Science Spies: JWST finds ancient galaxy may actually be two galaxies merging

Saturday, November 5, 2022

JWST finds ancient galaxy may actually be two galaxies merging

The James Webb Space Telescope has cast its eye on one of the oldest known galaxies, formed just 400 million years after the big bang, and it might be two galaxies merging

Space

31 October 2022

By Alex Wilkins

Light from MACS0647-JD is bent and magnified by the massive gravity of this cluster of galaxies, called MACS0647, causing it to appear in multiple places (highlighted in boxes)
NASA, ESA, CSA, STScI ,UT, JHU

One of the oldest known galaxies might actually be two galaxies in the process of merging, according to new observations from the James Webb Space Telescope (JWST).

In 2012, the Hubble Space Telescope spotted what appeared to be the oldest galaxy we had found, MACS0647-JD, thanks to the gravity from a cluster of galaxies called MACS0647 bending and magnifying its light, a phenomenon called gravitational lensing. MACS0647-JD formed about 13.3 billion years ago, or just 400 million years after the big bang, although JWST has since found galaxies that are potentially older.

Now, Tiger Yu-Yang Hsiao at Johns Hopkins University in Maryland and his colleagues have captured light from MACS0647-JD using JWST’s powerful infrared sensors and found that it appears to be either two galaxies or a galaxy with two distinct clumps of stars, with a third satellite galaxy nearby.

Galaxies so far away from us, about 13.3 billion light years away, are usually too dim to observe. “Due to gravitational lensing from the galaxy cluster, the light will bend and magnify [MACS0647-JD] by eight times, so it’s actually eight times brighter than it was,” says Hsiao.

One of the clumps of stars in MACS0647-JD looks larger and bluer than the other, which indicates that it formed more recently than the redder clump, because the longer it takes light to travel to us, the more its colouration shifts to redder spectral hues. Hsiao and his team don’t have enough data to say whether the galaxies are merging or have formed beside one another, but if confirmed to be a merger, it would be the oldest known example of one.

Even if MACS0647-JD is just one galaxy with two distinct clumps, it could provide important information on how the first galaxies formed. It came into being in the so-called reionisation era of the universe, just after the first stars began to form.

The universe has vastly expanded since these early galaxies formed, so they are all very far away. “Although other galaxies have been observed out to these early times, you can count them on one hand,” says Simon Dye at the University of Nottingham, UK. “They are rare and therefore each one gives us valuable insight into galaxy formation in the very early universe.”

However, we will need a larger sample of such objects across the sky to work out exactly when and how the first galaxies began to form, he adds.

Hsiao and his team hope to learn more about MACS0647-JD with another JWST observation in January next year. On that pass, it will take photos at two longer wavelengths of light and gather spectroscopic data for MACS0647-JD, which will allow better estimates of age and composition.

Reference: arxiv.org/abs/2210.14123

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