Quasars are the brightest persistent objects in the universe. They represent the brightest examples of a somewhat more common object: an active galactic nucleus (AGN). As the name implies, an AGN exists at the center of some galaxies. Most quasars, and even most AGN, are not found in the nearby universe, since they are especially luminous for a relatively brief time compared to the age of the universe, perhaps 50 or 100 million years, and also because the conditions under which they develop were more common in the early universe.
Because most AGN are quite distant, it’s very difficult to observe them in detail. However, persistent efforts of astrophysicists – based on separate observation of many objects, some of which are relatively close to us – have resulted in a reliable theoretical model of the structure of an AGN. At the center of an AGN is a supermassive black hole that may be as large as several billion solar masses. When there is a sufficient amount of galactic gas and dust available, it will be sucked into the vicinity of the black hole and form a relatively thin accretion disk that is heated hot enough to radiate vigorously at wavelengths far into the ultraviolet.
Clouds of somewhat cooler dust form in a toroidal ring outside of the disk. These clouds present an additional factor that makes it difficult to observe the accretion disk itself. Only if our line of sight is almost perpendicular to the disk is it possible to get an unobstructed view, and even then the large distance of most AGNs prevents seeing much detail.
But there are certain very rare instances when we get lucky and can see more – when a quasar is magnified by a gravitational lens formed by another galaxy that is directly between us and the quasar. In research just published, the Hubble Space Telescope was used to study six different quasars that were lensed by intervening galaxies. One objective of the study was to use light from the quasars as a way to probe the amount of dust present in the lensing galaxy.
However, in one case conditions were just right to allow astrophysicists to determine some properties of the quasar itself. The quasar in question is designated HE1104−1805. The image above shows two copies of HE1104−1805 that resulted from the lensing effect of the somewhat dimmer lens galaxy ([WKK93] G) itself (between the two images of HE1104−1805). The redshift of the quasar is z=2.32 and that of the lensing galaxy is z=0.73. These correspond to light travel time distances of 10.8 and 6.5 billion light-years, respectively.
The double image is caused by lensing due to the mass of the entire galaxy. However, individual stars within the galaxy can also cause lensing (“microlensing”) if they lie precisely between us and the quasar. Although that is not visible in the image, the effect can be detected from very subtle variations in the light due to motion of the lensing stars.
A careful analysis of the data yields results never before obtained – direct measurements of both the size of the accretion disk of a quasar and the temperature of some locations on the disk.
[T]here were clear signs that stars in the intervening galaxy were passing through the path of the 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 disc as they pass through the path of the quasar’s light.
By recording the variation in color, the team were able to reconstruct the color 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 colors 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 center.
They found that the disc is between four and eleven light-days across (approximately 100 to 300 billion kilometers). 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.
|Muñoz, J., Mediavilla, E., Kochanek, C., Falco, E., & Mosquera, A. (2011). A STUDY OF GRAVITATIONAL LENS CHROMATICITY WITH THE HUBBLE SPACE TELESCOPE|
The Astrophysical Journal, 742 (2) DOI: 10.1088/0004-637X/742/2/67