Galaxies in gravitational cosmic lens not as many as they seem

An optic effect called “strong gravitational lensing” is distorting the number and shape of the universe’s most distant galaxies – making it seem like there are more galaxies than are actually there, according to a new study led by the University of Melbourne.

Researchers say this discovery could change astronomers’ views on how the earliest galaxies were formed. It will also factor into how researchers can effectively use the planned James Webb Space Telescope to search for very distant galaxies.

What is gravitational lensing?
The gravity of an object in space warps the space-time around it, thus bending the light around the object. This effect is normally extremely small, but when light passes close to a very massive object, such as a galaxy or a supermassive black hole, the bending of the light rays becomes more noticeable.

Gravitational lensing of distant galaxies. Photo: HubbleSite

Like a cosmic hall of mirrors, gravitational lensing can distort the way the most distant galaxies appear when viewed from Earth. Imagine that you’re looking at two galaxies that, from Earth, appear in alignment. The gravity of the closer, foreground galaxy will bend the light from the distant galaxy, acting like a sort of magnifying lens.

The study’s lead author, Professor Stuart Wyhite of the University of Melbourne’s School of Physics, said when light from a very distant galaxy passes a galaxy much closer to us, it can detour around the foreground galaxy, thus magnifying the light from the more distant galaxy directly behind it.

“There are only a few direct lines-of-sight to very distant objects in space. Our finding shows images from the earliest galaxies reach us more often via a gravitationally bent path. What you see is not exactly what is there,” he said.

Another of the study’s authors, Haojing Yan, postdoctoral fellow at the Centre for Cosmology and Astro-Particle Physics at Ohio State University, said on HubbleSite that gravitational lensing is not unfamiliar to astronomers – in fact, researchers use it to find distant objects that would otherwise be invisible.

“We predict that many galaxies in the most remote universe will only ever be visible to us because they are magnified in this way,” Yan said.

However, this has been assumed to be a rare occurrence and would not affect galaxy surveys.

Galaxy image distortions
There are a few ways that gravitational lensing can affect the images of distant galaxies. It can cause light from these faraway galaxies, which is usually too faint to detect, to appear brighter than they really are. It can also distort the images of these galaxies, making them appear near foreground galaxies. Gravitational lensing can also split the light from the distant galaxy, making it look like there are two or more galaxies around the lens, when there is actually only one.

Professor Wyithe said that a key goal of NASA’s Hubble Space Telescope over the past decade was to look for the most distant galaxies and answer the questions of how and when the first generation of galaxies formed in the universe.

This study, however, makes the case that the Hubble Space Telescope’s existing view of the most distant galaxies is already significantly modified by gravitational lensing, and that future galaxy surveys will need to account for a significant gravitational lensing bias in high-redshift galaxies at these distances.

“Only the James Webb Space Telescope, if finished as designed, can ultimately make sense out of this gravitationally biased view of the distant universe, because it will have exquisite resolution and sensitivity at longer wavelengths to disentangle these very distant objects from the foreground lensing galaxies,” Professor Wyithe said.

The study, published in the journal Nature, was undertaken together with scientists from Ohio State University, Arizona State University and the University of Manchester.
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Main image shows a colour composite image of the Hubble Ultra Deep Field. Green circles mark the locations of lower-redshift candidate galaxies (z~8). Higher-redshift candidates are circled in red. The estimated distances to these candidates have not been confirmed spectroscopically. About 20 to 30 per cent of these high-z galaxy candidates are very close to foreground galaxies, consistent with the prediction that a significant fraction of galaxies at very high redshifts are gravitationally lensed by individual foreground galaxies. This will help as a guide for future observations planned for the James Webb Space Telescope when it is launched. (Source: HubbleSite)