Credit: X-ray(NASA/CXC/Stanford/S.Allen); Optical/Lensing(NASA/STScI/UC Santa Barbara/M.Bradac)
[/caption]
A powerful collision of galaxy clusters captured by NASA’s Hubble Space Telescope and Chandra X-ray Observatory provides evidence for dark matter and insight into its properties. Observations of the cluster known as MACS J0025.4-1222 indicate that a titanic collision has separated dark matter from ordinary matter. The images also provide an independent confirmation of a similar effect detected previously in a region called the Bullet Cluster. Like the Bullet Cluster, this newly studied cluster shows a clear separation between dark and ordinary matter.
MACS J0025 formed after an enormously energetic collision between two large clusters. Using visible-light images from Hubble, the team was able to infer the distribution of the total mass — dark and ordinary matter. Hubble was used to map the dark matter (colored in blue) using a technique known as gravitational lensing. The Chandra data enabled the astronomers to accurately map the position of the ordinary matter, mostly in the form of hot gas, which glows brightly in X-rays (pink).
As the two clusters that formed MACS J0025 (each almost a whopping quadrillion times the mass of the Sun) merged at speeds of millions of miles per hour, the hot gas in the two clusters collided and slowed down, but the dark matter passed right through the smashup. The separation between the material shown in pink and blue therefore provides observational evidence for dark matter and supports the view that dark-matter particles interact with each other only very weakly or not at all, apart from the pull of gravity.
The bright blue blob is an ancient galaxy cluster. Credits: ESA XMM-Newton/EPIC, LBT/LBC, AIP (J. Kohnert)
[/caption]
A massive cluster of galaxies seen in the distant universe by ESA’s orbiting XMM-Newton x-ray observatory is so big that astronomers believe there can only be a few of them that far away in space and time. “Such massive galaxy clusters are thought to be rare objects in the distant Universe,” said Georg Lamer, Astrophysikalisches Institut in Potsdam, Germany. “They can be used to test cosmological theories. Indeed, the very presence of this cluster confirms the existence of a mysterious component of the Universe called dark energy.†The astronomers compared the rare find to a cosmic ‘needle in a haystack.’
The newly-discovered monster, known by the catalogue number 2XMM J083026+524133, is 7.7 thousand million light-years distant and is estimated to contain as much mass as a thousand large galaxies. Much of it is in the form of 100-million-degree hot gas. The bright blue blob of gas was found during a systematic analysis of catalogued objects as Lamer and his team were looking for patches of X-rays that could either be nearby galaxies of distant clusters of galaxies.
Based on 3,500 observations performed with XMM-Newton’s European Photon Imaging Camera (EPIC) covering about 1% of the entire sky, the catalogue contains more than 190,000 individual X-ray sources. J083026+524133 stood out because it was so bright. While checking visual images from the Sloan Digital Sky Survey, the team could not find any obvious nearby galaxy in that location. So they turned to the Large Binocular Telescope in Arizona and took a deep exposure, which found a cluster of galaxies in that location.
The astronomers were surprised to find the cluster contains a thousand times the mass of our own Milky Way Galaxy.
No one knows what dark energy is, but it is causing the expansion of the Universe to accelerate. This hampers the growth of massive galaxy clusters in more recent times, indicating that they must have formed earlier in the Universe. “The existence of the cluster can only be explained with dark energy,†says Lamer.
Yet he does not expect to find more of them in the XMM-Newton catalogue. “According to the current cosmological theories, we should only expect to find this one cluster in the 1% of sky that we have searched,†says Lamer.
Complete with tentacles, a supermassive black hole and x-ray emitting gas, a monster of a galaxy has been found by NASA’s Hubble Space Telescope, and is helping astronomers answer a long-standing puzzle. The very active galaxy NGC 1275 has giant but beautiful and delicate filaments influenced and shaped by a beastly-strong extragalactic magnetic field. But how the delicate structures such as those found in this galaxy can withstand the hostile, high-energy environment has been a mystery. But researchers say the beauty and the beast co-exist and are dependent on each other for survival.
