Galaxy Cluster Collision Creates a Dark Matter Core

2007-0816collision.thumbnail.jpg

This strange photograph is a composite image of Abell 520, a massive cluster of galaxies in the process of colliding with one another – it’s one of the most massive structures in the Universe. Several different instruments and observatories came together to produce the image, and the final result gave astronomers a big mystery: its dark matter is behaving strangely.

When galaxies collide, three ingredients come into play: individual galaxies and their billions of stars, hot gas in between the galaxies, and the mysterious dark matter that actually makes up the bulk of the mass. Optical telescopes can see the light from the stars in the galaxies, and X-ray observatories, like Chandra can see the radiation pouring out of the superheated gas. But the presence of dark matter has to be calculated by the way it warps light from more distant objects.

During gigantic collisions like this, astronomers believed that the dark matter and galaxies should stay together, even during the most violent collisions. And this was seen in another galaxy collision: the so-called Bullet Cluster. But in the collision of Abell 520, something surprising was seen.

They found a dark matter core, containing hot gas, but no galaxies. For some reason, the galaxies were stripped away from the densest part of the dark matter. Here’s what Dr. Hendrik Hoekstra, from the University of Victoria had to say:

“It blew us away that it looks like the galaxies are removed from the densest core of dark matter. This would be the first time we’ve seen such a thing and could be a huge test of our knowledge of how dark matter behaves.”

In addition to this core, they also found a corresponding “light region”, which had galaxies, but little or no dark matter. Somehow this collision separated the dark matter from the regular matter.

So what could have stripped these two apart? One possibility is that the galaxies and dark matter were torn apart by a series of gravitational slingshots. Unfortunately, the researchers weren’t able to come up with a realistic computer simulation that had gravitational interactions powerful enough to do this.

Here’s the stranger possibility: we know that dark matter is affected by gravity, but maybe there’s also some kind of unknown interaction between particles of dark matter. This would be extremely difficult to detect since we can’t even see the stuff.

The astronomers have secured time with the Hubble Space Telescope, and will come back and take another look with its powerful gaze. This should help answer some of the mysteries they’ve unearthed.

Original Source: Chandra News Release

No Stars Shine in This Dark Galaxy

2007-0613darkgalaxy1.thumbnail.jpg

An international team of astronomers have conclusive new evidence that a recently discovered “dark galaxy” is, in fact, an object the size of a galaxy, made entirely of dark matter. Although the object, named VIRGOHI21, has been observed since 2000, astronomers have been slowly ruling out every alternative explanation.

In a new research paper, entitled 21-cm synthesis observations of VIRGOHI 21 – a possible dark galaxy in the Virgo Cluster, researchers provide updated evidence about this mysterious galaxy.

They have now performed a high resolution observations of VIRGOHI21 using the Westerbork Synthesis Radio Telescope (WSRT), to better pin down the quantities of neutral hydrogen gas. They also did followup observations with the Hubble Space Telescope, looking for any evidence of stars.

Astronomers first suspected there was an invisible galaxy out there when they spied galaxy NGC 4254. This unusual-looking galaxy appears to be one partner in a cosmic collision. All the normal evidence is there: gas is being siphoned away into a tenuous stream, and one of its spiral arms is being stretched out.

But the other partner in this collision is nowhere to be seen.

The researchers’ calculated that an object with 100 billion solar masses must have careened past NGC 4254 within the last 100 million years, creating the gas stream, and tearing at one of its arms. This was the clue that an invisible dark matter galaxy might be lurking nearby.

A detailed search turned up a mysterious object called VIRGOHI21, located about 50 million light-years from Earth. Were it a normal galaxy, you would be able to see it in a powerful amateur telescope. But there’s nothing there. Even in the Hubble Space Telescope, not a single star is shining from this massive region of space.

It was only visible in radio telescopes, which could detect the radio emissions from neutral hydrogen gas located in the cloud.

