The "cinnamon bun" weirdness doesn't end with the moniker

[Nightfall]

NGC 12588 was a cosmic nobody till its image started showing up in SciTech Daily & all The Regular Suspects earlier today. Have any of you logged it, and what did you see?

So now that it's a somebody, what is it? It appears to have an unusual hexagonal morphology of 6 vestigial arms visible that emerge from a featureless surface only near the outer Lindblad resonance, where most gassy galaxies consist mainly of atomic hydrogen and shouldn't have young molecular HII clouds making scads of blue stars. It also appears to have a higher surface density in the 3:00 o'clock to 8 o'clock sector, possibly a remnant of ancient interaction with a galaxy not readily discernible here. This is a hard critter to track down—none of the papers in the 24 cited in Simbad provide any more than positional, proper/peculiar motion, and association data. The only published motion data is from 1988. No spectra available. The galaxy appears to have low H-alpha emission density and a marked absence of dust features. It could be an SO in mid-evolution between dusty spiral and featureless lenticular. If X-ray images were available we might see the telltale blue trail of ram pressure stripping. If it's an outlier member of the Perseus SSC (as its sky position suggests) it may be in rapid free-fall toward NGC 1275. However, the required velocity would have to have been imparted during a multi-body interaction in which 12588 was ejected at speed as the lowest-mass member. Its 2D sky position isn't its 3D position and there's no constraining distance data available. It might be one of the triple systems cited in KARACHENTSEVA V.E.1988, but that paper apparently hasn't been translated & available on ADS.

Any clues from your sightings and research?

[Ciel Extreme]

Both “NGC 1258” and “NGC 1288” are southern galaxies, and there is no “NGC 12588” as the NGC ends at 7840. Could you clarify which galaxy you are discussing?

[Paul Alsing]

I think you actually mean UGC 12588...

https://asterisk.apod.com/viewtopic....41156&start=25

[Steve Gottlieb]

I couldn't track down any connection between the HST image and recent papers by Brent Tully or Gagandeep Anand (a graduate student working with Tully). For what it's worth, I made an observation 7 years back of UGC 12588, but didn't see much in terms of detail.

24" (10/4/13): fairly faint, slightly elongated ~SW-NE, 45"x35". No distinct core but contains a small brighter nucleus. A mag 15.2 star is near the NE end and a mag 14 star is off the SW side, 1' from center.

Looking now at the HST image, I have feeling the "nucleus" I mentioned may be a superimposed star.

[Jimi Lowrey]

Hubble Image inverted. Looks like lots of star forming knots. Must be gas rich?

36314715-786A-4853-A2B4-4FC515FCA8D8.jpeg

[Nightfall]

Thank you for the inverted version of UGC 12588, Jimi. Was it a direct inversion from the HST 3.6 MB jpg used in SciTech Daily? Your rendering brings out structural features obscured by crowding in the HST 3.6 original. 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. 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. OK, another theory bites the dust. 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 Jimi uses for the 3.6 MB version, 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 post a 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.

Now back to business. The term ‘co-rotation radius’ has 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 that goes on in that 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. WikiSky 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.

Steve G mentioned the 2017 Kourkchi & Tully paper, which catalogs galaxies out to a recessional velocity of –3500 km/sec. Simbad catalogs UGC 12588 at a piffling –419 km/sec (z=0.0014), which positions it as about 32 million light years out and definitely not a Perseus-Pisces SSC object, since those inhabit the sniffy realms of –4000 to –5000 km/sec. Thanks for the head’s-up, Steve, and too bad for my earlier view that it was. Most visual astronomers are unaware how fast we imposter astrophysicists’ wastebaskets get full of wadded up paper balls. (DIY amateur astrophysics buffs are derided as ‘imposter astrophysicists’ in snooty, snobby MA-student circles. Those noses in the air have no idea how full of visual aroma the night skies are compared with all those exposed underarms that populate the classrooms.)

If UGC's enigmatic mixed-morphology structure wasn't pain enough in the neck, its UGC classification doesn't help. There’s no PGC cross-classification, and most PhD theses use PGC classifications because of their positional accuracy. (The way this paper is written suggests it is Kourkchi's PhD thesis and Tully was the advisor.) The Kourkchi paper is a membership identity paper, not a galactic assembly study, so it doesn'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.

Hence 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 (see Kourkchi & Tully above).

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 the messy dual-morphology appearance that you lucky large-glass lads can see in your 30-inch and larger scopes. Next time you take another peek at IC 1613, you are looking at what UGC 12588 might have looked about 100 million years ago. 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. Jimi, do you feel like donating some observatory time so we all find out? A 48-inch would probably do.

