Quasar Microlensing May Reveal New Details About the Internal Structure of Quasars

New Quasar Microlensing Technique

Images of a quasar from the Sloan Digital Sky Survey in 2005 and then made with the Liverpool Telescope in 2012. The quasar has brightened dramatically as a result of microlensing. Credit: A. Lawrence and the Liverpool Telescope.

Research presented at the 2013 RAS National Astronomy Meeting describes a new quasar microlensing technique that should allow astronomers to better map out the internal structure of quasars, revealing details that would otherwise simply be impossible to detect.

An international team of astronomers may have found a new way to map quasars, the energetic and luminous central regions typically found in distant galaxies. Team leader Prof. Andy Lawrence of the Royal Observatory 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 center of a galaxy, it will be shredded by the strong gravitational field. This should produce a flare-up in the brightness of an otherwise normal looking galaxy that then fades over a few months. In a large scale survey Prof. Lawrence and his team studied galaxies to search for this effect, finding flare-ups but with very different behavior to predictions.

Instead of seeing a fade over months, the objects they found look like ‘normal’ quasars, regions in the center of galaxies where material is swirling around a giant black hole in a disk. The quasars in the survey were not seen a decade ago, so must be at least ten times brighter than before. They are also changing slowly, fading over a timescale of years rather than months.

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.

A New Way to Map Quasars

Illustration of the effect of gravitational microlensing on a distant quasar. Credit : Jason Cowan, Astronomy Technology Center; adapted from a figure made by NASA

In the new survey, the quasars are typically around 8 billion light years away, whereas the galaxies that host them are 3.4 billion light years distant. It could be that the estimated galaxy distances are wrong and that the black holes in the center of the galaxies have flared up dramatically. 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.”

Source: Royal Astronomical Society

Images: A. Lawrence and the Liverpool Telescope; Jason Cowan, Astronomy Technology Center; adapted from a figure made by NASA

3 Comments on "Quasar Microlensing May Reveal New Details About the Internal Structure of Quasars"

  1. Therese Holman | July 2, 2013 at 10:33 pm | Reply

    Prochter and Prochaska analyzed 15 GRBs in the new study and found strong absorption signatures indicating the presence of galaxies along 14 GRB sightlines. They had previously used data from the Sloan Digital Sky Survey (SDSS) to determine the incidence of galaxies along the sightlines to quasars. Based on the quasar study, they would have predicted only 3.8 galaxies instead of the 14 detected along the GRB sightlines.

  2. Harrison Gregory | July 5, 2013 at 2:53 am | Reply

    According to new research, a galaxy with a quasar in the middle is not a good place to grow up. As active galactic nuclei (AGN) evolve, they pass through a “quasar phase”, where the accretion disk surrounding the central black hole blasts intense radiation into space. The quasar far outshines the entire host galaxy. After the quasar phase, when the party is over, it is as if there is no energy left and star formation stops. AGN are the compact, active and bright central cores to active galaxies. The intense brightness from these active galactic cores is produced by the gravitationally driven accretion disk of hot matter spinning and falling into a supermassive black hole at the centre. During the lifetime of an AGN, the black hole/accretion disk combo will undergo a “quasar phase” where intense radiation is blasted from the superheated gases surrounding the black hole. Typically quasars are formed in young galaxies. Although the quasar phase is highly energetic and tied with young galaxy formation, according to new results from the Sloan Digital Sky Survey, it also marks the end for any further star birth in the galaxy. These findings will be presented today (Friday 4th April) at the RAS National Astronomy Meeting in Belfast, Northern Ireland, by Paul Westoby having just completed a study of 360 000 galaxies in the local Universe. He carried out this research with Carole Mundell and Ivan Baldry from the Astrophysics Research Institute of Liverpool, John Moores University, UK. This study was proposed to understand the relationship between accreting black holes, the birth of stars in galactic cores and the evolution of galaxies as a whole. The results are astonishingly detailed.

  3. Jimmy Collins | July 5, 2013 at 2:06 pm | Reply

    In the new survey, the quasars are typically around 8 billion light years away, whereas the galaxies that host them are 3.4 billion light years distant. It could be that the estimated galaxy distances are wrong and that the black holes in the centre of the galaxies have flared up dramatically.

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