HubbleSite: Dark Matter Core Defies Explanation

[bystander]

Dark Matter Core Defies Explanation
NASA | STScI | HubbleSite | 2012 Mar 02

Astronomers observed what appeared to be a clump of dark matter left behind during a bizarre wreck between massive clusters of galaxies. The dark matter collected into a "dark core" containing far fewer galaxies than would be expected if the dark matter and galaxies hung together. Most of the galaxies apparently have sailed far away from the collision. This result could present a challenge to basic theories of dark matter, which predict that galaxies should be anchored to the invisible substance, even during the shock of a collision.

The initial observations, made in 2007, were so unusual that astronomers shrugged them off as unreal, due to poor data. However, new results obtained in 2008 from NASA's Hubble Space Telescope confirm that dark matter and galaxies parted ways in the gigantic merging galaxy cluster called Abell 520, located 2.4 billion light-years away. Now, astronomers are left with the challenge of trying to explain dark matter's seemingly oddball behavior in this cluster.

Merging Galaxy Cluster Abell 520

This composite image shows the distribution of dark matter, galaxies, and hot gas in the core of the merging galaxy cluster Abell 520, formed from a violent collision of massive galaxy clusters.

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The natural-color image of the galaxies was taken with NASA's Hubble Space Telescope and with the Canada-France-Hawaii Telescope in Hawaii.

Superimposed on the image are "false-colored" maps showing the concentration of starlight, hot gas, and dark matter in the cluster. Starlight from galaxies, derived from observations by the Canada-France-Hawaii Telescope, is colored orange. The green-tinted regions show hot gas, as detected by NASA's Chandra X-ray Observatory. The gas is evidence that a collision took place. The blue-colored areas pinpoint the location of most of the mass in the cluster, which is dominated by dark matter. Dark matter is an invisible substance that makes up most of the universe's mass. The dark-matter map was derived from the Hubble Wide Field Planetary Camera 2 observations by detecting how light from distant objects is distorted by the cluster of galaxies, an effect called gravitational lensing.

The blend of blue and green in the center of the image reveals that a clump of dark matter resides near most of the hot gas, where very few galaxies are found. This finding confirms previous observations of a dark-matter core in the cluster. The result could present a challenge to basic theories of dark matter, which predict that galaxies should be anchored to dark matter, even during the shock of a collision.

Abell 520 resides 2.4 billion light-years away.

Credit: NASA, ESA, CFHT, CXO, M.J. Jee (University of California, Davis), and A. Mahdavi (San Francisco State University)

It was the result no one wanted to believe. Astronomers observed what appeared to be a clump of dark matter left behind during a bizarre wreck between massive clusters of galaxies.

The dark matter collected into a "dark core" containing far fewer galaxies than would be expected if the dark matter and galaxies hung together. Most of the galaxies apparently have sailed far away from the collision. This result could present a challenge to basic theories of dark matter, which predict that galaxies should be anchored to the invisible substance, even during the shock of a collision.

The initial observations, made in 2007, were so unusual that astronomers shrugged them off as unreal, due to poor data. However, new results from NASA's Hubble Space Telescope confirm that dark matter and galaxies parted ways in the gigantic merging galaxy cluster called Abell 520, located 2.4 billion light-years away.

Now, astronomers are left with the challenge of trying to explain dark matter's seemingly oddball behavior in this cluster.

"This result is a puzzle," said astronomer James Jee of the University of California, Davis, leader of the Hubble study. "Dark matter is not behaving as predicted, and it's not obviously clear what is going on. Theories of galaxy formation and dark matter must explain what we are seeing."

A paper reporting the team's results has been accepted for publication in The Astrophysical Journal and is available online.

First detected about 80 years ago, dark matter is thought to be the gravitational "glue" that holds galaxies together. The mysterious invisible substance is not made of the same kind of matter that makes up stars, planets, and people. Astronomers know little about dark matter, yet it accounts for most of the universe's mass.

They have deduced dark matter's existence by observing its ghostly gravitational influence on normal matter. It's like hearing the music but not seeing the band.

One way to study dark matter is by analyzing smashups between galaxy clusters, the largest structures in the universe. When galaxy clusters collide, astronomers expect galaxies to tag along with the dark matter, like a dog on a leash. Clouds of intergalactic gas, however, plow into one another, slow down, and lag behind the impact.

