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Distant Galaxies Are In The Red

April 03, 2001

According to scientists from the Institute of Astronomy, University of Cambridge, red is the colour favoured by distant galaxies. But the reason for this is still not clear.

Working with astronomers in California and Canada, the Cambridge team used a special infrared-sensitive camera to carry out a large-scale survey of distant galaxies. The main goal of the project was to study the Universe when it was 7 billion years old or around half its current age.

On Wednesday 4 April at the UK National Astronomy Meeting in Cambridge, graduate student Andrew Firth will present the first surprising results from the Cambridge InfraRed Survey Instrument (CIRSI).


CLUMPY RED GALAXIES

The recently completed infrared sky survey has detected over 50,000 galaxies in a patch of sky covering roughly the area of a full Moon.

Although the Cambridge team has so far analysed only one fifth of the data, they have already found that there are three times as many very red galaxies as expected.

One possibility is that these galaxies have more old stars in them than expected. Old stars tend to be large and relatively cool - hence the red colour.

A second possibility is that the galaxies are very dusty. Just as dust in the atmosphere creates red sunsets, so dust clouds in galaxies scatter red light and change the light they emit.

A second significant result is the discovery that these red galaxies seem to clump together much more than galaxies in the nearby Universe. One possible explanation is that these red galaxies are merging with each other to form single more massive galaxies.

This merging process would explain why the astronomers are seeing more galaxies in the past than they expected. If galaxies merge, their total number will decrease to the present-day value.

Later this month, team member Andrew Firth will travel to Hawaii to use the Keck telescope in an effort to measure the distances to some of these faint galaxies and find out whether they are, indeed, merging.


A STATE-OF-THE-ART INFRARED CAMERA

The survey was made with the Cambridge InfraRed Survey Instrument (CIRSI), a special camera that is sensitive to infrared radiation. Unlike most astronomical instruments that work at optical wavelengths, this innovative camera contains four highly advanced electronic detectors that are sensitive to infrared radiation.

The camera is so powerful that when it was mounted on the 2.5m Isaac Newton Telescope, the Cambridge team was able to detect infrared radiation more than 50 times faster than the giant 10m Keck telescope on Hawaii. In fact, the idea for this supersensitive instrument came to the team leader, Dr Richard McMahon, when he was visiting the Keck telescope in 1995.

The Cambridge team has also carried out observations with the same camera on the UK-Dutch 2.5m Isaac Newton Telescope and 4.2m William Herschel Telescope on La Palma, in the Canary Islands.

The camera takes a picture every 20 seconds, so in a single night over 2000 pictures may be taken. The nightly data volume is gigantic and amounts to 30,000 MegaBytes of data (i.e. enough to fit on 60 CD-ROMs).

At the heart of the camera are four highly sensitive infrared arrays from the Rockwell International Science Centre (USA), built using a hybrid of Mercury-Cadmium-Telluride semiconductors. Each detector is 19mm x 19mm in size and consists of 1024 x 1024 pixels or picture elements, each 18.5 x 18.5 microns (approximately 1/50 of a millimetre) in size. The detectors are cooled to a temperature of about -196 degrees centigrade, using liquid nitrogen that is enclosed in a large evacuated metal vessel like a vacuum flask.

The camera has been built using a generous donation made by the Raymond and Beverly Sackler Foundation under a new "Deep Sky Initiative" proposed by the Institute of Astronomy in 1995.

The Cambridge team is led by Dr Richard McMahon, Dr Ofer Lahav and graduate student Andrew Firth. Other members of the Cambridge team are Dr. Craig Mackay, Dr. Chris Sabbey, Dr Rachel Somerville and Prof. Richard Ellis. The team also includes Dr Pat McCarthy from the Observatories of the Carnegie Institute of Washington and Dr Ray Carlberg from University of Toronto.


AlphaGalileo Foundation