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Tag Archives: Chandra X-ray Observatory

Galaxy Clusters Probe Dark Energy

Rich (large) clusters of galaxies are significant celestial X-ray sources. In fact, large clusters of galaxies typically contain around 10 times as much mass in the form of very hot gas as is contained in their constituent galaxies.

Moreover, the dark matter content of clusters is even greater than the gas content; typically it amounts to 80% to 90% of the cluster mass. In fact, the first detection of dark matter’s gravitational effects was made by Fritz Zwicky in the 1930s. His measurements indicated that the galaxies were moving around much faster than expected from the known galaxy masses within the cluster.

clusters_1280.abell1835.jpg

Image credit: X-ray: NASA/CXC/Univ. of Alabama/A. Morandi et al; Optical: SDSS, NASA/STScI (X-ray emission is shown in purple)

The dark matter’s gravitational field controls the evolution of a cluster. As a cluster forms via gravitational collapse, ordinary matter falling into the strong gravitational field interacts via frictional processes and shocks and thermalizes at a high temperature in the range of 10 to 100 million degrees (Kelvins). The gas is so hot, that it emits X-rays due to thermal bremsstrahlung.

Recently, Drs. Morandi and Sun at the University of Alabama have implemented a new test of dark energy using the observed X-ray emission profiles of clusters of galaxies. Since clusters are dominated by the infall of primordial gas (ordinary matter) into dark matter dominated gravitational wells, then X-ray emission profiles – especially in the outer regions of clusters – are expected to be similar, after correcting for temperature variations and the redshift distance. Their analysis also considers variation in gas fraction with redshift; this is found to be minimal.

Because of the self similar nature of the X-ray emission profiles, X-ray clusters of galaxies can serve as cosmological probes, a type of ‘standard candle’. In particular, they can be used to probe dark energy, and to look at the possibility of the variation of the strength of dark energy over multi-billion year cosmological time scales.

The reason this works is that cluster development and mass growth, and corresponding temperature increase due to stronger gravitational potential wells, are essentially a tradeoff of dark matter and dark energy. While dark matter causes a cluster to grow, dark energy inhibits further growth.

This varies with the redshift of a cluster, since dark energy is constant per unit volume as the universe expands, but dark matter was denser in the past in proportion to (1 + z)^3, where z is the cluster redshift. In the early universe, dark matter thus dominated, as it had a much higher density, but in the last several billion years, dark energy has come to dominate and impede further growth of clusters.

The table below shows the percentage of the mass-energy of the universe which is in the form of dark energy and in the form of matter (both dark and ordinary) at a given redshift, assuming constant dark energy per unit volume. This is based on the best estimate from Planck of 68% of the total mass-energy density due to dark energy at present (z = 0). Higher redshift means looking farther back in time. At z = 0.5, around 5 billion years ago, matter still dominated over dark energy, but by around z = 0.3 the two are about equal and since then (for smaller z) dark energy has dominated. It is only since after the Sun and Earth formed that the universe has entered the current dark energy dominated era.

Table: Total Matter & Dark Energy Percentages vs. z 

Redshift

Dark Energy percent

Matter percent

0

68

32

0.25

52

48

0.5

39

61

0.75

28

72

1.0

21

79

1.5

12

88

The authors analyzed data from a large sample consisting of 320 clusters of galaxies observed with the Chandra X-ray Observatory. The clusters ranged in redshifts from 0.056 up to 1.24 (almost 9 billion years ago), and all of the selected clusters had temperatures measured to be equal to or greater than 3 keV (above 35 million Kelvins). For such hot clusters, non-gravitational astrophysical effects, are expected to be small.

Their analysis evaluated the equation of state parameter, w, of dark energy. If dark energy adheres to the simplest model, that of the cosmological constant (Λ) found in the equations of general relativity, then w = -1 is expected.

The equation of state governs the relationship between pressure and energy density; dark energy is observed to have a negative pressure, for which w < 0, unlike for matter.

Their resulting value for the equation of state parameter is

w = -1.02 +/- 0.058,

equal to -1 within the statistical errors.

