---

---

---

---

Science Current Events and Science News RSS Feeds

---

---

Email a Friend Printer Friendly

First Image and Spectrum of a Dark Matter Object

December 05, 2001

HST and VLT Identify MACHO as a Small and Cool Star

An international team of astronomers 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.

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.

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 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.

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.

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 (ESO 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 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.

Read the full text of ESO Press Release 28/01 (with two photos and all
weblinks), available at

http://www.eso.org/outreach/press-rel/pr-2001/pr-28-01.html

European Southern Observatory (ESO)