That theory was supported by visible-light and X-ray observations of a colossal collision between two galaxy clusters called the Bullet Cluster. The galactic grouping has become a textbook example of how dark matter should behave.

But studies of Abell 520 showed that dark matter's behavior may not be so simple. The original observations found that the system's core was rich in dark matter and hot gas but contained no luminous galaxies, which normally would be seen in the same location as the dark matter. NASA's Chandra X-ray Observatory detected the hot gas. Astronomers used the Canada-France-Hawaii and Subaru telescopes atop Mauna Kea to infer the location of dark matter by measuring how the mysterious substance bends light from more distant background galaxies, an effect called gravitational lensing.

The astronomers then turned Hubble's Wide Field Planetary Camera 2 to help bail them out of this cosmic conundrum. Instead, to their chagrin, the Hubble observations helped confirm the earlier findings. Astronomers used Hubble to map the dark matter in the cluster through the gravitational lensing technique.

"Observations like those of Abell 520 are humbling in the sense that in spite of all the leaps and bounds in our understanding, every now and then, we are stopped cold," explained Arif Babul of the University of Victoria in British Columbia, the team's senior theorist.

Is Abell 520 an oddball, or is the prevailing picture of dark matter flawed? Jee thinks it's too soon to tell.

"We know of maybe six examples of high-speed galaxy cluster collisions where the dark matter has been mapped," Jee said. "But the Bullet Cluster and Abell 520 are the two that show the clearest evidence of recent mergers, and they are inconsistent with each other. No single theory explains the different behavior of dark matter in those two collisions. We need more examples."

The team has proposed a half-dozen explanations for the findings, but each is unsettling for astronomers. "It's pick your poison," said team member Andisheh Mahdavi of San Francisco State University in California, who led the original Abell 520 observations in 2007. One possible explanation for the discrepancy is that Abell 520 was a more complicated interaction than the Bullet Cluster encounter. Abell 520 may have formed from a collision between three galaxy clusters, instead of just two colliding systems in the case of the Bullet Cluster.

Another scenario is that some dark matter may be what astronomers call "sticky." Like two snowballs smashing together, normal matter slams into each other during a collision and slows down. But dark matter blobs are thought to pass through each other during an encounter without slowing down. This scenario proposes that some dark matter interacts with itself and stays behind when galaxy clusters collide.

A third possibility is that the core contained many galaxies, but they were too dim to be seen, even by Hubble. Those galaxies would have to have formed dramatically fewer stars than other normal galaxies. Armed with the Hubble data, the group hopes to create a computer simulation to try to reconstruct the collision, hoping that it yields some answers to dark matter's weird behavior.

Abell 520: Dark Matter and Galaxies Part Ways in Collision between Hefty Galaxy Clusters
NASA | SAO | Chandra X-ray Observatory | 2012 Mar 02

This composite image shows the distribution of dark matter, galaxies, and hot gas in the core of the merging galaxy cluster Abell 520, formed from a violent collision of massive galaxy clusters that is located about 2.4 billion light years from Earth.

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Data from NASA's Chandra X-ray Observatory show the hot gas in the colliding clusters colored in green. The gas provides evidence that a collision took place. Optical data from NASA's Hubble Space Telescope and the Canada-France-Hawaii Telescope (CFHT) in Hawaii are shown in red, green, and blue. Starlight from galaxies within the clusters, derived from observations by the CFHT and smoothed to show the location of most of the galaxies, is colored orange.

The blue-colored areas pinpoint the location of most of the mass in the cluster, which is dominated by dark matter. Dark matter is an invisible substance that makes up most of the universe's mass. The dark-matter map was derived from the Hubble observations, by detecting how light from distant objects is distorted by the cluster galaxies, an effect called gravitational lensing. The blend of blue and green in the center of the image reveals that a clump of dark matter (which can be seen by mousing over the image) resides near most of the hot gas, where very few galaxies are found.

This finding confirms previous observations of a dark-matter core in the cluster announced in 2007. The result could present a challenge to basic theories of dark matter, which predict that galaxies should be anchored to dark matter, even during the shock of a powerful collision.

