Universe Today

Image credit: UCSC

A team of astronomers from the University of Cambridge have been researching a rare group of galaxies, known as dwarf spheroidal galaxies, which seem to have few stars but massive amounts of “dark matter”. The team analyzed one such galaxy and found that the stars in the outer edges were moving so quickly that the galaxy could only stay together if it had 100 times more dark matter than the mass of the stars alone. This research will help astronomers understand how galaxies are formed and how dark matter plays into their composition.

New research on dwarf spheroidal galaxies by a team of astronomers at the University of Cambridge promises a real astronomical first: detection, for the first time, of the true outer limits of a galaxy.

The team is presenting today (23 July 2003) at the 25th General Assembly of the International Astronomical Union (IAUXXV) in Sydney, Australia. The research could provide the key to understanding how larger galaxies were formed, including our own Milky Way galaxy.

The rare dwarf spheroidal galaxies display few stars but contain massive amounts of ‘dark matter’ or matter that does not emit radiation that can be observed by astronomers. The team studied these galaxies in detail using some of the largest optical telescopes on earth in order to probe their dark secrets. Dwarf spheroidal galaxies are widely believed to be the building blocks from which galaxies were formed.

By studying the motion of many stars the scientists have created a picture of how the mass of the galaxy is arranged. Surprisingly, when the Cambridge team looked at the stars at the edge of one such galaxy, Draco, they found that the outer stars were moving so quickly that the galaxy could only stay together if it contained 100 times more dark matter than the mass of the stars alone. Using detailed models of the motions of stars in a galaxy containing large quantities of dark matter, the group was able to demonstrate their observations could only be understood if the galaxy was surrounded by a large halo of dark matter.

Observations of the Ursa Minor dwarf spheroidal galaxy presented a new complication in the study. The team found an unexpected clump of slow-moving stars interpreted as the dead remains of one of the pure star systems, a globular cluster. The cluster should have been scattered across the galaxy, but it was still held together. The team realised this was only possible if the dark matter were arranged in a manner very differently from standard galaxies.

In May 2003, further research into Ursa Minor showed the stars in the very outermost parts are not moving quickly like the stars at the edge of Draco. Several theories are being investigated including dark matter from edge of Ursa Minor has been snatched away from the galaxy by its massive parent, the Milky Way, allowing some stars to wander gently away from their parent. Or they could be stars which wandered too close to other stars in the centre of the galaxy and were slung out to the edge of the galaxy as a result.

Whatever the explanation, the findings promise a real astronomical first: detection, for the first time, of the true outer limits of a galaxy.

Gerry Gilmore, Professor of Experimental Philosophy at the Institute of Astronomy at the University of Cambridge, said:

“This research, utilising some of the largest optical telescopes on earth, has provided us with insight to the makeup of these rare dwarf galaxies. This research helps astronomers better understand how galaxies were formed, and help take into account dark matter in all galaxies.”

Original Source: Cambridge University News Release

Image credit: SDSS

Since the discovery several years ago of a mysterious force, called dark energy, which seems to be accelerating the Universe, astronomers have been searching for additional evidence to either support or discount this theory. Astronomers from the Sloan Digital Sky Survey have found fluctuations in cosmic background radiation that match the repulsive influence of dark energy.

Scientists from the Sloan Digital Sky Survey announced the discovery of independent physical evidence for the existence of dark energy.

The researchers found an imprint of dark energy by correlating millions of galaxies in the Sloan Digital Sky Survey (SDSS) and cosmic microwave background temperature maps from NASA’s Wilkinson Microwave Anisotropy Probe (WMAP). The researchers found dark energy’s “shadow” on the ancient cosmic radiation, a relic of cooled radiation from the Big Bang.

With the combination of results from these two large sky surveys, this discovery provides physical evidence for the existence of dark energy; a result that complements earlier work on the acceleration of the universe as measured from distant supernovae. Observations from the Balloon Observations of Millimetric Extragalactic Radiation and Geophysics (BOOMERANG) of Cosmic Microwave Background (CMB) were also part of the earlier findings.

Dark energy, a major component of the universe and one of the greatest conundrums in science, is gravitationally repulsive rather than attractive. This causes the universe’s expansion to accelerate, in contrast to the attraction of ordinary (and dark) matter, which would make it decelerate.

“In a flat universe the effect we’re observing only occurs if you have a universe with dark energy,” explained lead researcher Dr. Ryan Scranton of the University of Pittsburgh’s Physics and Astronomy department. “If the universe was just composed of matter and still flat, this effect wouldn’t exist.”

“As photons from the cosmic microwave background (CMB) travel to us from 380,000 years after the Big Bang, they can experience a number of physical processes, including the Integrated Sachs-Wolfe effect. This effect is an imprint or shadow of dark energy on microwaves. The effect also measures the changes in temperature of cosmic microwave background due to the effects of gravity on the energy of photons”, added Scranton.

