JWST Sees Multiple Gravitational Lenses in a Massive Cluster: “The Fishhook” and “The Thin One”

We’ve been getting plenty of spectacular images from the James Webb Space Telescope since it began operations last year. Fraser even covered everything we learned from it in a video a few weeks ago. But the news keeps coming, and recently a science team known as the Prime Extra-Galactic Areas for Reionization and Lensing Science (PEARLS) team released a series of four papers describing Webb’s observations of a galaxy cluster known as El Gordo (“the fat one” in Spanish). But what’s more – they also released another absolutely stunning picture.

To fully understand the picture shown as the banner, but also provided in greater detail in the link below, it’s essential first to understand gravitational lensing. We’ve discussed the general concept many times before and even shown some fascinating images of objects known as Einstein rings. The critical thing to understand is that large objects, such as the El Gordo galaxy cluster formed about 6.8 billion years ago, can magnify objects appearing directly behind them by bending the light around their mass, creating an effect called “gravitational lensing.”

This is precisely why the Webb team turned their attention towards this massive galaxy cluster, which is thought to be the biggest that existed at that point in the universe’s life. Hubble, Webb’s predecessor as an amazing astronomical picture-taker, also took some images of El Gordo, but it wasn’t sensitive enough to capture the full grandeur of the scene. 

Video released along with the papers touring the El Gordo Cluster.
Credit – JWST YouTube Channel

A bonus of those fantastic images is astronomers can use them for science. One of the four papers from the PEARLS team, led by Brenda Fyre of the University of Arizona, discusses the overall effect of the gravitational lensing effect offered by El Gordo, but the other three look at specific features of the image.

First, we have one of the most noticeable features of the image – in the upper right, there appears to be a galaxy that is curved like a fishhook, which is what one of the graduate students on the PEARLS team nicknamed it. In Webb’s infrared image, it appears red, partially due to dust inside the galaxy itself but also due to the redshift caused by the light traveling 10.6 billion years to get to us.

The Fishhook is a relatively small disk galaxy that is only about ¼ the size of the Milky Way. It even appears to be dying out over 10 billion years ago, as the star-forming region in the center was rapidly declining, also known as “quenching” in astronomical jargon. That discovery was the subject of the second paper, led by Patrick Kamieneski of Arizona State University.

Fraser’s description of gravitational lensing

Another feature that some observers may even mistake as an imaging artifact is the line in the middle left of the image. That is, in fact, another galaxy being lensed by El Gordo that is even further away from Earth at 11 billion years. Known as La Flaca (“the thin one” in Spanish). While that galaxy itself is simply cool looking, a more subtle feature of the image intrigues scientists.

Near La Flaca, there is a red dwarf star that is the first of its kind to be observed beyond 1 billion years from Earth. Now named Quyllur, the Quechua word for star, it was only possible to see because of El Gordo’s gravitational lensing and Webb’s ultrasensitive sensors. Typically far away individual stars have to be “blue” so that the redshift caused by being so far away doesn’t move their light out of the observed spectra. But with Webb’s instruments, Quyllur will likely only be the first of many the telescope will find. Its discovery was discussed in the third paper, led by Jose Diego of the Instituto de Fisica de Cantabria in Spain.

There are other, less notable features of the image that also attract the interest of scientists. One is a miniature galaxy cluster formed 12.1 billion years ago, while another is a set of “ultra-diffuse” galaxies. The stars are spread far apart in these, making them difficult to image as a “galaxy” per se. But Webb was able to, at a here-to-fore unheard of distance of 7.2 billion light years. The farther-away diffuse galaxies even look different than the ones that are closer to home, as discussed in the fourth paper by Timothy Carleton, also of Arizona State.

Overall, this series of papers brings together a wealth of scientific knowledge with the inspiration of one of the most amazing astronomical photographs ever taken. And more will surely keep coming as Webb continues its operations into its second year.

Learn More:
Webb Space Telescope – Webb Spotlights Gravitational Arcs in ‘El Gordo’ Galaxy Cluster
B Fyre. et al. – The JWST PEARLS View of the El Gordo Galaxy Cluster and of the Structure It
Magnifies
Kamieneski et al. – Are JWST/NIRCam color gradients in the lensed z=2.3 dusty star-forming galaxy El Anzuelo due to central dust attenuation or inside-out galaxy growth?
J. M. Diego et al. – JWST’s PEARLS: A new lens model for ACT-CL J0102?4915, “El Gordo,” and the first red supergiant star at cosmological distances discovered by JWST
T. Carleton et al. – PEARLS: Low Stellar Density Galaxies in the El Gordo Cluster Observed with JWST
UT – What is Gravitational Lensing?
UT – This JWST Image Shows Gravitational Lensing at its Finest

Lead Image:
JWST image of the El Gordo galaxy cluster, and the galaxies that are gravitational lensed behind it.
Credit – NASA, ESA, CSA, Alyssa Pagan, Jake Summers, Jordan C. D’Silva, Anton M. Koekemoer, Aaron Robotham, Rogier Windhorst

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