Optical (Red, Green, Blue); X-ray (Green); Mass (Blue); Luminosity (Orange)

A Study of the Dark Core in A520 with the Hubble Space Telescope: The Mystery Deepens - M. J. Jee et al

• Astrophysical Journal 747(2) 96 (2012 Mar 10) DOI: 10.1088/0004-637X/747/2/96
  arXiv.org > astro-ph > arXiv:1202.6368 > 28 Feb 2012

A Dark Core in Abell 520 - A. Mahdavi et al

• Astrophysical Journal 668(2) 806 ((2007 Oct 20) DOI: 10.1086/521383
  arXiv.org > astro-ph > arXiv:0706.3048 > 21 Jun 2007

Dark Matter Core Defies Explanation
Canada France Hawaii Telescope | 2012 Mar 02

Dark matter's odd behavior baffles astronomers
University of California, Davis | 2012 Mar 02

Cosmic Mystery Deepens
University of Victoria | 2012 Mar 02

Mystery deepens around dark core in cosmic collision
San Francisco State University | 2012 Mar 02

Hubble Spots Mysterious Dark Matter ‘Core’
Universe Today | Ray Sanders | 2012 Mar 02
http://asterisk.apod.com/viewtopic.php?t=27610#p170932

[bystander]

Journal Club – Aberrant Dark Matter
Universe Today | Steve Nerlich | 2012 Mar 10

Today’s journal article is about dark matter being in the wrong place at the wrong time.

So, some might remember the Bullet Cluster – a seemingly clinching proof of dark matter, where two galactic clusters had collided in the past and what we see post-collision is that most of the mass of each cluster has passed straight through and out the other side. The only material remaining at the collision site is a huge jumbled clump of intergalactic gas.

This means that each galactic cluster, that has since moved on, has been stripped of much of its intergalactic gas. But lo and behold the seemingly empty intergalactic space within each of these stripped galactic clusters continues to distort the background field of view (a phenomenon known as weak gravitational lensing).

This seemed strong proof that the intergalactic spaces of each cluster must be filled with gravitationally-inducing, but otherwise invisible, stuff. In other words, dark matter. It makes sense that this dark matter would have moved straight on through the collision site because it is weakly interacting – whereas the gas caught up in the collision was not.

[attachment=0]arXiv.1202.6368.figure.4.jpg[/attachment][/i]

So, a cool finding and almost identical findings were discovered within the cluster collisions MACS J0025.4-1222, Abell 2744 and a couple of others. But now along comes Abell 520 with a completely counter example. Two or more galaxy clusters have collided, most of the visible contents have passed straight through, but back at the collision point is an apparent big clump of invisible stuff creating weak gravitational lensing – i.e. dark matter. It is the region labelled 3 on figure 4 at page 5 of the article.

This finding requires us to consider that we had naively concluded that the Bullet Cluster’s post-collision appearance was easily interpretable and that its outcome would surely be repeated in any equivalent collision of galaxy clusters.

But in the wake of Abell 520 we now may need to consider that the outcome of a collision between rapidly moving and utterly gargantuan collections of mass is much more complex and unpredictable than we had initially assumed. This doesn’t mean that the dark matter hypothesis has been debunked, it just means that the Bullet Cluster might not have been the clinching proof that we thought it was.

If we subsequently find fifty new Bullet cluster analogues and no more Abell 520 analogues, we might then assume that Abell 520 is just a weird outlier, which can be dismissed as an unrepresentative anomaly. But with only five or six such collision types known, one of which is Abell 520 – we can’t really call it an outlier at the moment.

So… comments? The authors offers six possible scenarios to explain this finding – got a seventh? Did we jump to conclusions with the Bullet Cluster?

<< Previous Journal Club

[geckzilla]

Bullet cluster, maybe, bitches.

[bystander]

[bystander]

‘Dark Core’ May Not Be So Dark After All
NASA | ESA | STScI | HubbleSite | University of Ohio | 2012 Nov 29

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These composite images, taken by two different teams using the Hubble Space Telescope, show different results concerning the amount of dark matter in the core of the merging galaxy cluster Abell 520.

Dark matter is an invisible form of matter that astronomers deduce is the underlying gravitational "glue" that holds galaxies together.

Top Image: Observations of the cluster, taken by D. Clowe with the Advanced Camera for Surveys, map the amount of dark matter in Abell 520. The map reveals an amount of dark matter astronomers expect based on the number of galaxies in the core. The dark-matter densities are marked in blue, and the dotted circle marks the dark-matter core. The map is superimposed onto visible-light images of the cluster.

