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The Case of the Disappearing Galaxies

Posted on: Monday, 3 March 2003, 06:00 CST

Cambridge and California Astronomers Use Hubble Space Telescope and Keck to Discover an Extremely Distant Galaxy - But Why Aren't There More?

Royal Astronomical Observatory -- UK astronomers Elizabeth Stanway, Andrew Bunker and Richard McMahon at the Institute of Astronomy in Cambridge, England, have used three of the most powerful telescopes in existence to identify some of the furthest galaxies yet seen.

But the astronomers have found a cosmic conundrum: there seem to be fewer galaxies forming stars early in the history of the Universe than there are in the more recent past.

In a paper accepted for publication in Monthly Notices of the Royal Astronomical Society, Stanway, Bunker and McMahon have used the unique power of the Hubble Space Telescope, by analyzing publicly-available images taken in the direction of the southern hemisphere constellation of Fornax (the Oven) with the new Advanced Camera for Surveys as part of the `Great Observatory Origins Deep Survey' (the GOODS project).

The Cambridge astronomers have found half a dozen objects with very red colors, and show that these are likely to be 95 per cent of the way across the observable Universe.

These galaxies are so far away that light from them has taken about 13 thousand million (13 billion) years to reach us (a `redshift' of 6 in the jargon of astronomers).

These galaxies - which existed when the Universe was less than a billion years old and seven billion years before the Earth and Sun formed - have had their light at visible wavelengths absorbed as it passes through gas on the way to us. We can, however, still see the infrared light - and it is these red colors which leads us to believe that they lie at these immense distances.

The astronomers have also used infrared images taken with one of the 8-metre Very Large Telescopes (VLT) at the European Southern Observatory (ESO) in Chile to study these galaxies.

"The infra-red pictures taken with the ESO telescopes allowed us to separate very distant galaxies - which lie at the edge of the observable Universe - from red objects nearby", said graduate student Elizabeth Stanway, who has identified these objects as part of her research for a doctorate in astrophysics at Cambridge.

Having found these candidate very distance objects using the Space Telescope, the astronomers then turned to the largest telescope in the World: the Keck telescope, at the top of the 14000ft mountain of Mauna Kea in Hawaii.

Working with California astronomers Professor Richard Ellis (Caltech) and Dr Patrick McCarthy (Carnegie Observatories) they took a spectrum of one of these red objects - spreading its light out into different wavelengths.

They saw that the hydrogen gas of this object is glowing as it is illuminated by hot, newly-born stars, and the wavelength of this glow showed that the redshift of this object is 5.78 (meaning that the Universe has stretched by a factor of seven since the light left this galaxy, in the continuing expansion after the Big Bang).

"This galaxy is in the process of giving birth to stars - each year it converts a mass of gas more than 30 times that of our sun into new stars", according to research astronomer Dr. Andrew Bunker. "Inside stars like these, the chemical elements that are in our bodies were formed" added Dr Richard McMahon.

"Using the largest optical telescope, Keck, was very important as it showed that this population of objects discovered by the Hubble Space Telescope really are incredibly distant", said Andrew Bunker, who was part of the team which did the observing in Hawaii.

Spreading the light from the distant galaxy into its component wavelengths (a spectrum) shows strong emission at a particular wavelength - this is hydrogen gas `glowing' is it absorbs ultraviolet light from young stars. Credit: Institute of Astronomy

The wavelength of the line emission tells us how much the Universe has expanded since light left the galaxy (a factor of seven), and from this we estimate that the galaxy is 13 billion light years away. We are look 95percent of the way back in time to the Big Bang. Credit: Institute of Astronomy

These results from the Keck telescope have recently been submitted to Monthly Notices of the Royal Astronomical Society (Bunker, Stanway, Ellis, McMahon and McCarthy). "The galaxy we have proved to be very distant is only 1000 light years across. This is very small compared to our own galaxy, the Milky Way, which is 100 times larger" said Elizabeth Stanway.

But the Cambridge team have found a cosmic puzzle: they know how many galaxies there are which are forming stars at a rapid rate, and they can compare the average we calculate in the very distant universe (redshift 6) with previous work done at redshifts below 4, on closer samples of galaxies. It seems that there are fewer of these galaxies early in the history of the Universe, compared to more recent times.

Theoretical predictions for the star formation history of the Universe are highly unceratin, which is why this observational work is essential. "It could be that we are seeing some of the first galaxies to be born", said Richard McMahon, "The light from these first stars to ignite could have ended the Dark Age of the Universe as the galaxies `turn on', and might have caused the gas between the galaxies to be blasted by starlight - the `reionization' which has recently been detected in the microwave background (the echo of the Big Bang) by the WMAP satellite".

This team of astronomers are currently building a new instrument in Cambridge called `DAZLE', which will probe even earlier in the history of the Universe and shed new light on the `Dark Ages'.

Authors: Elizabeth Stanway, Andrew Bunker, Richard McMahon (Institute of Astronomy, Cambridge), Richard Ellis (Caltech) and Patrick McCarthy (Carnegie Observatories)

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