Hubble directly observes the disk around a black hole

ScienceDaily (Nov. 4, 2011) — A team of scientists has used the NASA/ESA Hubble Space Telescope to observe a quasar accretion disc -- a brightly glowing disc of matter that is slowly being sucked into its galaxy's central black hole. Their study makes use of a novel technique that uses gravitational lensing to give an immense boost to the power of the telescope. The incredible precision of the method has allowed astronomers to directly measure the disc's size and plot the temperature across different parts of the disc.

An international team of astronomers has used a new technique to study the bright disc of matter surrounding a faraway black hole. Using the NASA/ESA Hubble Space Telescope, combined with the gravitational lensing effect of stars in a distant galaxy [1], the team measured the disc's size and studied the colours (and hence the temperatures) of different parts of the disc. These observations show a level of precision equivalent to spotting individual grains of sand on the surface of the Moon.

While black holes themselves are invisible, the forces they unleash cause some of the brightest phenomena in the Universe. Quasars -- short for quasi-stellar objects -- are glowing discs of matter that orbit supermassive black holes, heating up and emitting extremely bright radiation as they do so.

"A quasar accretion disc has a typical size of a few light-days, or around 100 billion kilometres across, but they lie billions of light-years away. This means their apparent size when viewed from Earth is so small that we will probably never have a telescope powerful enough to see their structure directly," explains Jose Muñoz, the lead scientist in this study.

Until now, the minute apparent size of quasars has meant that most of our knowledge of their inner structure has been based on theoretical extrapolations, rather than direct observations.

The team therefore used an innovative method to study the quasar: using the stars in an intervening galaxy as a scanning microscope to probe features in the quasar's disc that would otherwise be far too small to see. As these stars move across the light from the quasar, gravitational effects amplify the light from different parts of the quasar, giving detailed colour information for a line that crosses through the accretion disc.

The team observed a group of distant quasars that are gravitationally lensed by the chance alignment of other galaxies in the foreground, producing several images of the quasar.

They spotted subtle differences in colour between the images, and changes in colour over the time the observations were carried out. Part of these colour differences are caused by the properties of dust in the intervening galaxies: the light coming from each one of the lensed images has followed a different path through the galaxy, so that the various colours encapsulate information about the material within the galaxy. Measuring the way and extent to which the dust within the galaxies blocks light (known to astronomers as the extinction law) at such distances is itself an important result in the study.

For one of the quasars they studied, though, there were clear signs that stars in the intervening galaxy were passing through the path of the light from the quasar [2]. Just as the gravitational effect due to the whole intervening galaxy can bend and amplify the quasar's light, so can that of the stars within the intervening galaxy subtly bend and amplify the light from different parts of the accretion disc as they pass through the path of the quasar's light.

By recording the variation in colour, the team were able to reconstruct the colour profile across the accretion disc. This is important because the temperature of an accretion disc increases the closer it is to the black hole, and the colours emitted by the hot matter get bluer the hotter they are. This allowed the team to measure the diameter of the disc of hot matter, and plot how hot it is at different distances from the centre.

They found that the disc is between four and eleven light-days across (approximately 100 to 300 billion kilometres). While this measurement shows large uncertainties, it is still a remarkably accurate measurement for a small object at such a great distance, and the method holds great potential for increased accuracy in the future.

"This result is very relevant because it implies we are now able to obtain observational data on the structure of these systems, rather than relying on theory alone," says Muñoz. "Quasars' physical properties are not yet well understood. This new ability to obtain observational measurements is therefore opening a new window to help understand the nature of these objects."

Notes:

1] Gravity bends the structure of spacetime, and hence deflects beams of light. When the alignment is right, with one object directly behind another, the foreground object's gravity 'bends' the light like a lens, a process called gravitational lensing. Gravitational lenses typically produce multiple, distorted images of the distant object.

The most dramatic effects from gravitational lensing are the amplification and distortion of light from distant galaxies as it passes through massive galaxy clusters.

This effect also takes place on smaller scales, with galaxies at an intermediate distance lensing the light of distant quasars, producing multiple images of them that are visible through the lens galaxy.

Individual stars can also lens light, although this effect, called gravitational microlensing, is much more subtle and can only be detected by measuring how the lensing effect increases the source's brightness.

This study makes use of gravitational microlensing by stars in a foreground galaxy to study the accretion disc of a quasar in the background. It also uses the interplay of quasar light and gravitational lensing to probe the gas and dust content of intermediate galaxies.

[2] The lens galaxy in which this phenomenon was observed is called [WKK93] G; the lensed quasar is called HE 1104-1805.

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Journal Reference:

J.A. Mun ~oz, E. Mediavilla, C.S. Kochanek, E.E. Falco and A.M. Mosquera. A Study of Gravitational Lens Chromaticity with the Hubble Space Telescope. Astrophysical Journal, December 1, 2011

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Hubble directly observes the disk around a black hole

ScienceDaily (Nov. 4, 2011) — A team of scientists has used the NASA/ESA Hubble Space Telescope to observe a quasar accretion disc -- a brightly glowing disc of matter that is slowly being sucked into its galaxy's central black hole. Their study makes use of a novel technique that uses gravitational lensing to give an immense boost to the power of the telescope. The incredible precision of the method has allowed astronomers to directly measure the disc's size and plot the temperature across different parts of the disc.

