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NAM 2013: Gravitational microlensing and quasars
BY KEITH COOPER
ASTRONOMY NOW

Posted: 1 July 2013


Stars in faraway galaxies may be magnifying the light of even more distant quasars, providing a serendipitous new technique for mapping these black hole powerhouses. That's the conclusion of new results presented for the first time today at the Royal Astronomical Society's National Astronomy Meeting (NAM) at the University of St Andrews.


How a passing star in a foreground galaxy bends light from a distant quasar, magnifying it. Image: Jason Cowan, Astronomy Technology Centre/NASA. See larger version.
 
Quasars are active supermassive black holes in the cores of galaxies. The black holes are feeding on so much matter that a disc of gas queuing up to be devoured grows so hot that it glows brightly while magnetic fields within the disc whip up white hot plasma at millions of degrees, firing beams of charged particles many hundreds of thousands of light years away from the black hole. When we look down one of these beams, we see a brilliant quasar that outshines the rest of its host galaxy.

Although there is a handful of quasars relatively nearby, most existed around ten billion years ago and their distance makes them somewhat more difficult to study. One way to get around the extreme distances is to observe them during an event in which a quasar can brighten, such as when they shred and consume an entire star.

Using PanSTARRS, the Panoramic Survey Telescope and Rapid Response System on the volcanic mountain peak of Haleakalā on Maui in the Hawaiian Islands, a team led by Professor Andy Lawrence of the University of Edinburgh set about hunting across millions of galaxies for these brightening events. However, as Lawrence describes during Monday’s presentation at NAM, the brightening events they did find were not what they expected.

Instead, PanSTARRS turned up galaxies with quasars that had not appeared to be quasars in similar surveys made ten years ago, but which now appear ten times brighter. Furthermore, follow-up observations made by the Liverpool Telescope on La Palma in the Canary Islands showed that they are slowly fading over a timescale of years, as opposed to an expected timescale of months if they were flares ignited by a star being gobbled up.


Comparison images of one of the quasars. The image on the left was taken by the Sloan Digital Sky Survey in 2005. The one on the right is provided by the Liverpool Telescope in 2012. The quasar has brightened substantially during the intervening years. Image: Andy Lawrence/Liverpool Telescope. See larger version.
 
Even more puzzling are the distance measurements. Because they are so bright, it is possible to clearly measure the redshift of the quasars. Redshift is the degree to which an object's light has been stretched towards longer, redder wavelengths by the expansion of the Universe. The more distant an object is, the faster cosmological expansion is carrying it away from us, and hence the greater the redshift. For the quasars in the PanSTARRS survey the results are unequivocal - their light set out ten billion years ago. As we have seen, that fits with what we know about quasars, but the galaxies that appear to be hosting the quasars are not playing ball; Lawrence's team have measured their distance to be only three billion light years. How can that be if the host galaxy and the quasar are two parts of the same object? One explanation, says Lawrence, is that they are not the same object at all and that what we are seeing is the more distant quasars located behind closer, but still faraway, foreground galaxies.

"When we measure distances with redshifts they are never that far out, but we don’t have redshifts for the intervening galaxies," Lawrence tells Astronomy Now. "All we know are the broadband colours of the galaxies."

Colour is affected by redshift - the higher the redshift, the redder the object seems to be, but given that the galaxies all have different intrinsic colours, there is no way to calibrate the method. Nevertheless, colour can be good for a first approximation. "This method is usually rough but reasonable, although it does sometimes go completely wrong" says Lawrence. "We would have to be unlucky, but it could happen."

Assuming nothing has gone wrong, Lawrence's team postulates that single stars in the foreground galaxies are magnifying the light of the more distant quasars through gravitational lensing. When relatively small foreground objects that we cannot individually see are doing the lensing, the effect is called microlensing. It is a phenomenon that in the past has allowed astronomers to find exoplanets as the briefly pass in front of a background star, their gravity bending and magnifying the starlight and causing it to brighten for a few days.

In the case of the quasars, because both the foreground galaxies and the background quasars are many billions of light years away, the parallax angle between them and us is quite large, meaning that these microlensing events last years instead of days, tallying with the slow fade that the Liverpool Telescope has observed. If Lawrence's interpretation is correct, then over these long timescales the deep intergalactic sky should appear to shimmer and scintillate like the twinkling of stars, as quasars move in and out of lensing alignments, providing a new method by which to catalogue them.

The Planets
From tiny Mercury to distant Neptune and Pluto, The Planets profiles each of the Solar System's members in depth, featuring the latest imagery from space missions. The tallest mountains, the deepest canyons, the strongest winds, raging atmospheric storms, terrain studded with craters and vast worlds of ice are just some of the sights you'll see on this 100-page tour of the planets.
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Hubble Reborn
Hubble Reborn takes the reader on a journey through the Universe with spectacular full-colour pictures of galaxies, nebulae, planets and stars as seen through Hubble's eyes, along the way telling the dramatic story of the space telescope, including interviews with key scientists and astronauts.
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3D Universe
Witness the most awesome sights of the Universe as they were meant to be seen in this 100-page extravaganza of planets, galaxies and star-scapes, all in 3D!
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