Swinburne University of Technology - Melbourne Australia

Galaxy hunters stalk invisible prey

Story by Julian Cribb

Swinburne University of Technology's Mike Murphy is a galaxy hunter, one of an elite tribe dedicated to seeking out and capturing some of the most elusive prey in the universe.

Over the past year, he and his international colleagues have used a remarkable new instrument on the European Southern Observatory's (ESO) Very Large Telescope (VLT) in northern Chile to discover no fewer than 14 new galaxies, reaching halfway back in time to the Big Bang itself.

What made their task particularly challenging - and improbable - was that the galaxies were hiding.

One of the few places in the universe where an object as huge and brilliant as a galaxy can hide is in front of something brighter still: a quasar, one of the most energetic objects known to exist in the firmament. The comparatively feeble light from a few hundred billion suns in the galaxy is easily overwhelmed by the violent glare of a quasar behind it, making it as hard to see as a pocket torch held in front of a searchlight.

"For 20 years astronomers have been trying to spot galaxies in front of quasars because our sky surveys only show up those that are visible against a dark background," Dr Murphy explains. "But it's incredibly difficult to do because quasars are so bright they dominate everything else in the line of sight.

"A quasar is probably a supermassive black hole, a thousand or more times greater than the one at the heart of our own Milky Way galaxy. These are extraordinarily violent - and probably fairly short-lived - events that light up as they suck in vast amounts of matter. Paradoxically, they are black holes that 'shine'."

The answer to the challenge of peering into this optical maelstrom was SINFONI, a new instrument attached to the giant ESO telescope, which images small patches of sky at many different wavelengths, enabling Dr Murphy's team, led by Dr Nicolas Bouché from the Max Planck Institute for Extraterrestrial Physics in Germany, to look into even the extreme brilliance of quasar light, effectively separating it from galaxy light.

"We thought it was going to be an extraordinarily difficult, even impossible, task to discern the signal of a galaxy against the backdrop of all that energy from the quasar," Dr Murphy recalls. "In the end, it proved to be surprisingly easy. We'd embarked on the project thinking it would be high-risk science, but it has proved otherwise and we're getting some very good results."

The galaxy's presence is revealed by absorption lines - dips in the spectrum of the quasar - caused by the absorption of light at specific wavelengths. Rather than actually seeing the galaxy directly, the astronomers infer its existence and distance from Earth from the specific pattern of shadows it casts on the quasar's spectrum.

The team used huge catalogues of quasars - the SDSS and 2QZ databases - to select those with the most promising dips in their spectra. They then used SINFONI to scan patches of sky around these quasars for galaxies lying in the foreground, in the line of sight from Earth.

In just a couple of nights' worth of observing, the team identified no fewer than 14 of these galaxies at a redshift of one, which makes their light about six or seven billion years old, half the age of the universe itself. This compares with most visible galaxies, which have a redshift of 0.1, and are barely a sixth of this age.

The new galaxies are by no means average in appearance - in fact many of them appear to be hyperactive 'starburst' galaxies: large nurseries of young suns that are forming vigorously at rates of up to 20 new stars per Earth year.

"It's been the equivalent of finding needles in a haystack," Dr Murphy says "For a long time, astronomers weren't able to study the galaxies associated with the dips in quasar spectra because they were so distant and faint compared with the bright quasars. But now we can find the galaxies with SINFONI and can analyse the link between them and the absorption lines they cause."

By being able to observe more galaxies of this age, the team is adding to our rapidly growing understanding of galactic evolution and the processes that took place during a much earlier phase in the development of galaxies and the universe in general.

What has pleased the team is that SINFONI's analysis of the spectral dips is providing sufficient information for them to make inferences about the mass and state of the hidden galaxies - their rates of star formation and other data revealing their condition. "For example, contrary to what you'd think, we've found some evidence that the stronger the absorption line is, the less massive the galaxy."

SINFONI stands for Spectrograph for INtegral Field Observations in the Near Infrared and is a super-cooled imaging spectrometer operating at the near infrared end of the spectrum. It was designed and developed by German and Dutch researchers at the Max Planck Institute and NOVA, the Netherlands Research School for Astronomy, and takes an image consisting of many pixels across a broad expanse of sky. Light from these is directed onto a grating where its full spectrum can be analysed and reconstructed using special software, at many different wavelengths. It is this that gives SINFONI the extraordinary power to envision the otherwise invisible - a power that has some international astronomers in raptures.

The device was first mounted on the giant ESO telescope on top of Cerro Paranal, northern Chile, in 2005 and underwent testing and work-up from 2006 by an international team which included Dr Murphy, who was then at Cambridge University. The collaboration continued after his return to Australia to join the cosmology team at Swinburne's Centre for Astrophysics and Supercomputing.

His continued involvement in the research helps keep Australia at the global forefront of inquiry into cosmology and the study of the early universe.