The Bullet Group

This image shows the Bullet Group, a group of galaxies also known as SL2S J08544-0121.

The galaxies belonging to the group are visible in the central part of the image, embedded in the diffuse dark matter (shown in blue). Hot gas, which fills the space between galaxies and comprises the bulk of ordinary (baryonic) matter in the group, is shown in pink, as imaged by ESA's XMM-Newton X-ray observatory.

The light from galaxies and hot gas belonging to the group, which lies at a redshift of z=0.351, has been traveling for almost four billion years before reaching us. Other galaxies, either in the foreground or background of the Bullet Group, are sprinkled across the image, as well as bright foreground stars that belong to our Galaxy.

The group's components appear to be clearly separated, with the hot gas partitioned from the rest of the mass within the group. This is the smallest object ever found to show such an effect, which was caused by a merger in the group's past.

Astronomers were able to map the extent of the Bullet Group's dark matter through its gravitational lensing of background galaxies. This effect is particularly evident in the center-right part of this image, where a round, bright galaxy that belongs to the Bullet Group is circled by curious arcs of light – the distorted image of another galaxy lying much farther away.

By exploring the contents of these cosmic wrecks, astronomers can learn more about the properties of dark matter. In particular, from the split between the dark matter and the hot gas, they can constrain how much dark matter does – or does not – interact with normal matter.

This image is a composite of an X-ray image (shown in pink) from ESA's XMM-Newton observatory, a three-color (red, green, blue) optical image from the Canada-France-Hawaii Telescope (CFHT), and a dark matter overlay (indicated in blue) based on data from CFHT, the NASA/ESA Hubble Space Telescope, and the W. M. Keck Observatory.

Image credit: ESA / XMM-Newton / F. Gastaldello (INAF/IASF, Milano, Italy) / CFHTLS

Note: For more information, see Cosmic Collision in the Bullet Group and Cosmic collision in the Bullet Group.

Black Hole Flare in Abell 1795

Abell 1795: A bright flare seen with Chandra provides evidence for a black hole having torn a star apart.

A bright, long duration flare may be the first recorded event of a black hole destroying a star in a dwarf galaxy. The dwarf galaxy is located in the galaxy cluster Abell 1795, about 800 million light years from Earth. A composite image of the cluster shows Chandra data in blue and optical data from the Canada-France-Hawaii Telescope in red, green and blue. An inset centered on the dwarf galaxy shows Chandra data taken between before and after 2005. The X-ray flare provides evidence that a large black hole has pulled in debris from a star that was torn apart by tidal forces.

Scale: Image is 6.8 arcmin on a side (about 1.5 million light years).

Image credit: X-ray: NASA/CXC/Univ. of Alabama/W.P.Maksym et al & NASA/CXC/GSFC/UMD/D.Donato, etal; Optical: CFHT

Note: For more information, see Abell 1795: Death By Black Hole In Small Galaxy?

Update on Near-Earth Objects 2013 UQ4, C/2013 US10 (Catalina), and 2013 UP8

Two surprisingly large Near-Earth Asteroids have been discovered in just the last week or so, as well as a third moderately large asteroid which, surprisingly, has also gone undetected until now, even though it can pass close enough to Earth to be classified as "potentially hazardous." Not since 1983 has any near-Earth asteroid been found as large as the approximately 12-mile (20-kilometer) size of the two new large ones. In fact, there are only three other known near-Earth asteroids that are of comparable size or larger than the two new large ones.

It is important to note that none of these three new large near-Earth asteroids can come close enough to Earth to represent a near-term threat to our planet.

The first of the new large near-Earth asteroid discoveries is named 2013 UQ4, and it is perhaps the most unusual. This approximately 12-mile (19-kilometer) wide object was spotted by the Catalina Sky Survey on October 23 when the asteroid was 270 million miles (435 million kilometers) away from Earth. Not only is this object unusually large, it follows a very unusual, highly inclined, retrograde orbit about the sun, which means it travels around the sun in the opposite direction of all the planets and the vast majority of asteroids.

The only objects usually found in retrograde orbits are comets, which suggests that 2013 UQ4 may be the remains of an old comet that no longer possesses the near-surface ices required for it to become active while near the sun. Comets that have exhausted most, or all, of their volatile ices do not spew dust during sweeps through the inner-solar system like their less-seasoned, more hyperactive space kin. Without the telltale comet tails or atmospheres, dead comets look like, and in fact for all practical purposes are, asteroids.