Bottom Image: A second team, led by James Jee of the University of California, Davis, used the Wide Field Planetary Camera 2 and found an unusual overabundance of dark matter in the cluster's core, denoted by the bright blue color at image center. The observation was surprising because astronomers expect that dark matter and galaxies should be anchored together, even during a collision between galaxy clusters.

This discrepancy between the two results requires further observation and analysis, say researchers.

The two dark-matter maps were made by detecting how light from distant objects is distorted by the galaxy clusters, an effect called gravitational lensing.

Abell 520 is located 2.4 billion light-years away.

Credits: NASA/ESA; [top] D. Clowe (Ohio University), HST ACS;
[bottom] J. Jee (University of California, Davis), HST WFPC2

Astronomers were puzzled earlier this year when NASA’s Hubble Space Telescope spotted an overabundance of dark matter in the heart of the merging galaxy cluster Abell 520. This observation was surprising because dark matter and galaxies should be anchored together, even during a collision between galaxy clusters.

Astronomers have abundant evidence that an as-yet-unidentified form of matter is responsible for 90 percent of the gravity within galaxies and clusters of galaxies. Because it is detected via its gravity and not its light, they call it “dark matter.”

Now, a new observation of Abell 520 from another team of astronomers using a different Hubble camera finds that the core does not appear to be over-dense in dark matter after all. The study findings were published in The Astrophysical Journal.

“The earlier result presented a mystery. In our observations we didn’t see anything surprising in the core,” said study leader Douglas Clowe, an associate professor of physics and astronomy at Ohio University. “Our measurements are in complete agreement with how we would expect dark matter to behave.”

Hubble observations announced earlier this year by astronomers using Hubble’s Wide Field Planetary Camera 2 suggested that a clump of dark matter was left behind during a clash between massive galaxies clusters in Abell 520, located 2.4 billion light-years away. The dark matter collected into a “dark core” that contained far fewer galaxies than would be expected if the dark and luminous matter were closely connected, which is generally found to be the case.

Because dark matter is not visible, its presence and distribution is found indirectly through its gravitational effects. The gravity from both dark and luminous matter warps space, bending and distorting light from galaxies and clusters behind it like a giant magnifying glass. Astronomers can use this effect, called gravitational lensing, to infer the presence of dark matter in massive galaxy clusters. Both teams used this technique to map the dark matter in the merging cluster.

Clowe’s team used Hubble’s Advanced Camera for Surveys (ACS) to measure the amount of dark matter in the cluster. ACS observed the cluster in three colors, allowing the astronomers to distinguish foreground and background galaxies from the galaxies in the cluster. From this observation, the team made an extremely accurate map of the cluster’s dark matter. “With the colors we got a more precise selection of galaxies,” Clowe said.

The astronomers estimated the amount of dark matter in the cluster by measuring the amount of gravitational “shear” in the Hubble images. Shear is the warping and stretching of galaxies by the gravity of dark matter. More warping indicates the presence of more gravity than is inferred from the presence of luminous matter, therefore requiring the presence of dark matter to explain the observation. “The WFPC2 observation could have introduced anomalous shear and not a measure of the dark matter distribution,” Clowe explained.

Using the new camera, Clowe’s team measured less shear in the cluster’s core than was previously found. In the study the ratio of dark matter to normal matter, in the form of stars and gas, is 2.5 to 1, which is what astronomers expected. The earlier WFPC2 observation, however, showed a 6-to-1 ratio of dark matter to normal matter, which challenged theories of how dark matter behaves.

“This result also shows that as you improve Hubble’s capabilities with newer cameras, you can take a second look at an object,” Clowe said.

His team is encouraging other scientists to study its data and conduct their own analysis on the cluster.

Clowe’s co-authors on the paper are Maxim Markevitch of NASA Goddard Space Flight Center, Greenbelt, Md.; Marusa Bradac of the University of California, Davis; Anthony H. Gonzalez and Sun Mi Chung of the University of Florida, Gainesville; Richard Massey of Durham University, Durham, England; and Dennis Zaritsky of the University of Arizona’s Steward Observatory, Tucson, Ariz.

On Dark Peaks and Missing Mass: A Weak-lensing Mass Reconstruction of the Merging Cluster System A520 - Douglas Clowe et al

• Astrophysical Journal 758(2) 128 (2012 Oct 20) DOI: 10.1088/0004-637X/758/2/128
  arXiv.org > astro-ph > arXiv:1209.2143 > 10 Sep 2012