Hubble Sees Graceful Dance Of Two Interacting Galaxies

A pair of galaxies, known collectively as Arp 87, is one of hundreds of interacting and merging galaxies known in our nearby Universe. Arp 87 was originally discovered and catalogued by astronomer Halton Arp in the 1970s. Arp's Atlas of Peculiar Galaxies is a compilation of astronomical photographs using the Palomar 200-inch Hale and the 48-inch Samuel Oschin telescopes.The resolution in the Hubble image shows exquisite detail and fine structure that was not observable when Arp 87 was first discovered in the 1970's.
The two main players comprising Arp 87 are NGC 3808 on the right (the larger of the two galaxies) and its companion NGC 3808A on the left. NGC 3808 is a nearly face-on spiral galaxy with a bright ring of star formation and several prominent dust arms. Stars, gas, and dust flow from NGC 3808, forming an enveloping arm around its companion. NGC 3808A is a spiral galaxy seen edge-on and is surrounded by a rotating ring that contains stars and interstellar gas clouds. The ring is situated perpendicular to the plane of the host galaxy disk and is called a "polar ring."
As seen in other mergers similar to Arp 87, the corkscrew shape of the tidal material or bridge of shared matter between the two galaxies suggests that some stars and gas drawn from the larger galaxy have been caught in the gravitational pull of the smaller one.
The shapes of both galaxies have been distorted by their gravitational interaction with one another.
Interacting galaxies often exhibit high rates of star formation. Many lines of evidence -- colours of their starlight, intensity of emission lines from interstellar gas, far-infrared output from heated interstellar dust -- support this fact. Some merging galaxies have the highest levels of star formation we can find anywhere in the nearby Universe.
A major aspect of this excess star formation could be properly revealed only when Hubble turned its imaging capabilities toward colliding galaxies. Among the observatory's first discoveries was that galaxies with very active star formation contain large numbers of super star clusters -- clusters more compact and richer in young stars than astronomers were accustomed to seeing in our galactic neighbourhood.
Arp 87 is in the constellation Leo, the Lion, approximately 300 million light-years away from Earth. These observations were taken in February 2007 with the Wide Field Planetary Camera 2. Light from isolated blue, green, red, and infrared ranges was combined to form this colour image.

Mars With Ice: Detailed Picture Of Frigid Red Planet Emerges

Mars, like Earth, is a climate-fickle water planet. The main difference, of course, is that water on the frigid Red Planet is rarely liquid, preferring to spend almost all of its time traveling the world as a gas or churning up the surface as ice. That's the global picture literally and figuratively coming into much sharper focus as various Mars-orbiting cameras send back tomes of unprecedented super high-resolution imagery of ever vaster tracts of the planet's surface.What were just a few years ago small hints about Mars' water and climate, as seen in a few "postage-stamp" high-resolution images and topography, have given way to broader theory that explains not only the features seen on the planet today, but imply a dynamic history of Martian climate change.
"When you have postage stamps, it's like studying a hair on an arm instead of the whole arm," said Mars researcher James Head III of Brown University. Head will present the latest integrated global view of Martian surface features and how they fit with Martian climate models on October 28, 2007, at the Geological Society of America Annual Meeting in Denver.
The pictures now reveal a range of ice-made features that show a strong preference to certain latitudes, Head explains. As on Earth, latitude-dependent features can mean only one thing: latitude-dependent climate.
The signs of water ice are obvious today at Mars' poles. But as you move towards the equator, there is plenty of evidence of water ice having shaped the surface in different ways not so long ago.
Not far from either pole, for instance, widespread bumpy polygonal patterned ground suggests the contraction and expansion of icy permafrost ground — very similar to that seen in Earth's Arctic and Antarctic. Next, between 30 and 60 degrees latitude in both hemispheres, the patterned ground gives way to a pervasive pitted texture of once ice-rich dust deposits. Even closer to the equator on the flanks of Mars' equatorial volcanoes are compelling signs of large glaciers, almost exactly like those of Earth. There are also craters which seem to be filled with glacial debris and small valleys which drop precipitously into canyons — which on Earth is usually a strong indicator that a glacier once filled and widened the canyon.
As for where all the ice went, much of it was sublimed away and deposited at the poles. The ice rules the more temperate latitudes only when the tilt of Mars' spin axis is far more extreme than today — up to 45 degrees. That tilt, or obliquity, exposed the poles to a lot more sun during the course of a Martian year, according to climate models, evaporating the ice caps. That same water refroze on the surface in the then darker and colder equatorial and middle latitudes, hence all the evidence of ice and glaciers.
"It's a quest to understand the Martian water cycle," said Head describing his work.
Among the instruments used to study Mars are the Mars Global Surveyor's Laser Altimeter (MOLA) and Camera (MOC), the Mars Reconnaissance Orbiter's Context Camera (CTX) and High Resolution Imaging Science Experiment (HiRISE), and the Mars Express's High Resolution Stereo Camera (HRSC).

