If individual star systems show us a wide variety of formation scenarios – and we just examined recent ESO work on circumstellar disks in different star-forming regions – the variety in galaxy evolution is even more spectacular. I’m reminded here of an unusual find when my uncle Roland died unexpectedly and I became his executor. Clearing out his house preparatory to sale, I discovered a series of astronomy photographs that he had blown up to huge scale. An image of M31, the great spiral of Andromeda, was fully six feet long and gorgeously mounted. I remembered nights as a child when he would visit from Florida and point out celestial objects for me to observe with my 3-inch reflector. M31, he told me then, was considerably wider than the Moon in the sky.

When I checked, I found that Andromeda had an angular size of 3 degrees, as opposed to about half a degree for the Moon. Even so, our spectacular sister galaxy is actually a difficult catch, with only its brighter central region visible to the naked eye, and even there tricky to find depending on local conditions of light pollution. Here I chuckle, remembering that I inherited from my uncle his eight-inch Celestron. The bane of his life was his backyard neighbor, who would power up huge outdoor security lights at the most inappropriate times. No luck seeing M31 under those conditions!

Image: M31 through a small telescope, with the Moon’s size shown for reference. Credit: Caradon Observatory.

Timothy Ferris produced a spectacular book called, simply, Galaxies, published by Random House in 1988. If you’re a deep sky devotee, it’s worth seeking out in a used book store, as it’s a coffee-table volume with spectacular photography. The following passage captures some of the grandeur verbally, though it’s from Ferris’ equally valuable Seeing in the Dark (Simon & Schuster, 2002):

The very concept of space is inadequate for dealing with galaxies; one must invoke time as well. The Andromeda galaxy is steeply inclined to our line of sight, only fifteen degrees from edge-on. Since the visible part of its disk is roughly one hundred thousand light years in diameter, the starlight reaching our eyes from its more distant side is about one hundred thousand years older than the light we simultaneously see coming from the near side. When the starlight from the far side of Andromeda started its journey, Homo habilis, the first true humans, did not yet exist. By the time the near-side light started out, they did. So within that single field of view lies a swath of time that brackets our ancestors’ origins – and that, like the incomplete dates in a biographical sketch of a living person (1944-?), inevitably raises the question of our destiny as a species. When the light leaving Andromeda tonight reaches Earth, 2.25 million years from now, who will be here to observe it? We think of Einstein’s spacetime as an abstraction, but to observe a galaxy is to sense its physical reality.

Image: An ultraviolet look at Andromeda, from NASA’s Galaxy Evolution Explorer. Credit: NASA. I owe the reminder for the Ferris quote to the blog Ten Minute Astronomy.

How much more stunning, then, to think about the galaxy recently observed by the team doing the JWST Advanced Deep Extragalactic Survey (JADES). I will mercifully shorten its designation to JADES-GS-z7-01-QU, as the authors of the paper in Nature do. This is a so-called ‘quiescent’ galaxy, meaning that for a variety of possible reasons, star formation within it has all but ceased. The authors describe it as “a compact, discy galaxy.” And as the work of the large team led by Tobias Looser (University of Cambridge) now shows, it is also the first galaxy beyond redshift z=5 to be confirmed and characterized. Indeed, the redshift calculated for this object is z=7.3. The light from this particular galaxy would have been emitted some 13 billion years ago, a ‘mere’ 700 million years after the Big Bang.

We could also look at this galaxy in terms of its ‘comoving distance.’ The latter term is used to accommodate the fact that the universe is expanding, necessary to consider here because if an object is so far away that the light from it has traveled for most of the age of the visible universe, then during that time cosmic expansion has continued. Doing the math on this is beyond my skill set, but my research indicates that at z=7.3, the comoving distance of JADES-GS-z7-01-QU should be in the range of 30 billion light years. Mathematically inclined readers might want to fine-tune that figure.

That ‘chill up the spine’ feeling of encountering deep time/distance never quite goes away. In terms of its significance, though, we can focus in on quiescence, which is a measure of how star formation ceases in a galaxy. The galaxy in question, as observed now, has stopped forming new stars (which means it did that over 13 billion years ago). The paper indicates that the quenching period occurred 10 to 20 million years ago. Star formation seems to have been fast, ending abruptly, but what we don’t know is whether this condition is permanent. Indeed, as the paper on this work points out, how star formation is regulated in galaxies is one of the key open problems in astrophysics.

The authors run through the possibilities for slowing or stopping star formation, which include gas being expelled from galaxies by supermassive black holes or rapid star formation heating the ‘circumgalactic medium,’ thereby preventing the accretion of fresh gases. Low-mass galaxies (this is one) can be affected by feedback mechanisms that deplete the medium within galactic clusters. These differing processes operate over varying timescales, making the significance of JADES-GS-z7-01-QU clear, as noted by Roberto Maiolino (University of Cambridge), a co-author on the paper:

“We’re not sure if any of those scenarios can explain what we’ve now seen with Webb. Until now, to understand the early universe, we’ve used models based on the modern universe. But now that we can see so much further back in time, and observe that the star formation was quenched so rapidly in this galaxy, models based on the modern universe may need to be revisited.”

Image: False-colour JWST image of a small fraction of the GOODS South field, with JADES-GS-z7-01-QU highlighted. Credit: JADES Collaboration.

Thus we have the first of what should become many opportunities to learn about galaxy growth and transformation in the early universe. About the mass of the Small Magellanic Cloud, which continues to form new stars, this galaxy is dead as of the time of observation, but we can’t know what occurred in the 13 billion years before JWST was turned on it. Is quenching a widespread phenomena in the early universe, but a temporary one, so that later epochs see galactic rejuvenation? The scope of future work with JWST is beginning to take shape as we examine finds like these.

The paper is Looser et al. “A recently quenched galaxy 700 million years after the Big Bang,” Nature (06 March 2024). Abstract.