Credits: NASA/ISAS/DSS/A. Simionescu et al.; inset: NASA/CXC/A. Fabian et al.
[/caption]
The Japanese Suzaku X-ray telescope has just taken a close look at the Perseus galaxy cluster, and revealed it’s got a bit of a spare tire.
Suzaku explored faint X-ray emission of hot gas across two swaths of the Perseus Galaxy Cluster. The resulting images, which record X-rays with energies between 700 and 7,000 electron volts in a combined exposure of three days, are shown in the two false-color strips above. Bluer colors indicate less intense X-ray emission. The dashed circle is 11.6 million light-years across and marks the so-called virial radius, where cold gas is now entering the cluster. Red circles indicate X-ray sources not associated with the cluster.
The results appear in today’s issue of Science.
The Perseus cluster (03hh 18m +41° 30‘) is the brightest extragalactic source of extended X-rays.
Lead author Aurora Simionescu, an astrophysicist at Stanford, and her colleagues note that until now, most observations of galaxy clusters have focused on their bright interiors. The Suzaku telescope was able to peer more closely at the outskirts of the Perseus cluster. The resulting census of baryonic matter (protons and neutrons of gas and metals) compared to dark matter offers some surprising observations.
It turns out the fraction of baryonic matter to dark matter at Perseus’s center was consistent with measurements for the universe as a whole, but the baryonic fraction unexpectedly exceeds the universal average on the cluster’s outskirts.
“The apparent baryon fraction exceeds the cosmic mean at larger radii, suggesting a clumpy distribution of the gas, which is important for understanding the ongoing growth of clusters from the surrounding cosmic web,” the authors write in the new paper.
X-ray emission in the COSMOS field (XMM-Newton/ESA)
[/caption]
Galaxy density in the Cosmic Evolution Survey (COSMOS) field, with colors representing the redshift of the galaxies, ranging from redshift of 0.2 (blue) to 1 (red). Pink x-ray contours show the extended x-ray emission as observed by XMM-Newton.
Dark matter (actually cold, dark – non-baryonic – matter) can be detected only by its gravitational influence. In clusters and groups of galaxies, that influence shows up as weak gravitational lensing, which is difficult to nail down. One way to much more accurately estimate the degree of gravitational lensing – and so the distribution of dark matter – is to use the x-ray emission from the hot intra-cluster plasma to locate the center of mass.
And that’s just what a team of astronomers have recently done … and they have, for the first time, given us a handle on how dark matter has evolved over the last many billion years.
COSMOS is an astronomical survey designed to probe the formation and evolution of galaxies as a function of cosmic time (redshift) and large scale structure environment. The survey covers a 2 square degree equatorial field with imaging by most of the major space-based telescopes (including Hubble and XMM-Newton) and a number of ground-based telescopes.
Understanding the nature of dark matter is one of the key open questions in modern cosmology. In one of the approaches used to address this question astronomers use the relationship between mass and luminosity that has been found for clusters of galaxies which links their x-ray emissions, an indication of the mass of the ordinary (“baryonic”) matter alone (of course, baryonic matter includes electrons, which are leptons!), and their total masses (baryonic plus dark matter) as determined by gravitational lensing.
To date the relationship has only been established for nearby clusters. New work by an international collaboration, including the Max Planck Institute for Extraterrestrial Physics (MPE), the Laboratory of Astrophysics of Marseilles (LAM), and Lawrence Berkeley National Laboratory (Berkeley Lab), has made major progress in extending the relationship to more distant and smaller structures than was previously possible.
To establish the link between x-ray emission and underlying dark matter, the team used one of the largest samples of x-ray-selected groups and clusters of galaxies, produced by the ESA’s x-ray observatory, XMM-Newton.
Groups and clusters of galaxies can be effectively found using their extended x-ray emission on sub-arcminute scales. As a result of its large effective area, XMM-Newton is the only x-ray telescope that can detect the faint level of emission from distant groups and clusters of galaxies.
“The ability of XMM-Newton to provide large catalogues of galaxy groups in deep fields is astonishing,” said Alexis Finoguenov of the MPE and the University of Maryland, a co-author of the recent Astrophysical Journal (ApJ) paper which reported the team’s results.
Since x-rays are the best way to find and characterize clusters, most follow-up studies have until now been limited to relatively nearby groups and clusters of galaxies.
“Given the unprecedented catalogues provided by XMM-Newton, we have been able to extend measurements of mass to much smaller structures, which existed much earlier in the history of the Universe,” says Alexie Leauthaud of Berkeley Lab’s Physics Division, the first author of the ApJ study. COSMOS-XCL095951+014049 (Subaru/NAOJ, XMM-Newton/ESA)
Gravitational lensing occurs because mass curves the space around it, bending the path of light: the more mass (and the closer it is to the center of mass), the more space bends, and the more the image of a distant object is displaced and distorted. Thus measuring distortion, or ‘shear’, is key to measuring the mass of the lensing object.
In the case of weak gravitational lensing (as used in this study) the shear is too subtle to be seen directly, but faint additional distortions in a collection of distant galaxies can be calculated statistically, and the average shear due to the lensing of some massive object in front of them can be computed. However, in order to calculate the lens’ mass from average shear, one needs to know its center.
“The problem with high-redshift clusters is that it is difficult to determine exactly which galaxy lies at the centre of the cluster,” says Leauthaud. “That’s where x-rays help. The x-ray luminosity from a galaxy cluster can be used to find its centre very accurately.”