When they first published their research a few years ago, the astronomy community was understandably skeptical, and proposed several alternative theories to explain the mysterious object.

For example, there could be additional mass associated with VIRGOHI21, and not just dark matter. The discovery of red giant stars in the region would give some indication that this was a more normal interaction. But Hubble turned up nothing.

Dr. Robert Minchin, lead researcher from the Arecibo Observatory, said, “not even the power of Hubble has been able to see any stars in it.”

It’s possible that VIRGOHI21 has always been this way, formed from primordial dark matter and neutral hydrogen after the Big Bang. It’s been cruising the Universe ever since, disrupting galaxies as it goes.

However, there do seem to be ways that galaxies and their dark matter can be separated. Only a few months ago, a ring of dark matter was found surrounding a group of colliding galaxy clusters by the Hubble Space Telescope. Perhaps VIRGOHI21 is the wreckage from one of these cluster collisions; a shred of dark matter hurled out into space.

It could be that there are many of these dark galaxies out there. A new sky survey, carried out with the 305-metre (1000-foot) Aricebo radio telescope in Puerto Rico should tease out more of these objects in the future. The survey is called the Arecibo Galaxy Environment Survey (AGES).

This most recent paper has been accepted for publication in the Astrophysical Journal.

Dark Matter Annihilation at the Centre of the Milky Way

2007-0529milkyway.thumbnail.jpg

Most of the Universe is a complete and total mystery. And one of these mysteries is dark matter. It’s out there, and astronomers are slowly teasing out its characteristics, but it’s not giving up its secrets easily. The problem is, dark matter only interacts with regular matter through gravity (and maybe through the weak nuclear force). It doesn’t shine, it doesn’t give off heat or radio waves, and it passes through regular matter like it isn’t there. But when dark matter is destroyed, it might give astronomers the clues they’re looking for.

Researchers have theorized that one productive way to search for dark matter might not be to search for it directly, but to look for the resulting particles and energy which are emitted when it’s destroyed. In the environment around the centre of our galaxy, dark matter might be dense enough that particles regularly collide, releasing a cascade of energy and additional particles; which could be detected.

And this theory could help account for a strange result gathered by the Wilkinson Microwave Anisotropy Probe (WMAP), a NASA spacecraft which is mapping the temperature of the Cosmic Microwave Background Radiation (CMBR). This background radiation was supposed to be roughly even across the entire sky. But for some reason, the satellite turned up an excess of microwave emission around the centre of our galaxy.

Perhaps this microwave radiation is the glow of all that dark matter getting annihilated.

This conclusion was reached by a team of US astronomers: Dan Hooper, Douglas P. Finkbeiner and Gregory Dobler. Their work is published in a new research paper called Evidence Of Dark Matter Annihilations In The WMAP Haze.

The excess microwave radiation around our galactic centre is known as the WMAP Haze, and was originally thought to be the emissions from hot gas. Astronomers set about trying to confirm this theory, but observations in other wavelengths failed to turn up any evidence.

According to the researchers, the microwave haze could be explained by annihilating particles of dark matter, like the interaction between matter and antimatter. As dark matter particles collide they could give off any number of detectable particles and radiation, including gamma-rays, electrons, positrons, protons, antiprotons and neutrinos.

The size, shape and distribution of the haze matches the central region of our galaxy which should also have a high concentration of dark matter. And if the dark matter particles are within a certain range of mass – 100 to 1000s of times the mass of a proton – they could release a torrent of electrons and positrons that nicely match the microwave haze.

In fact, their calculations precisely match one of the most attractive dark matter particle candidates: the hypothetical neutralino which is predicted in supersymmetry models. When annihilated, these would produce heavy quarks, gauge bosons or the Higgs boson, and would have the right mass and particle size to produce the microwave haze observed by WMAP.

