NASA's Hubble Space Telescope is back in business

The telescope captured an image of Arp 147.

Provided by the Space Telescope Science Institute

This Hubble image shows interacting galaxies Arp 147. NASA/ESA/M. Livio (STScI) October 31, 2008 Just a couple of days after the orbiting observatory was brought back online, Hubble aimed its prime working camera, the Wide Field Planetary Camera 2 (WFPC2), at a pair of gravitationally interacting galaxies called Arp 147. The galaxy pair was photographed October 27-28, 2008. Arp 147 lies in the constellation Cetus, and it is more than 400 million light-years away Earth. The image shows the camera functions as it did before going offline. The two galaxies happen to be oriented so that they appear to mark the number 10. The left-most galaxy, or the "one" in the image, is relatively undisturbed apart from a smooth ring of starlight. It appears nearly on edge to our line of sight. The right-most galaxy, resembling a "zero," exhibits a clumpy, blue ring of intense star formation. The blue ring probably formed after the galaxy on the left passed through the galaxy on the right. Just as a pebble thrown into a pond creates an outwardly moving circular wave, a propagating density wave was generated at the point of impact and spread outward. As this density wave collided with material in the target galaxy that was moving inward due to the gravitational pull of the two galaxies, shocks and dense gas were produced, stimulating star formation. The dusty reddish knot at the blue ring's lower left probably marks the location of the original nucleus of the galaxy that was hit. WFPC2 used three separate filters to capture the picture. The blue, visible-light, and infrared filters are represented by the colors blue, green, and red, respectively.

Hungry, hungry black holes

Large and small black holes may have similar feeding habits, according to a new study.

Provided by the Chandra X-Ray Center

This NASA image of M81 includes X-ray data from the Chandra X-ray Observatory (blue), optical data from the Hubble Space Telescope (green), infrared data from the Spitzer Space Telescope (pink) and ultraviolet data from GALEX (purple). The inset shows the center of M81, a supermassive black hole that is about 70 million times more massive than the Sun. NASA/CXC/Wisconsin/D.Pooley/CfA/A.Zezas/ESA/JPL Caltech/J.Huchra et al.]

June 19, 2008
The biggest black holes may feed just like the smallest ones, according to data from NASA's Chandra X-ray Observatory and ground-based telescopes. This discovery supports the implication of Einstein's relativity theory that black holes of all sizes have similar properties, and will be useful for predicting the properties of a conjectured new class of black holes.

The conclusion comes from a large observing campaign of the spiral galaxy M81, which is about 12 million light-years from Earth. In the center of M81 is a black hole that is about 70 million times more massive than the Sun, and generates energy and radiation as it pulls gas in the central region of the galaxy inwards at high speed.

In contrast, so-called stellar mass black holes, which have about 10 times more mass than the Sun, have a different source of food. These smaller black holes acquire new material by pulling gas from an orbiting companion star. Because the bigger and smaller black holes are found in different environments with different sources of material to feed from, a question has remained about whether they feed in the same way.

Using these new observations and a detailed theoretical model, a research team compared the properties of M81's black hole with those of stellar mass black holes. The results show that either big or little, black holes indeed appear to eat similarly to each other, and produce a similar distribution of X-rays, optical and radio light.

One of the implications of Einstein's theory of General Relativity is that black holes are simple objects and only their masses and spins determine their effect on space-time. The latest research indicates that this simplicity manifests itself in spite of complicated environmental effects.

"This confirms that the feeding patterns for black holes of different sizes can be very similar," says Sera Markoff of the Astronomical Institute, University of Amsterdam in the Netherlands, who led the study. "We thought this was the case, but up until now we haven't been able to nail it."

The model that Markoff and her colleagues used to study the black holes includes a faint disk of material spinning around the black hole. This structure would mainly produce X-rays and optical light. A region of hot gas around the black hole would be seen largely in ultraviolet and X-ray light. A large contribution to both the radio and X-ray light comes from jets generated by the black hole. Multi-wavelength data is needed to disentangle these overlapping sources of light.

"When we look at the data, it turns out that our model works just as well for the giant black hole in M81 as it does for the smaller guys," says Michael Nowak, a coauthor from the Massachusetts Institute of Technology. "Everything around this huge black hole looks just the same except it's almost 10 million times bigger."

Among actively feeding black holes the one in M81 is one of the dimmest, presumably because it is "underfed". It is, however, one of the brightest as seen from Earth because of its relative proximity, allowing high quality observations to be made.

