Galaxy Clusters Enveloped by Cigar-Shaped Dark Matter Halos

Image comment: A photo of the Bullet Cluster, one of the 20 such structures that was analyzed for the new investigation
Image credits: NASA / STScI / Magellan / U.Arizona / D.Clowe et al

A series of recent experiments has revealed that, more often than not, the halos of dark matter surrounding massive galaxy clusters are flattened and shaped like a cigar. Until now, astrophysicists believed that the mysterious stuff, which is believed to be five times more abundant than regular matter around the Universe, would clump up in rounded spheres. However, observations appear to paint a different picture, and experts are currently working on models that would help explain that. The discovery could finally lead to studies that would result in the direct detection of the peculiar type of matter, whose existence can only be inferred from the gravitational pull it exerts on normal matter around it. “There are clear theoretical predictions that we expect dark mater halos to be flattened like this. It's a very beautiful, very clean and direct measurement of that,” explains expert Graham P. Smith, who is based at the University of Birmingham, in the United Kingdom. He is also a coauthor of the new study, which will appear in an upcoming issue of the esteemed scientific publication Monthly Notices of the Royal Astronomical Society. In the new studies, the investigators looked at about 20 galaxy clusters, which are massive collections of galaxies, held together by strong gravitational interactions. In order to see the effect dark matter has on the largest organized structures in the Universe, the researchers used gravitational lensing. This observations technique analyzes how much light is bent when mass wraps time-space in order to determine the mass of celestial objects beyond. The Mauna Kea, Hawaii-based Subaru Telescope was used for the study, and the team took advantage of the Prime Focus Camera above all other instruments. “What we're probing with these gravitational lensing observations is the dark matter distribution, because the dark matter dominates the mass on these large scales,” Smith says. The research team in charge of the study was led by National Astronomical Observatory of Japan expert Masamune Oguri and University of Tokyo scientist Masahiro Takada. The cigar-like shapes of these dark matter halos have been predicted in computer models of the cold dark matter theory, but thus far they have not been evidenced in practice in such a large number of galaxy clusters, Space reports.

Dark matter may give neutron stars black hearts

DARK matter may be prompting black holes to appear spontaneously in the hearts of distant exotic stars. If so, this could hint at the nature of dark matter.

Arnaud de Lavallaz and Malcolm Fairbairn of King's College London wondered what would happen when dark matter - which makes up most of the mass of galaxies - is sucked into the heart of neutron stars. These stars, the remnants of supernova explosions, are the densest known stars in the universe. It turns out that the outcome depends on the nature of dark matter.

Most of the favoured theories of dark matter suggest each particle of the stuff is also an antiparticle, meaning that they should annihilate each other when they meet. But Fairbairn and de Lavallaz considered a dark matter particle of a different type, which is not also its antiparticle.

The pair calculated what would happen if dark matter particles like these were attracted by the intense gravity of neutron stars. Because they would not annihilate each other, the dark matter particles would end up forming a smaller, dense star at the heart of the neutron star. If the neutron star were near the centre of the galaxy, for example, and surrounded by an abundance of dark matter, then it would continue to accrete dark matter.

Eventually, the mass of the dark matter star would exceed its "Chandrashekar limit" - beyond which a star cannot withstand gravitational pressure. The dark matter star would collapse into a black hole. "Then the neutron star won't be able to survive anymore, and it'll collapse too," says Fairbairn. "It would be pretty catastrophic."

Their calculations show that if a neutron star collapsed in this way the result would be a burst of gamma rays, which could be spotted from Earth (arxiv.org/abs/1004.0629).

Various underground experiments back on Earth have been trying to detect dark matter, using different techniques. While none of the major experiments have seen anything yet, physicists running the Dark Matter (DAMA) experiment inside the Gran Sasso mountain in Italy have been saying for some time that dark matter particles are hitting their detector. Most physicists are sceptical of the DAMA results because it doesn't sit well with favoured theories on the nature of dark matter.

Fairbairn says that the DAMA experiment could be sensitive to dark matter particles that do not self-annihilate, which might explain why it is seeing something and others are not.

Source: NewScientist.com

How Dark Matter Behaves Around Black Holes

The standard explanation of how our Universe is set up states that roughly one quarter (23 percent) of everything is made out of dark matter, a form of matter that cannot be readily observed through conventional means. In fact, it has never been observed at all, although scientists tried out a wide variety of methods for detecting it. But now, two experts advance a new role for dark matter that proposes the stuff altered the amounts of galaxies in the Universe, and that it did so to a significant margin, AlphaGalileo reports.

Astronomers Dr Xavier Hernandez and Dr William Lee, both of whom are based at the National Autonomous University of Mexico (UNAM), say that their study was prompted by their curiosity in learning how dark matter would interact with the supermassive black holes that form at the cores of impressively-large galaxies. The team says that its models provided them with a new basis for calculating both these interactions and the rate at which dark matter would get sucked in through the event horizon.

The group determined the existence of a threshold in these rates. If the immediate surroundings of the black hole are laden with dark matter in amounts higher than 7 Sun masses per cubic light-year, then the initial black hole was found to be capable of growing at extremely fast rates. Their investigation also revealed that, if such a growth spur was activated, then the black hole would be fueled even further by dark matter, eventually consuming the galaxy around it. If the galaxy would survive this ordeal, the group says, then it would be altered beyond any possible recognition when compared to its original shape.

“Over the billions of years since galaxies formed, such runaway absorption of dark matter in black holes would have altered the population of galaxies away from what we actually observe,” Hernandez says. The greatest implication of their analysis is the fact that the amounts of dark matter present at the cores of galaxies needs must tend for a constant value, if the entire structure is to remain stable. An additional implication is that dark matter may be behaving in ways previously not thought-of. This means that current models aimed at explaining its behavior – and which therefore underline efforts to find the stuff – may have to be thought over.