Caught in the act: Forming galaxies captured in the young Universeby HST, VLT & Spitzer (Forwarded)

Royal Astronomical Society
London, U.K.

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CONTACTS:

Dr Aprajita Verma
University of Oxford
Tel: +44 (0) 1865 273319

Dr Malcolm Bremer
University of Bristol
Tel: +44 (0) 117 928 8764
Mob: +44 (0)7796 904612
E-mail:

From 16 to 20 April, Dr. Verma and Dr Bremer can be contacted via the NAM
press office (see above).

Professor Robert Kennicutt
University of Cambridge
Tel: +44 (0)1223 765844

PRESS INFORMATION NOTE: RAS PN 07/13 (NAM 09)

EMBARGOED FOR 00:01 BST, TUESDAY 17 APRIL 2007

CAUGHT IN THE ACT: FORMING GALAXIES CAPTURED IN THE YOUNG UNIVERSE BY HST,
VLT & SPITZER

A team of UK, French and German astronomers have discovered that the
majority of the most distant galaxies so far identified are very young,
undergoing their first extremely vigorous bursts of star formation. This
discovery allows the astronomers to study the first important stages in
the formation of the kind of galaxies we see in the Universe today. One of
the scientists involved in the study, Dr Malcolm Bremer of the University
of Bristol will present the team's findings in his talk on Tuesday 17
April at the Royal Astronomical Society National Astronomy Meeting in
Preston. Full details of the study will soon appear as a paper in the
journal Monthly Notices of the Royal Astronomical Society.

According to Dr Bremer: "Our new systematic survey shows that the majority
of these distant galaxies are undergoing their first significant episodes
of star formation at the epoch at which we observe them, thereby allowing
us to directly observe this key moment in galaxy evolution."

The light that we see from these galaxies was emitted when the Universe
was about 10 per cent of its present age (or just over a billion years
old). They are forming stars at a very high rate (up to a hundred times
the rate at which our own Galaxy, the Milky Way, is currently forming
stars). The duration of these intense star formation events is short
astronomically-speaking, comparable to the time it would take for a star
to cross one of these galaxies (a few tens of millions of years). This
indicates that we are seeing in these galaxies one of their first major
star formation events, and are therefore watching the earliest stages of
galaxy formation in the young Universe.

The team of astronomers discovered that the galaxies have a very high
density of stars, the like of which is seen in only the centres of the
most massive galaxies today. The stars that are forming in these young
galaxies will end up in the biggest galaxies seen in the Universe today.
Previous analysis of the light emitted by massive galaxies close to our
own indirectly suggested that most stars in these galaxies formed just 1-2
billion years after the Big Bang. The new results give direct evidence for
this, the observed galaxies are captured in the first major phases of
their star formation. The lead author of the study, Dr Aprajita Verma of
Oxford University noted: "It is exciting to think that by analysing the
light from these very distant galaxies we can directly study the first
star formation episodes that happened so soon after the Universe began".

The data allowed the astronomers to determine further characteristics for
the galaxies. In particular they were able to compare these very distant
sources to star forming galaxies seen when the Universe was a billion
years older. They found that the later galaxies were physically larger,
more massive, more chemically enriched by heavier elements (created
through nuclear fusion in the earliest stars) and had endured far longer
episodes of star formation. The scientists are seeing direct evidence for
the evolution of galaxies as the Universe ages. Team member Dr Matt
Lehnert comments: "The differences between the two samples are exactly
what is expected. As time goes on, galaxies grow from mergers of smaller
systems and they can sustain longer bursts of star formation. These create
multiple generations of stars that go on to enrich the galaxy with more
and more elements heavier than hydrogen and helium".

Robert Kennicutt, the Plumian Professor of Astronomy and Experimental
Philosophy at the University of Cambridge, commenting on this work said,
"These results suggest that we are already able to observe some of the
first building blocks of present-day galaxies. Furthermore, these results
predict that many of the galaxies observed should have relatively
primitive chemical compositions. In the coming decade it should be
possible to test this prediction, by measuring the heavy element content
of these galaxies with the next generation giant ground-based telescopes
such as ESO's Extremely Large Telescope and with the successor to the
Hubble Space Telescope, the James Webb Space Telescope".

How did the astronomers carry out this work?

In 2003 Lehnert and Bremer showed that samples of very distant galaxies
could be reliably identified in a set of deep optical images by their
unique colours. While the technique relied upon the galaxies containing
some young stars, it could not determine how long star formation had
continued in the galaxies. In the current work, the team led by Aprajita
Verma observed similar objects in infrared light, enabling them to better
characterise the galaxies' emission and thereby determining for how long
star formation had been taking place.

The astronomers combined pre-existing data of an area of sky from several
telescopes in order to identify many distant galaxies and then to
determine the mix of stars within those galaxies. They used Hubble Space
Telescope imaging to explore their properties in visible light, together
with the ground-based ESO VLT (in Chile) and the orbiting NASA Spitzer
telescope to determine their brightnesses in the infra red. Because these
galaxies are so far away, their light is dramatically reddened by the
expansion of the Universe that has occurred between the time the light was
emitted by the galaxies and when it is received by us.

By determining the relative brightness of each galaxy in visible and
infrared light, the team of astronomers were able to determine the ages of
the stars within the galaxies. In common with several more limited
studies, they found that a few of the most distant galaxies have
moderately old stellar populations indicating that they had been forming
stars for several hundred million years. However, the comprehensive nature
of this study showed that the majority of the galaxies had been forming a
significant amount of stars for a far shorter period. In essence the
galaxies were being seen in their first flush of youth.

What happens to the galaxies subsequently is an ongoing topic of study. It
is not clear whether these objects cease forming stars on a timescale of a
few tens of millions of years or whether they continue but become
enshrouded in dust produced as part of the ongoing star formation process
and are effectively rendered invisible to the telescopes used for these
studies. Only further observations will make this clear.

NOTES FOR EDITORS

The 2007 RAS National Astronomy Meeting is hosted by the University of
Central Lancashire. It is sponsored by the Royal Astronomical Society and
the UK Science and Technology Facilities Council.

This year the NAM is being held together with the UK Solar Physics (UKSP)
and Magnetosphere, Ionosphere and Solar-Terrestrial (MIST) spring
meetings. 2007 is International Heliophysical Year.

The preprint of the paper to be published by MNRAS can be found at:
http://arxiv.org/pdf/astro-ph/0701725
"Lyman-break galaxies at z~5 -I. First significant stellar mass assembly
in galaxies that are not simply z~3 LBGs at higher redshift" by Verma, A,
Lehnert, M.D., Foerster-Schreiber, N., Bremer, M.N., Douglas, L.

IMAGES

These are available at:
http://www-astro.physics.ox.ac.uk/~a...ase/index.html