One of the closest giant elliptical galaxies, NGC 1275 hosts a supermassive black hole. Energetic activity of gas swirling near the black hole blows bubbles of material into the surrounding galaxy cluster. Long gaseous filaments stretch out beyond the galaxy, into the multimillion-degree, X-ray–emitting gas that fills the cluster. Astronomers thought these delicate filaments should have heated up, dispersed, and evaporated by now, or collapsed under their own gravity to form stars.
These filaments are the only visible-light manifestation of the intricate relationship between the central black hole and the surrounding cluster gas. They provide important clues about how giant black holes affect their surrounding environment.
Using Hubble’s view, a team of astronomers led by Andy Fabian from the University of Cambridge, UK, have for the first time resolved individual threads of gas that make up the filaments. The amount of gas contained in a typical thread is around one million times the mass of our own Sun. They are only 200 light-years wide, are often very straight, and extend for up to 20,000 light-years. The filaments are formed when cold gas from the core of the galaxy is dragged out in the wake of the rising bubbles blown by the black hole.
A new study published in the August 21 Nature magazine proposes that magnetic fields hold the charged gas in place and resist the forces that would distort the filaments. This skeletal structure is strong enough to resist gravitational collapse.
“We can see that the magnetic fields are crucial for these complex filaments – both for their survival and for their integrity,” said Fabian.
Similar networks of filaments are found around other more remote central cluster galaxies. However, they cannot be observed with comparable resolution to the view of NGC 1275. In future observations, the team will apply the understanding of NGC 1275 to interpret what they see in other, more distant galaxies.
Map of distribution of galaxies. Credit: M. Blanton and the SDSS.
[/caption]
Cosmic voids really are devoid of matter. Astronomers have found that even the pervasive ‘dark matter’ which accounts for about 80% of the mass of the universe is not present in these voids, which are areas of vast emptiness in space that can be tens of millions of light-years across. “Astronomers have wondered for a quarter-century whether these voids were ‘too big’ or ‘too empty’ to be explained by gravity alone,” said University of Chicago researcher Jeremy Tinker, who led the new study using data from the Sloan Digital Sky Survey II (SDSS-II). “Our analysis shows that the voids in these surveys are exactly as big and as empty as predicted by the ‘standard’ theory of the universe.”
The largest 3-dimensional maps of the universe show that galaxies lie in filamentary superclusters interlaced by cosmic voids that contain few or no bright galaxies. Researchers using SDSS-II and the
Two-Degree Field Galaxy Redshift Survey (2dFGRS) have concluded that these voids are also missing the “halos” of invisible dark matter that bright galaxies reside in.
A central element of the standard cosmological theory is cold dark matter, which exerts gravity but does not emit light. Dark matter is smoothly distributed in the early universe, but over time gravity pulls it into filaments and clumps and empties out the spaces between them. Galaxies form when hydrogen and helium gas falls into collapsed dark matter clumps, referred to as “halos,” where it can form luminous stars.
But astronomers were not sure if the areas that are devoid of galaxies were also devoid of dark matter, or if the dark matter was there, but for some reason stars just didn’t form in these voids.
The research team used bright galaxies to trace the structure of dark matter and compared it with computer simulations to predict the number and sizes of voids.
Princeton University graduate student Charlie Conroy measured the sizes of voids in the SDSS-II maps. “When we used galaxies brighter than the Milky Way to trace structure, the biggest empty voids we found were about 75 million light years across,” said Conroy. “And the predictions from the simulations were bang-on.”
The sizes of voids are ultimately set, Conroy explained, by the small variations in the primordial distribution of dark matter, and by the amount of time that gravity has had to grow these small variationsinto large structures.
The agreement between the simulations and the measurements holds for both red (old) and blue (new) galaxies, said Tinker. “Halos of a given mass seem to form similar galaxies, both in numbers of stars and in the ages of those stars, regardless of where the halos live.”
Tinker presented his findings today at an international symposium in Chicago, titled “The Sloan Digital Sky Survey: Asteroids to Cosmology.” A paper detailing the analysis will appear in the September 1 edition of The Astrophysical Journal, with the title “Void Statistics in Large Galaxy Redshift Surveys: Does Halo Occupation of Field Galaxies Depend on Environment?”
