A previously undiscovered candidate for a cosmic gravitational phenomenon has been uncovered, in a surprise discovery made while searching through publicly available James Webb Space Telescope observations.
Detailed in a new paper appearing on the preprint server arXiv.org, researcher Homer Dávila Gutiérrez now reports the discovery of a previously uncatalogued gravitational arc—a phenomenon that occurs when light from a distant celestial object is dramatically warped and magnified by the gravity of a closer, extremely massive object positioned between it and Earth.
The new candidate, which has been designated A1, was spotted in JWST NIRCam observations of MACS J0308.9+2645, an immense galaxy cluster at a redshift of z = 0.356. According to Gutiérrez, the discovery was made during a systematic search for elongated cosmic objects, whose orientations and positions suggested they could be the products of gravitational lensing.
Warped Space Between Sources and Observers
Predicted by Einstein’s theory of general relativity, the phenomenon known as gravitational lensing occurs when massive objects in space distort the spacetime around them, not unlike the way that light is modified when it passes through a concave lens, bending parallel light rays outward and causing them to spread apart from each other.
When gravitational lensing occurs, the tremendous mass exerted by celestial objects closer to Earth warps light coming from more distant objects, such as galaxies positioned behind them, allowing astronomers to observe a distorted perspective of these objects. This phenomenon gives rise to a range of interesting features that include what are known as Einstein rings, Einstein crosses, and gravitational arcs.
In the case of A1, this object appears as a strikingly elongated object, roughly comprising a space of approximately 5.1 arcseconds across, and a length-to-width ratio of approximately 6.5. Resting at about 51 arcseconds from the X-ray center of MACS J0308.9+2645, this feature is tangentially aligned with its respective cluster, as astronomers would expect for a gravitationally lensed arc.
A Galaxy That Initially Appeared Much More Distant
Discovering this unique candidate gravitational arc required Gutiérrez to sift through 54 publicly available JWST/NIRCam fields, comprising a total of 1,591 potential sources. Based on preliminary photometric analysis, a compelling possibility emerged: the presence of a redshift of approximately z = 4.4, which would potentially place the galaxy within the first roughly 1.5 billion years of cosmic history.
However, complicating things had been the fact that the catalog photometry that was used for the initial estimate had been designed primarily for unresolved sources, rather than extended structures like A1. Since measurement apertures only managed to capture a small fraction of the total light emanating from this suspected gravitational arc, and changed size at different wavelengths, the result seemed to be an effect where the galaxy was made to appear significantly redder than it really is.
That would be important, if correct, since the superficial coloration could also mean that the object would appear to be far more distant than it really is.
To assess the situation, Gutiérrez employed several methods that are better suited for extended objects, which, combined with archival observations collected in the past by the Hubble Space Telescope, revealed a “corrected” redshift of approximately z = 1.4, with a likely range between 1.2 and 1.7.
Additionally, a detection of the object in Hubble’s F435W filter provided additional evidence against the earlier z = 4.4 interpretation, since at that distance, a galaxy should effectively vanish at the wavelengths in question due to absorption associated with what is called the Lyman limit.
A Cosmic Magnifying Glass
Further comparisons were made between A1 and existing models of the gravitational field produced by MACS J0308.9+2645, pointing to the likelihood that at a redshift of approximately z = 1.4, A1 would be positioned near the cluster’s critical curve. This would place it in a region where gravitational lensing could strongly stretch the galaxy, but without necessarily creating additional visible copies of it.
Significantly, if A1 were indeed located at the redshift of z = 4.4, another gravitational lensed image of it should have been produced—yet no additional lensed “copies” were evident.
“The most probable interpretation,” Gutiérrez writes, “is a singly lensed galaxy at z ≈ 1.4 behind the cluster.” Additionally, the arc displays a concave curvature in the direction of the cluster center, featuring a radius of curvature equivalent to around 19 arcsec, although “its statistical significance is marginal and method-dependent once correlated noise is properly accounted for,” Gutiérrez adds.
Taken together, all the evidence points to the fact that A1 is indeed most likely a closer galaxy whose appearance has been significantly warped by the massive cluster positioned between it and Earth. Even beyond the identification of a candidate gravitational arc, the discovery also demonstrates the remarkable scientific potential that remains untapped within the growing archive of astronomical observations in Webb’s already enormous body of collected data.
The recent work by Gutiérrez also demonstrates that a host of additional discoveries await as soon as discerning astronomers are willing to put in the time to sift through the vast amounts of Webb data that are already publicly available.
The recent study, “Discovery of a gravitational arc candidate at zphot ≈ 1.4 in MACS J0308.9+2645 from a catalogue-based search of JWST imaging,” was uploaded to the preprint server arXiv.org and can be read online.
Micah Hanks is the Editor-in-Chief and Co-Founder of The Debrief. A longtime reporter on science, defense, and technology with a focus on space and astronomy, he can be reached at micah@thedebrief.org. Follow him on X @MicahHanks, and at micahhanks.com.