One of the predictions made in this paper is for the upcoming Gamma Ray large Area Space Telescope (GLAST), due to launch in December, 2007. If they’re correct, GLAST will be able to detect a glow of gamma rays coming from the Galactic Centre, matching the microwave haze, and even put an upper limit of the mass of dark matter particles. The upcoming ESA Planck mission will give an even more precise look at the microwave haze, providing better data.

It might still be mysterious, but dark matter is revealing its secrets slowly but surely.

Original Source: Arxiv (PDF)

Ring of Dark Matter Discovered Around a Galaxy Cluster

2007-0515darkmatter.thumbnail.jpg

Astronomers using the Hubble Space Telescope have turned up a ghostly ring of dark matter, surrounding the aftermath of a collision between two galaxy clusters. This is one of the strongest pieces of evidence ever found for the existence of dark matter; a shadowy substance that only interacts with regular matter through gravity.

Researchers discovered the ring while they were mapping the distribution of dark matter inside the galaxy cluster Cl 0024+17, which is located about 5 billion light-years from Earth. The ring itself is 2.6 million light-years across.

Since dark matter is invisible, the researchers discovered the ring by its gravitational influence on background galaxies. The more dark matter concentrated into an area, the more the light from background objects is distorted, like ripples on a pond of water. We’re fortunate that the head-on collision between the galaxy clusters provided us with a perfect view from our perspective here on Earth.

So how did this ring form? Simulations have shown that when galaxy clusters collide, the dark matter falls into the centre of the combined cluster, and then sloshes back out. As it heads back out, mutual gravity slows it back down, and the dark matter piles up into a ring.

Original Source: Hubble News Release

How Dark Matter Might Have Snuffed Out the First Stars

2007-0508firststars.thumbnail.jpg

What role did dark matter play in the early Universe? Since it makes up the majority of matter, it must have some effect. A team of researchers is proposing that massive quantities of dark matter formed dark stars in the early Universe, preventing the first generations of stars from entering their main sequence stage. Instead of burning with hydrogen fusion, these “dark stars” were heated by the annihilation of dark matter.

And these dark stars might still be out there.

Just a few hundred thousand years after the Big Bang, the Universe cooled enough for first matter to coalesce out of a superheated cloud of ionized gas. Gravity took hold and this early matter came together to form the first stars. But these weren’t stars as we know them today. They contained almost entirely hydrogen and helium, grew to tremendous masses, and then detonated as supernovae. Each successive generation of supernovae seeded the Universe with heavier elements, created through the nuclear fusion of these early stars.

Dark matter dominated the early Universe too, hovering around normal matter in great halos, concentrating it together with its gravity. As the first stars gathered together inside these halos of dark matter, a process known as molecular hydrogen cooling helped them collapse down into stars.

Or, that’s what astronomers commonly believe.

But a team of researchers from the US think that dark matter wasn’t just interacting through its gravity, it was right there in the thick of things. Their research is published in the paper “Dark matter and the first stars: a new phase of stellar evolution“. Particles of dark matter compressed together began to annihilate, generating massive amounts of heat, and overwhelming this molecular hydrogen cooling mechanism. Hydrogen fusion was halted, and a new stellar phase – a “dark star” – began. Massive balls of hydrogen and helium powered by dark matter annihilation, instead of nuclear fusion.

If these dark stars are stable enough, it’s possible that they could still exist today. That would mean that an early population of stars never reached the Main Sequence stage, and still live in this aborted process, sustained by the annihilation of dark matter. As the dark matter is consumed in the reaction, additional dark matter from surrounding regions could flow in to keep the core heated, and hydrogen fusion might never get a chance to take over.

Dark stars might not be so long lasting, however. The fusion from regular matter might eventually overwhelm the dark matter annihilation reaction. Its evolution into a regular star wouldn’t be halted, only delayed.

How could astronomers search for these dark stars?

They would be very large, with a core radius larger than 1 AU (the distance from the Earth to the Sun), so they might be candidates for gravitational lensing experiments. These observations use the gravity from nearby galaxies to serve as an artificial telescope to focus the light from a more distant object. This is the best technique astronomers have to find the most distant objects.

