Tales from the radio

The nice thing about atomic hydrogen is that we think we understand the basic emission physics well enough that we can use it probe other things, like galaxy environments. All we have to do is measure the brightness and bam! we also get the mass, and can do more complex stuff like work out how galaxies are interacting and what's going on in clusters.

21cm emission is pretty unusual in that regard in radio astronomy. At lower frequencies, such as those described here, the emission mechanism is far less clear - and depends on all kinds of nasty relativistic effects which are not nice. Which either makes this stuff really interesting or obnoxiously difficult, depending on your point of view.

Here the authors describe a gigantic low-frequency radio source in a nearby cluster, about 140 Mpc distant and 900 kpc long, with the peak of the emission terminating at the galaxy IC 711. They have a nice shiny new map taken with India's Giant Metre-wave Radio Telescope which gives excellent resolution. Until recently, I'd heard that the GMRT worked quite well but only as long as you got one of the locals to do the data reduction for you, but lately I'm told it's producing some really good data even for external users. Certainly the map they show looks very impressive, so congratulations to them on that.

The feature itself is yuuuuge. It's extremely linear and well-collimated, though it has two distinct breaks where the angle changes very sharply, in one case by a neat right-angle. And it's not the only such feature known in this otherwise nondescript cluster. It also hosts another (much smaller) head-tail structured radio source and a double-lobed broader feature. Radio waves may be very low energy, but that doesn't mean they don't reveal some really weird stuff that's going on.

So what in the world is going on with this enormous feature ? It's not at all clear. It's presumably related to gas lost by the galaxy during its passage through the cluster. But given the length of stream and velocity dispersion of the galaxies, that indicates an age of about a billion years. Could it survive and remain linear in a cluster for that long ? It's unclear. The emission mechanism here is thought to be synchrotron radiation, but that should only last about 100 million years. They suggest that what's kept it going is the cosmic microwave background. Crazily, when the stream first started to form, the CMB would have been significantly brighter than it is today - it's THAT big.

What keeps it so narrow ? They suggest pressure confinement by the intracluster medium, which allows calculation of the ICM pressure. But what that implies for the ICM pressure throughout the cluster I'm not sure; it seems like an awfully nice coincidence would be necessary for it to have such a uniform thickness. Likewise it's not clear what's causing the two kinks. There's no way the galaxy's orbit could have changed so suddenly, but if it was due to local overdensities in the ICM, then the thickness of the tail should vary a lot more. Making sharp kinks with tidal fields is possible but difficult. And even the whole linear shape of the tail is a bit weird - you'd expect it to be at least a little bit curved as the galaxy moves through such a large orbit.

So what's going on ? No idea. The paper appears to be still at the submission stage after nearly three years, which is a bit concerning... though that could be because they occasionally use some rather strong phrases about other people's work being wrong. It definitely looks like one to keep an eye on.

GMRT observations of IC 711 -- The longest head-tail radio galaxy known

We present low-frequency, GMRT observations at 240, 610 and 1300 MHz of IC~711, a narrow angle tail (NAT) radio galaxy. The total angular extent of the radio emission, $\sim 22$ arcmin, corresponds to a projected linear size of $\sim 900$ kpc, making it the longest among the known head-tail radio galaxies.