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💫Inside Globular Cluster M22

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Piercing the heart of a globular star cluster with its needle-sharp vision, NASA's Hubble Space Telescope has uncovered tantalizing clues to what could potentially be a strange and unexpected population of wandering, planet-sized objects. In results published this week in NATURE, the international science journal, Kailash Sahu (Space Telescope Science Institute, Baltimore, MD) and colleagues report six unusual microlensing events inside the globular cluster M22. Microlensing occurs when a background star brightens momentarily as a foreground object drifts by. The unusual objects thought to cause these events are far too dim to be seen directly, but instead were detected by the way their gravitational field amplifies light from a distant background star in the huge central bulge of our galaxy. Microlensing has been used before to search for low-mass objects in the disk and halo of our galaxy, but Hubble's sharp vision is essential to probe the interiors of globular clusters further.




In addition to the microlensing event caused by the dwarf star, Sahu and his team recorded six even more interesting, unexpectedly brief events where a background star jumped in brightness by as much as a factor of two for less than 20 hours before dropping back to normal brightness. This means that the microlensing object must have been much smaller than a normal star. These microlensing events were unusually brief, indicating that the mass of the intervening object could be as little as 80 times that of Earth. So what are they? Theoretically they might be planets that were gravitationally torn away from parent stars in the cluster. However, they are estimated to make up as much as 10 percent of the cluster's mass — too numerous to be wandering, "orphaned" planets. The results are so surprising, the astronomers caution that these preliminary observations must be confirmed by follow-up Hubble observations. If verified, these dark denizens could yield new insights about how stars and planets formed in the early universe. "Hubble's excellent sharpness allowed us to make this remarkable new type of observation, successfully demonstrating our ability to see very small objects," says Sahu. "This holds tremendous potential for further searches for dark, low-mass objects."

"Since we know that globular clusters like M22 are very old, this result opens new and exciting opportunities for the discovery and study of planet-like objects that formed in the early universe," adds co-investigator Nino Panagia (European Space Agency and Space Telescope Science Institute). "This initial observation shows that our microlensing method works beautifully," states co-investigator Mario Livio (Space Telescope Science Institute). As microlensing events are brief, unpredictable and rare, astronomers improve their chances of observing one by looking at many stars at once — much like a person buying several lottery tickets at once. Most microlensing searches have been aimed at the central bulge of our galaxy or out towards the Magellanic Clouds — the densest observable regions of stars in the sky. In general these surveys cover areas of sky larger than the full Moon and look for foreground objects lying somewhere between us and the background population of stars. Sahu and his team took advantage of Hubble's superb resolution and narrow field of view to aim the telescope directly through the center of a globular star cluster lying between Earth and the galactic bulge.

This gave the team a very dense stellar region to probe for drifting low-mass foreground objects and a very rich background field of stars to be lensed. Only Hubble's resolution is sharp enough to actually peer through the crowded center of the cluster and see the far more distant stars in the galactic bulge. As the lensing objects were part of the cluster, the astronomers also had an accurate distance (8,500 light-years) and velocity for these objects. In a normal lensing event, a background star brightens and dims for a length of time depending on the mass of the lensing body. The short, "spurious" events seen by the team are shorter than the interval between the Hubble observations, leading to an upper estimate for the mass of an object of one quarter Jupiter's mass. To confirm these extraordinary, but tentative results, Sahu and colleagues next plan to monitor the center of the globular cluster continuously over a seven-day interval. They expect to detect 10 to 25 short-duration microlensing events, which will be well-sampled enough to yield direct measurements of the true masses of the small bodies.


Credit: NASA, ESA, and K. Sahu (STScI)


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💫The region around MWC 147


Wide field image taken by Stéphane Guisard (ESO) with a 200 mm lens from Paranal, showing the region of the sky with the NGC 2247 star forming complex and containing the Herbig Ae/Be object MWC 147. Visible in the image are the rich, colourful Cone nebula region (at the centre-left of the image) and the Rosette Nebula (at the top right). They are both located in the Monoceros constellation, very close to the better known Orion constellation. The star MWC 147 is close to a dark nebula, in the top left part of the image, and belongs to an association of massive stars, the Monoceros OB1 association. The close-up reveals that MVC 147 is surrounded by some nebulosity. The image is a colour-composite based on exposures through R, V, B, and H-alpha filters, for a total exposure time of more than 16 hours.

Credit:
ESO/S. Guisard (www.eso.org/~sguisard)


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💫ACS COSMOS Tile

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An international team of astronomers using Hubble Space Telescope has created the first three-dimensional map of the large-scale distribution of dark matter in the universe. Dark matter is an invisible form of matter whose total mass in the universe is more than five times that of "normal" matter (i.e., atoms). The nature of dark matter is still unknown. Its presence in the universe is inferred from its current influence within galaxies and clusters of galaxies, and the gravitational effect it has had on the evolution of structure in the universe. The first direct detection of dark matter was made this past year through observations of the Bullet Cluster of galaxies. This new map provides the best evidence to date that normal matter, largely in the form of galaxies, accumulates along the densest concentrations of dark matter. The map reveals a loose network of filaments that grew over time and intersect in massive structures at the locations of clusters of galaxies.




