First results from study of Hubble Deep Field: Remote galaxies seen by Hubble Telescope defy classification

Circulated by: The Royal Astronomical Society (28 Feb)

A team of astronomers from Cambridge (UK), Australia and Canada have produced the first published analysis of galaxy images in the Hubble Deep Field &emdash; the Hubble Space Telescope (HST) picture released in January that contains the faintest galaxies ever seen. Their results, which will appear in the Letters pages of the Monthly Notices of the Royal Astronomical Society in April 1996, show that many of the most remote galaxies cannot be slotted into the basic classification system for galaxy shapes devised by Edwin Hubble.

The astronomers have been able to come up with results quickly because they were ready and waiting for the Hubble Deep Field observations. Dr Roberto Abraham, Dr Nial Tanvir, Professor Richard Ellis and Dr Basilio Santiago, all of the Institute of Astronomy, University of Cambridge, along with Dr Karl Glazebrook at the Anglo-Australian Observatory and Professor Sidney van den Bergh at the Dominion Astrophysical Observatory, Canada, applied methods already developed for analysing previous HST images. They used a sophisticated computerized technique to classify the images of about 300 individual galaxies in a selected range of brightness according to their size and how symmetrical they appear. To be sure of making a fair comparison with nearer galaxies, they also analysed images of bright, nearby galaxies which had been processed by computer to simulate how they would have appeared in the Hubble Deep Field.

The results reveal a markedly higher proportion of irregular galaxies among the faint images of remote galaxies in the Hubble Deep Field in comparison with the galaxies populating the universe nearer to us. Regular spirals and ellipticals account for a smaller proportion.

"Whichever way we looked at it, we found that at least 30 or 40 per cent of the faint galaxies appear extremely unusual and distorted compared to only a few per cent of such irregular galaxies in the local universe," said Dr Roberto Abraham. "Indeed, even the relatively normal galaxies are less regular than their counterparts in the nearby universe. The differences are dramatic."

Ground-based telescopes just cannot see clearly enough to reveal the structure in these extremely distant galaxies with such tiny images. Other pictures taken by the HST have already shown an increase in the numbers of irregular galaxies as the observations penetrate deeper into the farthest reaches of the universe but the present study goes about ten times fainter than any done before and shows the trend towards irregularity increasing significantly.

The light we are now receiving from these faint, distant galaxies has taken billions of years to reach us, so we see them as they existed billions of years in the past. By looking very far away, we are also seeing things as they were long ago.

"It is rather like viewing a cross-section through the history of the universe," commented Dr Nial Tanvir. "We are now in a similar situation to palaeontolgists who can see how evolution took place by probing fossils in ever deeper layers of the Earth," said Prof. Sydney van den Bergh. This is the first time astronomers have been able to see large numbers of the galaxies which are the forebears of the galaxies we consider 'normal' today.

Just how far back are we looking? It is difficult to be precise, say the astronomers, but the indications are that many of the galaxies in the Hubble picture are being seen as they were 8 to 10 billion years ago, at a time when the universe was only about 30 per cent of its present age. There is also evidence, based on an analysis of the colours of the galaxies, that some belong to an even earlier epoch, only one or two billion years after the Big Bang took place. Probing to such early times suggests that the irregular shapes being detected may be of galaxies in the process of formation and collisions between galaxies. "This is very exciting," said Prof. Richard Ellis, "and will take us a step closer to understanding how galaxies like our own formed in the first place."

Edwin Hubble (1889-1953), after whom the Hubble Space Telescope is named, drew up his classification scheme for galaxies in the 1920s. The galaxies nearer to us, on which Hubble based his work, are mainly regular and symmetric. Most are either roughly spheroidal in form (rather like squashed or elongated ball), or else, like the our own Milky Way galaxy, they are flattened disks with spiral arms winding out from a central bulge or bar. So Hubble designated his main categories: elliptical, spiral and barred spiral. A handful of 'misfits' he termed 'irregular'. The scheme is no longer thought to relate to the evolution of galaxies as Hubble supposed, but is still widely used as a means of describing the shapes of galaxies. Now, it seems, the Hubble classification system can no longer be regarded as a good description of the range of galaxy types in the universe.

To obtain its 'Deep Field', the Hubble Space Telescope was pointed at an essentially 'blank' patch of sky, well away from any nearby bright stars or galaxies. By doing this, the astronomers could be sure that the very faint objects they were really interested in would not be obscured by brighter and nearer objects. To reveal structural details in these faint images, the HST's detectors were exposed for a record-breaking total of 6.5 days, using 4 different filters to provide information on the colours of the galaxies.

Unlike most other HST data, which belongs to a particular observer or team for a year before anyone else can make use of it, the Hubble Deep Field data was released only three weeks after it was acquired so that any reasearchers who wished could study it immediately. The observations were acquired by a team under the leadership of Dr Robert Williams of the Space Telescope Science Institute.

Images: Images to accompany this release can downloaded from the AAO Homepage:


Hubble Deep Field images

HST Images - AAO press release 6 March 1996(tiff)or (gif) or (jpeg)
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More HST Images - AAO press release 6 March 1996(tiff) or (gif) or (jpeg)
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