Archive for ‘Galaxy clusters’

March 5, 2012

Puzzle remains around dark core in cosmic collision

Galaxy clusters, like the universe as a whole, are composed of both baryonic (“ordinary”) matter and dark matter, with about 1 part in 6 of the former, and five parts in 6 of the latter. A curious thing happens – usually – when two clusters collide. Both the visible baryonic matter, consisting mainly of luminous galaxies, and the dark matter, continue on their way after the collision. They show almost no signs of the encounter, although they may be fated, ultimately, to merge.

However, there appears to be substantial interaction of the intergalactic gas that existed before the encounter in both clusters. As a result, the gas loses most of its momentum and is left behind, between the two departing clusters, after the collision. This gas is very hot, and is easily detected at X-ray wavelengths. The best known example of this phenomenon is the Bullet Cluster, about which research was reported in 2006. (News articles: here, here, here, here; Research: here).

Yet now there is confirmation of a galaxy cluster collision – Abell 520 (A520) – in which a large quantity of dark matter appears to have been left behind. This is surprising, since the main reason given to explain why this should not happen is that dark matter is expected to interact very little with itself. The magnitude of this interaction is expressed as the ratio of the interaction “cross section” to mass. In the Bullet Cluster, the upper limit for this was found to be at most 1 cm2g-1. But in A520 this same measure is estimated at ~3.8 cm2g-1.

A520 seems to be the exceptional case, rather than the the Bullet Cluster, since results similar to the latter have been found in other cluster collisions, such as MACS J0025.4-1222 (News article: here; Research: here), Abell 2744 (News article: here; Research: here), and DLSCL J0916.2+2951.

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January 15, 2012

Primordial galaxy cluster is earliest ever seen

At what point did stars, galaxies, and galaxy clusters make their first appearance in the universe? There’s now good evidence that galaxy clusters were starting to form about 650 million years after the big bang. So galaxies must have begun forming earlier than that, and the first stars even earlier.

The evidence consists of the discovery that a known bright galaxy at z≈8 has 4 dimmer companions within a radius of about 10 million light-years. The known galaxy was originally found using the Wide Field Camera 3 (WFC3) of the Hubble Space Telescope, by means of the Lyman-break technique. The was done as part of a search for z≈8 galaxies, known as the Brightest of Reionizing Galaxies (BoRG) survey. The region in which the bright galaxy was found was designated BoRG58.

The Lyman-break technique is based on the fact that photons having wavelengths less than the Lyman-α length of 121.6 nm (medium ultraviolet) are easily absorbed by neutral hydrogen gas. Consequently, most of the light from hot young stars is extinguished by neutral hydrogen, since such stars are brightest at wavelengths shorter than Lyman-α. The wavelength of Lyman-α photons emitted at z≈8 is stretched by a factor of z+1 to 1094 nm, in the infrared range. So in order to identify bright galaxies at z≈8, images are made through a series of filters that pass photons on either side of 1094 nm. Objects that seem to disappear in images made through filters that pass only light with wavelengths below 1094 nm are probably at the desired redshift.

Four bright objects presumed to be at z≈8 were identified early in the survey process. Computer simulations have indicated that at that redshift the most massive galaxies (which are presumably the brightest) should have a number of less massive, less luminous companions. So the search was extended around each of the four candidates, using longer exposure times to detect fainter objects. 17 less luminous objects were detected with substantial confidence. Field BoRG58, which had the best original candidate, turned out to have 4 less luminous companions of the candidate at the best level of confidence.

It hasn’t yet been possible to obtain direct spectroscopic evidence for the actual redshift of the detected objects. This is probably because so much of the available light from those objects has been extinguished by neutral hydrogen. However, the detection of fainter z≈8 dropout objects near the original candidate provides additional evidence that the objects are not spurious dropouts (which can occur with low probability).

The computer simulations suggest that the halo mass (which includes dark matter) around the brightest object should be in the range 400 to 700 billion M. (The Milky Way’s halo is estimated to be more than 1 trillion M.) There should also be additional smaller halos nearby about 100 billion M, hosting the fainter dropouts. By the present time the system should have grown into a cluster with halo mass 200 trillion M, a fairly typical cluster size.

