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Posted: Feb 20, 2013

3-D Observations of the outflow from an active galactic nucleus

(Nanowerk News) Quasars are bright central regions of some distant galaxies. Their luminosities are often hundreds of times greater than those of their host galaxies (Note 2). Scientists believe that their light source is a very bright gaseous disk surrounding a supermassive black hole at the center of the galaxy. Gas streams called “outflows” move outward from the disk and have a substantial influence on surrounding interstellar/intergalactic regions. However, because quasars at large distances look like mere stars, their internal structures are not easy to investigate.
The current team used the large light-gathering power of the 8.2-meter Subaru Telescope mounted with its high-resolution spectrograph HDS (High Dispersion Spectrograph) to observe the quasar SDSS J1029+2623 (from now on referred to as “J1029”) and examine its structure. This quasar is ~10 billion light-years distant from Earth (Note 1) toward the constellation Leo, and a massive cluster of galaxies, ~5 billion light-years away, lies between the quasar and Earth. Because astronomical objects are usually very distant, they are difficult to study from different angles. Nevertheless, gravitational lensing opens up this possibility.
quasar
An artist's rendition of the central region of the quasar. A gaseous disk surrounds a central black hole. The outflow is gas streaming from the disk outward along the curved mesh, which indicates the distortion of space/time, and is distinguished from a jet that is blowing vertically. (Iamge: Shinshu University and the National Astronomical Observatory of Japan)
If a cluster of galaxies lies along the line of sight to a distant quasar, then part of the light from the distant quasar (the “lensed quasar”) bends around the intervening cluster (the “lensing cluster”), and observers will see more highly resolved and brighter images of the now magnified background quasar. Due to the gravitational lensing by the cluster intervening between J1029 and Earth, there is significant distortion in the light path from the quasar, and it splits into three images: A, B, and C (Figure 3, Note 3). The maximum separation angle, ~22”.5 (Note 4), between images A and B is a current record; it is larger than the typical separation of quasar images lensed by a single galaxy. The team hypothesized that each lensed image could contain information on the outflow from the quasar when viewed from different angles ("pectroscopy along Multiple, Lensed Sight Lines through Outflowing Winds in the Quasar SDSS J1029+2623").
The team used Subaru Telescope’s HDS to perform spectroscopic observations of the brightest two images A and B (Note 5), and their results supported their hypothesis. Any absorber between the quasar and Earth provides absorption features in the spectra of the quasar images. While most absorption features originate from foreground objects that are physically unrelated to the quasar, some show clear evidence of origins from the outflow, such as partial coverage by clouds (Note 6). Those features show a clear difference between the images A and B, although they are generally similar (Figure 4). This result supports the idea that the sight lines are going through different areas of the outflow from different directions. When viewed through one eye alone, an object appears to be two-dimensional, but viewing with both eyes yields a 3-D image that provides multi-directional information. This process is analogous to what occurred in the observations (Figure 2, Note 7).
It is surprising that the absorption profiles arising in the outflow show clear differences between them, despite the small separation angle of ~22”.5. Misawa commented on this discovery saying, “The outflow may not necessarily be homogeneous, but could instead have a complex internal structure with a number of clumpy gas clouds like cirrocumulus clouds in Earth’s atmosphere. The team plans to observe the area in image C in more detail.” Direct observation of a clumpy structure in tandem with theoretical analysis will contribute to revealing the mysterious formation history of these outflows.
The team has also explored other explanations for the outflows. Because the light paths of the images A and B are different, they have a substantial time difference between them when they reach Earth (Note 8). If the internal structure of the outflow varies with time, the two images deliver information about different epochs even if they pass through the same region of the outflow. The astronomers intend to conduct observations with the Subaru Telescope in March 2013 to test the “time-variation” scenario.
Notes
1. It corresponds to a redshift of z~2.197. A “z” or redshift value measures how much the expansion of space has stretched the light from an object. Generally, the greater the observed z value for a galaxy, the more distant it is in time and space from Earth.
2. Because their appearance is star-like, they are called “quasi-stellar objects” and abbreviated as quasars.
3. An international team led by Naohisa Inada and Masamune Oguri (both are members of the current research team) discovered the first quasar (SDSS J1004+4112) that is lensed by a cluster of galaxies (http://www.sdss.org/news/releases/20031217.lensing.html). Only three quasars that are lensed by a cluster of galaxies have been discovered so far (SDSS J1004+4112, SDSS J1029+2623, and SDSS J2222+2745). Among them, SDSS J1029+2623 has the largest separation angle.
4. 1 arcsec (1”) is a unit of angle, defined as 1/3600 of 1 degree. Human eyes cannot distinguish such a small angle.
5. The observed flux ratio of the three lensed images is A:B:C ~ 0.95:1.00:0.24.
6. This means that an absorber only partially covers the background light source toward the sight line. Because foreground interstellar or intergalactic media are larger than the light source of the quasar by more than several orders, only small gas clouds in the vicinity of the quasar can reproduce a partial coverage.
7. Similar observation has been also performed for the other lensed quasar SDSS J1004+4112 (Green, P. 2006, The Astrophysical Journal, vol. 644, pp.733-741).
8. The image A leads the image B by 744 days (Fohlmeister, J. et al., 2013, the Astrophysical Journal, vol. 764, 186): http://dx.doi.org/10.1088/0004-637X/764/2/186
Source: National Astronomical Observatory of Japan
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