The discovery is “a physical detection of dark energy, and highly complementary to other detections of dark energy” added Dr. Bob Nichol, an SDSS collaborator and associate professor of physics at Carnegie Mellon University in Pittsburgh. Nichol likened the Integrated Sachs-Wolfe effect to looking at a person standing in front of a sunny window: “You just see their outline and can recognize them from just this information. Likewise the signal we see has the right outline (or shadow) that we’d expect for dark energy,” said Nichol.

“In particular the color of the signal is the same as the color of the cosmic microwave background, proving it is cosmological in origin and not some annoying contamination,” added Nichol.

“This work provides physical confirmation that one needs dark energy to simultaneously explain both the CMB and SDSS data, independent of the supernovae work. Such cross-checks are vital in science,” added Jim Gunn, Project Scientist of the SDSS and Professor of Astronomy at Princeton University.

Dr. Andrew Connolly of the University of Pittsburgh explained that photons streaming from the cosmic microwave background pass through many concentrations of galaxies and dark matter. As they fall into a gravitational well they gain energy (just like a ball rolling down a hill). As they come out they lose energy (again like a ball rolling up a hill). Photographic images of the microwaves become more blue (i.e. more energetic) as they fall in toward these supercluster concentrations and then become more red (i.e. less energetic) as they climb away from them.

“In a universe consisting mostly of normal matter one would expect that the net effect of the red and blue shifts would cancel. However in recent years we are finding that most of the stuff in our universe is abnormal in that it is gravitationally repulsive rather than gravitationally attractive,” explained Albert Stebbins, a scientist at the NASA/Fermilab Astrophysics Center Fermi National Accelerator Laboratory, an SDSS collaborating institution. “This abnormal stuff we call dark energy.”

SDSS collaborator Connolly said if the depth of the gravitational well decreases while the photon travels through it then the photon would exit with slightly more energy. “If this were true then we would expect to see that the cosmic microwave background temperature is slightly hotter in regions with more galaxies. This is exactly what we found.”

Stebbins added that the net energy change expected from a single concentration of mass is less than one part in a million and researchers had to look at a large number of galaxies before they could expect to see the effect. He said that the results confirm that dark energy exists in relatively small mass concentrations: only 100 million light years across where the previously observed effects dark energy were on a scale of 10 billion light years across. A unique aspect of the SDSS data is its ability to accurately measure the distances to all galaxies from photographic analysis of their photometric redshifts. “Therefore, we can watch the imprint of this effect on the CMB grow as a function of the age of the universe,” Connolly said. “Eventually we might be able to determine the nature of the dark energy from measurements like these, though that is a bit in the future.”

“To make the conclusion that dark energy exists we only have to assume that the universe is not curved. After the Wilkinson Microwave Anisotropy Probe results came in (in February, 2003), that’s a well-accepted assumption,” Scranton explained. “This is extremely exciting. We didn’t know if we could get a signal so we spent a lot of time testing the data against contamination from our galaxy or other sources. Having the results come out as strongly as they did was extremely satisfying.”

The discoveries were made in 3,400 square degrees of the sky surveyed by the SDSS.

“This combination of space-based microwave and ground-based optical data gave us this new window into the properties of dark energy,” said David Spergel, a Princeton University cosmologist and a member of the WMAP science team. “By combining WMAP and SDSS data, Scranton and his collaborators have shown that dark energy, whatever it is, is something that is not attracted by gravity even on the large scales probed by the Sloan Digital Sky Survey.

“This is an important hint for physicists trying to understand the mysterious dark energy,” Spergel added.

In addition to principal investigators Scranton, Connolly, Nichol and Stebbins, Istavan Szapudi of the University of Hawaii contributed to the research. Others involved in the analysis include Niayesh Afshordi of Princeton University, Max Tegmark of the University of Pennsylvania and Daniel Eisenstein of the University of Arizona.

ABOUT THE SLOAN DIGITAL SKY SURVEY (SDSS)
The Sloan Digital Sky Survey (sdss.org) will map in detail one-quarter of the entire sky, determining the positions and absolute brightness of 100 million celestial objects. It will also measure the distances to more than a million galaxies and quasars. The Astrophysical Research Consortium (ARC) operates Apache Point Observatory, site of the SDSS telescopes.

SDSS is a joint project of The University of Chicago, Fermilab, the Institute for Advanced Study, the Japan Participation Group, The Johns Hopkins University, the Los Alamos National Laboratory, the Max-Planck-Institute for Astronomy (MPIA), the Max-Planck-Institute for Astrophysics (MPA), New Mexico State University, University of Pittsburgh, Princeton University, the United States Naval Observatory, and the University of Washington.

