Quasars found acting as gravitational lenses

Posted by Kate Taylor

Astronomers using NASA's Hubble Space Telescope have found several  quasars acting as gravitational lenses, making it easier to study the galaxies in which they're contained.

Quasars, which are powered by supermassive black holes, are some of the brightest objects in the universe, far brighter than the total starlight of their host galaxies. Their great mass means they can act as gravitational lenses, amplifying and distorting images of galaxies aligned behind them.

To find them, a team of astronomers led by Frederic Courbin at the Ecole Polytechnique Federale de Lausanne picked out 23,000 quasar spectra in the Sloan Digital Sky Survey (SDSS).

They looked for the spectral imprint of galaxies at much greater distances that happened to align with foreground galaxies. When they found some, they used Hubble's sharp view to look for the gravitational arcs and rings that would be produced by gravitational lensing.

Quasar host galaxies are hard - often impossible - to see because the central quasar far outshines the galaxy; and this means it's difficult to estimate the host galaxy's mass by looking at the collective brightness of its stars.

Gravitational lensing, though, makes it possible, as the amount of distortion in the lens can be used to estimate a galaxy's mass.

The next step for the researchers is to catalog all their 'quasar-lenses'  so that they can work out masses for a statistically significant number of quasar host galaxies and compare them with galaxies without quasars.

What with all the wide-field surveys now in operation or expected to kick off in the near future, there should be hundreds of thousands of quasars to examine.