If you’d like to know how to rule out intergalactic war on extremely large scales, then you’ll need to wander over to my Discovery blog.
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.
While yesterday morning’s talks focused on the general population of stars in the Milky Way, the last two talks of the day told us a little of what can be learnt by looking at more unusual stars. David Lai of UC Santa Cruz took us through the results of his studies of some of the Milky Way’s most metal-poor stars.
Talking about metals in stars sounds counterintuitive, but when an astronomer talks about ‘metals’ they mean any element heavier than helium. These elements are made in the cores of previous generations of stars (the origin of the wonderful imagery of the phrase ‘we are stardust’), and so the most metal-poor stars must be made of the material in the galaxy which is closest to primordial. In fact, Lai told us that his results were consistent with these stars having formed from gas enriched by only one previous generation of stars, which would have exploded in a supernova a long time ago. What we have, he said, is ‘a window into early environments’, thanks to what are literally ‘stellar relics’ of that time. Many such stars have been identified with the SDSS survey, and work is underway to follow up the best candidates in the outer regions of the Galaxy with an instrument attached to the Keck telescopes on Mauna Kea, Hawai’i. One observing night has produced data for nine stars, and their next run is in two weeks; watch this space.
The last talk looked at stars which were unusual in other ways; as the name suggests, high velocity stars are notable thanks to their rapid motion through space. They’d been predicted since astronomer Jack Hills realised in the 1980s that the close approach of a binary star (two stars in orbit around each other) to the supermassive black hole that lurks at the Milky Way’s centre would result in one of the stars being rapidly ejected from the galactic centre at high speed. Identifying them will then allow us to probe the dynamics of and star formation in the Milky Way’s heart. To give just one example, it’s impossible for us to see low mass stars at the distance of the galactic centre, so the only way we’ll know they’re there is to see them once they’ve been thrown out.
They’re therefore of great interest besides being, as speaker Jana Kollmeier of Carnegie Observatory both ‘super important’ and ‘awesome’ (it’s always good to have an enthusiastic speaker who realises that we need to be kept awake at the end of the day!). Finding them amongst the rest of the stars in the galaxy has proved a difficult challenge, though – literally like looking for a needle in a haystack. To improve the odds, astronomers have, until now, looked for blue needles in a red haystack. The vast majority of the native stars in the outer part of the Milky Way’s halo are old and therefore red. Young, massive, blue interlopers stand out like, well, still like a blue star amongst many red ones.
The first few were identified as the by-products of studies of blue stars. The first, discovered by Brown et al. was clocked at 710 km per second (more than one and a half million miles per hour – or about 70,000 times faster than the winner of the men’s Olympic 100 metres) heading rapidly away from the centre of the galaxy. In 2005 they were followed by two more. Hirsch et al’s escapee was moving slightly faster (720 km per second), but the star discovered by Edelmann et al. is even stranger. It’s moving at 550 km per second, but not away from the Milky Way’s centre, but apparently from the Large Magellanic Cloud. As the LMC doesn’t contain a supermassive black hole how this could have happened is something of a mystery.
Blue stars aren’t the solution to all of the Milky Way centre’s mysteries. They don’t live long enough for us to see fast moving stars heading on the inward parts of their orbits. Using this technique with the data that the Sloan survey provides has brought the total up considerably.
Dr Kollmeier only shared her preliminary results with the audience on the condition that we all stopped noting things down, so I’ll wait until the paper comes out before writing about it. The next phase of the Sloan survey, the third, is going to include specific follow-up of fast moving stars and it sounds like the fastest moving objects in the Milky Way will have plenty of competition in the years to come.