Model of a our galaxy showing trails of stars torn from disrupted satellite galaxies. Credit: K. Johnston, J. Bullock
[/caption]
The halo of stars that envelops the outer Milky Way galaxy is like a “jumble of pasta” said one researcher, describing criss-crossed patterns of stellar streams revealed in new data from the Sloan Digital Sky Survey (SDSS). These stars appear to have been ripped away from the dwarf galaxies that are companions to our own galaxy, creating messy, spaghetti-like streams of stars in the outer edge of the Milky Way. The SEGUE (Sloan Extension for Galactic Understanding and Exploration) of the Sloan Survey is mapping the structure and stellar makeup of the Milky Way Galaxy and has found numerous new small streams of stars mixed and tangled among larger streams that had been mapped out over the last decade. It appears the Milky Way’s thievery is creating quite a mess.
While the center of galaxy is quite orderly, the outer Milky Way is a cluttered mess. Kathryn Johnston from Columbia University explained how dwarf galaxies that pass close to the Milky Way can be stretched by gravitational tides into spaghetti-like strands, which wind around the Galaxy as stars trace out the same orbital paths at different rates.
“In the center of the Galaxy, these stellar strands crowd together and you just see a smooth mix of stars,” said Johnston. “But as you look further away you can start to pick out individual strands, as well as features more akin to pasta shells that come from dwarfs that were on more elongated orbits.” Johnston described the new smaller strands recently detected as “angel hair” that came from smaller dwarf or ones that were destroyed longer ago.
Heidi Newberg of Rensselaer Polytechnic Institute and her thesis student Nathan Cole have been trying to follow some of the larger strands as they weave across the sky. “It’s a big challenge to piece things together,” said Cole, “because the stream from one dwarf galaxy can wrap around the Galaxy and pass through streams of stars ripped from other dwarf galaxies.”
Toward the constellation Virgo, where SDSS images revealed an excess of stars covering a huge area of sky, there are at least two superposed structures, and possibly three or more. The SEGUE velocity measurements can separate systems that overlap in sky maps, Newberg explained. “Part of what we see toward Virgo is a tidal arm of the Sagittarius dwarf galaxy, whose main body lies on the opposite side of the Milky Way, but we don’t know the origin of the other structures. There really aren’t enough pasta varieties to describe all the structures we find.”
“The SDSS has taught us a huge amount about the Milky Way and its neighbors,” said Johnston. “But we’re still just beginning to map the Galaxy in a comprehensive way, and there’s a trove of discoveries out there for the next generation of surveys, including the two new Milky Way surveys that will be carried out in SDSS-III,” the next set of surveys slated for Sloan.
COSMOS Survey. Credit: NASA, ESA, K. Sheth, P. Capak and N. Scoville
[/caption]
Two galaxies walk into a bar. The young, regular spiral galaxy and the mature, barred spiral both order a drink. But the bartender only gives a drink to the barred spiral galaxy. The regular spiral galaxy says, “Hey, why didn’t I get my drink?” The bartender replies, “You’re too young, plus we don’t serve your type.”
Extremely lame joke, I know. But now that I have your attention, one of the latest studies conducted by the Hubble Space Telescope show that barred spiral galaxies were less plentiful 7 billion years ago than they are today. This confirms the idea that bars are a sign of galaxies getting older and reaching full maturity; they are no longer in their “formative years.” Using Hubble’s Advanced Camera for Surveys, astronomers say this study of the history of bar formation provides clues to understanding when and how spiral galaxies form and evolve over time.
And if anyone can come up with a better “two galaxies walk into a bar” joke, post it in the comments below. The winner gets a free subscription to Universe Today.
Hubble looked at more than 2,000 spiral galaxies in the Cosmic Evolution Survey (COSMOS). A team led by Kartik Sheth of the Spitzer Science Center at the California Institute of Technology discovered that only 20 percent of the spiral galaxies in the distant past possessed bars, compared with nearly 70 percent of their modern counterparts.
Bars have been forming steadily over the last 7 billion years, more than tripling in number. “The recently forming bars are not uniformly distributed across galaxy masses, however, and this is a key finding from our investigation,” said Sheth. “They are forming mostly in the small, low-mass galaxies, whereas among the most massive galaxies, the fraction of bars was the same in the past as it is today.”
