Chris Lintott’s Universe

An American Astronomical Society tradition is the party held on the Wednesday evening. I’m sorry to let you down by not bringing you all the gossip- what happens at the AAS party stays at the AAS party – but it’s reassuring to see that the lecture hall is busy this morning for the first talk of the day, so let me talk about that instead.

It’s been given by Steve Ritz, one of the team behind Fermi, NASA’s new gamma-ray space telescope which was launched in June last year. Fermi was known, before its launch, as GLAST – the Gamma-Ray Large Area Space Telescope. Gamma-rays mark the most energetic end of the electromagnetic spectrum, and so Fermi sees some of the most dramatic events and most energetic objects anywhere in the Universe.

The gamma-ray sky looks very different from that our eyes can see; the lecturer pointed out that Fermi operates in the only region of the spectrum where the Moon is brighter than the Sun, for example! Satellites have looked at the sky at these short wavelengths before; the most famous was the Compton gamma-ray observatory, and it’s with these previous results that Ritz began his talk. For starters, there’s an obvious background glow – the source of which is unknown – in front of which we see emission from our galaxy (the long stripe across the all-sky map seen below). There are some individual sources, too; nearly 300 of them, but the identity of half of these is unknown.

EGRET all-sky gamma ray map

Fermi is a much more sophisticated spacecraft than Compton was, and even after only six months there are plenty of new results for astronomers to chew over. It carries two instruments; the Large Area Telescope can view more than 20% of the sky at once, while the Gamma-ray burst monitor scans the whole sky for dramatic sudden changes. Instead of pointing the spacecraft at specific objects, Fermi normally scans across the sky repeatedly, and in total each pixel on the sky has already been viewed for 2 million seconds. As this process continues, the team will see fainter and fainter objects or watch the brighter ones change.

Faint means faint, here, too. Ritz mentioned that one of the goals of the mission is to monitor pulsars – the spinning remnants of massive stars which shine out boldly in the radio. By contrast, Fermi might be lucky to receive ten individual photons of light a day, and yet they’ve managed to draw conclusions from their observations. One of the most remarkable first results is a pulsar (CTA1) that only shines in gamma-rays – not in any other part of the spectrum. This strange behaviour is most likely a result of our particular viewing angle, but much more work needs to be done.

Perhaps the most dramatic feature of the gamma-ray sky is the presence of gamma-ray bursts, ‘the biggest bangs since the Big one’. Fermi is on course to detect roughly 250 of these each and every year, and the selection it’s already found include the burst with the highest apparent energy ever seen. These events remain mysterious, despite the sterling work in discovery provided by the Swift satellite over the last few years, and Fermi’s ability to look in many different wavelengths at once will be very important here.

There’s plenty in the data set for those of us that study galaxies, too. Fermi sees the most active black holes in the nearby Universe, lurking in the centre of active galactic nuclei (AGN). These objects can and do change extremely rapidly, flaring and sputtering as (presumably) the fuel drops into the black hole, and there will be much discussion of variations in brightness seen in the data, particularly in the 20 or so bright galaxies that Fermi will be concentrating on.

The talk was clearly an early and whirlwind tool through a huge host of topics. Ritz closed by reminding astronomers that they are invited to apply to use the observatory for their own projects, and I suspect keeping up with Fermi’s progress will be essential for years to come. As the lecture’s final slide said – ‘join the fun’!

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.

View from the JCMT's SCUBA camera

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.

The CARMA mm array.

They were able to sidestep the need for radio, and go directly to Hubble data which covered the field.

The Antennae Galaxies

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.

I’m in the Sunday morning session of the Sloan Digital Sky Survey conference (let it never be said that astronomers don’t work hard, even on weekends), and the first talk of the day is by Simon White of the Max Planck Institute in Garching, Germany.

Simon is usually worth listening to, and was introduced by today’s chairman as someone who’d talked at literally every conference he can remember, so in the absence of structured writing I will try and keep you updated here.

He’s started by reminding us that looking at the largest scale structures was the original primary goal of the survey. In the early 90s, as the Sloan was being designed, it was still a relatively new proposition that the structures we see all formed by gravity acting on the tiny fluctuations that we observe in the early Universe, which were themselves detected in our earliest glimpse of the Universe, the cosmic microwave background, by the COBE satellite in 1992.

9.07 : The survey has done this and more – for example, gravitational lensing was considered only to be a curiosity.

9.11 : The world changed rapidly between this point and the survey producing data. For example, astronomers studying distant supernovae realised the Universe’s expansion is speeding up, rather than slowing down. Perhaps more importantly, we were able to measure the geometry of the Universe, and discovered that it is effectively ‘flat’; this is a measure of the energy density in the Universe.

9.13 : The simulation included in the proposal turns out to be not too bad, despite the fact that they had to randomly scatter the galaxies in the absence of other information. By 1996, the resolution of the simulation was good enough to allow us to model the formation of the galaxies themselves – although as with most of these simulations we’re only talking about following the evolution of the dominant dark matter.

9.22 : Simon’s working his way through a list of (fairly technical) observations from the Sloan which compare the distribution of galaxies, and groups of galaxies with the predictions of our standard model of cosmology. Although agreement is currently good, the results are surprisingly sensitive and (at least according to Simon) offer the possibility of distinguishing between the presence of dark matter versus theories which change gravity.

9.24 : Scratch the last bit; a new paper has made him think again about truly testing dark matter.

9.25 : What about looking for the shapes of dark matter halos by fitting profiles to the observed systems? And now we’re on to looking at beautiful simulations. One simulation of the Milky Way’s dark matter has what I think is hundreds of millions of particles whose position and movement are being modelled. This being a science conference, he skipped the movie. Boo!

9.28 : The conclusions from the simulation are many and varied, but include the prediction that the dark matter in our local neighbourhood will essentially be smooth. It will actually be in streams, but there are hundreds of thousands of them and so we won’t be able to distinguish between what the models predict and a truly smooth distribution.