Massive Black Hole Discovered

Using two NASA satellites, astronomers have discovered the heftiest known black hole to orbit a star. The new black hole, with a mass 24 to 33 times that of our Sun, is more massive than scientists expected for a black hole that formed from a dying star.The newly discovered object belongs to the category of "stellar-mass" black holes. Formed in the death throes of massive stars, they are smaller than the monster black holes found in galactic cores. The previous record holder for largest stellar-mass black hole is a 16-solar-mass black hole in the galaxy M33, announced on October 17.
"We weren’t expecting to find a stellar-mass black hole this massive," says Andrea Prestwich of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., lead author of the discovery paper in the November 1 Astrophysical Journal Letters. "It seems likely that black holes that form from dying stars can be much larger than we had realized."
The black hole is located in the nearby dwarf galaxy IC 10, 1.8 million light-years from Earth in the constellation Cassiopeia. Prestwich’s team could measure the black hole’s mass because it has an orbiting companion: a hot, highly evolved star. The star is ejecting gas in the form of a wind. Some of this material spirals toward the black hole, heats up, and gives off powerful X-rays before crossing the point of no return.
In November 2006, Prestwich and her colleagues observed the dwarf galaxy with NASA’s Chandra X-ray Observatory. The group discovered that the galaxy’s brightest X-ray source, IC 10 X-1, exhibits sharp changes in X-ray brightness. Such behavior suggests a star periodically passing in front of a companion black hole and blocking the X-rays, creating an eclipse. In late November, NASA’s Swift satellite confirmed the eclipses and revealed details about the star’s orbit. The star in IC 10 X-1 appears to orbit in a plane that lies nearly edge-on to Earth’s line of sight, The Swift observations, as well as observations from the Gemini Telescope in Hawaii, told Prestwich and her group how fast the two stars go around each other. Calculations showed that the companion black hole has a mass of at least 24 Suns.
There are still some uncertainties in the black hole’s mass estimate, but as Prestwich notes, "Future optical observations will provide a final check. Any refinements in the IC 10 X-1 measurement are likely to increase the black hole’s mass rather than reduce it."
The black hole’s large mass is surprising because massive stars generate powerful winds that blow off a large fraction of the star’s mass before it explodes. Calculations suggest massive stars in our galaxy leave behind black holes no heavier than about 15 to 20 Suns.
The IC 10 X-1 black hole has gained mass since its birth by gobbling up gas from its companion star, but the rate is so slow that the black hole would have gained no more than 1 or 2 solar masses. "This black hole was born fat; it didn’t grow fat," says astrophysicist Richard Mushotzky of NASA Goddard Space Flight Center in Greenbelt, Md., who is not a member of the discovery team.
The progenitor star probably started its life with 60 or more solar masses. Like its host galaxy, it was probably deficient in elements heavier than hydrogen and helium. In massive, luminous stars with a high fraction of heavy elements, the extra electrons of elements such as carbon and oxygen "feel" the outward pressure of light and are thus more susceptible to being swept away in stellar winds. But with its low fraction of heavy elements, the IC 10 X-1 progenitor shed comparatively little mass before it exploded, so it could leave behind a heavier black hole.
"Massive stars in our galaxy today are probably not producing very heavy stellar-mass black holes like this one," says coauthor Roy Kilgard of Wesleyan University in Middletown, Conn. "But there could be millions of heavy stellar-mass black holes lurking out there that were produced early in the Milky Way’s history, before it had a chance to build up heavy elements."

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