Old galaxies stick together in the young Universe (Forwarded)

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CONTACTS

Will Hartley
Centre for Astronomy and Particle Theory
School of Physics and Astronomy
University of Nottingham
University Park
Nottingham NG7 2RD
United Kingdom
Tel: +44 (0)115 846 8829

Dr Omar Almaini
Centre for Astronomy and Particle Theory
School of Physics and Astronomy
University of Nottingham
University Park
Nottingham NG7 2RD
United Kingdom
Tel: +44 (0)115 846 7901

EMBARGOED UNTIL 0001 BST, 1 April 2008

Ref.: PN 08/13 (NAM 04)

Old galaxies stick together in the young Universe

Using the most sensitive images ever obtained with the United Kingdom
Infra-Red Telescope (UKIRT), astronomers have found convincing evidence
that galaxies which look old early in the history of the Universe reside
in enormous clouds of invisible dark matter and will eventually evolve
into the most massive galaxies that exist in the present day.

University of Nottingham PhD student Will Hartley, who led the study, will
speak at the RAS National Astronomy Meeting in Belfast on Tuesday 1 April.

The distant galaxies identified in the UKIRT images are considered elderly
because they are rich in old, red stars. However, because the light from
these systems has taken up to 10 billion years to reach Earth, they are
seen as they appeared in the very early Universe, just 4 billion years
after the Big Bang. The presence of such fully evolved galaxies so early
in the life of the cosmos is hard to explain and has been a major puzzle
to astronomers studying how galaxies form and evolve.

Hartley and collaborators used the deep UKIRT images to estimate the mass
of the dark matter surrounding the old galaxies by measuring how strongly
the galaxies cluster together. All galaxies are thought to form within
massive halos of dark matter which collapse under their own gravity from a
smooth distribution of matter after the Big Bang.

These halos are invisible to normal telescopes but their mass can be
estimated through analysis of galaxy clustering.

Hartley explains: "Luckily, even if we don't know what dark matter is, we
can understand how gravity will affect it and make it clump together. We
can see that the old, red galaxies clump together far more strongly than
the young, blue galaxies, so we know that their invisible dark matter
halos must be more massive.²

The halos surrounding the old galaxies in the early Universe are found to
be extremely massive, containing material which is one hundred thousand
billion times the mass of our Sun. In the nearby Universe, halos of this
size are known to contain giant elliptical galaxies, the largest galaxies
known.

"This provides a direct link to the present day Universe," says Hartley,
"and tell us that these distant old galaxies must evolve into the most
massive but more familiar elliptical-shaped galaxies we see around us
today. Understanding how these enormous elliptical galaxies formed is one
of the biggest open questions in modern astronomy and this is an important
step in comprehending their history."

IMAGES

Image of the elderly galaxies,
http://www.nottingham.ac.uk/astronomy/UDS/UDS_files/bigger_pBzK3.jpg
(83KB)

Image caption: The white arrows point to a few of the old, massive
galaxies at a distance of 10 billion light years, discovered in the UKIDSS
Ultra-Deep survey. This cut-out image represents just 1/150th of the full
survey. (Credit: UKIDSS UDS survey team)

FURTHER INFORMATION

* The UKIDSS Ultra-Deep Survey
http://www.nottingham.ac.uk/astronomy/UDS/
* UKIRT
http://www.jach.hawaii.edu/UKIRT/
* Nottingham Astronomy group
http://www.nottingham.ac.uk/astronomy/

NOTES FOR EDITORS

The old galaxies were identified from images taken as part of the
Ultra-Deep Survey (UDS), one element of a five-part project, the UKIRT
Infrared Deep Sky Survey (UKIDSS), which commenced in 2005. UKIRT is the
world's largest telescope dedicated solely to infrared astronomy, sited
near the summit of Mauna Kea, Hawaii, at an altitude of 4194 metres (13760
feet) above sea level.

The RAS National Astronomy Meeting (NAM 2008) is hosted by Queen's
University Belfast. It is principally sponsored by the RAS and the Science
and Technology Facilities Council (STFC). NAM 2008 is being held together
with the UK Solar Physics (UKSP) and Magnetosphere, Ionosphere and
Solar-Terrestrial (MIST) spring meetings.


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