Gravitational lens helps Hubble and Keck discover galaxy building block

A very small, faint galaxy – possibly one of
the long sought `building blocks’ of present-day galaxies –
has been discovered by a collaboration between the Hubble
Space Telescope and the Keck Telescopes at a tremendous
distance of 13.4 billion light-years (based on the estimate
of 14 billion years as the age of the Universe). The
discovery was made possible by examining small areas of sky
viewed through massive intervening clusters of galaxies.
These act as a powerful gravitational lens, magnifying
distant objects and allowing scientists to probe how
galaxies assemble at very early times. This has profound
implications for our understanding of how and when the
first stars and galaxies formed in the Universe.

A unique systematic search for very distant objects – among
the most distant known – using the NASA/ESA Hubble Space
Telescope and the 10 metre Keck Telescopes has produced a
remarkable result. Benefiting from the magnifying power of
a foreground giant cluster of galaxies, a team of European
and American scientists has discovered one of the smallest
and most distant galaxies known to date by pushing both
telescopes to their limits.

Abell 2218 is a rich galaxy cluster composed of thousands
of galaxies and a mass equivalent to ten thousand galaxies
interspersed throughout the cluster. The cluster is located
relatively nearby – at a distance of 2 billion light-years
(redshift 0.18). Redshift is the stretching of light waves
as they travel across expanding space. The longer they
travel, the more they are stretched, and the higher the
measured redshift.

The gravitational field from this huge concentration of
matter distorts and magnifies the light from distant
galaxies according to Einstein’s General Theory of
Relativity. Gravitational lenses acting as `natural
telescopes’ give researchers a unique tool with which to
learn more about the detailed physics of the first galaxies
in the Universe.

The recently discovered `baby-galaxy’ has a redshift of
5.58, corresponding to a distance of about 13.4 billion
light-years. The galaxy’s light has been magnified more
than 30 times by Abell 2218 and split into two `images’ by
the uneven distribution of matter in the cluster.

The team, led by Richard Ellis from the California
Institute of Technology (Caltech), USA, determined the
amount of matter in the object to be astoundingly low for a
galaxy – only a few million times the mass of our Sun, or
about one hundred thousand times less than the amount of
matter in our own Milky Way galaxy. The object is only
around 500 light-years across, as compared to the 100 000
light-year diameter of the Milky Way.

Many galaxy clusters were investigated before such a
clear-cut candidate for a very distant galaxy building
block was found. Images taken from the Hubble archive and
spectroscopy carried out by one of the ground-based Keck
Telescopes revealed that this galaxy is one of the most
distant found so far.

Ellis explains, “Without the benefit of the powerful cosmic
lens, the source would not even have been detected in the
Hubble Deep Fields, historic deep exposures taken in 1995
and 1998.”

Jean-Paul Kneib from the Observatoire Midi-Pyrenees,
France, an expert in gravitational lensing, describes the
excitement of the team: “It took two observing runs with
the Keck Telescope before we had gathered enough light from
this feeble object to determine its distance and thus
confirm the discovery. When we realised what we had found,
we literally jumped up and down.”

Research team member Konrad Kuijken from the Kapteyn
Institute, the Netherlands, continues: “We are very
excited. We are looking at something very small and very
young. A two million year old, one million solar mass,
galaxy-like object consisting of young hot stars is the
best fit to the observations. We believe it is one of the
galaxy building blocks that join together and make up
larger galaxies later in the history of the Universe. With
this discovery, we may finally be witnessing the
circumstances in which this first generation of stars was
born.”

The first galaxies in the Universe hold invaluable clues
that shed light on the period known as the cosmic `Dark
Ages’, a period that lasted possibly up to one billion
years after the Big Bang and ended when the first
generation of stars appeared.

ESA is planning two different observatories to probe these
distant regions of space and time: the Herschel Space
Observatory and the Next Generation Space Telescope (NGST –
in collaboration with NASA and CSA, the Canadian Space
Agency). ESA’s project scientist for NGST, Peter Jakobsen
(ESA-ESTEC, the Netherlands) states: “The gravitational
lensing provided by the foreground galaxy cluster is
clearly extremely powerful – but a bit hard to point in
other directions! This young, faint galaxy is precisely the
kind of object we hope to study in more detail with NGST –
all over the sky.” NGST is planned for launch around 2009.

The team’s research paper will appear in the Astrophysical
Journal Letters.

Credit: ESA, NASA, Richard Ellis (Caltech, USA) and
Jean-Paul Kneib (Observatoire Midi-Pyrenees, France)

Notes for editors

The Hubble Space Telescope is a project of international
co-operation between ESA and NASA.

The W.M. Keck Observatory is operated as a scientific
partnership among the California Institute of Technology,
the University of California and NASA.

Members of the team of scientists include: Jean-Paul Kneib
(Observatoire Midi-Pyrenees, France), Richard Ellis and
Mike Santos (Caltech, USA) and Konrad Kuijken (Kapteyn
Institute, the Netherlands).

This news release is issued jointly by ESA (Hubble European
Space Agency Information Centre), NASA (STScI/Office of
Public Outreach), Caltech/the W.M. Keck Observatory and
NOVA (the Netherlands Research School for Astronomy).

Acknowledgements: The original images of Abell 2218 were
obtained by A. Fruchter and the ERO team (STScI, ST-ECF) as
part of the Hubble Servicing Mission 3A Early Release
Observations.

Contacts

Lars Lindberg Christensen

Hubble European Space Agency Information Centre, Garching,
Germany

Phone: +49-89-3200-6306 (089 in Germany)

Cellular (24 hr): +49-173-38-72-621 (0173 in Germany)

E-mail:lars@eso.org

Ray Villard

Space Telescope Science Institute, Baltimore, USA

Phone: +1-410-338-4514

E-mail:villard@stsci.edu

Robert Tindol

California Institute of Technology (Caltech), Pasadena, USA

Phone: +1-626-395-3631

E-mail: tindol@caltech.edu

James Beletic

W.M. Keck Observatory, Hawaii, USA

Phone: +1-808-885-7887

E-mail: jbeletic@keck.hawaii.edu

Contact details for the scientists:

Jean-Paul Kneib

Observatoire Midi-Pyrenees, Toulouse, France

Phone: +33-5-61-33-28-24 and +33-5-61-33-29-29 (05 in
France)

E-mail: kneib@ast.obs-mip.fr

Richard Ellis

Caltech, USA

Phone: +1-626-395-2598

E-mail: rse@astro.caltech.edu

Konrad Kuijken

Kapteyn Institute, Groningen, the Netherlands

Phone: +31-50-363-4055/4073

E-mail: kuijken@astro.rug.nl