An international team of astronomers (Note)
from Taiwan, England, and Japan has used the Subaru Telescope to
measure the distribution of dark matter in fifty galaxy clusters and
found that its density gradually decreases from the center of these
cosmic giants to their diffuse outskirts. This new evidence about the
mysterious dark matter that pervades our Universe conforms to the
predictions of cold dark matter theory, known as "CDM".
Few scientists seriously doubt the existence of dark
matter, which researchers discovered almost eighty years ago.
Nevertheless, astronomers cannot directly see dark matter in the night
sky, and particle physicists have not yet identified a dark matter
particle in their experiments. "What is dark matter?" is a big
unanswered question facing astronomers and particle physicists,
especially because invisible dark matter probably makes up 85% of the
mass of the Universe.
The current team, led by Dr. Nobuhiro Okabe (Academia
Sinica, Taiwan) and Dr. Graham Smith (University of Birmingham,
England), used the Subaru Prime Focus Camera (Suprime-Cam) to
investigate the nature of dark matter by measuring its density in fifty
galaxy clusters, the most massive objects in the Universe. "A galaxy
cluster is like a huge city viewed from above during the night", said
Smith. "Each bright city light is a galaxy, and the dark areas between
the lights that appear to be empty during the night are actually full of
dark matter. You can think of the dark matter in a galaxy cluster as
being the infrastructure within which the galaxies live," he explained.
The team wanted to use a large sample of galaxy clusters to find out how
the density of dark matter changes from the center of a typical galaxy
cluster to its outskirts.
The density of dark matter depends on the properties
of the individual dark matter particles, just like the density of
everyday materials depends on their components. CDM, the leading theory
about dark matter to date, asserts that dark matter particles only
interact with each other and with other matter via the force of gravity;
they do not emit or absorb electromagnetic radiation and are difficult
if not impossible to see. Therefore, the team chose to observe dark
matter by using gravitational lensing, which detects its presence
through its gravitational interactions with ordinary matter and
radiation. According to Einstein's theory of relativity, light from a
very distant bright source bends around a massive object, e.g., a
cluster of galaxies, between the source object and the observer. It
follows from this principle that the dark matter in cosmic giants like
galaxy clusters alters the apparent shape and position of distant
galaxies. Lead author Okabe enthused, "The Subaru Telescope is a
fantastic instrument for gravitational lensing measurements. It allows
us to measure very precisely how the dark matter in galaxy clusters
distorts light from distant galaxies and gauge tiny changes in the
appearance of a huge number of faint galaxies." (Figure 1)
CDM theory describes how dark matter in galaxy
clusters changes from its dense center to its lower density edges in two
ways. One is a simple measure of the galaxy cluster's mass, the amount
of matter that it contains. The other is a concentration parameter,
which is a single measurement of the cluster's average density, how
compact it is. CDM theory predicts that central regions of galaxy
clusters have a low concentration parameter while individual galaxies
have a high concentration parameter.
The team combined measurements from observations of
fifty of the most massive known galaxy clusters to calculate their
concentration parameter. The average mass map (Figure 2)
is remarkably symmetrical with a pronounced mass peak. The mass density
distribution for individual clusters shows a wide range of densities.
They found that the density of dark matter increases from the edges to
the center of the cluster, and that the concentration parameter of
galaxy clusters in the near Universe aligns with CDM theory. Past
research based on a small number of clusters found that they had large
concentration parameters and did not conform to CDM theory. In contrast,
measurement of the average concentration parameter from a large number
of clusters yielded a different result, which supports CDM theory. Okabe
commented on the team's findings, based on a larger sample of galaxy
clusters: "This is a very satisfying result, which is based on a very
careful analysis of the best available data".
