DLSCL J0916.2+2951: The Musketball Cluster

DLSCL J0916.2+2951: A merger of galaxy clusters about 5.2 billion light years from Earth.

This composite image shows Chandra (red) and Hubble (yellow and white) data of the galaxy cluster system that has been nicknamed the "Musket Ball" cluster. Astronomers call it this because the newly discovered cluster is older and slower than the Bullet Cluster, a famous system in which "normal" matter has been wrenched apart from dark matter. Chandra detects the normal matter as hot gas, while optical emission reveals the presence of dark matter through the effect of gravitational lensing (blue). DLSCL J0916.2+2951 is further along in its evolution than the Bullet Cluster, giving scientists valuable insight into a different phase of how galaxy clusters -- the largest known objects held together by gravity -- grow and change after major collisions.

Scale: Image is 6.4 arcmin across. (8 million light years across.)

Photo credit: X-ray: NASA/CXC/UCDavis/W.Dawson et al; Optical: NASA/STScI/UCDavis/W.Dawson et al.

Note: For more information, see DLSCL J0916.2+2951: Discovery of the Musket Ball 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.

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.

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

Note: For more information, see Abell 520: Dark Matter and Galaxies Part Ways in Collision between Hefty Galaxy Clusters .

Galaxy Cluster ACT-CL J0102−4915 - "El Gordo"

This picture of the galaxy cluster ACT-CL J0102−4915 combines images taken with ESO’s Very Large Telescope with images from the SOAR Telescope and X-ray observations from NASA’s Chandra X-ray Observatory. The X-ray image shows the hot gas in the cluster and is shown in blue. This newly discovered galaxy cluster has been nicknamed El Gordo — the "big" or "fat one" in Spanish. It consists of two separate galaxy subclusters colliding at several million kilometers per hour, and is so far away that its light has traveled for seven billion years to reach the Earth.

Photo credit: ESO/SOAR/NASA

Note: For more information, see El Gordo — A “Fat” Distant Galaxy Cluster. Also, see El Gordo (ACT-CL J0102-4915):
NASA's Chandra Finds Largest Galaxy Cluster in Early Universe.

600 Mysteries in the Night Sky

NASA's Fermi team recently released the second catalog of gamma-ray sources detected by their satellite's Large Area Telescope (LAT). Of the 1873 sources found, nearly 600 are complete mysteries. No one knows what they are.

"Fermi sees gamma rays coming from directions in the sky where there are no obvious objects likely to produce gamma rays," says David Thompson, Fermi Deputy Project Scientist from Goddard Space Flight Center.

Gamma rays are by their very nature heralds of great energy and violence. They are a super-energetic form of light produced by sources such as black holes and massive exploding stars. Gamma-rays are so energetic that ordinary lenses and mirrors do not work. As a result, gamma-ray telescopes can't always get a sharp enough focus to determine exactly where the sources are.

For two thirds of the new catalog's sources the Fermi scientists can, with at least reasonable certainty, locate a known gamma ray-producing object*, such as a pulsar or blazar, in the vicinity the gamma-rays are coming from. But the remaining third – the "mystery sources" -- have the researchers stumped, at least for now. And they are the most tantalizing.

"Some of the mystery sources could be clouds of dark matter – something that's never been seen before," speculates Thompson.

About 85% of the gravitational mass of the universe is dark matter. The stuff we see makes up the rest. Dark matter is something that pulls on things with the force of its gravity but can't be detected in any other way. It doesn't shine – doesn't emit or scatter light – hence the adjective "dark."

Astronomers cannot detect dark matter directly using optical or radio telescopes. But dark matter just might shine in gamma rays.

"We've been using Fermi to search for dark matter for a long time," says the principal investigator for the Large Area Telescope, Peter Michelson of Stanford University.

Some researchers believe that when two dark matter antiparticles bump into each other, they will annihilate, producing gamma rays. Concentrated clouds of dark matter could form a gamma ray source at specific wavelengths detectable by Fermi.