They could also be detectable by the annihilation products of the dark matter. If the nature of dark matter matches the Weakly Interacting Massive Particles theory, its annihilation would give off very specific radiation and particles in large quantities. Astronomers could look for gamma-rays, neutrinos, and antimatter.

A third way to detect them would be to search for a delay in the transition to the Main Sequence stage for the early stars. The dark stars could have interrupted this stage for millions of years, leading to an unusual gap in stellar evolution.

Perhaps these dark stars will give astronomers the evidence they need to finally know what dark matter really is.

Original Source: Dark matter and the first stars: a new phase of stellar evolution

Missing Matter Could Be Clouds of Gas

NASA’s Chandra X-ray Observatory has discovered two huge intergalactic clouds of diffuse hot gas. These clouds are the best evidence yet that a vast cosmic web of hot gas contains the long-sought missing matter – about half of the atoms and ions in the Universe.

Various measurements give a good estimate of the mass-density of the baryons – the neutrons and protons that make up the nuclei of atoms and ions – in the Universe 10 billion years ago. However, sometime during the last 10 billion years a large fraction of the baryons, commonly referred to as “ordinary matter” to distinguish them from dark matter and dark energy, have gone missing.

“An inventory of all the baryons in stars and gas inside and outside of galaxies accounts for just over half the baryons that existed shortly after the Big Bang,” explained Fabrizio Nicastro of the Harvard-Smithsonian Center for Astrophysics, and lead author of a paper in the 3 February 2005 issue of Nature describing the recent research. “Now we have found the likely hiding place of the missing baryons.”

Nicastro and colleagues did not just stumble upon the missing baryons – they went looking for them. Computer simulations of the formation of galaxies and galaxy clusters indicated that the missing baryons might be contained in an extremely diffuse web-like system of gas clouds from which galaxies and clusters of galaxies formed.

These clouds have defied detection because of their predicted temperature range of a few hundred thousand to a million degrees Celsius, and their extremely low density. Evidence for this warm-hot intergalactic matter (WHIM) had been detected around our Galaxy, or in the Local Group of galaxies, but the lack of definitive evidence for WHIM outside our immediate cosmic neighborhood made any estimates of the universal mass-density of baryons unreliable.

The discovery of much more distant clouds came when the team took advantage of the historic X-ray brightening of the quasar-like galaxy Mkn 421 that began in October of 2002. Two Chandra observations of Mkn 421 in October 2002 and July 2003, yielded excellent quality X-ray spectral data. These data showed that two separate clouds of hot gas at distances from Earth of 150 million light years and 370 million light years were filtering out, or absorbing X-rays from Mkn 421.

The X-ray data show that ions of carbon, nitrogen, oxygen, and neon are present, and that the temperatures of the clouds are about 1 million degrees Celsius. Combining these data with observations at ultraviolet wavelengths enabled the team to estimate the thickness (about 2 million light years) and mass density of the clouds.

Assuming that the size and distribution of the clouds are representative, Nicastro and colleagues could make the first reliable estimate of average mass density of baryons in such clouds throughout the Universe. They found that it is consistent with the mass density of the missing baryons.

Mkn 421 was observed three times with Chandra’s Low-Energy Transmission Grating (LETG), twice in conjunction with the High Resolution Camera (May 2000 and July 2003) and once with the Advanced CCD Imaging Spectrometer (October 2002). The distance to Mkn 421 is 400 million light years.

NASA’s Marshall Space Flight Center, Huntsville, Ala., manages the Chandra program for NASA’s Office of Space Science, Washington. Northrop Grumman of Redondo Beach, Calif., formerly TRW, Inc., was the prime development contractor for the observatory. The Smithsonian Astrophysical Observatory controls science and flight operations from the Chandra X-ray Center in Cambridge, Mass.