The results from combining three other experiments, namely

  1. Planck satellite cosmic microwave background (CMB) measurements
  2. WMAP satellite CMB polarization measurements
  3. optical observations of Type 1a supernovae

yield a value

w = -1.09 +/- 0.19,

also consistent with a cosmological constant. And combining both the X-ray cluster results with the CMB and optical results yields a tight constraint of

w = -1.01 +/- 0.03.

Thus a simple cosmological constant explanation for dark energy appears to be a sufficient explanation to within a few percent accuracy.

The authors were also able to constrain the evolution in w and find, for a model with

w(z) = w(0) + wa * z / (1 + z), that the evolution parameter is zero within statistical errors:

wa = -0.12 +/- 0.4.

This is a powerful test of dark energy’s existence, equation of state, and evolution, using hundreds of X-ray clusters of galaxies. There is no evidence for evolution in dark energy with redshift back to around z = 1, and a simple cosmological constant model is supported by the data from this technique as well as from other methods.

References:

  1. Morandi, M. Sun arXiv:1601.03741v3 [astro-ph.CO] 4 Feb 2016, “Probing dark energy via galaxy cluster outskirts”
  2. http://chandra.harvard.edu/photo/2016/clusters/
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X-raying Dark Matter

I was at the dentist this week. Don’t ask, but they took 3 digital X-Rays.

One of the most significant methods by which we detect the presence of dark matter is through the use of X-ray telescopes. The energy associated with these X-rays is typically around an order of magnitude less than those zapped into your mouth when you visit the dentist.

Around 50 years ago scientists at American Science and Engineering flew the first imaging X-ray telescope on a small rocket. At a later date, I worked part-time at AS&E, as we called it, while in graduate school. One major project was a solar X-Ray telescope mounted in SkyLab, America’s first space station. This gave me the wonderful opportunity to work in the control rooms at the NASA Johnson Space Center in Houston.

X-rays are absorbed in the Earth’s atmosphere, so today X-ray astronomy is performed from orbiting satellites. X-ray telescopes use the principle of grazing incidence reflection; the X-rays impinge at shallow angles onto gold or iridium-coated metallic surfaces and are reflected to the focal plane and the detector electronics.

cxcmirrors-72

Schematic of grazing incidence mirrors used in the Chandra X-ray Observatory. Credit NASA/CXC/SAO; obtained from chandra.harvard.edu.

How does dark matter result in X-rays being produced? Indirectly, as a consequence of its gravitational effects.

One of the main mechanisms for X-ray production in the universe is known as thermal bremsstrahlung. Bremsstrahlung is a German word meaning ‘decelerated radiation’. A gas which is hot enough to give off X-rays will be ionized. That is, the electrons will be stripped from the nuclei and move about freely. As electrons fly around near ions (protons and helium nuclei primarily) their mutual electromagnetic attraction will result in some of the electrons’ kinetic energy being transferred to radiation.

The speed at which the electrons are moving around determines how energetic the produced photons will be. We talk about the temperature of such an ionized gas, and that is proportional to the square of the average speed of the electrons. A gas with a temperature of 10 million degrees will give off approximately 1 kiloVolt X-rays (hereafter we use the KeV abbreviation), and a gas with a temperature of 100 million degrees will radiate 10 KeV X-rays. One eV converts to 11,605 degrees Kelvin (or we can just say Kelvins).

ChandrainColumbiabay

Chandra X-ray Observatory prior to launch in the Space Shuttle Columbia in 1999. NASA image.

So how can we produce gas hot enough to give off X-Rays by this mechanism? Gravity, and lots of it. The potential energy of the gravitational field is proportional to the amount of matter (total mass) coalesced into a region and inversely proportional to the characteristic scale of that region. GM/R, simple Newtonian mechanics, is sufficient; no general relativistic calculation is needed at this point. G is the gravitational constant and M and R are the cluster mass and characteristic radius, respectively.

