Robert, you old fart, where the hell have you been hiding? I've been hoping to see your handle on this forum for years, wondering all this time what the dickens happened to you. And now that you DO turn up, what do I find? You've forsaken the glories of galaxies on high for ... looking down at your toenail fungus? Or whatever.
Could you do us all a great boon and look back up for awhile? I haven't had a halfway decent astronomical colloquy since you vanished into your Jurassic Park or whatever you call it. For heaven's sakes, my friend, lift thy eyes back to the skies. You'll hear bells pealing all over Australia, or at least the bells near Coonbarbaran or Melbourne. As I recall, you are from Melbourne, right? Well, that's at least one good thing I can say about the place.
Anyway, you got the picture. It's about time you showed up, and if it takes cosmic cinnamon buns to break you out of the woodwork, let's nibble on UGC 12588 and count its raisins.
Go to the HST website and download the 3.6 MB version of UGC 12588. Invert it to a inverted image of UGC 12588 extracted from the HST website 3.6 MB JPG. Inverting the image brings out structural features obscured by crowding in the HST 3.6 original. (This is why so many early astronomers made their notes directly on the glass plates of yore.) In the case of UGC 12588 one such feature is an apparent bar-like overdensity close to the 0°–180° axis. For another, there is no sign of a bulge or core/cusp central concentration. I tested both by fiddling with the image contrast settings and suspected what appears to be a coreless bar. So the next step up the learning ladder is a multiband
ugriz and micron-to-microwave sweep. As Robert has shown us, optically, the
PanSTARRS image set griz + y gives us a handful of mysteries to sort through. First, there no trace of a core or cusp-like centre—and also no bar. The pseudo-bar in the 606 nm HST image is possibly a processing artifact when the HST team reduced it from the HST 24.5 MB TIFF original. That leaves us with a diffuse cuspless core light profile. These are common in dwarf spheroidals, which is what that soft core does look like visually. A cross-check of the
Aladin Lite image set basically confirms the PanSTARRS images, with the additional fillip that the DSS2/red image reveals a red, rather bright diffuse central luminosity with no traces of H-alpha activity. That too would be the signature of a dSph that hasn't formed new stars for multi-billions of years. Such a galaxy should also exhibit a large population of >2.2 solar mass red giants. If we invert the
HST 24.5 MB TIFF using the same method, we find a central region peppered with a myriad of tiny low-luminosity dots—the low-mass red giant population we are looking for. Ergo, no doubt about it: this thing is dwarf spheroidal. So why the dickens do we have that expansive and very young starburst outside the corotation radius suddenly lighting up a huge reservoir of gas that has apparently snoozed for billions of years in an atomic hydrogen halo surrounding that dwarf, until now?
I have make confession here: My first post misidentified the diffuse boundary between the red starless centre and the active 6-arm spiral as 'outer Lindblad resonance' when I meant 'co-rotation circle'. My bad. Another wad of paper makes it to the wastebasket.
The term ‘co-rotation radius’ has real meaning here because in spiral galaxies it marks the point where the rotating mass of gas and stars on galaxy-wide scales moves as a unit body with respect to the forward velocity of a spiral arm density wave. Inside the corotation radius the stars move faster than the wave, which is why in most spirals the OB associations are on the inside curl of the spiral arm and reach full maturation by the time they cross the middle of the arm and go off as supernovae. That is why we find HII regions predominantly in the middle of spiral arms and not the outer edges. Inside a corotation circle it takes roughly 10 to 25 million years for a star-forming cloud complex to cross a spiral arm. At the corotation radius itself the wave and gas/stars move in step, so these regions are relatively quiescent. Lucky for us, because we live near the middle of the Milky Way’s co-rotation circle. Move us into the Scutum Cloud and we probably wouldn’t be here given the fulminous amount of shock turbulence, door slamming, yelling, boozing, and general mayhem that goes on in that cloud-dark of a household
UGC 12588’s starbursts are occurring outside the corotation radius, where the spiral waves move faster than the gas-star mass. We see evidence for this in UGC 12588’s spiral arms, where most of the bright O-B systems are near the front edge to middle of the arms—albeit with a couple of exceptions in the arm at 90° (west). For now we can overlook that anomaly because what we are really after is why so many stars have burst into life in such a rather brief recent era.
Something banged. What?
It would be awfully helpful at this point to have UV and X-ray bands which demonstrate copious low-mass protostar formation, and dust-revealing 3 to 6 micron bands which would reveal a long history of stellar outgassing. A lot of dust suggests multiple star-forming episodes; a dearth of it suggests we are looking a youngish initial outburst. That is pretty much what we are suspecting at this point—UGC 12588 is an old, lonesome dwarf spheroidal with a quiescent massive halo has suddenly come to life and we are keen to learn what happened. Alas for us, this galaxy has not been studied at high rez in the higher UV and X-ray bands (the Spitzer website draws a blank with this ID in the search box), nor in the dust-revealing micron bands that mm-to-micron systems like ALMA were built for.
That leaves us with the rest of the galaxy to explain and few tools to do it with. How do we reconcile a galaxy with a dSph light profile with a very recent >50 million year O-B star-forming burst in an unusual and hard to explain 6-arm spiral configuration if all we have is a simple visual-band image set?
To start with, large spiral arm systems usually presume a large core mass to anchor a density wave structure, plus a large gas mass to populate an entire galaxy arm structure in the sub-50 million year age bin of a large O-B starforming region. For the central mass, our earlier cross-check of the Aladin Lite image set basically confirmed the PanSTARRS results, with the additional fillip that the DSS2/red image revealed a red central luminosity with no traces of H-alpha activity (a sure sign of a dSph morphology). We already knew this, so no news there. Resorting to the higher authority of the
HST 24.5 MB hi-rez TIFF, we see that OB formation does extend into the central region, albeit at less furious a rate. We also see the central region peppered with a multitude of low-mass red giants, as one would expect in a middle-aged dSph. Even at this high a resolution the featureless matte of a dSph is unmistakable—but so is the very young blaze of rapid-onset star formation across the entire galaxy.
Back to Square One, then: how does a multi-billion year old quiescent galaxy hold onto enough gas reserve to suddenly burst forth into a youngish spiral with 3 times the radius and perhaps half the mass of the old galaxy? Moreover, what happened to spontaneously initiate this level of star formation? UGC 12588 exists in near isolation in the outskirts of the Local Group. The WikiSky field of this galaxy shows just how lonely it is out there. There just ain't nothin' to hit out on those realms of nowhere in particular. Can’t go Boom without a fuse.
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