"It seems like the underfed black holes are the simplest in practice, perhaps because we can see closer to the black hole," says Andrew Young of the University of Bristol in England. "They don't seem to care too much where they get their food from."

This work should be useful for predicting the properties of a third, unconfirmed class called intermediate mass black holes, with masses lying between those of stellar and supermassive black holes. Some possible members of this class have been identified, but the evidence is controversial, so specific predictions for the properties of these black holes should be very helpful.

In addition to Chandra, three radio arrays (the Giant Meterwave Radio Telescope, the Very Large Array and the Very Long Baseline Array), two millimeter telescopes (the Plateau de Bure Interferometer and the Submillimeter Array), and Lick Observatory in the optical were used to monitor M81. These observations were made simultaneously to ensure that brightness variations because of changes in feeding rates did not confuse the results. Chandra is the only X-ray satellite able to isolate the faint X-rays of the black hole from the emission of the rest of the galaxy.

The winding Milky Way

The winding Milky Way

A new map reveals a complicated outer halo in our galaxy.

Provided by Sloan Digital Sky Survey

A theoretical model of a galaxy like the Milky Way, showing trails of stars torn from disrupted satellite galaxies that have merged with the central galaxy. The region shown is about 1 million light-years on a side; the Sun is just 25,000 light-years from the center of the galaxy and would appear close to the center of this picture. K. Johnston/J. Bullock

August 18, 2008
The halo of stars that envelops the Milky Way is like a river delta crisscrossed by stellar streams large and small, according to new data from the Sloan Digital Sky Survey (SDSS-II). While the largest rivers of this delta have been mapped out over the last decade, analysis of the new SDSS-II map shows that smaller streams can be found throughout the stellar halo, says Kevin Schlaufman, a graduate student at the University of California at Santa Cruz.

Schlaufman reported his results Saturday at an international symposium in Chicago, titled "The Sloan Digital Sky Survey: Asteroids to Cosmology." Over the last 3 years, Schlaufmann explains, the SEGUE survey of SDSS-II has measured the motions of nearly a quarter million stars in selected areas of the sky. A careful search for groups of stars at the same velocity turned up 14 distinct structures, 11 of them previously unknown.

"Even with SEGUE, we are still only mapping a small fraction of the galaxy," says Schlaufman, "so 14 streams in our data implies a huge number when we extrapolate to the rest of the Milky Way." If each velocity structure were a separate stream, Schlaufman explains, there would be close to 1,000 in the inner 75,000 light-years of the galaxy. However, these structures could arise from a smaller number of streams that are seen many times in different places.

"A jumble of pasta" is the way Columbia University researcher Kathryn Johnston described her theoretical models of the Milky Way's stellar halo. In a review talk at the symposium, Johnston 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," says 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. By looking at faint features, Kevin may be finding some of the 'angel hair' that came from smaller dwarfs, 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," says Cole, "because the stream from one dwarf galaxy can wrap around the Milky Way 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, Newberg finds that 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 in her symposium talk. "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."

In addition to stellar streams, astronomers searching the SDSS data have found 14 surviving dwarf companions of the Milky Way, including two new discoveries announced Saturday at the symposium by Gerard Gilmore of Cambridge University. These satellite galaxies are orbiting within the halo of invisible dark matter whose gravity holds the Milky Way itself together. Most of them are much fainter than the ten satellites known prior to the SDSS.

Because even the SDSS can only detect these ultra-faint dwarfs if they are relatively nearby, there could be several hundred more of them further out in the Milky Way's dark halo, according to independent analyses by graduate students Sergey Koposov, of the Max Planck Institute for Astronomy in Heidelberg, Germany, and Eric Tollerud, of the University of California at Irvine. "Even so," says Koposov, "we expect that the number of dark matter clumps is much larger than that, so something must prevent the smaller clumps from gathering gas and forming stars."

The SDSS dwarfs have far fewer stars than the previously known satellites, notes Gilmore, but they have similar spatial extents, and the stars within them move at similar speeds. "I think the internal dynamics of these tiny galaxies may be hard to explain with our conventional ideas about dark matter," says Gilmore.

"The SDSS has taught us a huge amount about the Milky Way and its neighbors," says Johnston, who is pleased to see some of the predictions of her models confirmed by the new data. "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."

