Light from a quasar has been delayed for seven years as its path to us was diverted by the gravity from a cluster of galaxies. It is the longest recorded delay of its kind so far.
When an object in space is massive enough, it can act as a gravitational lens that bends light around it. For those watching from Earth, it can produce multiple images of things behind the big object because light for each image takes a different curved path, arriving at different times.
at the University of Valencia in Spain and his colleagues have now measured the light from four images of a quasar 鈥� a supermassive black hole with a bright disc of material circling it 鈥� that had been bent by a cluster of galaxies called SDSS J1004+4112 over a period of more than 14 years, from the Fred Lawrence Whipple Observatory in Arizona.
Advertisement
Three of the images, A, B and C, had been measured by previous groups, but the fourth image, D, hadn鈥檛. By taking repeated light readings of all four images and performing statistical analyses, Mu帽oz鈥檚 team confirmed previous image measurements and determined a delay time for D: about 2458 days, or nearly seven years.
鈥淔or a system with such a long delay, you get many more data points and can measure the delay more accurately, and they have ended up with a beautiful data set,鈥� says at Imperial College London. The accuracy of the delay is unlike anything we have for other lenses from cluster galaxies, he adds.
Accurately measuring delays can help calibrate models for predicting delays for other galaxy cluster lenses. Such predictions are difficult, says at Durham University, UK, due to the many different possible distributions of mass that can exist in an object acting as a gravitational lens. 鈥淭here is a lot of space for improvement when it comes to precise and accurate measurements of time delays from lens mass modelling predictions,鈥� she says.
By calculating the delay between these different images, astronomers can measure the Hubble constant, a number that represents how fast the expansion of the universe is accelerating.
All but one of the previous delay predictions for image D were incorrect. However, these errors aren鈥檛 surprising, says Jauzac. 鈥淭he fact that at least one of the models managed to encompass the direct measurement presented in this paper is already quite a step.鈥�
Taking measurements for such a long delay takes many years, so working out a cluster galaxy delay with this level of accuracy is a feat unlikely to be repeated for some time, says Warren. The next accurate measurement of an image from a gravitational lens will probably come a number of years after the opening of the Vera C. Rubin Observatory, previously called the called the Large Synoptic Survey Telescope, in Chile in 2023.
Journal reference:
Sign up to Lost in Space-Time, a free monthly newsletter on the weirdness of reality
