James Webb captured a nearly perfect Einstein ring

James Webb captured a nearly perfect Einstein ring

An observation that is very rare today, but could become much more common thanks to the exceptional performance of the telescope.

The James Webb Space Telescope has recently shown up with a remarkable new image; impressively surprised with a galaxy whose brightness took 12 billion years to reach us. What makes it interesting is that it appears to us in a rather particular form, namely an almost perfect Einstein ring.

This is not the first time this galaxy, named SPT-S J041839-4751.8, has been photographed. The venerable Hubble and even the Webb have already drawn his portrait several times. But recently, JWST looked back to examine it with another instrument, namely its Mid-Infrared Instrument (MIRI).

According to ScienceAlert, the raw data has since been published on the MAST portal. They were later recovered by Spaceguy44, a Reddit resident who is also a Ph.D. student in astronomy. He compiled these elements to produce a nice composite image spotted by ScienceAlert; he highlights the most striking feature of this little corner of heaven, namely Einstein’s famous ring.

©NASA/ESA/Spaceguy44 via Reddit

What is an Einstein ring?

In the center of the image, we can see a light source that seems to be enclosed in the middle of a strange circular halo. In fact, they are two very different galaxies and very far from each other.

The blue dot in the center of this structure corresponds to the closest galaxy to us. The second is located far back, but on exactly the same axis, a bit like the Moon and Sun during a total eclipse, but at a much greater distance.

Therefore, one would expect that the second one, namely SPT-S J041839-4751.8, would be completely hidden by the first one. However, it is clearly visible; it even appears as an almost perfect circle. And to understand the origin of this phenomenon, we must quickly look at general relativity formalized by Einstein, hence its name.

This theory states that objects generate a gravitational influence proportional to their mass. In the case of extremely massive objects such as galaxies, this generates a significant distortion of space-time in the vicinity of the celestial body in question.

And when light passes through this perimeter, its path is bent to follow the curvature of space-time. A finding that gave Einstein pause; in his works that paved the way for general relativity, we find mention of a “gravitational lens.”

A galactic-scale “lens”

In optics, a lens is a transparent device whose surfaces have a carefully calibrated curvature. They allow the path of light rays to be manipulated with great precision thanks to the phenomenon of refraction.

Functionally speaking, gravitational lenses are quite similar to physical lenses; Although they involve different physical principles, astronomers can also use them to observe invisible objects with a good level of precision. It is because of this phenomenon that SPT-S J041839-4751.8 appears to us as a very visible and well-defined circle.

The phenomenon of gravitational lensing outlined by the ESA. The grid represents the curvature of space-time. © NASA, ESA & L. Calçada

Note that this term should be taken with a grain of salt due to a very important difference; Unlike a standard lens, a gravitational lens does not have a focal point, but rather a focal axis. This means that the distance between the telescope, the “lens” and the object to be observed does not matter in this case.

But this is still a very rare sighting, and for good reason; anyway, all three objects must be perfectly alignedwhich is quite unlikely given the distances involved. Einstein himself believed that it would be impossible to get close enough to the center line, and that we would probably never have an instrument with sufficient resolution to observe… but the JWST decided otherwise!

And without this gravitational lensing effect, SPT-S J041839-4751.8. probably indistinguishable ; at best, it would take the form of a tiny point of light from which it would be difficult to extract meaningful data.

Anyway, it is a new image that comes to adorn the already well-stocked hunting table of the JWST. And given the outstanding contributions this engineering marvel has already brought us in just a few short months, it’s quite possible that sightings of Einstein’s rings will become more frequent in the future.

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