Figure 2: Dark
matter maps for a sample of fifty individual galaxy clusters (left), an
average galaxy cluster (center), and those based on dark matter theory
(right). The density of dark matter increases in the order of blue,
green, yellow, and red colors. The white horizontal line represents a
scale of one million light-years. The map based on predictions from CDM
theory (right, middle) is a close match to the average galaxy cluster
observed with the Subaru Telescope. (Credit: NAOJ/ASIAA/School of
Physics and Astronomy, University of Birmingham/Kavli IPMU/Astronomical
Institute, Tohoku University)
What does the future hold for the team's continued
research on dark matter? Smith noted, "We don't stop here. For example,
we can improve our work by measuring dark matter density on even smaller
scales, right in the center of these galaxy clusters. Additional
measurements on smaller scales will help us to learn more about dark
matter in the future."
Team member Professor Masahiro Takada (Kavli
Institute for the Physics and Mathematics of the Universe, The
University of Tokyo, Japan) is also excited about the future: "Combining
lensing observations of many galaxy clusters into a single measurement
like this is a very powerful technique. Japanese astronomers are
preparing to use Subaru Telescope's new Hyper Suprime-Cam (HSC) to
conduct one of the biggest surveys of galaxies in human history. Our new
results are a beautiful confirmation of our plan to use HSC for
gravitational lensing studies."
Note:
The members of this research team are also team members of the "Local Cluster Substructure Survey (LoCuSS)", an international consortium of astronomers studying galaxy clusters, as part of the global research effort to answer big, open questions about the cosmos, including the nature of dark matter. More information about the LoCuSS consortium is available from Dr. Graham Smith, and at http://www.sr.bham.ac.uk/locuss.
The members of this research team are also team members of the "Local Cluster Substructure Survey (LoCuSS)", an international consortium of astronomers studying galaxy clusters, as part of the global research effort to answer big, open questions about the cosmos, including the nature of dark matter. More information about the LoCuSS consortium is available from Dr. Graham Smith, and at http://www.sr.bham.ac.uk/locuss.
References:
The research paper on which this article is based was published online in the May 17, 2013 edition of the Astrophysical Journal Letters: N. Okabe et al., "LoCuSS: The Mass Density Profile of Massive Galaxy Clusters at z=0.2", Volume 769, Number 2, Article ID. 35 (2013). The paper's authors are as follows:
The research paper on which this article is based was published online in the May 17, 2013 edition of the Astrophysical Journal Letters: N. Okabe et al., "LoCuSS: The Mass Density Profile of Massive Galaxy Clusters at z=0.2", Volume 769, Number 2, Article ID. 35 (2013). The paper's authors are as follows:
- Dr. Nobuhiro Okabe, Institute of Astronomy and Astrophysics, Academica Sinica, Taiwan
- Dr. Graham P. Smith, School of Physics and Astronomy, University of Birmingham, England
- Dr. Keiichi Umetsu, Institute of Astronomy and Astrophysics, Academica Sinica, Taiwan
- Professor Masahiro Takada, Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU), The University of Tokyo, Japan
- Professor Toshifumi Futamase, Astronomical Institute, Tohoku University, Japan
Acknowledgements:
This research was based in part on the following:
This research was based in part on the following:
- Observations at the Subaru Observatory obtained under the Time Exchange Program operated between the Gemini North Observatory and the Subaru Telescope.
- Data from the Subaru Telescope obtained from the data archive of NAOJ telescopes, SMOKA (Subaru-Mitaka Okayama-Kiso Archive System), which is operated by the Astronomy Data Center, the National Astronomical Observatory of Japan.
This research was supported in part by funding from:
- A Grant-in-Aid for Scientific Research on Priority Area No. 467 "Probing the Dark Energy through an Extremely Wide & Deep Survey with Subaru Telescope"
- World Premier International Research Center Initiative (WPI Initiative), MEXT, Japan
- The FIRST program “Subaru Measurements of Images and Redshifts” (SuMIRe)
- The Royal Society, United Kingdom
- Science and Technology Facilities Council, United Kingdom
- The National Science Council of Taiwan (grant NSC100-2112-M-001-008-MY3)
- The Academica Sinica Career Development Award
