Hubble image of galaxy cluster MACS J1206
Lensing cluster Abell 383
Brightest galaxy in Abell 2261
Galaxy cluster MACS J1720+35
Wide-field image of Abell S1063 (ground-based image)
Wide field view of MACS 1206 (ground-based image)
Videos
[1] The study was performed using archive data from Hubble. The observations were originally made for the CLASH and LoCuSS surveys.
David Harvey
Pan across the galaxy cluster Abell S1063
Pan across Abell 383
Observations may hint at nature of dark matter
Using the NASA/ESA Hubble Space
Telescope, astronomers have discovered that the brightest galaxies
within galaxy clusters “wobble” relative to the cluster’s centre of
mass. This unexpected result is inconsistent with predictions made by
the current standard model of dark matter. With further analysis it may
provide insights into the nature of dark matter, perhaps even indicating
that new physics is at work.
Dark matter
constitutes just over 25 percent of all matter in the Universe but
cannot be directly observed, making it one of the biggest mysteries in
modern astronomy. Invisible halos of elusive dark matter enclose
galaxies and galaxy clusters
alike. The latter are massive groupings of up to a thousand galaxies
immersed in hot intergalactic gas. Such clusters have very dense cores,
each containing a massive galaxy called the “brightest cluster galaxy”
(BCG).
The standard model of dark matter (cold dark matter model)
predicts that once a galaxy cluster has returned to a “relaxed” state
after experiencing the turbulence of a merging event, the BCG does not
move from the cluster’s centre. It is held in place by the enormous
gravitational influence of dark matter.
But now, a team of Swiss, French, and British astronomers have
analysed ten galaxy clusters observed with the NASA/ESA Hubble Space
Telescope, and found that their BCGs are not fixed at the centre as
expected [1].
The Hubble data indicate that they are “wobbling” around the centre
of mass of each cluster long after the galaxy cluster has returned to a
relaxed state following a merger. In other words, the centre of the
visible parts of each galaxy cluster and the centre of the total mass of
the cluster — including its dark matter halo — are offset, by as much
as 40 000 light-years.
“We found that the BCGs wobble around centre of the halos,” explains David Harvey, astronomer at EPFL, Switzerland, and lead author of the paper. “This
indicates that, rather than a dense region in the centre of the galaxy
cluster, as predicted by the cold dark matter model, there is a much
shallower central density. This is a striking signal of exotic forms of
dark matter right at the heart of galaxy clusters.”
The wobbling of the BCGs could only be analysed as the galaxy
clusters studied also act as gravitational lenses. They are so massive
that they warp spacetime enough to distort light from more distant
objects behind them. This effect, called strong gravitational lensing,
can be used to make a map of the dark matter associated with the
cluster, enabling astronomers to work out the exact position of the
centre of mass and then measure the offset of the BCG from this centre.
If this “wobbling” is not an unknown astrophysical phenomenon and in
fact the result of the behaviour of dark matter, then it is inconsistent
with the standard model of dark matter and can only be explained if
dark matter particles can interact with each other — a strong
contradiction to the current understanding of dark matter. This may
indicate that new fundamental physics is required to solve the mystery
of dark matter.
Co-author Frederic Courbin, also at EPFL, concludes: “We’re
looking forward to larger surveys — such as the Euclid survey — that
will extend our dataset. Then we can determine whether the wobbling of
BGCs is the result of a novel astrophysical phenomenon or new
fundamental physics. Both of which would be exciting!”
Notes
[1] The study was performed using archive data from Hubble. The observations were originally made for the CLASH and LoCuSS surveys.
More Information
The Hubble Space Telescope is a project of international cooperation between ESA and NASA.
This research was presented in a paper entitled “A detection of
wobbling Brightest Cluster Galaxies within massive galaxy clusters” by
Harvey et al., which appeared in the Monthly Notices of the Royal
Astronomical Society.
The international team of astronomers in this study consists of David
Harvey (Laboratoire d’Astrophysique EPFL, Switzerland), F. Courbin
(Laboratoire d’Astrophysique EPFL, Switzerland), J.P. Kneib (Laboratoire
d’Astrophysique EPFL, Switzerland; CNRS, France), and Ian G. McCarthy
(Liverpool John Moores University, UK).
Image credit: NASA, ESA, J. Lotz (STScI), M. Postman (STScI), J.
Richard (CRAL) and J.-P. Kneib (LAM), T. Lauer (NOAO), S. Perlmutter (UC
Berkeley, LBNL), A. Koekemoer (STScI), A. Riess (STScI/JHU), J. Nordin
(LBNL, UC Berkeley), D. Rubin (Florida State), C. McCully (Rutgers
University) and the CLASH Team
Links
Contacts
David Harvey
Laboratoire d’Astrophysique EPFL
Versoix, Switzerland
Tel: +41 22 37 92277
Email: david.harvey@epfl.ch
Frederic Courbin
Laboratoire d’Astrophysique EPFL
Versoix, Switzerland
Tel: +41 22 37 92418
Email: frederic.courbin@epfl.ch
Jean-Paul Kneib
Laboratoire d’Astrophysique - EPFL
Versoix, Switzerland
Tel: +41 79 733 21 11
Email: jean-paul.kneib@epfl.ch
Mathias Jäger
ESA/Hubble Public Information Officer
Garching bei München, Germany
Cell: +49 176 62397500
Email: mjaeger@patner.eso.org
Source: ESA/Hubble/News