Additional information and images are available at: http://chandra.harvard.edu and http://chandra.nasa.gov

Original Source: Chandra News Release

Dark Matter Halo Puzzles Astronomers

Dark matter continues to confound astronomers, as NASA’s Chandra X-ray Observatory demonstrated with the detection of an extensive envelope of dark matter around an isolated elliptical galaxy. This discovery conflicts with optical data that suggest a dearth of dark matter around similar galaxies, and raises questions about how galaxies acquire and keep such dark matter halos.

The observed galaxy, known as NGC 4555, is unusual in that it is a fairly large, elliptical galaxy that is not part of a group or cluster of galaxies. In a paper to be published in the November 1, 2004 issue of the Monthly Notices of the Royal Astronomical Society, Ewan O’Sullivan of the Harvard-Smithsonian Center for Astrophysics in Cambridge, MA and Trevor Ponman of the University of Birmingham, United Kingdom, use the Chandra data to show that the galaxy is embedded in a cloud of 10-million-degree-Celsius gas.

This hot gas cloud has a diameter of about 400,000 light years, about twice that of the visible galaxy. An enormous envelope, or halo, of dark matter is needed to confine the hot cloud to the galaxy. The total mass of the dark matter halo is about ten times the combined mass of the stars in the galaxy, and 300 times the mass of the hot gas cloud.

A growing body of evidence indicates that dark matter – which interacts with itself and “normal” matter only through gravity – is the dominant form of matter in the universe. According to the popular “cold dark matter” theory, dark matter consists of mysterious particles left over from the dense early universe that were moving slowly when galaxies and galaxy clusters began to form.

“The observed properties of NGC 4555 confirm that elliptical galaxies can posses dark matter halos of their own, regardless of their environment,” said O’Sullivan. “This raises an important question: what determines whether elliptical galaxies have dark matter halos?”

Most large elliptical galaxies are found in groups and clusters of galaxies, and are likely the product of the merger of two spiral galaxies. In such an environment, the dark matter halos can be stripped away by gravitational tidal force and added to other galaxies or the group as a whole. Therefore, it is difficult to determine how much dark matter the original galaxies had, and how much they have lost to the group as a whole through interactions with their environment.

The importance of the issue of the intrinsic amount of dark matter associated with an elliptical galaxy has recently increased owing to a report by an international team of astronomers led by Aaron Romanowsky of the University of Nottingham, United Kingdom. This team found little, if any evidence of dark matter in three relatively nearby elliptical galaxies. Two of these were in loose galaxy groups, and one was isolated. Their result, based on optical data from the 4.2 meter William Herschel Telescope on the Spanish island of La Palma, is in clear conflict with the X-ray data on NGC 4555. The optical technique used to search for dark matter in the nearby elliptical galaxies could not be applied to NGC 4555 because it is more than 3 times as far away from Earth.

Either the galaxies observed by Romanowsky and colleagues have lost their dark matter halos through earlier interactions with other galaxies, or their dark matter halos are much more extended, or they formed without dark matter halos. The first option is possible for the galaxies in groups, but very unlikely for the isolated galaxy. The second and third options are still open, but would require a modification – perhaps a major modification – of the cold dark matter theory of galaxy formation.

“This is clearly a question which deserves further consideration,” said O’Sullivan. “It seems likely that much more theoretical and observational work on elliptical galaxies will be required before this issue can be resolved.”

Chandra observed NGC 4555 with its Advanced CCD Imaging Spectrometer (ACIS) in February 2003. NASA’s Marshall Space Flight Center, Huntsville, Ala., manages the Chandra program for NASA’s Office of Space Science, Washington. Northrop Grumman of Redondo Beach, Calif., formerly TRW, Inc., was the prime development contractor for the observatory. The Smithsonian Astrophysical Observatory controls science and flight operations from the Chandra X-ray Center in Cambridge, Mass.

Additional information and images are available at:

http://chandra.harvard.edu
and
http://chandra.nasa.gov

Original Source: Chandra News Release