An international team of astronomers has used a new technique to study the bright disc of matter surrounding a faraway black hole. Using the NASA/ESA Hubble Space Telescope, combined with the gravitational lensing effect of stars in a distant galaxy [1], the team measured the disc's size and studied the colours (and hence the temperatures) of different parts of the disc. These observations show a level of precision equivalent to spotting individual grains of sand on the surface of the Moon.

While black holes themselves are invisible, the forces they unleash cause some of the brightest phenomena in the Universe. Quasars -- short for quasi-stellar objects -- are glowing discs of matter that orbit supermassive black holes, heating up and emitting extremely bright radiation as they do so.

"A quasar accretion disc has a typical size of a few light-days, or around 100 billion kilometres across, but they lie billions of light-years away. This means their apparent size when viewed from Earth is so small that we will probably never have a telescope powerful enough to see their structure directly," explains Jose Muñoz, the lead scientist in this study.

Until now, the minute apparent size of quasars has meant that most of our knowledge of their inner structure has been based on theoretical extrapolations, rather than direct observations.

The team therefore used an innovative method to study the quasar: using the stars in an intervening galaxy as a scanning microscope to probe features in the quasar's disc that would otherwise be far too small to see. As these stars move across the light from the quasar, gravitational effects amplify the light from different parts of the quasar, giving detailed colour information for a line that crosses through the accretion disc.

The team observed a group of distant quasars that are gravitationally lensed by the chance alignment of other galaxies in the foreground, producing several images of the quasar.

They spotted subtle differences in colour between the images, and changes in colour over the time the observations were carried out. Part of these colour differences are caused by the properties of dust in the intervening galaxies: the light coming from each one of the lensed images has followed a different path through the galaxy, so that the various colours encapsulate information about the material within the galaxy. Measuring the way and extent to which the dust within the galaxies blocks light (known to astronomers as the extinction law) at such distances is itself an important result in the study.

For one of the quasars they studied, though, there were clear signs that stars in the intervening galaxy were passing through the path of the light from the quasar [2]. Just as the gravitational effect due to the whole intervening galaxy can bend and amplify the quasar's light, so can that of the stars within the intervening galaxy subtly bend and amplify the light from different parts of the accretion disc as they pass through the path of the quasar's light.

By recording the variation in colour, the team were able to reconstruct the colour profile across the accretion disc. This is important because the temperature of an accretion disc increases the closer it is to the black hole, and the colours emitted by the hot matter get bluer the hotter they are. This allowed the team to measure the diameter of the disc of hot matter, and plot how hot it is at different distances from the centre.

They found that the disc is between four and eleven light-days across (approximately 100 to 300 billion kilometres). While this measurement shows large uncertainties, it is still a remarkably accurate measurement for a small object at such a great distance, and the method holds great potential for increased accuracy in the future.

"This result is very relevant because it implies we are now able to obtain observational data on the structure of these systems, rather than relying on theory alone," says Muñoz. "Quasars' physical properties are not yet well understood. This new ability to obtain observational measurements is therefore opening a new window to help understand the nature of these objects."

Notes:

1] Gravity bends the structure of spacetime, and hence deflects beams of light. When the alignment is right, with one object directly behind another, the foreground object's gravity 'bends' the light like a lens, a process called gravitational lensing. Gravitational lenses typically produce multiple, distorted images of the distant object.

The most dramatic effects from gravitational lensing are the amplification and distortion of light from distant galaxies as it passes through massive galaxy clusters.

This effect also takes place on smaller scales, with galaxies at an intermediate distance lensing the light of distant quasars, producing multiple images of them that are visible through the lens galaxy.

Individual stars can also lens light, although this effect, called gravitational microlensing, is much more subtle and can only be detected by measuring how the lensing effect increases the source's brightness.

This study makes use of gravitational microlensing by stars in a foreground galaxy to study the accretion disc of a quasar in the background. It also uses the interplay of quasar light and gravitational lensing to probe the gas and dust content of intermediate galaxies.

[2] The lens galaxy in which this phenomenon was observed is called [WKK93] G; the lensed quasar is called HE 1104-1805.

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and Google +1:

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Story Source:

The above story is reprinted from materials provided by ESA/Hubble Information Centre.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

J.A. Mun ~oz, E. Mediavilla, C.S. Kochanek, E.E. Falco and A.M. Mosquera. A Study of Gravitational Lens Chromaticity with the Hubble Space Telescope. Astrophysical Journal, December 1, 2011

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

View the original article here

Scientist searches for moons around asteroids

ScienceDaily (Oct. 7, 2011) — Most people know that some planets have moons but would be surprised to know that some asteroids do, too. According to Joshua Emery, assistant professor of earth and planetary sciences at the University of Tennessee, Knoxville, about 20 percent of them do.

