3-D View of a Quasar with a Galactic Lens


I get excited when I see astronomers using novel techniques to study more distant and more difficult problems in astronomy. In this case, we have a galaxy cluster acting as a lens and some high-resolution spectroscopy to get a 3-dimensional view of a quasar.

A typical quasar is dominated by a point of light coming from the very center of the distant galaxy, specifically from the accretion disk around a supermassive black hole that is pulling matter into it. That bright central quasar can often act as a background light for probing what is in front of that quasar along our line of sight, including the structures within the very same galaxy.

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For this study, a group led by Toru Misawa looks at the broad absorption line features in the spectra of one such quasar, J1029+2623. Such features are not uncommon in quasar spectra as they are a result of some outflow of material away from the accretion disk. Yes, there are things falling in AND things flowing away from the black hole; it’s a very turbulent place. In fact, these outflows are key to transporting angular momentum away from the black hole so that more material can fall in. They also can have the side effect of shutting down star formation in other parts of the galaxy. So, they are quite important to understanding the whole evolution of a quasar.

Artist conception, where paths A, B, and C correspond to the image at top.

However, typically, a quasar only allows one, piercing line of sight through the outflow region. J1029+2623 is different because it is being lensed by a whole galaxy cluster that lies somewhere in front of it along our line of sight. So there are three different images of the quasar widely separated apart. This means, as the diagram above shows, you get several different lines of sight through the outflow and can begin to discern its shape and properties.

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The astronomers confirmed this by doing high resolution spectroscopy of quasar with the 8.2-meter Subaru Telescope in Hawaii. The shape of the broad absorption lines were different from each lensed image of the quasar. They narrowed this down to being caused by two scenarios: either they were seeing different parts of the outflow cloud of material, or they were seeing changes over a large size scale over a short period of time, since the lensed images take slightly different paths to get to us. Further observations have been scheduled on the telescope to further distinguish between the two possibilities.

Although this method had been tried before, this is the most widely separated quasar lens known, so this had the best chance of succeeding. That means there probably won’t be many more examples that can be studied, so what astronomers learn here from continued observations will serve as just one model to try with other quasars.

It is now thought that many galaxies went through an active stage at some point in their evolution that affected what galaxies will look like today. Also, it has already been theorized that most active galaxies, in all their myriad manifestations, are generally the same type of object, though some differences exist that make one “unification” model too simple to explain every known active galaxy.

The canonical active galaxy unification scheme, as described by Antonucci 1993.

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Dare I say, active galaxies such as this one will remain an active area of research as astronomers continue to unravel the mysteries of galaxy evolution with bigger telescopes to come.

This research was published in Astronomical Journal, and the preprint is available at arXiv.org.

Images: Top - Color image of the region around SDSS J1029+2623, taken with Hubble Space Telescope. Quasar images (marked with A, B, and C) are gravitationally lensed by a foreground cluster of galaxies. Three galaxies of the lensing cluster (marked as G1a, b and G2) are visible. Credit: Shinshu University, the National Astronomical Observatory of Japan, and Kavli Institute for the Physics and Mathematics of the Universe. Middle - Schematic drawing of the gravitational lensing. Credit: Shinshu University and the National Astronomical Observatory of Japan. Bottom – The “zoo” of active galaxy types as explained by a “unification scheme” described in Antonucci 1993

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