Quasar’s Light Echoes After 6.73 Years

Sep 24, 2022 | Daily Space, Quasar

Quasar’s Light Echoes After 6.73 Years
IMAGE: Image of the lens cluster SDSS J1004+4112 whose gravitational field has bent the light rays from a single distant quasar into four images that can be seen as very bright white dots around the centre of the cluster. CREDIT: Universitat de València

If you’ve been watching for a while, you know that gravity can act like a lens and direct light that might otherwise be headed to a different part of the universe toward us instead. Like a really badly made magnifying glass being used to focus the Sun’s light into a super bright blob on a sidewalk, this isn’t exactly a perfect process. Massive galaxy clusters, with their knots of material and large distribution of mass, actually have the ability to focus a single distant galaxy into multiple blobs of light. The wild thing is, the light from each of those blobs will have traveled a different distance as it shines from the source, passes some point in the cluster, and is then focused into place.

This has the somewhat wild consequence that since each blob comes from a different light path, we can simultaneously see distant galaxies at two points in their history.

This is because light travels at a finite speed. If one light path is 8 billion light-years long, and another is 7.999999999 billion light-years long, that tiny difference means we might see events, depending on how many nines are in there, anywhere from minutes to years apart in time.

Most galaxies are pretty placid, and while we might know we are seeing the same galaxy several different times as a galaxy cluster bends its light hither and yon, we will never know how different those light paths are if that galaxy doesn’t flicker or flare for some reason.

And astronomers eagerly watch for any change in brightness that might allow us to get this precious distance information.

We watch eagerly, and sometimes we watch for a really long time.

Researchers led by Jose A. Muñoz used the 1.2-meter Whipple Observatory to follow the brightness of a lensed galaxy for 14.5 years. This particular galaxy was split into four different images, with four different light paths. From this extraordinary run of observations, they were able to confirm that events seen in the lens with the shortest light path arrive almost seven years before the light arrives along the longest path.

Having seen the exact same events on repeat in these four different versions of the same galaxy’s light, astronomers now have the capacity to use these measurements to map out things like dark matter in the galaxy cluster responsible for all the light bending, and – with a whole lot of math that still needs to be published, and may still need to be done – researchers will also be able to use these repeating events to measure traits about our universe, including its expansion rate.

The time delay between light arriving along the shortest and farthest paths is 6.73 years. This is the longest known path distance of all gravitationally lensed objects so far studied. With that large difference will one day come great results. When they are published, we’ll bring them to you here on the Daily Space.

More Information

University of Valencia press release

The Longest Delay: A 14.5 yr Campaign to Determine the Third Time Delay in the Lensing Cluster SDSS J1004+4112,” J. A. Muñoz et al., 2022 September 22, The Astrophysical Journal

A Mass Model for the Lensing Cluster SDSS J1004+4112: Constraints from the Third Time Delay,” R. Forés-Toribio et al., 2022 September 22, The Astrophysical Journal

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