star formation and the slow build-up of a stellar population that could form diffuse and metal-rich dwarf galaxies in the Leo ring.
Methods. We mapped nebular lines in three dense HI clumps and complemented the data with archival stellar continuum observations from the Galaxy Evolution Explorer and the Hubble Space Telescope, as well as with population synthesis models.
Results. We detect a sparse population of stars in the main body of the ring, with individual young stars as massive as O7-types powering some HII regions. The average star-formation rate density in the ring is on order of 10^–5M_☉/yr/kpc² and proceeds with local bursts a few hundred parsecs in size, where loose stellar associations of 500-1000M_☉ occasionally host massive outliers. The far ultraviolet-to-Hα emission ratio in nebular regions implies recent stellar bursts, from 2 to 7Myr ago. The relation between the local HI gas density and the star-formation rate in the ring is similar to what is found in dwarfs and outer disks with gas depletion times as long as 100 Gyr. We find a candidate planetary nebula in a compact and faint Hα region with [OIII]/Hα line enhancement, which is consistent with the estimated mean stellar surface brightness of the ring. The presence of a 1kpc partial ring that is emitting weak Hα lines around the brightest and youngest HII region suggests that local shocks might triggers future star-forming sites.