Today's paper is about a galaxy so faint that after reading the discovery paper back in 2018, I must have immediately forgotten all about it. Which was a mistake, because it's in Leo. Since I did a whole paper on optically dark gas clouds and stuff in Leo but didn't cite these authors, I now feel a little less annoyed that they don't cite me either.
Anyway, this little galaxy's claim to fame is being bloody dim. It is in fact, they say, the faintest object every detected by its optical emission. It's embedded right in the heart of the Leo I group, home to the extraordinary Leo Ring, which is just a little to the north of this object. In fact, the galaxy is in a bridge of HI connecting the Ring to the giant spiral galaxy M96, whose own HI emission is rather distorted.
How does such an incredibly faint, diffuse object form ? They suggest two scenarios. It could be a really extreme but basically "normal" galaxy, having managed to maintain an incredibly low star formation rate over the whole lifetime of the Universe. This would be really interesting because it would then be unclear how it ever formed any stars at all, with models not predicting stuff like this : it seems to be just too diffuse to do anything (this might be a fun connection to other Ultra Diffuse Galaxies found elsewhere). Or, perhaps less excitingly, it could be a tidal dwarf galaxy, having only just recently formed from the gas stripped away from M96.
If it was the first scenario, then it should be expected to have an old stellar population. So here they present Hubble observations indicating pretty unequivocally that it doesn't. They can detect the occasional older star but nothing above background contamination levels, with the stars in this object clearly dominated by younger ones. It all suggests a brief starburst took place about 300 Myr ago and then stopped, with pretty much nothing happening today at all based on the Halpha emission. All their measurements emphatically support this : the distribution of stars, their metallicity, modelling of these parameters by different techniques, everything. Circumstantially, the distortion of the gas in M96 also supports this interpretation (though we still have no idea how the Leo Ring itself, which is connected to M96, actually formed).
So this is pretty cut and dried. The tidal dwarf model means that unfortunately this doesn't really tell us much about galaxies more generally, since this would be enormously atypical of the galaxy population in general. And in fact they describe this galaxy as "failing", being unable to sustain further star formation and likely to be tidally disrupted. What seems to have happened was that the initial compression of the tidal field triggered a brief burst of star formation, but as the whole larger system expands, this has already stopped.
This all raises two questions for me. What does this imply for the optically dark clouds in the same region ? Remember one such "cloud" is actually optically bright, and not especially diffuse by any standards, either optically or in terms of gas. It's an open question whether that object is in any way related to the other, truly dark clouds or just a coincidence that it's so close to them and so similar in terms of its gas content. Could it be a similar case to this galaxy but far more extreme ? I dunno.
The second question is, why isn't there a third option ? Why can't it be a hybrid of the two ? Rather than a normal but extremely diffuse galaxy "recovering" from a tidal encounter, could it not instead be a once-dark galaxy that has only just lit up thanks to the tidal encounter ? I can't think of anything here that would rule this out, but nor can I think of a good way to test this. Still, maybe things aren't quite as clear-cut as they at first appear.
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