A lot of mass in a confined region – how about large groups of clusters and galaxies? It turns out we need of order 1000 galaxies for a rich cluster and this will do the trick. But only because there is dark matter as well as ordinary matter. There are three main matter components to consider: galaxies, hot intracluster gas found between galaxies, and dark matter. The cluster forms from gravitational self-collapse from a region that was of above average density in the early universe. All the over dense regions are subject to collapse.

darkmatter.bulletcluster

The “Bullet Cluster” is actually two colliding clusters. The bluish color shows the distribution of dark matter as determined from the gravitational lensing effect on background galaxy images. The reddish color depicts the hot X-ray emitting gas measured by the Chandra X-ray Observatory.

(X-ray: NASA/CXC/CfA/M.Markevitch Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/D.Clowe)

The optically visible galaxies are the least important contributor to the cluster mass, only around 1%! Galaxy clusters are made of dark matter much more than they are made out of galaxies. And secondarily, they are made out of hot gas. The ordinary matter contained within galaxies is only the third most important component. The table below gives the typical 90 / 9 / 1 proportions for dark matter, hot gas, and galaxies, respectively.

Three main components of a galaxy cluster (Table derived from Wikipedia article on galaxy clusters)

Component                      Mass fraction             Description

Galaxies                           1%                         Optical/infrared observations

Intergalactic gas              9%                         High temperature ionized gas – thermal bremsstrahlung

Dark matter                     90%                        Dominates, inferred through gravitational interactions

The intracluster gas has two sources. A major portion of it is primordial gas that never formed galaxies, but falls into the gravitational potential well of the cluster. As it falls in toward the cluster center, it heats. The kinetic energy of infall is converted to random motions of the ionized gas. An additional portion of the gas is recycled material expelled from galaxies. It mixes with the primordial gas and heats up as well through frictional processes. The gas is supported against further collapse by its own pressure as the density and temperature increase in the cluster core.

The temperature which characterizes the X-ray emission is a measure of gravitational potential strength and proportional to the ratio of the mass of the cluster to its size. Typical X-Ray temperatures measured for rich clusters are around 3 to 12 KeV, which corresponds to temperatures in the range of 30 to 130 million Kelvins.

There is another way to measure the strength of the cluster’s gravitational potential well. That is by measuring the speed of galaxies as they move around in somewhat random fashion inside the cluster. The assumption, which is valid for well-formed clusters after they have been around for billions of years, is that the galaxies are not just falling into the center of the cluster, but that their motions are “virialized”. This is the method used by Fritz Zwicky in the 1930s for the original discovery of dark matter. He found that in a certain well known cluster, the Coma cluster, that the average speed of galaxies relative to the cluster centroid was of order 1000 kilometers/sec, much higher than the expected 300 km/sec based on the visible light from the cluster galaxies. This implied 10 times as much dark matter as galactic matter. This early, rather crude measurement, was on the right track, but fell short of the actual ratio of dark matter to galactic matter since we now know that galaxies themselves have large dark matter halos. The X-ray emission from clusters was discovered much later, starting in the 1970s.

The two methods of measuring the amount of dark matter in Galaxy clusters generally agree. Both the galaxies and the hot intracluster gas are acting as tracers of the overall mass distribution, which is dominated by dark matter. Galaxy clusters play a major role in increasing our understanding of dark matter and how it affects the formation and evolution of galaxies.

In fact if dark matter was not 5 times as abundant by mass as ordinary matter, most galaxy clusters would never have formed, and galaxies such as our own Milk Way would be much smaller.

References

Wikipedia article “galaxy clusters”.

“X-ray Temperatures of Distant Clusters of Galaxies”, S. C. Perrenod,  J. P. Henry 1981, Astrophysical Journal, Letters to the Editor, vol. 247, p. L1-L4.

“The X-ray Luminosity – Velocity Dispersion Relation in the REFLEX Cluster Survey”, A. Ortiz-Gil, L. Guzzo, P. Schuecker, H. Boehringer, C.A. Collins 2004, Mon.Not.Roy.Astron.Soc. 348, 325; http://arxiv.org/abs/astro-ph/0311120v1