... Can’t go Boom without a fuse ...
If UGC's enigmatic mixed-morphology structure wasn't pain enough in the neck as it is, its UGC classification rubs salt into it. There’s no PGC cross-classification, and most PhD theses use PGC classifications because of their positional accuracy. All the 24 Simbad citations referencing this galaxy are membership identity papers, not a galactic assembly studie. They don't address the question of how dwarf galaxies can hold on to huge gas reserves in their halos until some impetus like a swerve-by shock or acoustic wave from a void-to-filament turbulent infall triggers the kind of rapid-onset formation we seem to see in UGC 12588.
My best guess at this point is that UGC 12588, traveling at about 400 km/sec as part of a local density field, encountered one or more of the multitude of
high-mass atomic hydrogen clouds known to inhabit the Local Group and Andromeda M31 dark matter haloes. Such clouds are not rare, but receive scant notice because of their extreme low luminosity in the 21 cm band and absence of any other markers such as dust density or CO emission. They lack oomph in our world where oomph is usually what gets PhD students’ attention via the beaming approval of their advisors
One analogous situation does come to mind: the dSph IC 1613, which coasted along placidly in the LG outskirts for 10 billion years until about 150 million years ago it encountered a succession of 2 and possibly 3 massive atomic hydrogen clouds that may have been primordial overdensities that were never shocked into free fall to become star-forming molecular clouds. IC 1613’s outer H1 halo
sideswiped the first of these between 100 and 150 million years ago, then more abruptly into the second and possibly a third. That is why we see IC 1613’s distantly offset ferocious HII collapse fronts some 6 kpc from the old dSph core. Those collisional shocks in turn triggered secondary collect-and-collapse shocks closer to the IC 1613 core, which is why the galaxy today has its messy dual-morphology appearance.
If you want to know how common or not-common these oddball dwarf spheroidal morphologies (and therefore histories) can be, check
Dan Weisz's 2014 paper, Fig. 2.
My current sheet of paper awaiting the wastebasket says that the UGC 12588 chance interaction with a primeval remnant cloud occurred as a pass-through direct- or near-direct collision. That would also explain the somewhat lopsided density differences between the 2nd and 4th quadrants as we look at it. For an example of head-on cloud-galaxy collisions,
see this simulation from the CLUES Project.
Nice theory. I wonder if it's true.
Over to you, Mr. Lang ...
And hey, stick around, OK?
=Dana in S Africa