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 the NASA/ ESA 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.
This research was funded by the Science and Technology Facilities
Council.
Issued by RAS Press Officers:
Robert Massey
Tel: +44 (0)20 7734 4582
Mobile: +44 (0)794 124 8035
E-mail:
rm@ras.org.uk|
National Astronomy Meeting Press Room (16 - 20 April only):
Tel: +44 (0)1772 892 613 or 892 475 or 892 477
CONTACT(s):
Dr Aprajita Verma
University of Oxford
Tel: +44 (0) 1865 273319
Mob: +44 (0) 7758 520294
Dr Malcolm Bremer
University of Bristol
Tel: +44 (0) 117 928 8764
Mob: +44 (0)7796 904612
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
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
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.
"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.
