More information is available in ESO
Press Release 28/01 at: http://www.eso.org/outreach/press-rel/pr-2001/pr-28-01.html

Summary

An international team of astronomers [2] has observed a Dark Matter object
directly for the first time.

Images and spectra of a MACHO microlens – a nearby dwarf star that
gravitationally focuses light from a star in another galaxy – were taken
by the NASA/ESA Hubble Space Telescope (HST) and the European Southern
Observatory’s Very Large Telescope (VLT).

The result is a strong confirmation of the theory that a large fraction of
Dark Matter exists as small, faint stars in galaxies such as our Milky
Way.

PR Photo 35a/01: HST image of a MACHO.

PR Photo 35b/01: VLT spectrum of a MACHO.

The Riddle of Dark Matter

The nature of Dark Matter is one of the fundamental puzzles in astrophysics
today. Observations of clusters of galaxies and the large scale structure of
individual galaxies tell us that no more than a quarter of the total amount
of matter in the Universe consists of normal atoms and molecules that make
up the familiar world around us. Of this normal matter, no more than a
quarter emits the radiation we see from stars and hot gas. So, a large
fraction of the matter in our Universe is dark and of unknown composition.

For the past ten years, active search projects have been underway for
possible candidate objects for Dark Matter. One of many possibilities is
that the Dark Matter consists of weakly interacting, massive sub-atomic
sized particles known as WIMPs. Alternatively, Dark Matter may consist of
massive compact objects (MACHOs), such as dead or dying stars (neutron stars
and cool dwarf stars), black holes of various sizes or planet-sized
collections of rocks and ice.

The MACHOs

In 1986, Bohdan Paczynski from Princeton University (USA) realised that if
some of the Dark Matter were in the form of MACHOs, its presence could be
detected by the gravitational influence MACHOs have on light from distant
stars.

If a MACHO object in the Milky Way passes in front of a background star in a
nearby galaxy, such as the Large Magellanic Cloud (LMC), then the
gravitational field of the MACHO will bend the light from the distant star
and focus it into our telescopes. The MACHO is acting as a gravitational
lens, increasing the brightness of the background star for the short time it
takes for the MACHO to pass by.

Depending on the mass of the MACHO and its distance from Earth, this period
of brightening can last days, weeks or months. The form and duration of the
brightening caused by the MACHO – the microlensing “light curve” – can be
predicted by theory and searched for as a clear signal of the presence of
MACHO Dark Matter.

MACHOs are described as “microlenses” since they are much smaller than other
known cases of gravitational lensing, such as those observed around clusters
of galaxies, cf. ESO PR 19/98. Observations of microlensing events have been
done on many occasions with ESO telescope with intersting results, e.g., the
recent detection of a corona of a distant star in the Milky Way (ESO PR
17/01).

The MACHO Project

Astronomers from the Lawrence Livermore National Laboratory, the Center for
Particle Astrophysics in the United States and the Australian National
University joined forces to form the “MACHO Project” in 1991. This team [2]
used a dedicated telescope at the Mount Stromlo Observatory in Australia to
monitor the brightness of more than 10 million stars in the Large Magellanic
Cloud (LMC) over a period of eight years.

The team discovered their first gravitational lensing event in 1993 and have
now published approximately twenty instances of microlenses in the direction
of the Magellanic Clouds. These results demonstrate that there is a
population of MACHO objects in and around the Milky Way galaxy that could
comprise as much as 50% of the Milky Way total (baryonic/normal-matter) Dark
Matter content.

Hubble obtains the first direct image of a MACHO

[ESO PR Photo 35a/01] ESO PR Photo ESO PR Photo 35a/01 is based on
35a/01 three exposures from the WFPC2
camera at the NASA/ESA Hubble Space
Telescope, obtained in the V-, R-
[Preview – JPEG: 400 x 387 pix – and I-bands (shown as blue, green
36k] and red, respectively). It shows the
[Normal – JPEG: 800 x 774 pix – 87k] first image of a Dark Matter object
– a MACHO (a massive compact
object). It is the red object that
is indicated with an arrow and very
near to the upper left (at 2
o’clock) of a blue background star.
This MACHO is a nearby red dwarf
star that gravitationally focused
light from the blue background star
in another galaxy in a so-called
microlensing event. Since the event
six years ago, the MACHO has moved
0.134 arcseconds on the sky and can
now be clearly separated in the
Hubble image.

In order to learn more about each microlensing event, the MACHO team has
used the Hubble Space Telescope (HST) to take high-resolution images of the
lensed stars.

One of these images showed a faint red object within a small fraction of an
arcsecond from a blue, normal (main-sequence) background star in the Large
Magellanic Cloud (ESO PR Photo 35a/01).

The image was taken by Hubble 6 years after the original microlensing event,
which had lasted approximately 100 days. The brightness of the faint red
star and its direction and separation from the star in the Large Magellanic
Cloud are completely consistent with the values indicated 6 years earlier
from the MACHO light curve data alone.

