It’s difficult, as someone who uses telescopes which work in the sub-mm – effectively short-wave radio – it’s difficult not to be jealous of optical astronomers, many of whom are busy in the exhibit hall giving out beautiful posters of their latest hits. Instead, we end up often with spectra, or at best with a picture like this, obtained by the state of the art SCUBA camera on the JCMT.
I remember SCUBA-guru Rob Ivison’s description of the field on the Sky at Night as ‘blobology’ which hits the nail on the head. When these blobs were first discovered a decade or so ago, no-one knew what they were.
Worse, this simple question turns out to be harder to answer than you might think because the resolution in the sub-mm is so poor. When we look at a sub-mm image, we’re looking at a blurred view of the Universe. If you pointed a big optical telescope at the blob in the image above, the odds are that you’d see many separate galaxies within the one blob. Which of them is responsible for the blob? We wouldn’t know. Rather than jump straight to the optical, therefore, astronomers look deep in longer wavelength radio. Anything we see in the radio is then assumed to correspond to the sub-mm blob, and we’ll have a precise enough position to go chasing the thing with optical telescopes, allowing us to measure the distance.
This technique works for about half the sources, and the orthodox conclusion is that these are rapidly star-forming massive galaxies, the likely precursors of today’s old red and dead ellipticals. What about the other half? My assumption had always been that these were much like the others, but were radio quiet or otherwise difficult to pin down.
In a talk by Kartik Sheth of Caltech here at the American Astronomical Society meeting this morning, I realised I might have to rethink. Sheth’s group used the CARMA array to look directly at one sub-mm galaxy which hadn’t been matched with a counterpart.
They were able to sidestep the need for radio, and go directly to Hubble data which covered the field.
To their surprise, the system was much nearer than other sub-mm galaxies. The best analogue seems to be a merging system, rather like the well-known Antennae which are pictured above. Is this a one-off? Are other nearby galaxies lurking among the distant blobs SCUBA sees? We’ll need, it seems much more data before we can say for sure.