Emery is part of an international team of planetary astronomers, led by Franck Marchis of the Carl Sagan Center of the SETI Institute in Mountain View, Calif., searching for moons around asteroids. The discovery of moons around asteroids is important because it can provide clues to the asteroid's formation.

Emery and his team's research has focused on the triple asteroid Minerva, the fourth asteroid located in the main-belt -- which houses most of the solar system's asteroids -- known to possess two moons.

"Minerva was thought to be a pretty typical, unremarkable asteroid until we discovered its two moons," said Emery. "Now, interest in this system has grown, and through a lot of new observations from both ground-based and space-based telescopes, we have developed a much more detailed understanding of Minerva and its moons."

The team studied the asteroid in detail using the large W.M. Keck telescope in Hawaii and a small robotic telescope at Kitt Peak in Arizona. By piecing together old and new observations, the astronomers were able to make precise determinations of the moons' orbits. With shape, size, and mass in hand, the scientists then derived the asteroid's density -- determining that Minerva is different than the other large asteroids in the main-belt.

"All other large main-belt asteroids with one or more moons are very porous," said Emery. "Such high porosity strongly suggests that they are piles of rubble held together by gravity rather than solid rocks. Imagine an asteroid being completely blasted apart in a collision, then the pieces coalescing back together--this is how we think most of these large, multiple asteroid systems form. From these glimpses into the interior structure of asteroids, we gain insight not only into the history and formation of multiple asteroid systems, but also the structure and origin of asteroids in general."

The results of the group's findings were released at the EPSC-DPS meeting in Nantes, France. Other members of the international team of planetary astronomers are J.E. Enriquez, of Carl Sagan Center at the SETI Institute, Calif.; P. Descamps, J. Berthier, and F. Vachier of the Institut de Mecanique Celeste et de Calcul des Ephemerides, France; J. Durech of Charles University, Prague, Czech republic; P. Dalba, UC Berkeley, Calif.; A.W. Harris of DLR, Berlin, Germany; J. Melbourne of Caltech, Pasadena, Calif.; A.N. Stockton and T.J. Dupuy of the University of Hawaii, Honolulu; and C.D. Fassnacht of the University of California at Davis, Calif.

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by University of Tennessee at Knoxville, via EurekAlert!, a service of AAAS.

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

View the original article here

Scientist searches for moons around asteroids

ScienceDaily (Oct. 7, 2011) — Most people know that some planets have moons but would be surprised to know that some asteroids do, too. According to Joshua Emery, assistant professor of earth and planetary sciences at the University of Tennessee, Knoxville, about 20 percent of them do.

Emery is part of an international team of planetary astronomers, led by Franck Marchis of the Carl Sagan Center of the SETI Institute in Mountain View, Calif., searching for moons around asteroids. The discovery of moons around asteroids is important because it can provide clues to the asteroid's formation.

Emery and his team's research has focused on the triple asteroid Minerva, the fourth asteroid located in the main-belt -- which houses most of the solar system's asteroids -- known to possess two moons.

"Minerva was thought to be a pretty typical, unremarkable asteroid until we discovered its two moons," said Emery. "Now, interest in this system has grown, and through a lot of new observations from both ground-based and space-based telescopes, we have developed a much more detailed understanding of Minerva and its moons."

The team studied the asteroid in detail using the large W.M. Keck telescope in Hawaii and a small robotic telescope at Kitt Peak in Arizona. By piecing together old and new observations, the astronomers were able to make precise determinations of the moons' orbits. With shape, size, and mass in hand, the scientists then derived the asteroid's density -- determining that Minerva is different than the other large asteroids in the main-belt.

"All other large main-belt asteroids with one or more moons are very porous," said Emery. "Such high porosity strongly suggests that they are piles of rubble held together by gravity rather than solid rocks. Imagine an asteroid being completely blasted apart in a collision, then the pieces coalescing back together--this is how we think most of these large, multiple asteroid systems form. From these glimpses into the interior structure of asteroids, we gain insight not only into the history and formation of multiple asteroid systems, but also the structure and origin of asteroids in general."

The results of the group's findings were released at the EPSC-DPS meeting in Nantes, France. Other members of the international team of planetary astronomers are J.E. Enriquez, of Carl Sagan Center at the SETI Institute, Calif.; P. Descamps, J. Berthier, and F. Vachier of the Institut de Mecanique Celeste et de Calcul des Ephemerides, France; J. Durech of Charles University, Prague, Czech republic; P. Dalba, UC Berkeley, Calif.; A.W. Harris of DLR, Berlin, Germany; J. Melbourne of Caltech, Pasadena, Calif.; A.N. Stockton and T.J. Dupuy of the University of Hawaii, Honolulu; and C.D. Fassnacht of the University of California at Davis, Calif.

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Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by University of Tennessee at Knoxville, via EurekAlert!, a service of AAAS.

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

View the original article here