Opening up the infrared sky

The infrared sky is expanding significantly for the world astronomical community with the first world release of data (DR1) from the UKIRT Infrared Deep Sky Survey (UKIDSS). UKIDSS DR1 has mapped a larger volume of the sky than any previous infrared survey. As the UKIDSS project progresses, it will gradually become the dominant source of information about the infrared sky, expanding its volume by a factor of 15 beyond DR1.

For the past two years, the United Kingdom Infrared Telescope (UKIRT) in Hawaii has been systematically scanning the heavens for five different 'colours' of faint infrared light. This allows astronomers to penetrate dark clouds where stars are currently forming, and to locate stars much less massive and much cooler than the Sun. Furthermore, our own Galaxy (the Milky Way) is transparent to the infrared, making it possible to see all the way to its centre and beyond. And finally, the expansion of the Universe stretches visible light from the most distant (and youngest) galaxies and quasars into the infrared part of the spectrum, and by observing this infrared light we can trace the evolution of galaxies from their youngest members. The first world release of these data makes all this information available to researchers everywhere.

Andy Lawrence from the University of Edinburgh, the UKIDSS Principal Investigator, said 'We are moving into new territory. This survey probes huge volumes of space, so that we can locate rare but important objects like the very coolest and least luminous stars and the most distant galaxies. Astronomers in Europe have started getting the science out, but this world release should really unleash the scientific potential of the dataset.'

The present release, large though it is, however, is just the beginning.

Andy Adamson, Associate Director of UKIRT, says 'WFCAM has recently taken its one millionth observation, and the UKIDSS survey is progressing strongly. UKIDSS will have surveyed a volume 15 times larger than the current release, DR1, by the time it is completed in 2012.'

Results from this world-leading effort are released in two stages - first to the member nations of the European Southern Observatory (ESO), and 18 months later to the world astronomical community. The data now being released worldwide were obtained in the first, intensive and exciting, WFCAM observing periods on the UKIRT telescope, up to January 2006. There will be new data releases approximately every six months over the coming five years.

Astronomers from the ESO nations have been busily following up on the early UKIDSS data for the past year. The survey has proved itself a rich source of exotic objects, exactly as expected. Steve Warren, the UKIDSS Survey Scientist, highlights the discovery of the coolest known brown dwarf in the Galaxy - ULAS J0034 for short - which, at an absolute temperature only just over twice that of the Earth, is fully 100 degrees cooler than any other known brown dwarf. This is likely one of the closest astronomical objects outside the Solar System, and was discovered in the shallow UKIDSS Large Area Survey (LAS). UKIDSS is also expected to discover some of the most distant objects known, and it appears to be well on the way to this goal. DR1 includes early data from the Ultra-Deep Survey (UDS), which aims to study the evolution of galaxies when the Universe was a fraction of its current age. This project is extraordinarily ambitious, requiring the telescope to revisit the same square-degree area of sky on hundreds of nights. 'A hundred thousand very distant galaxies are detected even in the earliest UDS data, and there is also a 'needle in a haystack' object - a quasar at a redshift just in excess of 6, meaning 12.7 billion light years from Earth,' says co-discoverer Ross McLure. 'The light we now see from this object is very, very old, having set off on its journey to the telescope only a billion years after the big bang.'

The first world release also contains large amounts of data on the Milky Way, with millions of stars, young stars and other objects seen clearly through the thick veils of dust which block the Milky Way to visible light, as illustrated in the accompanying images. Phil Lucas, head of the Galactic Plane Survey (GPS), notes that 'in terms of detected objects, the GPS dominates UKIDSS, with hundreds of millions of infrared stars in DR1 and many times that still to come. And with the science archive now hosting a large-scale image of the GPS so far, we're able to visualise the infrared Milky Way better than ever before.'

These results are among the motivations for carrying out surveys of the infrared sky. Comprising five separate surveys, some of which are highlighted here, UKIDSS has now scoured a larger volume of the Universe than any previous sky survey, and only slightly less than the largest visible light surveys. When the observations are completed in 2012, UKIDSS will have probed some 70 times deeper on average than the previous largest survey.

'The UKIDSS survey programme was expressly designed to capitalise on the unique technical capabilities of the UKIRT Wide-Field Camera' said Gary Davis, Director of the Joint Astronomy Centre in Hawaii, which operates the UKIRT. WFCAM was developed at the UK Astronomy Technology Centre in Edinburgh at a cost of GBP5M, and it is now the world's leading infrared panoramic camera. 'It is rewarding to see the effort and dedication of a large team of scientists and engineers over many years coming to fruition. The release of DR1 presages the huge impact that UKIRT will make on world astronomy over the next several years by probing deeper into the infrared universe than ever before.'

Images from the Wide Field Camera undergo processing at the Cambridge Astronomical Survey Unit (CASU), Cambridge, UK, and the science products are transferred to the WFCAM Science Archive operated by the Wide Field Astronomy Unit in the Institute for Astronomy at the University of Edinburgh. Astronomers from around the world will access the UKIDSS data from the Science Archive, which is bracing for the influx of new users.

A small preliminary release, of about 1/4 the size of DR1, has been scrutinised from all over the world since it was opened up in August 2007. Nigel Hambly, the scientist responsible for operation of the Science Archive, says that interest is likely to be intense. 'Followup of objects discovered in this data release within the ESO nations has already revealed the power of the UKIDSS survey to turn up unique objects and we expect the world community will want to quickly make the most of the data now becoming available.'