=Dana in S Africa

[Jimi Lowrey]

Dana the image I posted is from the Hubble Legacy Archive.

This is a link to the server. https://hla.stsci.edu/hlaview.html

[Nightfall]

Hi all, Jimi's HST Legacy image is in the F606W band, which corresponds to near the g in ugriz. The HST was programmed to acquire survey images in F606W and also F814W. The latter serves us better when trying to track down the stellar populations in the UGC 12588 core if we suspect it to be a dwarf spheroidal. dSph galaxies are commonly 11-13 Gyr old, which means that many of the sub-solar mass stars are now well up on the red giant branch. Being redder, they show up better in the F814W filter because it corresponds to roughly the r-i in ugriz. If this is truly a dSph the core composition will be roughly uniform all the way across becuse of the galaxy DM halo, unlike the concentrations we see in GCs, which have no DM. Hence, comparing the two in inverted form (which I unsharp masked somewhat to reduce star-dot PSF spreading), we get:

F606W
UGC 12588 HLA F606W inverted.jpg

F814W
UGC 12588 HLA F814W inverted.jpg

The F814W cleanly shows the relatively uniform density dispersion as well as the rather formidable number of stars in the RGB stage. This object is practically a textbook dSph.

While it's nice to confirm what we already suspected, the question is the back to Square 1, what shocked its quiescent halo of primordial H1 (retained thanks to the DM halo's gravitational potential) to suddenly blossom forth with O-B starbursts all across the disc? Here are comparative images of the same galaxy in normal mono to reveal the rather modest presence of glowing HII gas:

UGC 12588 HLA F606W mono.jpg

UGC 12588 HLA F814W mono.jpg

For all of us who are so accustomed to O-B star-forming regions practically fulminating with glowing gas, the remarkable dearth of glowing gas in the F606W image is mute testimony that poor 12588 has used up nearly all its available gas reserves in one grand star bloom. Makes me think of those photos of mass flowerings in Death Valley when once-in-a-century rain conditions lead to a floral explosion. We have the same flower bursts in a place called Namaqualand once every few years.

It's rather nice to consider how many yummy nibbles we managed to get out of this one 'cinnamon bun' galaxy.

=Dana in S Africa

[Steve Gottlieb]

For what its worth, I notice that NED also has one redshift-independent distance of 9.3 Mpc (~30 million l.y.) based on Tully-Fischer. Using a diameter of 1.7' that distance translates to an actual diameter of roughly 15,000 l.y., in other words dwarf spheroidal size.

[Howard B]

I always enjoy your posts Dana, not only because you write so well but also because I invariably learn something.

[Nightfall]

I always enjoy your posts Dana, not only because you write so well but also because I invariably learn something.

Thank you for your comments, Howard. You aren't alone in learning something in these posts. I am learning, too, and usually only a few days ahead of you. I have no formal training in astronomy. I started out with an Orion 80 mm short-tube 12 years ago, discovered CN, and bought a really tall ladder. Been climbing ever since.

It is now over a month since the most recent UGC 12588 post. I've passed that month doing a full-blown research report on this little oddball galaxy. For such a modest little thing out there in the middle of not very much at all, UGC 12588's back story is one of the most involved I've ever gotten into. The report came in at 28 pretty dense pages. I just uploaded it onto Google Drive for public access here.

I hope you all enjoy the report. I also hope your 2021 turns out better than all of our respective 2020 experiences. Isn't it wonderful that only one person at a time can look through that eyepiece!

=Doug Bullis, editor ASSA Nightfall

[Dragan]

Very impressive report Doug! I can't imagine what you'd come up with on an object that is well known!

Thank you for your comments, Howard. You aren't alone in learning something in these posts. I am learning, too, and usually only a few days ahead of you. I have no formal training in astronomy. I started out with an Orion 80 mm short-tube 12 years ago, discovered CN, and bought a really tall ladder. Been climbing ever since.

It is now over a month since the most recent UGC 12588 post. I've passed that month doing a full-blown research report on this little oddball galaxy. For such a modest little thing out there in the middle of not very much at all, UGC 12588's back story is one of the most involved I've ever gotten into. The report came in at 28 pretty dense pages. I just uploaded it onto Google Drive for public access here.

I hope you all enjoy the report. I also hope your 2021 turns out better than all of our respective 2020 experiences. Isn't it wonderful that only one person at a time can look through that eyepiece!

=Doug Bullis, editor ASSA Nightfall

[Howard B]

Really impressive paper about UGC 12588. I've found that much the same thing happens with any object I search the literature for more information - there's always more to them than I suspect. Excellent job, and may you all have a happier 2021.