This Hubble observation further reveals that the MACHO is a small faint,
dwarf star at a distance of 600 light-years, and with a mass between 5% and
10% of the mass of the Sun.

The VLT adds spectral information

[ESO PR Photo 35b/01] ESO PR Photo ESO PR Photo 35b/01 shows a
35b/01 composite spectrum of the two very
close objects seen on the HST image
(PR Photo 35a/01). It is based on
[Preview – JPEG: 400 x 319 pix – four 1500-second exposures that were
37k] obtained with the FORS2 multi-mode

[Normal – JPEG: 1003 x 800 pix – instrument at the 8.2-m VLT KUEYEN
144k] telescope on February 2, 2001. The
presence of certain metal and alkali
resonance lines, in particular of
sodium (Na), is typical of a cool
stellar object. Telluric molecular
bands (from the Earth’s atmosphere)
are indicated with an earth-symbol.

To further confirm these findings, members of the MACHO team sent in a
special application for observing time on the FORS2 instrument on the ESO
8.2-m VLT KUEYEN Unit Telescope to obtain spectra of the object. ESO
responded swiftly and positively to the request. Although it was not
possible to separate the spectra of the MACHO and background star, the
combined spectrum (PR Photo 35b/01) showed the unmistakable signs in the red
spectral region of the deep absorption lines of a dwarf M star superimposed
on the spectrum of the blue main sequence star in the Large Magellanic
Cloud.

The nature of Dark Matter

The combination of the microlensing light curve from the MACHO project, the
high-resolution images from Hubble and the spectroscopy from the VLT has
established the first direct detection of a MACHO object, to be published in
the international science journal “Nature” on December 6, 2001.

Thanks to the HST and VLT observations, the astronomers now have a complete
picture of this particular MACHO: its mass, distance and velocity. The
result greatly strengthens the argument that a large fraction of the
‘normal’ Dark Matter in and around our galaxy exists in the form of MACHOs.
Thus this Dark Matter is not as dark as previously believed!

Future searches for MACHO-like objects will have the potential to map out
this form of Dark Matter and reach a greater understanding of the role that
Dark Matter plays in the formation of galaxies. These efforts will further
strengthen the drive to reveal the secrets of Dark Matter and take a large
step towards closing the books on the mass budget of the Universe.

Notes

[1]: This is a joint Press Release by the European Southern Observatory
(ESO) and the Hubble European Space Agency Information Centre. The Hubble
Space Telescope is a project of international co-operation between ESA and
NASA.

[2]: The MACHO collaboration is made up of: Kem H. Cook, Andrew J. Drake,
Stefan C. Keller, Stuart L. Marshall, Cailin A. Nelson and Piotr Popowski
(Lawrence Livermore National Laboratory, Livermore, CA, USA); Charles Alcock
and Matt J. Lehner (University of Pennsylvania, Philadelphia, PA, USA);
Robyn A. Allsman (Australian National Supercomputing Facility, Canberra,
ACT, Australia); David R. Alves (STScI, Baltimore, USA); Tim S. Axelrod, Ken
C. Freeman and Bruce A. Peterson (Mount Stromlo Observatory, Weston, ACT,
Australia); Andrew C. Becker (Bell Labs, Murray Hill, NJ, USA); Dave P.
Bennett (University of Notre Dame, IN, USA); Marla Geha (University of
California at Santa Cruz, CA, USA); Kim Griest and Thor Vandehei (University
of California, San Diego, CA, USA); Dante Minniti (P. Universidad Catolica,
Santiago de Chile); Mark R. Pratt, Christopher W. Stubbs and Austin B.
Tomaney (University of Washington, Seatlle, WA, USA); Peter J. Quinn
(European Southern Observatory, Garching, Germany); Will Sutherland
(University of Oxford, UK) and Doug Welch (McMaster University, Hamilton,
Ontario, Canada).

Contacts

Peter Quinn

European Southern Observatory

Garching, Germany

Tel.: +4989-3200-6509

email: pjq@eso.org

Cailin Nelson

Lawrence Livermore National Laboratory

USA

Tel.: +1-925-423-2852

email: cnelson@igpp.ucllnl.org

Dante Minniti

P. Universidad Catolica

Depto de Astronomia

Chile

Tel.: +56 2 686 4946

email: dante@astro.puc.cl

Tim Axelrod

Research School of Astronomy and Astrophysics

Australian National University

Australia

Tel.: +61-2-6125-0214

email: tsa@mso.anu.edu.au

Lars Lindberg Christensen

Hubble European Space Agency Information Centre

Garching, Germany

Tel.: +49-89-3200-6306 or +49-173-38-72-621

email: lars@eso.org

Richard West

European Southern Observatory

Garching, Germany

Tel.: +49-89-3200-6276

email: rwest@eso.org