That's
NGC 5249 (DR8 ObjId 1237668623557460046) [1]
Morphologically, it's a lenticular, S0 in
the Hubble/de Vaucouleurs classification scheme. Its distorted shape, and prominent dust lanes, would lead zooites to say it's likely a late-stage merger (you would think that, wouldn't you, you '100k+ GZ classifications' zooite?
)
And the bluish tinge suggests star-formation is not quite dead yet; the spectrum can be introduced as evidence too (yes, it's a nice 'old' spectrum, but there are some 'hard to ignore' emission lines too):
Fair enough; while lenticulars are often very hard to tell from ellipticals, in this case it's not that hard, right?
NGC 2534 (DR8 ObjId 1237663916804538442) this time. Morphologically it's ... E1!
But clearly it's a
blue elliptical, and there are hints of shells - tell-tale signs of past merger activity - so add in the obvious dust lane, and it's very likely a late-stage merger too (despite its E1 morphology).
A completely unambiguous, completely
authentic typical elliptical, right?
NGC 4374 (DR7 ObjId 588017704006910002; a.k.a. M84) is classified as E1 ... but with these extras: 'LINER', 'Sy 2', 'FR I', ...
Yet while you may not be able to pick out any distinct dust lanes in that SDSS image, the central part is, in fact, packed with enough dust to
put the dust bunnies under a billion couches to shame tip the scales at many thousands or even tens of thousands of sols. This is an example of an elliptical with a 'nuclear' dust
lane morphology. Here is a Herschel image (
source [2]); at these far-infrared wavelengths, dust is what makes a galaxy shine:
I had intended to include an SDSS image of a large elliptical - preferably an NGC object - in which it is obvious that there is dust (in the elliptical itself); a non-blue one. However, while there are plenty of lenticulars with such dust - with classifications such as S0, S0/a - I couldn't find an elliptical.
Also, there are images of nuclear dust (e.g. in the van Dokkum & Franx paper referenced below), but they're not SDSS. Anyway, part of my motivation for doing this OotD is my
Not a dustlane? collection, in GZ Talk, which was inspired by some images of a brown splotch in the bright yellow part of some SDSS ETGs (early type galaxies; basically ellipticals and lenticulars). Here's the only one in that collection which
might, as of today, be a dustlane in an elliptical, and not an artifact (
AGZ0005b32; GZ4 image, then
SDSS Explore one):
Anyway, it turns out that dust is surprisingly common in ellipticals, as the first couple of sentences in this paper makes clear (
source):
Optical absorption patches, lanes, and filaments of dust have been seen in 50%80% of nearby bright elliptical galaxies (e.g., van Dokkum & Franx 1995). Observations at a range of other wavelengths have also revealed unexpected amounts of gas and dust in these galaxies (e.g., Roberts et al. 1991; Goudfrooij et al. 1994). In some objects, the dust masses estimated from optical extinction studies are a magnitude lower than masses implied by the IRAS far-infrared (FIR) fluxes, suggesting that elliptical galaxies may contain diffusely distributed dust not detected or properly accounted for in optical observations (e.g., Goudfrooij & de Jong 1995).
How did all this dust get there? Aren't ellipticals supposed to be 'dead and red' (the traditional ones, not the blue ones), where star-formation ceased long ago? And isn't it true that, left alone in the interstellar medium (ISM), dust will largely disappear after ~100 million years (a blink of a cosmic eye), being sputtered into a gas by the hot ISM plasma and cosmic rays? Yes, there will be some new dust created, as stars age and become red giants and blow smoke into
the room the ISM; but that can't ever amount to much. Sure, if the nucleus is active - an AGN - it may keep things well and truly stirred up, and keep creating new dust by any of several different processes.
Well, according to this recent paper, "Dust and Ionized Gas Association in E/S0 Galaxies with Dust Lanes: Clues to their Origin" (
Finkelman et al. 2012), a lot of different clues point to a somewhat surprising conclusion: the ionized gas (plasma) often seen outside the nuclear region in ellipticals is closely associated with the dust, and the ISM in these ellipticals may be surprisingly complex in its structure and morphology (not at all like the 'thin, uniform screen' often assumed in textbooks). The paper is fairly easy to read, and the conclusion a nice, calm summary:
We argue that these observed relations indicate that the ionized gas and the obscuring material have the same origin, are heated by the same sources and are well mixed. We conclude that an internal origin of the dust and ionized gas in E/S0 galaxies with dust lanes is highly unlikely; the hot gas content of E/S0 galaxies is quite heterogeneous and expected to affect differently the grain size distribution, mass content and dust distribution of individual galaxies, whereas our findings are independent of the hot gas content of each galaxy. We argue also that our results are consistent with the ‘evaporation flow’ hypothesis, albeit with some uncertainty as to the exact details of the process. If the dusty gas that we observe in the optical is part of the dense material arriving from outside during an accretion or merger event, than it could survive destruction even in hot and extreme environments. Relaxed gaseous discs and chaotic filamentary structures represent in this picture different states of similar events. The frequent detection of tidal features, atomic and molecular gas and kinematically decoupled gas components in E/S0 galaxies with dust lanes support this proposed view.
Hmm, I wonder what
Robaina and Masters would make of this? And maybe it might be worth somebody's while to collect elliptical-elliptical overlaps after all.
[1] All galaxies in this OotD have been posted many, many times before (except for AGZ0005b32), and I'm not even going to try to find who posted each first. As they're all (but one) NGC objects, the link is to the entry in the
NGC catalogue LIST[2] Did you hear that this most magnificent of space-based observatories
ran out of liquid helium, just last week, and so it's now basically just a big piece of space junk?
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), the probability of the the datasets being drawn from the same population is very high (98.9%, to be precise). At least, according to the KS statistic (or test?)