"If we see a bump in the gamma-ray spectrum -- a narrow spectral line at high energies corresponding to the energy of the annihilating particles – we could be the first to 'apprehend' dark matter,” says Michelson.

The team plans to continue observing the mystery sources. Fermi scans the entire sky ever three hours, and this ongoing sequence of observations "piles up" gamma rays for the researchers to analyze. So far, too few gamma rays have been collected from the mystery sources to form definite conclusions.

Another, less-dark possibility for some of the mystery sources is colliding galaxy clusters. According to Michelson and Thompson, clashes of such magnitude would generate super large scale shock waves that would accelerate particles. Others of the sources, they say, might be some brand new phenomenon, perhaps something involving galactic black holes.

When all is said and done, many of the mystery sources could prove to be familiar. "[They] will probably turn out to be members of known source classes – things we know but haven't recognized yet, like undiscovered pulsars, binary systems, and supernova remnants," says Michelson.

"Of course we're hoping for something really exotic like dark matter, but we have to look first at all the other options," says Thompson. "Fermi is an ongoing mission. We'll continue to search for answers to these puzzles and perhaps turn up even more surprises."

Will notorious dark matter finally be nabbed? Stay tuned!

Footnote: *Among the known classes in the second Fermi LAT catalog are almost 100 gamma-ray pulsars -- super dense neutron stars that are blinking in gamma rays. They are 1 ½ times the mass of our sun, but crushed to the size of a city. "Pulsars rotate rapidly and the light from them sweeps past us like light from a light house," explains Thompson. "Some of them rotate as fast as a kitchen blender -- hundreds of times per second!" The LAT team has rock-solid identification on 83 pulsars in our own galaxy. But the largest single class of gamma-ray sources the LAT "sees" is blazars from active galactic nuclei. Blazars make up over 1000 of the 1873 sources. "They are of course extra-galactic," says Michelson. "Only about 25 of these are rock-solid identifications, but we believe the others, because of their location, are likely associated with active galactic nuclei blazars too." Active galactic nuclei are the cores of galaxies. "The gamma rays are probably coming from the vicinity of black holes, which have 1 million to 1 billion times the mass of our sun," says Thompson. "A black hole pulls everything into itself by virtue of its unbelievable strong gravity. As it does so, enormous energy is released and squirted away from the black hole as a beam of particles and radiation moving at nearly the speed of light. It's called a blazar if the jet is aimed at us and we are looking down the barrel. But the jet of energy doesn't come out of the black hole itself; rather it is powered by material falling into the black hole." Binary systems constitute another known source class in the new catalog. They are made up of a neutron star or black hole orbiting a large normal star.

Credits: NASA

Abell 1758: Cluster Collisions Switch On Radio Halos

This is a composite image of the northern part of the galaxy cluster Abell 1758, located about 3.2 billion light years from Earth, showing the effects of a collision between two smaller galaxy clusters. Chandra X-ray data (blue) reveals hot gas in the cluster and data from the Giant Metrewave Radio Telescope (GMRT) in India (pink) shows huge "halos" generated by ultra-relativistic particles and magnetic fields over vast scales. Optical data from the Digitized Sky Survey are colored gold.

A study of this galaxy cluster and 31 others with Chandra and the GMRT shows that huge radio halos are generated during collisions between galaxy clusters. This result implies that galaxy clusters with radio halos are still forming, while clusters without this radio emission are not still accumulating large amounts of material. The result also implies that relativistic electrons are likely accelerated by turbulence generated by mergers between clusters.

Galaxy clusters are the largest structures in the Universe that are bound together by gravity. They form when smaller clusters or groups of galaxies collide and merge. Collisions between galaxy clusters, such as this one in Abell 1758 and its more famous cousin the Bullet Cluster, are the most energetic events in the Universe since the Big Bang. Their growth rate over the last 7 billion years has been slowed by the effects of dark energy, as shown by previous studies with Chandra.

Photo credit: X-ray (NASA/CXC/SAO/M.Markevitch); Radio (TIFR/GMRTSAO/INAF/R.Cassano, S.Giacintucci); Optical (DSS)