Further reading:

Hubble Pinpoints Furthest Protocluster of Galaxies Ever Seen

CU-led study pinpoints farthest developing galaxy cluster ever found

CU-Boulder finds farthest galaxy cluster ever seen

Hubble spies earliest galaxy cluster ever seen

Hubble shows images from record-breaking 13.1 billion light-years

Overdensities of Y-dropout Galaxies from the Brightest-of-Reionizing Galaxies Survey: A Candidate Protocluster at Redshift z≈8

January 15, 2012

Two galaxy clusters in an advanced stage of collision

DLSCL J0916.2+2951 is a recently discovered pair of galaxy clusters in an advanced stage of collision. Its redshift is z=0.53, corresponding to a distance of 5.2 billion light-years. The two clusters began to collide about 700 million years previously, making this the most advanced cluster collision discovered – from 2 to 5 times farther along than previous discoveries (of which there have been only 4 that are comparable). The 2 clusters together span a distance of about 6 million light-years.

The clusters have already passed through each other, though they will eventually merge completely. The galaxies and dark matter within each cluster were relatively unaffected by the encounter. However, the intergalactic gas contained in one cluster has interacted strongly with that of the other. Consequently, much of the gas has been separated from its original cluster. It now lies between the two original clusters and has a temperature around 6 million K – 1000 times hotter than the surface of the Sun. As a result, the gas is a strong emitter of X-rays whose photons have energies in the 0.5 to 2.0 keV range.

The amount of dark matter within each of the colliding clusters can be estimated by the technique of weak gravitational lensing. This is based on the fact that a very massive object, such as a galaxy or cluster of galaxies, bends light from objects on the far side of the massive object along the line of sight. The concentration of mass distorts the shape of the more distant objects – turning a circle into an ellipse, for example. A statistical analysis of the observed shapes, compared to what would be seen in the absence of the gravitational lens, yields an estimate of the amount of mass in the lens.

The total mass of one cluster is estimated to be about 2×1014 M and the other is 3×1014 M – a total of about 500 trillion solar masses for the two clusters together. Of that, 86% is dark matter, 12% very hot gas, and 2% visible stars. This ratio of dark matter to ordinary matter is close to the average for the universe as a whole – so this is a further confirmation of the abundance of dark matter.

Since the dark matter halos of the two original clusters have mostly passed through each other without combining the way that the intergalactic gas did, it has been possible to estimate that the probability of self-interaction between dark matter particles is relatively small. This “cross section” value can be used in simulations of how the universe has evolved on a large scale.

Further reading:

When galaxy clusters collide

Discovery of a Dissociative Galaxy Cluster Merger with Large Physical Separation

October 5, 2011

Galaxy Clusters Validate Einstein’s Theory – and Dark Matter Too

The theory of relativity – both special relativity and general relativity – has now been tested successfully so many ways ever since its early days that further tests seem almost redundant. (That little mix-up with superluminal neutrinos to the contrary notwithstanding – since it’s at most a higher order correction, but more likely an error in data interpretation.) But science never stops testing if there’s some creative new way to do so. And that’s what’s involved here.

One of the predictions of general relativity is that light photons lose energy when climbing out of a strong gravitational field. In that case, the light should be redshifted, in a way that doesn’t involve either a Doppler effect or the expansion of the universe.

It takes a very strong gravitational field to make this gravitational redshift apparent. Even a single galaxy isn’t large enough. But a cluster of galaxies will do, although the effect is almost masked by relative motion of galaxies in the cluster. However, in a cluster there’s a fairly straightforward way to check, since the shift should be larger with photons from the center of the cluster than from the periphery, as the gravitational field is stronger at the center.

Galaxy Clusters Validate Einstein’s Theory – ScienceNOW

Wojtak and his colleagues knew that measuring gravitational redshifting within a single galaxy cluster would be difficult because the effect is very small and needs to be teased apart from the redshifting caused by the orbital velocity of individual galaxies within the cluster and the redshifting caused by the expansion of the universe. The researchers approached the problem by averaging data collected from 8000 galaxy clusters by the Sloan Digital Sky Survey. The hope was to detect gravitational redshift “by studying the properties of the redshift distribution of galaxies in clusters rather than by looking at redshifts of individual galaxies separately,” Wojtak explains.

Sure enough, the researchers found that the light from the clusters was redshifted in proportion to the distance from the center of the cluster, as predicted by general relativity. “We could measure small differences in the redshift of the galaxies and see that the light from galaxies in the middle of a cluster had to ‘crawl’ out through the gravitational field, while it was easier for the light from the outlying galaxies to emerge,” Wojtak says.

In addition to further confirming relativity, this research may also rule out some alternative theories of gravity that have been proposed to avoid the hypotheses of dark matter and dark energy, which are needed with standard relativity to explain many astronomical observations – and do so very well.

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