Intergalactic magnifying glasses could help astronomers map galaxy centres

ESO/M. Kornmesser

An international team of astronomers may have found a new way to map quasars, the energetic and luminous central regions often found in distant galaxies. Team leader Prof. Andy Lawrence of the University of Edinburgh presents the new results on Monday 1 July at the RAS National Astronomy Meeting in St Andrews, Scotland.

If a star passes too close to a giant black hole found in the centre of a galaxy, it will be shredded by the strong gravitational field. This produces a flare-up in the brightness of an otherwise normal looking galaxy that then fades over a few months. In a large scale survey using the PanSTARRS telescope on Hawaii, Prof. Lawrence and his team studied millions of galaxies to search for this rare effect. They did find flare-ups but with very different behaviour to the ‘star shredding’ predictions.

Instead of seeing a fade over months, the objects they found look like ‘normal’ quasars, regions in the centre of galaxies where material is swirling around a giant black hole in a disk. But the quasars in the survey were not seen a decade ago, so must now be at least ten times brighter than before. Monitoring with the Liverpool Telescope on La Palma showed that they are also changing slowly, fading over a timescale of years rather than months.

The PanSTARRS telescope has a very wide field of view, so it can survey the sky repeatedly and look for things that change, as well as build up a deep picture of the sky over time. A variety of different science projects are being done with PanSTARRS, but my special interest is looking for Active Galactic Nuclei that vary dramatically. Together with Suvi Gezari in Maryland, we were looking for cases of a black hole shredding a star and then swallowing the gas. These are very rare, so we had to check millions of galaxies. We may have found some of those, but we also found these giant brightenings of background quasars." Prof. Andy Lawrence 

The biggest surprise however was that the quasars seemed to be at the wrong distance. Measuring the characteristic shift in lines found in the spectrum of the quasars allows astronomers to measure the speed at which they are moving away from the Earth. Knowing the way in which the universe is expanding enables scientists to deduce the distance to each object.

In the new survey, the quasars are typically around 10 billion light years away, whereas the galaxies that host them seem on average to be about 3 billion light years distant. The distances are rough estimates, so it could be that the estimated galaxy distances are completely wrong and that they are actually much further away. The black holes in their centres have then have flared up very dramatically, explaining why they seem so bright. But past studies of thousands of well known quasars have never shown events on this scale.

If however the estimated galaxy distances are right, then Prof. Lawrence and his team believe they are looking at a distant quasar through a foreground galaxy. Normally this has little effect on the light of the quasar, but if a single star in the foreground galaxy passes exactly in front of the quasar, it can produce a gravitational focusing of the light which makes the background quasar seem temporarily much brighter.

This "microlensing" phenomenon is well known inside our own Galaxy, producing a brightening when one star passes in front of another. (It is for example also now being used to detect exoplanets). Microlensing may also be the cause of low-level "flickering" seen in some quasars. But this is the first time it has been suggested to cause such giant brightening events.

Prof. Lawrence sees real potential in this newly-discovered effect. “This could give us a way to map out the internal structure of quasars in a way that is otherwise impossible, because quasars are so small. As the star moves across the face of the distant quasar, it is like scanning a magnifying glass across it, revealing details that would otherwise simply be impossible to detect.”

RAS National Astronomy Meeting (NAM 2013)

 Bringing together more than 600 astronomers and space scientists, the RAS National Astronomy Meeting (NAM 2013) will take place from 1-5 July 2013 at the University of St Andrews, Scotland. The conference is held in conjunction with the UK Solar Physics (UKSP: www.uksolphys.org) and Magnetosphere Ionosphere Solar Terrestrial (MIST: www.mist.ac.uk) meetings. NAM 2013 is principally sponsored by the RAS, STFC and the University of St Andrews and will form part of the ongoing programme to celebrate the University’s 600th anniversary.

Meeting arrangements and a full and up to date schedule of the scientific programme can be found on the official website at http://www.nam2013.co.uk

Royal Astronomical Society

The Royal Astronomical Society (RAS: www.ras.org.uk, Twitter: @royalastrosoc), founded in 1820, encourages and promotes the study of astronomy, solar-system science, geophysics and closely related branches of science. The RAS organises scientific meetings, publishes international research and review journals, recognizes outstanding achievements by the award of medals and prizes, maintains an extensive library, supports education through grants and outreach activities and represents UK astronomy nationally and internationally. Its more than 3500 members (Fellows), a third based overseas, include scientific researchers in universities, observatories and laboratories as well as historians of astronomy and others.

Pan-STARRS

The Pan-STARRS Project is led by the University of Hawaii Institute for Astronomy, and exploits the unique combination of superb observing sites and technical and scientific expertise available in Hawaii. Funding for the development of the observing system has been provided by the United States Air Force Research Laboratory.

The Pan-STARRS1 Surveys (PS1) have been made possible through contributions by the Institute for Astronomy, the University of Hawaii, the Pan-STARRS Project Office, the Max-Planck Society and its participating institutes, the Max Planck Institute for Astronomy, Heidelberg and the Max Planck Institute for Extraterrestrial Physics, Garching, The Johns Hopkins University, Durham University, the University of Edinburgh, the Queen's University Belfast, the Harvard-Smithsonian Center for Astrophysics, the Las Cumbres Observatory Global Telescope Network Incorporated, the National Central University of Taiwan, the Space Telescope Science Institute, and the National Aeronautics and Space Administration under Grant No. NNX08AR22G issued through the Planetary Science Division of the NASA Science Mission Directorate, the National Science Foundation Grant No. AST-1238877, and the University of Maryland.