Understanding the star formation rate (SFR) in a galaxy is critical to understanding the galaxy itself. Some galaxies are starburst galaxies with extremely high SFRs, some are quenched or quiescent galaxies with very low SFRs, and some are in the middle. Researchers used the JWST to observe a pair of galaxies at Cosmic Noon that are just beginning to merge to see how SFRs vary in different regions of both galaxies.
A rare alignment of massive objects in space allowed astronomers using the James Webb Space Telescope to observe a pair of distant, ancient galaxies that are just beginning to interact and merge. The JWST sees the galaxies as they were about seven billion years ago, near the end of the Universe’s Cosmic Noon. The Cosmic Noon was when star formation was at its peak.
One of the galaxies is a blue, face-on galaxy, and the other is a dusty red, edge-on galaxy. The JWST can only see them because of an intervening galaxy cluster named MACS-J0417.5-1154. It’s a gravitational lens that magnifies the light from the galaxy pair and smears the galaxies’ light into an arc.
Astronomers have found many gravitational lenses and regularly use them to observe objects that are otherwise nearly impossible to see. But this lens is different. It’s a hyperbolic umbilic gravitational lens and produces multiple images of the same objects, where each one has a different magnification and brightness.
“We know of only three or four occurrences of similar gravitational lens configurations in the observable universe, which makes this find exciting, as it demonstrates the power of Webb and suggests maybe now we will find more of these,” said astronomer Guillaume Desprez of Saint Mary’s University in Halifax, Nova Scotia. Desprez works with the CAnadian NIRISS Unbiased Cluster Survey (CANUCS), the team presenting the Webb results.
Not only does the cluster magnify the distant background galaxies, but it also warps their appearance and produces multiple copies. Together with an unrelated one, the galaxies combine to look like a question mark. They’ve been dubbed the Question Mark Galaxy Pair.
False colour images of the Question Mark Pair and MACS J0417.5?1154 (right-hand panel). The left two panels are zoom-in images of four of the multiply lensed images of the Question Mark Pair taken with HST and JWST. By comparing the JWST and HST images, we see how dusty the red edge-on galaxy is as it is barely visible in the HST/ACS imaging. Image Credit: Estrada-Carpenter et al. 2024. This isn’t the first time astronomers have observed these galaxies. The Hubble observed it previously. But the Hubble and the JWST see things differently. The JWST can see longer wavelengths of infrared light that pass through cosmic dust, while the Hubble only sees the wavelengths of light that get trapped in the dust. So, the Hubble couldn’t detect the question mark shape, whereas the JWST could.
“This is just cool looking. Amazing images like this are why I got into astronomy when I was young,” said astronomer Marcin Sawicki of Saint Mary’s University, one of the lead researchers on the team.
But the question mark shape is just an interesting visual curiosity. The research is about star formation, and these results highlight the JWST’s ability to identify star formation regions in distant galaxies.
“Knowing when, where, and how star formation occurs within galaxies is crucial to understanding how galaxies have evolved over the history of the universe,” said astronomer Vicente Estrada-Carpenter of Saint Mary’s University. Estrada-Carpenter used both Hubble’s ultraviolet and Webb’s infrared data to show where new stars are forming in the galaxies.
The researchers developed a new method to probe SFRs on different timescales of about ten million years and one hundred million years. The ten-million-year timescale relied on H-alpha emission line maps, and the one-hundred-million-year timescale relied on UV observations. H-alpha is sensitive to ten-million-year timescales because it stems from gas around massive, short-lived stars. UV is sensitive to one-hundred-million-year timescales because it originates from longer-lived stars.
Together, the ratio between the two can spatially resolve star formation burstiness.
They found that SFRs decrease at longer distances from the galactic center. That’s not surprising since star-forming gas tends to accumulate near galactic nuclei. However, they also found that overall, the SFR has increased by a factor of 1.6 over the last ~100 Myr, an indication that the galaxies are beginning to merge.
To better understand the merger aspect, the researchers broke the QMP down into segments: blue galaxy bulge and disc, red bulge and disc, and three types of clumps: bursting, equilibrium, and quenching.
This figure from the study shows how the researchers broke the QMP into segments to better understand it. Image Credit: Estrada-Carpenter et al. 2024.“Both galaxies in the Question Mark Pair show active star formation in several compact regions, likely a result of gas from the two galaxies colliding,” said Estrada-Carpenter. “However, neither galaxy’s shape appears too disrupted, so we are probably seeing the beginning of their interaction with each other.”
They identified twenty star-forming clumps in the galaxy pair, highlighting the JWST’s ability to spatially resolve star formation in distant galaxies. Of those 20, seven were experiencing bursty star formation, 10 were quenching, and three were in equilibrium. The blue face-on galaxy, especially its disk, is mostly in a quenching phase, which makes sense since the JWST is seeing the galaxy pair as they were near the end of Cosmic Noon.
Galaxies grow massive by merging, and one of the JWST’s science goals is to better understand mergers and how they affect star formation. The QMP could be beginning to merge which only increases its value as an observational target.
“What makes the QMP so interesting is that these galaxies are possibly at the beginning of an interaction (as their morphologies do not seem to be disturbed). An interaction between the galaxy pair could lead to a burst of star formation, and this may be the reason why the blue face-on galaxy contains so many clumpy star-forming regions,” the authors write in their paper.
These results are also giving us a look at what our own galaxy was like during Cosmic Noon.
“These galaxies, seen billions of years ago when star formation was at its peak, are similar to the mass that the Milky Way galaxy would have been at that time. Webb is allowing us to study what the teenage years of our own galaxy would have been like,” said Sawicki.
