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

Royal Astronomical Society
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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: m.bremer@xxxxxxxxxxxxx

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.

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