So.. some days back in an earlier post I waxed lyrical on how mapping the sky has been of profound cultural importance – setting our lives in context, and satisfying a deep desire to discover whats out there. But are surveys the right thing to do scientifically ? When we map the sky, do we not spend many nights of expensive telescope time collecting data “just in case its handy” ? Would it not be more cost effective to do much more targeted experiments ? Au contraire …(Pretentious ? Moi ?)
Surveys are cheap
We do the science in two steps. Surveys are a kind of summarised digest of the sky. Once we have this digest as a database we can do multiple experiments from the same database, without having to go back to the telescope. This could be anything from “I have an X-ray source – does it have an IR counterpart ?” to “I want to test my model by calculating the correlation function of a special subset of galaxies chosen to have parameters XYZ”. This multiple use is hugely productive and very cost effective. In modern times the prime example is the Sloan Digital Sky Survey (SDSS). The team themselves have written plenty of papers, but astronomers all over the world suck data out of the SDSS archive every day, and the net effect has been almost a thousand scientific papers using the survey.
Put another way, online survey maps and catalogues can be seen as a kind of virtual sky. It is no coincidence that people like me who like surveys are also pushing on the Virtual Observatory front… more of this another time.
You need big surveys
Often the science we do just needs big swathes of sky. There can be three typical reasons for this. First, the thing we are studying covers a large fraction of the sky – like the Milky Way for example, or the Local Supercluster. Sometimes it is because we are looking for needles in a haystack – very rare objects such as quasars at z=6 or the very nearest and coldest brown dwarfs – we expect a handful over the sky, but don’t know where they are .. Finally, it can be because we need a very large sample – millions of objects – to beat down the statistical errors. The classic example is measuring the power spectrum of galaxy clustering, where we want a few percent accuracy in every spatial frequency bin. Another good example is mapping out the dark matter by measuring the distortions in galaxy images caused by weak gravitational lensing. (As in the recent result from the COSMOS survey). To do this, we need to average over hundreds of galaxies in each spatial bin in our map.
Surveys are the engine of discovery
Since the 1950s, every time we survey the sky at a new wavelength, we have been taken by surprise – radio galaxies, X-ray binaries, gamma ray bursts, huge starbursts. (More examples in the earlier post).
Many people would take this aspect of surveys as the best value for money for the last fifty years. We have been through a Golden Age of discovery. But has it now ended ? SCUBA, the submillimetre camera, plugged the last wavelength gap. It was very successful, discovering debris disks round nearby stars, and so many high redshift starbursts, that this seems to be the way that half the stars were formed. SCUBA2 will be bigger and better .. but not rdaically different. So is that it ? Have all the windows have been opened…. ? I will take a look at this in part III.