The map stretches halfway back to the beginning of the universe and shows how dark matter has grown increasingly "clumpy" as it collapses under gravity. The dark matter map was constructed by measuring the shapes of half a million faraway galaxies. To reach Hubble, the light of the galaxies traveled through intervening dark matter. The dark matter deflected the light slightly as it traveled through space. Researchers used the observed, subtle distortion of the galaxies' shapes to reconstruct the distribution of intervening mass along Hubble's line of sight, a method called "weak gravitational lensing. For astronomers, the challenge of mapping dark matter in the universe has been similar to mapping a city from nighttime aerial snapshots showing only streetlights. Dark matter is invisible, so only the luminous galaxies can be seen directly. These new map images are equivalent to seeing a city, its suburbs and country roads in daylight for the first time. Major arteries and intersections become evident, and a variety of neighborhoods are visible. Mapping dark matter's distribution in space and time is fundamental to understanding how galaxies grew and clustered over billions of years. Tracing the growth of clustering in dark matter may eventually also shed light on dark energy, a repulsive form of gravity that would have influenced how dark matter clumps.

The research results appeared online today in the journal Nature and were presented at the 209th meeting of the American Astronomical Society in Seattle, Wash., by Richard Massey and Nick Scoville. Both researchers are from the California Institute of Technology, Pasadena, Calif. "It's reassuring how well our map confirms the standard theories for structure formation," said Massey. He calls dark matter the "scaffolding" inside of which stars and galaxies have been assembled over billions of years. Researchers created the map using the Hubble's largest survey to date of the universe, the Cosmic Evolution Survey, otherwise known as COSMOS. The survey covers an area of sky nine times the area of the Earth's moon. This allows for the large-scale filamentary structure of dark matter to be evident. To add 3-D distance information, the Hubble observations were combined with multicolor data from powerful ground-based telescopes, Europe's Very Large Telescope in Chile, Japan's Subaru telescope in Hawaii, the U.S.'s Very Large Array radio telescope, New Mexico, as well as the European Space Agency's orbiting XMM-Newton X-ray telescope.


Credit: NASA, ESA, and R. Massey (California Institute of Technology)


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💫NGC 3576


Located 9,000 light-years away, NGC 3576 is a gigantic region of glowing gas about 100 light-years across, where stars are currently forming. The intense radiation and winds from the massive stars are shredding the clouds from which they form, creating dramatic scenery. The black area in the right middle part of the image is dark because of the presence of very dense, opaque clouds of gas and dust. The data used to make this colour-composite images were taken with ISAAC on the VLT, in the framework of observing proposal 079.C-0203(A). The image processing was done by Yuri Beletsky (ESO) and Hännes Heyer (ESO). It is based on data taken through 4 different narrow-band filters centred around 1.21, 1.71, 2.09 and 3.28 microns.

Credit:
ESO


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💫Distant Spiral Galaxy NGC 4603, Home to Variable Stars

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Hubble Space Telescope (HST) view of the magnificent spiral galaxy NGC 4603, the most distant galaxy in which a special class of pulsating stars called Cepheid variables have been found. It is associated with the Centaurus cluster, one of the most massive assemblages of galaxies in the nearby universe. The Local Group of galaxies, of which the Milky Way is a member, is moving in the direction of Centaurus at a speed of more than a million miles an hour under the influence of the gravitational pull of the matter in that direction. Clusters of young bright blue stars highlight the galaxy's spiral arms. In contrast, red giant stars in the process of dying are also found. Only the very brightest stars in NGC 4603 can be seen individually, even with the unmatched ability of the Hubble Space Telescope to obtain detailed images of distant objects. Much of the diffuse glow comes from fainter stars that cannot be individually distinguished by Hubble. The reddish filaments are regions where clouds of dust obscure blue light from the stars behind them.




This galaxy was observed by a team affiliated with the HST Key Project on the Extragalactic Distance Scale. Because NGC 4603 is much farther away than the other galaxies studied with Hubble by the Key Project team, 108 million light-years, its stars appear very faint from the Earth, and so accurately measuring their brightness, as is required for distinguishing the characteristic variations of Cepheids, is extremely difficult. At this distance some non-variable stars may by chance appear to grow brighter and fainter in the same fashion as Cepheids due to the physical impossibility of perfect measurements of such dim objects. Determining the distance to the galaxy required an unprecedented statistical analysis based on extensive computer simulations.

Researchers found 36-50 Cepheids and used their observed properties to securely determine the distance to NGC 4603. These measurements indicate that when the expansion of the universe and the motion of the Local Group are accounted for, the Centaurus cluster is very nearly at rest compared with the surrounding regions. It is part of the cause of the rapid motions in the nearby universe, rather than being strongly pulled by other concentrations of matter. Observations of distant Cepheids such as those in NGC 4603 also help astronomers to precisely measure the expansion rate of the universe.


Credit: Jeffrey Newman (Univ. of California at Berkeley) and NASA


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💫The field around HD 87643


This image, showing a very rich field of stars towards the Carina arm of the Milky Way, is centred on the star HD 87643, a member of the class of B[e] stars. The amazing image is set ablaze by a flurry of stars of all colours and brightnesses, some of which are seen against a backdrop of clouds of dust and gas. The red emission nebula is RCW47 and was catalogued in 1960. HD 87643 has been extensively studied with a full range of ESO telescopes, including the Very Large Telescope Interferometer (VLTI). Surrounded by a complex, extended nebula that is the result of previous violent ejections, the star is shown to have a companion. Interactions in this double system, surrounded by a dusty disc, may be the engine fuelling the star's remarkable nebula. The image, taken with the Wide Field Imager on the MPG/ESO 2.2-metre telescope at La Silla, is based on data obtained through different filters: B, V and R. The field of view is 33 x 32 arc-minutes.

Credit:
ESO/F. Millour


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