A Direct Empirical Proof of the Existence of Dark Matter Page: 1 of 5
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DRAFT VERSION AUGUST 21, 2006
Preprint typeset using LATEX style emulateapj v. 05/04/06
A DIRECT EMPIRICAL PROOF OF THE EXISTENCE OF DARK MATTER *
DOUGLAS CLOWEI, MARUSA BRADA&2, ANTHONY H. GONZALEZ3, MAXIM MARKEVITCH4,5, SCOTT W. RANDALL4,
CHRISTINE JONES4, AND DENNIS ZARITSKYI
Draft version August 21, 2006
We present new weak lensing observations of 1E0657-558 (z 0.296), a unique cluster merger,
that enable a direct detection of dark matter, independent of assumptions regarding the nature of the
gravitational force law. Due to the collision of two clusters, the dissipationless stellar component and
the fluid-like X-ray emitting plasma are spatially segregated. By using both wide-field ground based
images and HST/ACS images of the cluster cores, we create gravitational lensing maps which show
that the gravitational potential does not trace the plasma distribution, the dominant baryonic mass
component, but rather approximately traces the distribution of galaxies. An 8o- significance spatial
offset of the center of the total mass from the center of the baryonic mass peaks cannot be explained
with an alteration of the gravitational force law, and thus proves that the majority of the matter in
the system is unseen.
Subject headings: Gravitational lensing Galaxies:
We have known since 1937 that the gravitational po-
tentials of galaxy clusters are too deep to be caused by
the detected baryonic mass and a Newtonian r-2 grav-
itational force law (Zwicky 1937). Proposed solutions
either invoke dominant quantities of non-luminous "dark
matter" (Oort 1932) or alterations to either the gravita-
tional force law (Bekenstein 2004; Brownstein & Moffat
2006) or the particles' dynamical response to it (Mil-
grom 1983). Previous works aimed at distinguishing be-
tween the dark matter and alternative gravity hypothe-
ses in galaxies (Buote et al. 2002; Hoekstra et al. 2004)
or galaxy clusters (Gavazzi 2002; Pointecouteau & Silk
2005) have used objects in which the visible baryonic
and hypothesized dark matter are spatially coincident,
as in most of the Universe. These works favor the dark
matter hypothesis, but their conclusions were necessar-
ily based on non-trivial assumptions such as symmetry,
the location of the center of mass of the system, and/or
hydrostatic equilibrium, which left room for counterar-
guments. The actual existence of dark matter can only
*BASED ON OBSERVATIONS MADE WITH THE NASA/ESA
HUBBLE SPACE TELESCOPE, OBTAINED AT THE SPACE
TELESCOPE SCIENCE INSTITUTE, WHICH IS OPERATED
BY THE ASSOCIATION OF UNIVERSITIES FOR RESEARCH
IN ASTRONOMY, INC., UNDER NASA CONTRACT NAS 5-
26555, UNDER PROGRAM 10200, THE 6.5 METER MAGEL-
LAN TELESCOPES LOCATED AT LAS CAMPANAS OBSER-
VATORY, CHILE, THE ESO TELESCOPES AT THE PARANAL
OBSERVATORIES UNDER PROGRAM IDS 72.A-0511, 60.A-
9203, AND 64.0-0332, AND WITH THE NASA CHANDRA X-
RAY OBSERVATORY, OPERATED BY THE SMITHSONIAN
ASTROPHYSICS OBSERVATORY UNDER CONTRACT TO
Electronic address: firstname.lastname@example.org
1 Steward Observatory, University of Arizona, 933 N Cherry Ave,
Tucson, AZ 85721
2 Kavli Institute for Particle Astrophysics and Cosmology,
P.O. Box 20450, M529, Stanford, CA 94309
3 Department of Astronomy, University of Florida, 211 Bryant
Space Science Center, Gainesville, FL 32611
4 Harvard-Smithsonian Center for Astrophysics, 60 Garden St.,
Cambridge, MA 02138
s Also Space Research Institute, Russian Acad. Sci., Profsoyuz-
naya 84/32, Moscow 117997, Russia
clusters: individual: 1E0657-558 dark matter
be confirmed either by a laboratory detection or, in an
astronomical context, by the discovery of a system in
which the observed baryons and the inferred dark mat-
ter are spatially segregated. An ongoing galaxy cluster
merger is such a system.
Given sufficient time, galaxies (whose stellar com-
ponent makes up ~ 1 - 2% of the mass (Kochanek
et al. 2003) under the assumption of Newtonian grav-
ity), plasma (~ 5 - 15% of the mass (Allen et al. 2002;
Vikhlinin et al. 2006)), and any dark matter in a typical
cluster acquire similar, centrally-symmetric spatial dis-
tributions tracing the common gravitational potential.
However, during a merger of two clusters, galaxies be-
have as collisionless particles, while the fluid-like X-ray
emitting intracluster plasma experiences ram pressure.
Therefore, in the course of a cluster collision, galaxies
spatially decouple from the plasma. We clearly see this
effect in the unique cluster 1E0657-558 (Tucker et al.
The cluster has two primary galaxy concentrations sep-
arated by 0.72 Mpc on the sky, a less massive (T ~ 6 keV)
western subcluster and a more massive (T ~ 14 keV)
eastern main cluster (Markevitch et al. 2002). Both con-
centrations have associated X-ray emitting plasma off-
set from the galaxies toward the center of the system.
The X-ray image also shows a prominent bow shock on
the western side of the western plasma cloud, indicat-
ing that the subcluster is currently moving away from
the main cluster at ~4700 km/s. As the line-of-sight ve-
locity difference between the components is only ~ 600
km/s (Barrena et al. 2002), the merger must be occur-
ring nearly in the plane of the sky and the cores passed
through each other ~ 100 Myr ago.
Two galaxy concentrations that correspond to the
main cluster and the smaller subcluster have moved
ahead of their respective plasma clouds that have been
slowed by ram pressure. This phenomenon provides an
excellent setup for our simple test. In the absence of
dark matter, the gravitational potential will trace the
dominant visible matter component, which is the X-ray
plasma. If, on the other hand, the mass is indeed domi-
Submitted to Astrophysical Journal Letters
Work supported in part by Department of Energy contract DE-ACO2-76SF00515
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Clowe, Douglas; Bradac, Marusa; Gonzalez, Anthony H.; Markevitch, Maxim; Randall, Scott W.; Jones, Christine et al. A Direct Empirical Proof of the Existence of Dark Matter, article, September 27, 2006; [Menlo Park, California]. (digital.library.unt.edu/ark:/67531/metadc882596/m1/1/: accessed October 15, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.