As reported on Circular No. 9262 of the International Astronomical Union, the Massachusett's Institute of Technology's Richard Binzel, David Polishook and Rachel Bowens-Rubin observed this object on October 31 with NASA's 3-meter Infrared Telescope Facility in Hawaii and determined this object belongs to the so-called X-type spectral class and exhibits no obvious comet-like activity. This implies about a 4 percent reflectivity, from which they estimate a diameter of approximately 12 miles (19 kilometers).

The second very large near-Earth object, named 2013 US10, was discovered on October 31 by the Catalina Sky Survey. While the reflectivity of this object has not yet been determined, and hence its diameter is still uncertain, it is also likely to be about 12 miles (20 kilometers) in size. Only three near-Earth asteroids (1036 Ganymed, 433 Eros and 3552 Don Quixote) are of comparable size or larger.

Why has it taken so long to discover these large near-Earth asteroids? The delay in discovering 2013 UQ4 is more easily understood because it has a very long orbital period that has kept it out of Earth's neighborhood for centuries. But the delayed discovery of 2013 US10 is a bit harder to explain, since current population models suggest that almost all near-Earth asteroids of this size and orbit should have already been found. A contributing factor may be that this object's orbit does not allow it to get closer than 50 million miles (80 million kilometers) of Earth's orbit, so the asteroid seldom gets close enough to Earth to become easily detectable. However, NASA-supported telescopic surveys are now covering more sky and looking "deeper" than they ever have before, and in fact, 2013 US10 was first detected where it spends much of its time, well beyond the orbit of Jupiter.

The third of the recent discoveries is the approximately 1.2-kilometer (two-kilometer) near-Earth asteroid 2013 UP8, found on October 25 by the Pan-STARRS group in Hawaii. This asteroid can approach quite close to Earth's orbit, within 3.4 million miles (5.5 million kilometers), which makes it a "potentially hazardous asteroid" (PHA). 2013 UP8 is in the top 5th percentile of the largest PHAs, most of which were found much earlier during NASA's asteroid survey program. Like the other new discoveries, this asteroid has gone undetected for a long time because it has not approached Earth closely for decades. But the increasingly capable NASA-supported asteroid surveys finally found this object while it was still at a large distance from Earth, well beyond the orbit of Mars.

Update: Near-Earth Object 2013 US10 is a Long-Period Comet

While initial reports from the Minor Planet Center in Cambridge, Massachusetts, categorized object 2013 US10 as a very large near-Earth asteroid, new observations now indicate that it is, in fact, a long-period comet, and it is now designated C/2013 US10 (Catalina). The comet was discovered by the Catalina Sky Survey near Tucson, Arizona, on Oct. 31, 2013, and linked to earlier pre-discovery Catalina observations made on September 12. The initial orbit suggested this object is a large, short period, near-Earth asteroid, as reported here yesterday. An updated orbit, issued today by the Minor Planet Center, removed the September 12 observations that belong to another object and include earlier pre-discovery August and September observations made by the Catalina Sky Survey, the ISON-HD observatory in Russia and Hawaii's Pan-STARRS group. The new orbit indicates that this object is in a long-period, near parabolic orbit about the sun. Furthermore, observations made last night at the Canada-France-Hawaii telescope indicate the object is showing modest cometary activity, which means that yesterday's rough estimate for the object's size (about 12 miles, or 20 kilometers) must now be completely revised. A new size estimate is not yet available, but the object could very well be much smaller than yesterday's estimate.

Illustration credit: NASA/JPL-Caltech

Green Bean Galaxy J2240

This view from the Canada-France-Hawaii Telescope shows thousands of galaxies in the distant Universe. But the one close to the center looks very odd — it is bright green. This very unusual object is known as J224024.1−092748 or J2240 and it is a bright example of a new class of objects that have been nicknamed green bean galaxies. Green beans are entire galaxies that are glowing under the intense radiation from the region around a central black hole. J2240 lies in the constellation of Aquarius (The Water Bearer) and its light has taken about 3.7 billion years to reach Earth.

Photo credit: CFHT/ESO/M. Schirmer

Note: For more information, see Galaxy-Wide Echoes From the Past.

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 .