The dirt in the discs

What, another paper on dust ? "But Rhys", I hear you say, "you said you hated dust !". Well, I do - it's awful. Outside of His Dark Materials, dust is not the least bit interesting. I just can't bring myself to become interested in where it comes from or what it does. Come on - it's feckin' dust. Dust is something you get rid of. Every time someone mentions it, I immediately remember an advertising slogan for a hoover : "compress your dust, compress your worries". Almost as good as a jingle on a children's magazine, "I love horses, they're my friends !"... but I digress.

Anyway, what caught my eye about this paper was not the dust, but the detection of spiral structure in elliptical galaxies. Since such structures have been found in early-type galaxies in Virgo, this is a potentially interesting avenue for exploring galaxy evolution as a function of environment.

This paper looks at a sample of field ellipticals, using two deliberately similar populations : one with and one without gas (both HI and CO). They use the MegaCam instrument on the CFHT to produce colour maps, using reddening as an indication of dust content after controlling for the underlying stellar population (I also find myself wondering why both this and the previous paper never mention Herschel or other direct dust measurements). This shows a variety of different dust morphologies. Here's their main figure using the SDSS optical images :

And here's the same sample (with no attempt at all at keeping the scaling constant) using their dust maps, taken directly from the paper :

N = no dust; D = disc; R = ring; Ir = Irregular

Now to be fair, you can see some hints of these structures in the conventional images, but they're far clearer in the dust maps. NGC 3626 I find particularly interesting : although it certainly doesn't look like a typical elliptical in the optical map, the spiral dust structure is quite different again. And if you'd just shown me it's dust map, I'd have assumed it was a quite typical spiral shown with a weird colour scheme.

The main trend they note is that there are clear differences in dust morphology depending on gas content. The presence of any gas anywhere in the galaxy seems to be an excellent predictor of dust in the central regions, which it would be nice to know a bit more about... what's the physical connection between outer gas and inner dust ? Perhaps they say something in the appendix, but that's a billion pages long so I'm not going to read it. They also show that gas-rich galaxies are dominated by dust morphologies of spiral and irregular types, with no rings and a very few discs. Gas-poor galaxies have dust morphologies dominated by discs, rings and irregulars. Gas-rich galaxies tend to have large dust structures, whereas small dust structures are found in gas-poor galaxies.

Understandably, they put a lot of effort into comparisons of their study with others, both in terms of similarities and differences. In particular, their sample only deals with field galaxies, so cluster galaxies (which are almost unanimously gas-poor) may be different. But what does this tell us about galaxy evolution ? When some apparently early-type galaxies look so strikingly similar to late-types by looking at their dust, I think we have to wonder if this indicates a connection between the two. They speculate that the origin of the dust could be due to internal production (by stars, despite the low rate of star formation) and brought in externally, but honestly, that's boring. I think this result is sufficient to warrant asking bigger questions about the connections between galaxies of different morphologies. Does one type evolve into the other ? Do they have more similar formation mechanisms than previously suspected ? I dunno, but that to me is way more interesting than any amount of bloomin' dust.

Cold gas and dust: Hunting spiral-like structures in early-type galaxies

Observations of neutral hydrogen (HI) and molecular gas show that 50% of all nearby early-type galaxies (ETGs) contain some cold gas. Molecular gas is always found in small gas discs in the central region of the galaxy, while neutral hydrogen is often distributed in a low-column density disc or ring typically extending well beyond the stellar body.