Webb Sees a Supercluster of Galaxies Coming Together

As a species, we’ve come to the awareness that we’re a minuscule part of a vast Universe defined by galaxy superclusters and the large-scale structure of the Universe. Driven by a healthy intellectual curiosity, we’re examining our surroundings and facing the question posed by Nature: how did everything get this way?

We only have incremental answers to that huge, almost infinitely-faceted question. And the incremental answers are unearthed by our better instruments, including space telescopes, which get better and more capable as time passes.

Enter the James Webb Space Telescope.

One of the reasons NASA and their partners built and launched the James Webb Space Telescope is to study the history of galaxy formation and to understand how they evolved into what we see today. That involves observing galaxies, galaxy clusters, galaxy superclusters, and the complex network of sheets, voids, and filaments that comprise the large-scale structure of the Universe. It also involves observing proto-clusters, the early stage of a galaxy cluster. They’re like building blocks for the cosmic web, which collapse and merge to form clusters and superclusters.

The Spiderweb protocluster is an ancient and well-studied object in the early Universe. More than 100 individual galaxies are forming a cluster at redshift z = 2.16, meaning it took more than 10 billion years for its light to reach us.

“We are observing the build-up of one the largest structures in the Universe, a city of galaxies in construction.”

Jose M. Pérez-Martínez, Instituto de Astrofísica de Canarias

Protoclusters are one key to understanding the Universe, and in two new papers, researchers present the results of the JWST’s observations of the Spiderweb protocluster. Among other things, the results show that gravity doesn’t play as large a role as thought in the formation of a cluster.

The difficulty in observing the Spiderweb is that it’s obscured by a healthy amount of cosmic dust. The dust blocks visible light but allows infrared light through. Since the JWST is an enormously powerful infrared telescope, its gaze has revealed things previously hidden from astronomers.

“We are observing the build-up of one the largest structures in the Universe, a city of galaxies in construction,” explained Jose M. Pérez-Martínez of the Instituto de Astrofísica de Canarias and the Universidad de La Laguna in Spain. “We know that most galaxies in local galaxy clusters (the biggest metropolises of the Universe) are old and not very active, whereas in this work we are looking at these objects during their adolescence. As this city in construction grows, their physical properties will also be affected. Now, Webb is giving us new insights into the build-up of such structures for the first time.”

The JWST can observe hydrogen gas more thoroughly than other telescopes. Astronomers often observe hydrogen-alpha (h-alpha) emissions to probe galaxies. h-alpha emissions are a specific type of light emitted when electrons transition between energy levels. However, there’s another type of infrared hydrogen emission called Paschen-beta emissions (Pa-beta) that the JWST can observe. It’s emitted by different electron transitions in hydrogen and is a valuable tracer of the star formation rate (SFR) in galaxies. While the JWST isn’t specifically designed to single out these emissions, it can observe the infrared wavelengths that include the Pa-beta line.

The two new papers based on the JWST observations are:

• JWST/NIRCam Pa-beta Narrowband Imaging Reveals Ordinary Dust Extinction for H? Emitters within the Spiderweb Protocluster at z = 2.16
• JWST/NIRCam Narrowband Survey of Pa-beta Emitters in the Spiderweb Protocluster at z = 2.16

These observations revealed the presence of new, previously undetected galaxies in the protocluster that were obscured by dust.

“As expected, we found new galaxy cluster members, but we were surprised to find more than expected,” explained Rhythm Shimakawa of Waseda University in Japan. “We found that previously-known galaxy members (similar to the typical star-forming galaxies like our Milky Way galaxy) are not as obscured or dust-filled as previously expected, which also came as a surprise.”

The characteristics of the dust show that gravitational interactions aren’t playing as large a role as thought. If there were gravity-driven mergers, the dust production would be higher as mergers trigger rapid SFRs. However, these observations show that the dust is being produced smoothly rather than abruptly.

“This can be explained by the fact that the growth of these typical galaxies is not triggered primarily by galaxy interactions or mergers that induce star-formation,” added Helmut Dannerbauer of the Instituto de Astrofísica de Canarias in Spain. “We now figure this can instead be explained by star formation that is fueled through gas accumulating at different locations all across the object’s large-scale structure.”

“These results support the scenario for which dust production within the main galaxy population of this protocluster is driven by secular star formation activities fueled by smooth gas accretion across its large-scale structure,” the authors write in the first paper. “This downplays the role of gravitational interactions in boosting star formation and dust production within the Spiderweb protocluster, in contrast with observations in higher redshift and less evolved protocluster cores.”

“We find no correlation between the dustiness of our sample of HAEs and their distribution in phase space (spectroscopic sample) or as a function of the projected clustercentric radius or local density,” the authors of the first paper explain. If gravity-driven mergers were behind the star and dust formation, it would be clumpy.

The second paper’s original goal was to make a deep-line survey aimed at Pa-beta emitters (PBEs). It used a unique narrow-band filter on the NIRCam that’s less sensitive to dust extinction. They ended up detecting new member candidates in the Spiderweb Protocluster. Interestingly, not all of the h? emitters are also Pa-beta emitters.

The researchers narrowed their Pa-beta emitters down to 41 sources. Only 17 of those are also confirmed as h? emitters. “The remaining 24 objects are considered to be unconfirmed candidates associated with the Spiderweb protocluster,” the authors write. “These PBE candidates are still at risk of foreground or background emitters other than PBEs; therefore, further follow-up studies are needed to establish that they are protocluster members.”

Finding more members of the Spiderweb protocluster and finding that gravity isn’t as important as thought is just a beginning. These are incremental answers on our path to understanding how the Universe evolved into what we see today. Science is a journey, and as is so often the case on the journey, more observations are the next step.

“Follow-up confirmations and characterizations of the PBE candidates will provide a better understanding of the total star formation rate in the Spiderweb protocluster, the environmental dependence of galaxy formation, and a transition process from a protocluster to a bona fide cluster of galaxies,” the authors of the second paper write in their conclusion.

The researchers intend to follow up this work with more spectroscopic observations form the JWST. Those observations should provide additional confirmation of the Spiderweb’s new members.