Knowing the centers of mass from the analysis of x-ray emission, Leauthaud and colleagues could then use weak lensing to estimate the total mass of the distant groups and clusters with greater accuracy than ever before.
The final step was to determine the x-ray luminosity of each galaxy cluster and plot it against the mass determined from the weak lensing, with the resulting mass-luminosity relation for the new collection of groups and clusters extending previous studies to lower masses and higher redshifts. Within calculable uncertainty, the relation follows the same straight slope from nearby galaxy clusters to distant ones; a simple consistent scaling factor relates the total mass (baryonic plus dark) of a group or cluster to its x-ray brightness, the latter measuring the baryonic mass alone.
“By confirming the mass-luminosity relation and extending it to high redshifts, we have taken a small step in the right direction toward using weak lensing as a powerful tool to measure the evolution of structure,” says Jean-Paul Kneib a co-author of the ApJ paper from LAM and France’s National Center for Scientific Research (CNRS).
The origin of galaxies can be traced back to slight differences in the density of the hot, early Universe; traces of these differences can still be seen as minute temperature differences in the cosmic microwave background (CMB) – hot and cold spots.
“The variations we observe in the ancient microwave sky represent the imprints that developed over time into the cosmic dark-matter scaffolding for the galaxies we see today,” says George Smoot, director of the Berkeley Center for Cosmological Physics (BCCP), a professor of physics at the University of California at Berkeley, and a member of Berkeley Lab’s Physics Division. Smoot shared the 2006 Nobel Prize in Physics for measuring anisotropies in the CMB and is one of the authors of the ApJ paper. “It is very exciting that we can actually measure with gravitational lensing how the dark matter has collapsed and evolved since the beginning.”
One goal in studying the evolution of structure is to understand dark matter itself, and how it interacts with the ordinary matter we can see. Another goal is to learn more about dark energy, the mysterious phenomenon that is pushing matter apart and causing the Universe to expand at an accelerating rate. Many questions remain unanswered: Is dark energy constant, or is it dynamic? Or is it merely an illusion caused by a limitation in Einstein’s General Theory of Relativity?
The tools provided by the extended mass-luminosity relationship will do much to answer these questions about the opposing roles of gravity and dark energy in shaping the Universe, now and in the future.
Sources: ESA, and a paper published in the 20 January, 2010 issue of the Astrophysical Journal (arXiv:0910.5219 is the preprint)
GEMS, the Gravity and Extreme Magnetism Small Explorer, will detect polarized X-rays from supernova remnants, neutron stars and black holes.
[/caption]
NASA has announced the development of a space-based observatory to give astronomers a new way to view X-rays from exotic objects such as black holes, neutron stars, and supernovae. Called the Gravity and Extreme Magnetism Small Explorer (GEMS), the mission is part of NASA’s Small Explorer (SMEX) series of cost-efficient and highly productive space-science satellites, and will be the first satellite to measure the polarization of X-rays sources beyond the solar system.
Polarization is the direction of the vibrating electric field in an electromagnetic wave. An everyday example of polarization is the attenuating effect of some types of sunglasses, which pass light that vibrates in one direction while blocking the rest. Astronomers frequently measure the polarization of radio waves and visible light to get insight into the physics of stars, nebulae, and the interstellar medium, but few measurements have every been made of polarized X-rays from cosmic sources.
“To date, astronomers have measured X-ray polarization from only a single object outside the solar system — the famous Crab Nebula, the luminous cloud that marks the site of an exploded star,” said Jean Swank, a Goddard astrophysicist and the GEMS principal investigator. “We expect that GEMS will detect dozens of sources and really open up this new frontier.”
Black holes will be high on the list of objects for GEMS to observe. The extreme gravitational field near a spinning black hole not only bends the paths of X-rays, it also alters the directions of their electric fields. Polarization measurements can reveal the presence of a black hole and provide astronomers with information on its spin. Fast-moving electrons emit polarized X-rays as they spiral through intense magnetic fields, providing GEMS with the means to explore another aspect of extreme environments.
“Thanks to these effects, GEMS can probe spatial scales far smaller than any telescope can possibly image,” Swank said. Polarized X-rays carry information about the structure of cosmic sources that isn’t available in any other way.
“GEMS will be about 100 times more sensitive to polarization than any previous X-ray observatory, so we’re anticipating many new discoveries,” said Sandra Cauffman, GEMS project manager and the Assistant Director for Flight Projects at Goddard.
Some of the fundamental questions scientists hope GEMS will answer include: Where is the energy released near black holes? Where do the X-ray emissions from pulsars and neutron stars originate? What is the structure of the magnetic fields in supernova remnants?
GEMS will have innovative detectors that efficiently measure X-ray polarization. Using three telescopes, GEMS will detect X-rays with energies between 2,000 and 10,000 electron volts. (For comparison, visible light has energies between 2 and 3 electron volts.) The telescope optics will be based on thin-foil X-ray mirrors developed at Goddard and already proven in the joint Japan/U.S. Suzaku orbital observatory.
GEMS will launch no earlier than 2014 on a mission lasting up to two years. GEMS is expected to cost $105 million, excluding launch vehicle.
Orbital Sciences Corporation in Dulles, Va., will provide the spacecraft bus and mission operations. ATK Space in Goleta, Calif., will build a 4-meter deployable boom that will place the X-ray mirrors at the proper distance from the detectors once GEMS reaches orbit. NASA’s Ames Research Center in Moffett Field, Calif., will partner in the science, provide science data processing software and assist in tracking the spacecraft’s development.
