Gaseous nebulae and massive stars in the giant H I ring in Leo

Abstract

Context. Chemical abundances in the Leo ring, the largest H I 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 H I 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 H II regions. The average star-formation rate density in the ring is on order of 10−5 M⊙ yr−1 kpc−2 and proceeds with local bursts a few hundred parsecs in size, where loose stellar associations of 500−1000 M⊙ occasionally host massive outliers. The far ultraviolet-to-Hα emission ratio in nebular regions implies recent stellar bursts, from 2 to 7 Myr ago. The relation between the local H I 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 1 kpc partial ring that is emitting weak Hα lines around the brightest and youngest H II region suggests that local shocks might triggers future star-forming sites.