Funding for the project has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Aeronautics and Space Administration, the National Science Foundation, the U.S. Department of Energy, the Japanese Monbukagakusho and the Max Planck Society.

The WILKINSON MICROWAVE ANISOTROPY PROBE (WMAP) is a NASA mission built in partnership with Princeton University and the Goddard Space Flight Center to measure the temperature of the cosmic background radiation, the remnant heat from the Big Bang. The WMAP mission reveals conditions as they existed in the early universe by measuring the properties of the cosmic microwave background radiation over the full sky. (http://map.gsfc.nasa.gov)

Original Source: SDSS News Release

Image credit: ESO

Astronomers from the European Southern Observatory have found a whole new population of massive newborn stars inside a giant molecular cloud near the centre of the Milky Way. Inside the cloud are four massive stellar clusters with young stars as large as 120 times the mass of our Sun. This region, called W49, is one of the most energetic star forming regions of the Milky Way, and the recent observations help astronomers better understand how these regions form.

Peering into a giant molecular cloud in the Milky Way galaxy – known as W49 – astronomers from the European Southern Observatory (ESO) have discovered a whole new population of very massive newborn stars. This research is being presented today at the International Astronomical Union’s 25th General Assembly held in Sydney, Australia, by ESO-scientist Jo?o Alves.

With the help of infrared images obtained during a period of excellent observing conditions with the ESO 3.5-m New Technology Telescope (NTT) at the La Silla Observatory (Chile), the astronomers looked deep into this molecular cloud and discovered four massive stellar clusters, with hot and energetic stars as massive as 120 solar masses. The exceedingly strong radiation from the stars in the largest of these clusters is “powering” a 20 light-year diameter region of mostly ionized hydrogen gas (a “giant HII region”).

W49 is one of the most energetic regions of star formation in the Milky Way. With the present discovery, the true sources of the enormous energy have now been revealed for the first time, finally bringing to an end some decades of astronomical speculations and hypotheses.

Giant molecular clouds
Stars form predominantly inside Giant Molecular Clouds which populate our Galaxy, the Milky Way. One of the most prominent of these is W49, which has a mass of a million solar masses. It is located some 37,000 light-years away and is the most luminous star-forming region known in our home galaxy: its luminosity is several million times the luminosity of our Sun. A smaller region within this cloud is denoted W49A – this is one of the strongest radio-emitting areas known in the Galaxy.

Massive stars are excessive in all ways. Compared to their smaller and ligther brethren, they form at an Olympic speed and have a frantic and relatively short life. Formation sites of massive stars are quite rare and, accordingly, most are many thousands of light-years away. For that reason alone, it is in general much more difficult to observe details of massive-star formation.

Moreover, as massive stars are generally formed in the main plane of the Galaxy, in the disc where a lot of dust is present, the first stages of such stars are normally hidden behind very thick curtains. In the case of W49A, less than one millionth of the visible light emitted by a star in this region will find its way through the heavy intervening layers of galactic dust and reach the telescopes on Earth.

And finally, because massive stars just formed are still very deeply embedded in their natal clouds, they are anyway not detectable at optical wavelengths. Observations of this early phase of the lives of heavy stars must therefore be done at longer wavelengths (where the dust is more transparent), but even so, such natal dusty clouds still absorb a large proportion of the light emitted by the young stars.

Because of this observational obstacle, nobody had ever looked deep enough into the central most dense regions of the W49A molecular cloud – and nobody really knew what was in there. That is, until Jo?o Alves and his colleague, Nicole Homeier decided to obtain “deep” and penetrating observations of this mysterious area with the SofI near-infrared camera on the 3.5-m New Technology Telescope (NTT) at the ESO La Silla Observatory (Chile).

A series of infrared images was secured during a spell of good weather and very good atmospheric conditions (seeing about 0.5 arcsec). They clearly show the presence of a cluster of stars at the centre of a region of ionized hydrogen gas (an “HII-region”) measuring 20 light-years across. In addition, three other smaller clusters of stars were detected in the image.

Altogether, the ESO astronomers were able to identify more than one hundred heavy-weight stars inside W49A, with masses greater than 15 to 20 times the mass of our Sun. Among these, about thirty are located within the 20 light-year central region and about ten in each of the three other clusters.

The discovery of these hot and massive stars solves a long-standing problem concerning W49A: the exceptional brightness (in astronomical terminology: “luminosity”) of the entire region requires the energetic output from about one hundred massive stars, and nobody had ever seen them. But here they are on the deep and sharp SofI images!
Formation scenarios

The presence of such a large number of very massive stars spread over the entire region suggests that star formation in the various regions of W49A must have happened rather simultaneously from different seeds and not, as some theories propose, by a “domino-type” chain effect where stellar winds of fast particles and the emitted radiation of newly formed massive stars trigger another burst of star formation in the immediate neighbourhood.

