Lord Of The Heavy Metal Rings

A two month break from reading papers is getting excessive, so let's address this with a nice little letter about Leo.

The Leo Ring is a gigantic, 200 kpc ring-shaped gas cloud in the Leo group, about 11 Mpc away. With a mass of HI of over 2 billion solar masses, this is one of those weird features that won't go away because someone made a calibration error or something daft like that. The Ring is by and large optically dark, with no obvious bright galaxies that you could point to and say, "yep, the gas probably came out of that one there".

Now some rings are not that complicated to explain : head-on collisions between galaxies can do the job nicely. But ring galaxies are collisional features which tend to be smaller and with more vigorous star formation, and tidal encounters usually have a clearer connection between the stripped galaxy and its lost material. While detecting star formation in a feature this large could be difficult just because it's so spread out, it certainly isn't happening at the level seen in other such collisions objects. It's not that there isn't any at all - UV observations have found some occurring in a few places in the recent past - just that there's not much happening right now.

This letter presents new Hα observations showing that there is ongoing star formation happening in at least a few parts of the Ring (coinciding with the previous UV detections, though I think this is by design). By itself, this isn't terribly interesting. It's not unexpected that parts of this gigantic structure could be collapsing and forming new stars under gravity, thought it's nice to know. What's more surprising is that the metallicity measurements indicate the chemical composition is similar to that of a typical galactic environment, and can't be explained by the enrichment due to the observed star formation.

The strange thing about that is that previous observations (from absorption lines in background quasars) showed that metallicity was very much lower. That would point to a primordial origin of the Ring, with the material condensing out of low-density material in the general field. I'm always skeptical of such claims of accretion : my question is always, "why are we seeing this happening here and not everywhere else ?". So an origin by some stripping mechanism, though it would have other problems, would at least knock this one on the head.

How come the previous estimates were so much lower ? They say it's because it relies on estimating the density of the HI material in the Ring from low-resolution observations, which underestimates the true density. So higher resolution observations could help with this.

Disclaimer : Leo is of particular interest to me because we have AGES data there. Unfortunately it isn't of any higher resolution than the existing observations, just of higher sensitivity. It doesn't reveal any smoking gun, but it's still interesting. Watch this space.

Heavy elements unveil the non primordial origin of the giant HI ring in Leo

Taking advantage of MUSE (Multi Unit Spectroscopic Explorer) operating at the VLT, we performed optical integral field spectroscopy of 3 HI clumps in the Leo ring where ultraviolet continuum emission has been found. We detected, for the first time, ionized hydrogen in the ring and identify 4 nebular regions powered by massive stars.