This picture shows a quasar that has been gravitationally lensed by a galaxy in the foreground, which can be seen as a faint shape around the two bright images of the quasar. Credit: NASA/ESA/J.A. Muñoz (University of Valencia)
By Hubble ESA, Garching, Germany
Published : November 4, 2011
An international team of astronomers has used a new technique to study the bright disk 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, the team measured the disk’s size and studied the colors and, hence, the temperatures of different parts of the disk. 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 disks of matter that orbit supermassive black holes, heating up and emitting extremely bright radiation as they do so.
“A quasar accretion disk has a typical size of a few light-days, or around 62 billion miles (100 billion kilometers) across, but they lie billions of light-years away,” said Jose Muñoz from the University of Valencia, Spain. “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.”
Until now, the minute 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.
They used the stars in an intervening galaxy as a scanning microscope to probe features in the quasar’s disk 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 color information for a line that crosses through the accretion disk.
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 color between the images as well as changes in color over the time the observations were carried out. The properties of dust in the intervening galaxies cause part of these color differences. The light coming from each one of the lensed images has followed a different path through the galaxy, so the various colors encapsulate information about the material within the galaxy. Measuring the way and extent to which the dust within the galaxies blocks light at such distances is 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 light from the quasar. 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 disk as they pass through the path of the quasar’s light.
By recording the variation in color, the team was able to reconstruct the color profile across the accretion disk. This is important because the temperature of an accretion disk increases the closer it is to the black hole, and the colors emitted by the hot matter get bluer the hotter they are. This allowed the team to measure the diameter of the disk of hot matter and plot how hot it is at different distances from the center.
They found that the disk is approximately 60-190 billion miles (100-300 billion km) across. 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,” said 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.”
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