Schoolteacher discovers 'cosmic ghost'

Galaxyzoo.org team members believe the gaseous object is the "light echo" of a quasar.

Provided by the University of Oxford

"Hanny's Voorwerp" is the green blob of gas (center) and is believed to be a "light echo" from the bright, stormy centerof a distant galaxy that has now gone dim. Dan Smith/Peter Herbert/Matt Jarvis/the ING [View Larger Image] August 7, 2008 A Dutch schoolteacher has discovered a mysterious and unique astronomical object through the Galaxy Zoo project, which enables members of the public to take part in astronomy research on-line. Hanny van Arkel, a primary schoolteacher from the Netherlands, came across the image of a strange gaseous object with a hole in the center that has been described as a "cosmic ghost" while using the galaxyzoo.org web site to classify images of galaxies. She posted about the image — which quickly became known as Hanny's "Voorwerp" after the Dutch word for "object" — on the Galaxy Zoo forum and the astronomers who run the site began to investigate. They soon realized the potential significance of what they think is a new class of astronomical object and will now use the Hubble Space Telescope to get a closer look at "Hanny's Voorwerp." "At first we thought it was a distant galaxy," said Dr. Chris Lintott of Oxford University, a galaxyzoo.org team member, "but we realized there were no stars in it so that it must be a cloud of gas." What was particularly puzzling to astronomers was that the gas was so hot — more than 18,000° Fahrenheit (10,000° Celsius) — when there were no stars in the vicinity to heat it up. "We now think that what we're looking at is light from a quasar — the bright, stormy center of a distant galaxy powered by a supermassive black hole," said Dr. Lintott. "The quasar itself is no longer visible to us, but its light continues to travel through space and the Voorwerp is a massive 'light echo' produced as this light strikes the gas." The black hole at the center of the galaxy, IC 2497, is now "turned off" — which is why the quasar has gone dim — but around 100,000 years ago the quasar was bright enough to have been visible from Earth through a small, inexpensive telescope. Dr. Lintott added: "From the point of view of the Voorwerp, the galaxy looks as bright as it would have done before the black hole turned off — it's this light echo that has been 'frozen in time' for us to observe. It's rather like examining the scene of a crime where, although we can't see them, we know the culprit must be lurking somewhere nearby in the shadows."

Hanny van Arkel, a Dutch primary schoolteacher, discovered a mysterious new astronomical object while on galaxyzoo.org. Edd Edmondson [View Larger Image]

"IC 2497 is so close that if the quasar was still shining today, on a good night you could probably see it with a small telescope," said galaxyzoo.org team member Kevin Schawinski of the Yale University who recently moved there from Oxford University. "The nearest active quasar, called 3C 273, is 1.7 billion light-years further away."

Smaller light echoes have been noted around supernovae before but never anything of the scale and shape of the Voorwerp. As yet nobody has a sensible explanation for the hole that runs through its center.

"It's amazing to think that this object has been sitting in the archives for decades and that amateur volunteers can help by spotting things like this on-line," said van Arkel. "It was a fantastic present to find out on my 25th birthday that we will get observational time on the Hubble Space Telescope to follow-up this discovery."

"This discovery really shows how citizen science has come of age in the Internet world," commented Professor Bill Keel of the University of Alabama, a galaxyzoo.org team member. "Hanny's attentiveness alerted us not only to a peculiar object, but to a window into the cosmic past, which might have eluded us for a long time otherwise. Trying to understand the processes operating here has proven to be a fascinating challenge, involving a whole range of astrophysical techniques and instruments around the world and beyond. This has also been some of the most rewarding astronomy I've done in years!"

Dr. Dan Smith of Liverpool John Moores University and Peter Herbert of the University of Hertfordshire were observing using the Isaac Newton Group of telescopes in La Palma, Spain, when word of the discovery filtered through. "When we got the news about Hanny's Voorwerp we were intrigued to find out what it was, and, fortunately, we were able to slew the telescopes round and get some great images and spectra to study it," said Dr Smith. "It was only later that we heard the story about how it had been discovered; it's inspirational that Hanny picked out this object from Galaxy Zoo in her spare time and nobody had ever seen anything like it before.'

During the last year, 50 million classifications of galaxies have been submitted on one million objects at www.galaxyzoo.org by more than 150,000 armchair astronomers from all over the world.

The next stage of Galaxy Zoo will ask volunteers for more detailed classifications, making it easier to identify more unusual objects such as Hanny's Voorwerp.