Jim Gunn from Princeton – one of the founders of the Sloan – is presenting the final conference summary. A large part of the design was influenced by the impact of people coming into astronomy from other parts of physics, doing things in the x-ray, the radio and the infrared and who were used to working with data, not with the pictures that were the stock in trade of the optical astronomer. Sloan was supposed to fill the gap, taking both images and data in the form of spectra.
The crucial technology were the fibre optic cables that allowed hundreds of spectrographs to be fed at the same time. Without them, the survey would have taken a thousand or so years assuming that everything worked with 100% efficiency. The new technology made much more rapid progress possible, and Sloan has touched every part of astrophysics.
Jim started his run down with the work on large scale structure. Among the surprises were the work on gravitational lenses, which it had never occurred to anyone might be possible. The net result is that our current cosmology is much better than we have any right to expect, and as Jim said ‘there are no obvious cracks – yet’. However, he warned that eventually as data improves it will crack, and said he’d be very surprised if it didn’t.
The study of galaxies themselves has been revolutionized by Sloan. The spectra were of a much greater quality than was necessary just to determine the distance to the galaxies SDSS targeted. Jim’s slide claims that establishing the properties of modern day galaxies was one of the primary purposes of the Sloan. The headline result is probably the discovery that large populations of galaxies can be divided into red and blue sub-populations. One of the current challenges driving research as a result is the need to explain how sufficient galaxies move quickly from the ‘blue cloud’ to the ‘red sequence’, let alone trying to work out what happens to them once they are there. The majority vote is that black holes have something to do with these mysteries, but the details are very sketchy.
Yet we have still made huge progress; as Jim said, when Sloan started we didn’t even know enough about galaxies to ask these questions, let alone to start answering them. Plenty of work has also been done on galaxies’ active cousins, the quasars, and on the Milky Way itself. The most striking thing – which has crept up on my consciousness for starters – is the acceptance of the idea that the Milky Way has grown by the accretion of smaller satellite galaxies.
Jim’s award for the most unsettling talk goes to ‘Pierre Bergeron’ whose talk on white dwarfs – the dense remnant of Sun-sized stars – lead to conclusion that the hydrogen atom, at least in these extreme conditions, is ‘not well understood’! In a more reassuring pocket lie some of the solar system work (the talks on which I missed by being busy elsewhere) where great strides have been made in identifying asteroids, ruling out threats to Earth.
Supernovae – exploding stars – have been the gold standard for cosmology for the last decade, but at current errors we’re getting close to the point where we have to understand the explosions themselves in order to make further progress. A task for the future, probably, but there are a new set of surveys on the horizon. What advice can the Sloan team offer them?
The most important is probably ‘do it right’ rather than take shortcuts. More interestingly, the lesson from Sloan is not to design surveys which are suited only for one task, but can do a whole host of things, most of which you haven’t thought of yet. Finally, one last discovery. It really is possible for hundreds of astronomers to work together across many institutions with very few constraints on who should do what. That’s something that just wasn’t known when Sloan started, and it’s illustrated by the fact that four of Jim’s academic grandchildren gave talks at this conference.
That’s a suitable end to a wide-ranging conference, so I shall leave it there. I hope you enjoyed my coverage.