NAM 2013: Gravitational microlensing and quasars
BY KEITH COOPER
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.
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.
"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.
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