Today's paper is a bit more technical than usual, but sometimes you've gotta tackle the hard stuff.
Ram pressure stripping is something we seem to understand pretty well on a large scale. When galaxies enter a massive cluster containing its own gas, pressure builds up that can push out the gas in the galaxy. If it's going fast enough, and/or the cluster gas is dense enough, then the galaxy can loose all of its gas pretty quickly. No ifs or buts, it just looses all its gas, stops forming stars, realises it's made incredibly poor life choices, and dies.
Yeah, literally, it dies. It's run out of fuel for star formation, which means all its remaining massive blue stars aren't replaced when they explode as supernovae in a few million years. Slowly it turns into a "red and dead" smooth, structureless, boring disc, and maybe eventually an elliptical. There's a wealth of evidence that ram pressure is the dominant mechanism of gas loss within clusters, and everything seems to just basically... work. Which is nice.
But, as ever, the details are where it gets interesting. In the extreme case, what you'll see is a galaxy with a big long tail of gas, one single plume stretching off until it's torn apart and dissolved in the chaos of the cluster.
Even here things can be complicated though. Some tails seem to have multiple components : extremely hot X-ray emitting gas, cooler neutral atomic hydrogen detectable with radio telescopes, intermediate temperature ionising gas that emits over very narrow "Hα" optical wavelengths, and very cold gas indeed that emits in the sub-mm regime. They may or may not have stars forming within the plume, and all of these different components can have radically different structures. Or they might all line up quite neatly. Sometimes all of these phases are present, sometimes just one or two.
And then, if a galaxy isn't in the extreme case, it can be even more complicated. If the ram pressure isn't enough to accelerate the gas to escape velocity, it can still be pushed out only to fall back in somewhere else in the disc. In short, it gets messy.
This paper attempts to understand one of those messy cases. It's part of the ALMA JELLY program, a large ALMA observing program run by my officemate Pavel Jachym (conflict of interest : declared ! BOX TICKED). Here they introduce the first analysis of one of their 28 target galaxies and tackle the important question (though they would never dare state it thus) :
Why does it look like the Playboy bunny rabbit ?
Wait, wait... why is it called ALMA JELLY ? It's not an acronym as far as I know. Instead, "jellyfish" galaxies have become a popular name for galaxies experiencing ram pressure stripping as some of them have distinct, narrow tails that look very much like the tentacles of a jellyfish. The term has become somewhat abused lately, often used for any ram-pressure stripping galaxy regardless of what its tail looks like. Here they attempt to take back control of the term and define it as galaxies which have stars forming it their stripped material. This often occurs in narrow tendrils so it's a pretty good proxy for jellyfish-like structures, and highlights the unusual physics at work in these cases.
And, why ALMA ? ALMA observes the cold molecular gas, which is generally agreed to be the main component of star formation. The target here already has many observations at other wavelengths, but the molecular gas has been traditionally tough to observe. Now they can fill in the gap, and with extreme resolution too.
So, the bunny rabbit. The first target for ALMA JELLY is NGC 4858. It's certainly a prime example of a jellyfish galaxy, with clear, bright tendrils of stars extending in one direction directly away from the centre of the Coma cluster in which it resides. It's also close to the cluster centre, where ram pressure ought to be very strong. Its got observations at a bunch of different wavelengths and it is, in short, a right proper mess. Really, it's the kind of thing I might be minded to throw up my hands and say, "hahahah no, I'm not touching that with a barge pole". Or, failing that, I might wave my hands furiously and say, "something something HYDRODYNAMICS !".
Hydrodynamic effects, the complicated interactions between two or more different fluids, are an easy get-out. Mixing of fluids causing extremely complex structures, so if something's a mess, it's a safe bet that hydrodynamics can explain it. Though, in that case you ought to run simulations to test if that really works or not.
Here they don't. Instead they try the much braver task of explaining it without any dedicated simulations, and even those simulations they do use don't have full hydrodynamic effects – just some very basic approximations of the major forces at work from the external gas. And yet they seem to have come up with a pretty convincing explanation.
It works like this. First, NGC 4858 is a grand design spiral, with two prominent spiral arms. As it rotates, each arm moves through a region where its subjected to varying ram pressure forces, which are greatest on the side rotating away from the cluster centre (where the gas is moving fastest away from the cluster, making it easiest to remove). A single, dense arm thus gives rise to a single, dense plume of gas – a tail. But this tail gas preserves some of the rotation it had around the galaxy's centre, so it doesn't just get blasted out into space – it keeps moving around the galaxy. This brings it into the shadow of the galaxy, protecting it from the wind of the cluster. Some of the gas is lucky enough that the greatly reduced ram pressure is now essentially impotent, and it falls back onto the galaxy.
Not all of it though. Some keeps going. If any makes it right around to the other side of the galaxy, it moves back into the zone of death and gets finally stripped away by the cluster gas once and for all. The key is that before it reaches this point, the gas gets compressed as it starts to hit the wind again. In the simulations they use as a reference, the galaxy doesn't have prominent spiral arms and shows a single prominent tail; they surmise that because NGC 4858 has two arms, this could naturally give rise to two tails (or ears).
Their observations also show direct evidence of gas returning to the galaxy. The ALMA observations allow them to make a velocity map of the gas, and there's one big feature which is discontinuous with the rest of the velocity structure. And again, that fits with the basic model of how they expect rotating gas to behave.
I've simplified and shortened this one quite a lot, missing out on any number of interesting details. And there's an awful lot more they could still do with this data. But to me, the first thing I wondered when I first saw the ALMA image was "why is it a bunny rabbit ?". I was expecting this to have a much more complex non-answer, featuring hand-waving and invocations to hydrodynamics galore, possibly involving a chicken sacrifice. As it is, they managed to come up with a decent explanation without any of that, which is no mean feat. Both the bunnies and the chickens can rest easy.
Now all they have to do is convince Playboy to give them a sponsorship deal...
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