The findings have important implications for galaxy evolution. “We know that evolution is generally faster for more massive galaxies: They form their stars early and fast and then fade into red disks. Low-mass galaxies are known to form stars at a slower pace, but now we see that they also made their bars slowly over time,” he said. Artist's illustration of the Milky Way. Credit; NASA
Our own Milky Way Galaxy was recently determined to have a central bar. Our galaxy is another massive barred spiral, and its central bar probably formed somewhat early, like the bars in other large galaxies in the Hubble survey. “Understanding how bars formed in the most distant galaxies will eventually shed light on how it occurred here, in our own backyard,” Sheth said.
COSMOS covers an area of sky nine times larger than the full Moon, surveying 10 times more spiral galaxies than previous observations. In support of the Hubble galaxy images, the team derived distances to the galaxies in the COSMOS field using data from Hubble and an assortment of ground-based telescopes.
Astronomers believe bars form when stellar orbits in a spiral galaxy become unstable and deviate from a circular path. “The tiny elongations in the stars’ orbits grow and they get locked into place, making a bar,” explained team member Bruce Elmegreen of IBM’s research Division in Yorktown Heights, N.Y. “The bar becomes even stronger as it locks more and more of these elongated orbits into place. Eventually a high fraction of the stars in the galaxy’s inner region join the bar.”
Bars are perhaps one of the most important catalysts for changing a galaxy. They force a large amount of gas towards the galactic center, fueling new star formation, building central bulges of stars, and feeding massive black holes.
“The formation of a bar may be the final important act in the evolution of a spiral galaxy,” Sheth said. “Galaxies are thought to build themselves up through mergers with other galaxies. After settling down, the only other dramatic way for galaxies to evolve is through the action of bars.”
Yes, there’s always lots of action in bars. Especially when two galaxies walk in.
There once was a galaxy known as ESO 593-IG 008. It was thought to be a relatively mild-mannered galaxy, even though scientists believed it was a collision of two different galaxies; one a barred spiral and the other an irregular galaxy. But now, an international team of astronomers has discovered that it actually is a stunning rare case of three interacting galaxies, with the third galaxy forming stars at a frantic rate.
Using adaptive optics on the European Southern Observatory’s (ESO) Very Large Telescope (VLT), astronomers were able to see through the all-pervasive dust clouds of the object that has been dubbed as “The Bird” because of its resemblance to a winged creature. With the adaptive optics of what’s called the NACO instrument, very fine details were able to be resolved.
“Examples of mergers of three galaxies of roughly similar sizes are rare,” says Petri Vaisanen, lead author of the paper which will appear in the journal of the Royal Astronomical Society. “Only the near-infrared VLT observations made it possible to identify the triple merger nature of the system in this case.”
NACO is the combination of NAOS – Nasmyth Adaptive Optics System that is equipped with both visible and infrared sensors, and CONICA, a Near-Infrared Imager and Spectrograph.
Looking like a bird or a cosmic Tinker Bell, the NACO images show two unmistakable galaxies that form the body and wings of “The Bird.” Astronomers were surprised with the new images that identify a third, clearly separate component that forms the head. This irregular, yet fairly massive galaxy is forming stars violently, at a rate of nearly 200 solar masses per year. It appears to be the major source of infrared luminosity in the system, even though it is the smallest of the three galaxies. The other two galaxies appear to be at a quieter stage of their interaction-induced star formation history. The object is 650 million light years distant but it is quite large with the “wings” alone extending more than 100,000 light-years, or the size of our own Milky Way.
Subsequent optical spectroscopy with the new Southern African Large Telescope, and archive mid-infrared data from the NASA Spitzer space observatory, confirmed the separate nature of the ‘head’, but also added further surprises. The ‘head’ and major parts of the ‘Bird’ are moving apart at more than 400 km/s (1.4 million km/h). Observing such high velocities is very rare in merging galaxies.
“The Bird” belongs to the prestigious family of luminous infrared galaxies, with an infrared luminosity nearly one thousand billion times that of the Sun. This family of galaxies has long been thought to signpost important events in galaxy evolution, such as mergers of galaxies, which in turn trigger bursts of star formation, and may eventually lead to the formation of a single elliptical galaxy.
The galaxy is also designated as IRAS 19115-2124. The ESO is more formally known as the European Organization for Astronomical Research in the Southern Hemisphere.