The present research results also imply that star formation in W49A began earlier and extends over a larger area than previously thought.

Jo?o Alves is sure that this news will be received with interest by his colleagues: “W49A has long been known to radio astronomers as one of the most powerful star-forming region in the Galaxy with 30 or so massive baby-stars of the O-type, very deeply embedded in their parental cloud. What we have found is in fact quite amazing: this stellar maternity ward is much bigger than we first thought and it has not stopped forming stars yet. We now have evidence for no less than more than one hundred such stars in this region, way beyond the few dozen known until now”.

Nicole Homeier adds: “Above all, we uncovered four massive clusters in there, with stars as massive as 120 times the mass of our Sun – real ‘beasts’ that bombard their surroundings with incredibly intense stellar winds and strong ultraviolet light. This is not a nice place to live – and imagine, this is all inside our so-called ‘quiet Galaxy’!”

Original Source: ESO News Release

Image credit: CfA

Astronomers have found a distant galaxy with a core shaped like a warped pancake around its central supermassive black hole. The disk contains 400,000 times the mass of the Sun, and got its strange shape because the black hole is spewing material in two broad cones. This is different from most black holes, which channel the outflow into a thin, fast-moving jet.

While a person’s shape can be affected by pancakes, especially if you eat too many, you may not expect the same to be true on a cosmic scale. As it turns out, at least for the Circinus spiral galaxy, a pancake can shape an entire galactic nucleus. Astronomer Lincoln Greenhill (Harvard-Smithsonian Center for Astrophysics) and colleagues have found direct evidence for a “pancake” of gas and dust at the center of Circinus — a thin, warped disk surrounding the galaxy’s central, supermassive black hole.

That disk shapes the galaxy’s nucleus. It shadows different regions from the “glare” of the black hole, a glare created by the glow of accreting gas. And when some of this material is blown away from the black hole, as by radiation, the disk channels it, leaving shadowed regions in relative peace. This idea stands in contrast to the prevailing wisdom that shadows and outflows are caused by vast, thick “doughnuts” of dust and gas.

“We caught the Circinus galaxy and its black hole red-handed,” said Greenhill. “Most astronomers think that the center of an active galaxy has an outflow directed and channeled by a doughnut-shaped torus of dust and gas. Our detailed radio images show that the culprit is a warped disk. And if that’s true for the Circinus galaxy, then the same may be true for other active galaxies.”

Greenhill and his fellow astronomers identified the disk using the Australia Telescope Long Baseline Array, which is a network of radio telescopes 600 miles across. Only radio imaging can reveal directly such tiny structures inside galactic nuclei. The Circinus disk in particular is so deeply buried in a jumble of stars, gas, and dust that no optical telescope can detect it. They estimate the disk contains enough mass to form perhaps as many as 400,000 stars like our Sun, were it given a chance.

The Australian array picked up microwave signals from clouds rich in water vapor within both the warped edge-on disk and the outflow. The locations and velocities of the clouds provide strong evidence that the disk is channeling ejected material into two broad cones extending above and below the galactic plane.

“Water masers have been observed in broad, wide-angle outflows in star formation regions within our Galaxy, but this is the first time they have been observed associated with the nuclear region of an active galaxy,” said Simon Ellingsen (University of Tasmania), a co-author of the study. “These observations also are the first to show that this wide-angle outflow originates within about a third of a light-year from the galactic nucleus.”

A black hole is a massive object so compact and with such a powerful gravitational field that nothing can escape its pull once past the black hole’s event horizon. However, material can and does escape from regions near the black hole due to radiation pressure and inefficiencies of the accretion flow, among other things. The escaping material carries away angular momentum, allowing the remaining matter to fall into the black hole. The black hole in Circinus presents a stark contrast to other supermassive black holes whose outflows are channeled into long, narrow jets of material that blast out from the galactic nucleus.

“In the center of the Circinus galaxy, we see a black hole that spews out gas and dust in a broad spray like clouds of vapor from a steam locomotive. This presents us with a paradox. X-ray radiation from the nucleus of Circinus — radiation driven by the black hole — is as intense as for black holes in other active galaxies. In that way, the Circinus black hole appears to be typical. However, while other black holes drive narrow relativistic jets of plasma, the Circinus black hole drives a comparatively meek wind — one that can support the formation of delicate molecules and dust,” said Greenhill.

Greenhill and his colleagues plan to continue studying the nucleus of the Circinus galaxy to investigate the mechanism responsible for generating the outflow.

Original Source: CfA News Release