Galaxies help zoom in on remote quasars

Astronomers believe they have found a new way to map the turbulent zones surrounding supermassive black holes in extremely distant galaxies.

A photo of the Pan-STARRS 1 telescope on Hawaii with an image atop Haleakala, Maui, with an image of the Trifid Nebula taken by the telescope, thrown in for colourful effect. Credit: PS1 Science Consortium.
A photo of the Pan-STARRS 1 telescope on Hawaii with an image atop Haleakala, Maui, with an image of the Trifid Nebula taken by the telescope, thrown in for colourful effect. Credit: PS1 Science Consortium.

Black holes are, by definition, invisible. But they reveal themselves because they are surrounded by energetic and luminous central regions of swirling material, called quasars.

Some of these quasars were observed to flare dramatically and with no obvious explanation, as an international team of astronomers was carrying out a large-scale survey of the sky.

The scientists suggest the “outbursts” are actually due to individual stars in closer foreground galaxies passing in front of the quasars.

Their gravitational pull acts like a magnifying glass on them, a microlensing effect predicted by Einstein, causing the galaxies with the quasars to brighten and then fade again over years.

The survey team, led by Professor Andy Lawrence, of the University of Edinburgh, used the PanSTARRS telescope on Hawaii to examine millions of galaxies, with follow-up work using the Liverpool Telescope on La Palma.

Illustration of the effect of gravitational microlensing on a distant quasar.
Illustration of the effect of gravitational microlensing on a distant quasar. Credit : Jason Cowan, ATC.

They were actually looking for a different flaring produced by the black holes themselves rather than any microlensing effect. They wanted to observe what happens on the rare occasions when a star comes too close to a giant black hole and is torn to pieces by its powerful gravitational field.

However, the flares they observed seemed to be longer lasting and to be occurring at a much closer distance than expected. The puzzle was that the quasars found in the survey were typically around 10 billion light-years away but the galaxies observed were on average only about 3 billion light-years distant.

Professor Lawrence’s team has deduced that these apparent discrepancies can be reconciled if the quasars are genuinely much further away and unconnected with the galaxies through which they are being viewed.

Reporting his results at the RAS National Astronomy Meeting in St Andrews, Scotland, this week, Professor Andrews said microlensing offered a method of learning more about incredibly remote quasars.

He said: “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.”

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Paul Sutherland

Paul Sutherland

I have been a professional journalist for nearly 40 years. I write regularly for science magazines including BBC Sky at Night magazine, BBC Focus, Astronomy Now and Popular Astronomy. I have also authored three books on astronomy and contributed to others.
Paul Sutherland

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Paul Sutherland

I have been a professional journalist for nearly 40 years. I write regularly for science magazines including BBC Sky at Night magazine, BBC Focus, Astronomy Now and Popular Astronomy. I have also authored three books on astronomy and contributed to others.

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