NASA’s two legendary space telescopes have unravelled a trove of celestial marvels, helping humanity solve many a puzzle and mystery of space and time. Recently, NASA’s James Webb Space Telescope (JWST) and Hubble Space Telescope together studied an expansive galaxy cluster known as MACS0416. The resulting panchromatic image combines visible and infrared light to assemble one of the most comprehensive views of the universe ever taken. Located about 4.3 billion light-years from Earth, MACS0416 is a pair of colliding galaxy clusters that will eventually combine to form an even bigger cluster.
The image, revealing a wealth of details, only possible to be captured by combining the power of both space telescopes, includes a bounty of galaxies outside the cluster and a sprinkling of sources that vary over time, likely due to gravitational lensing – the distortion and amplification of light from distant background sources.
This cluster was the first of a set of unprecedented, super-deep views of the universe from an ambitious, collaborative Hubble programme called the Frontier Fields, inaugurated in 2014. Hubble pioneered the search for some of the intrinsically faintest and youngest galaxies ever detected, while JWST’s infrared view significantly bolsters this deep look by going even farther into the early universe with its infrared vision.
“We are building on Hubble’s legacy by pushing to greater distances and fainter objects,” says Rogier Windhorst of Arizona State University, principal investigator of the PEARLS (Prime Extragalactic Areas for Reionization and Lensing Science), which took the JWST observations.
What the colours mean
To make the image, in general, the shortest wavelengths of light were colour-coded blue, the longest wavelengths red, and the intermediate wavelengths green. The broad range of wavelengths, from 0.4 to 5 microns, yields a particularly vivid landscape of galaxies. Those colours give clues to galaxy distances: The bluest galaxies are relatively nearby and often show intense star formation, as best detected by Hubble, while the redder galaxies tend to be more distant as detected by JWST. Some galaxies also appear very red because they contain copious amounts of cosmic dust that tends to absorb bluer colours of starlight. “The whole picture doesn’t become clear until you combine Webb data with Hubble data,” says Windhorst.
Christmas Tree Galaxy Cluster
While the new Webb observations contribute to this aesthetic view, they were taken for a specific scientific purpose. The team combined their three epochs of observations, each taken weeks apart, with a fourth epoch from the CANUCS (CAnadian NIRISS Unbiased Cluster Survey) research team. The goal was to search for objects varying in observed brightness over time, known as transients.
They identified 14 such transients across the field of view, 12 of which were located in three galaxies that are highly magnified by gravitational lensing, and may be individual stars or multiple-star systems that are briefly very highly magnified. The remaining two transients are within more moderately magnified background galaxies and are likely to be supernovae.
“We’re calling MACS0416 the Christmas Tree Galaxy Cluster, both because it’s so colourful and because of these flickering lights we find within it. We can see transients everywhere,” says Haojing Yan of the University of Missouri in Columbia, lead author of one paper describing the scientific results.
Finding so many transients with observations spanning a relatively short time frame suggests that astronomers could find many additional transients in this cluster and others like it through regular monitoring with Webb.
A Kaiju star
Among the transients the team identified, one stood out in particular. Located in a galaxy that existed about 3 billion years after the Big Bang, it is magnified by a factor of at least 4,000. The team nicknamed the star system “Mothra” in a nod to its “monster nature”, being both extremely bright and magnified. It joins another lensed star the researchers previously identified that they nicknamed “Godzilla”. (Both Godzilla and Mothra are giant monsters known as “kaiju” in Japanese pop culture).
Interestingly, Mothra is also visible in the Hubble observations that were taken nine years before. This is unusual because a very specific alignment between the foreground galaxy cluster and the background star is needed to magnify a star so greatly. The mutual motions of the star and the cluster should have eventually eliminated that alignment.
The most likely explanation is that there is an additional object within the foreground cluster that is adding more magnification. The team was able to constrain its mass to be between 10,000 and 1 million times the mass of our Sun. The exact nature of this so-called “milli-lens,” however, remains unknown.
“The most likely explanation is a globular star cluster that’s too faint for Webb to see directly,” states Jose Diego of the Instituto de Física de Cantabria in Spain, lead author of the paper detailing the finding. “But we don’t know the true nature of this additional lens yet,” he adds.