Intergalactic magnifying glasses could help map galaxy centers
Washington: A team of astronomers may have discovered a new way through which they can map quasars - the energetic and luminous central regions which are often found in distant galaxies.
In a large scale survey using the Pan-STARRS telescope on Hawaii, Andy Lawrence of the University of Edinburgh and his team studied millions of galaxies to search for this rare effect. They did find flare-ups but with very different behavior to the `star shredding` 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.
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 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 centers have then 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.
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 said that the new method could give them a way to map out the internal structure of quasars in a way that is otherwise impossible, as quasars are so small.
He added that 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.
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