Opinion We thought we understood how a galaxy forms — and the universe just proved us wrong
‘Alaknanda’ is the latest galaxy discovered by the James Webb telescope that challenges popular theories amongst astronomers
If we could view our galaxy from the outside, it would appear to have a thin disc with some distinct spiral “arms” that are brightened by stars. 
Architectural projects on a galactic scale cannot be fast-tracked. Galaxies like our Milky Way take a long time to assemble and form. That, at least, was the conventional wisdom of astronomers until a few rather unsettling discoveries were made of late. A recent discovery by two astronomers from the National Centre of Radio Astrophysics, Pune, appears to throw one more challenge at astronomers.
If we could view our galaxy from the outside, it would appear to have a thin disc with some distinct spiral “arms” that are brightened by stars. These “arms” are like a traffic jam of stars in the galactic highway. Stars, including our Sun, go around the centre of the galaxy, and their paths often lead to a piling up of matter. This is similar to what happens behind a relatively slow-moving lorry on a highway. Gas in these regions is compressed by the “traffic jam”, which leads to further star formation. But how these structures develop in the first place has puzzled astronomers for a long time. Although divided by various theories of their origin, one idea has been common among theorists: Such spiral galaxies are rather fragile and not easy to form in the first place.
The natural implication is that the ancient universe could not have hosted many Milky Way-type spiral galaxies. The universe would have had to wait for a few billion years for them to appear. Apart from the spiral design, large galaxies like ours take a considerable amount of time to gather mass. Galaxies are believed to have formed “bottom up”, much like heroes in films rise from rags to riches. Small galaxies are thought to have formed early on, which then merged to build larger galaxies as time progressed. To reach the massive scale of our Milky Way, along with forming spiral arms, has been neither simple nor quick, according to the favourite theories among astronomers.
A few recent discoveries have, however, challenged these theories. Earlier this year, a humongous galaxy was found to have existed when the universe was just about a billion years old. This galaxy, dubbed “Zhúlóng” (meaning “Torch Dragon” in Chinese), was in size and its wealth of stars, similar to our Milky Way. It was decorated with star-studded spiral arms, like our own galaxy. Scientists can estimate the stellar mass and age of a galaxy by inspecting the spectrum of light emerging from it, and comparing it with their theoretical models of how a population of stars of a certain age and total mass should look in different colours.
The discovery of another ancient galaxy, named “Big Wheel”, was announced a few months ago. This giant structure with spiral arms existed when the universe was less than two billion years old, when astronomers thought such objects should either not exist or, at least, be rare to find.
And yet, another such galaxy, named “Alaknanda”, after the Himalayan river, by the astronomers from Pune, has been added to the growing list of surprises. Researchers Rashi Jain and Yogesh Wadadekar have peered behind a cluster of galaxies, the enormous mass of which has warped space to the extent that empty space acts like a giant lens and magnifies the galaxies located behind it. Alaknanda, patterned with readily discernible spiral arms and with a mass comparable to that of the Milky Way, seems to have existed when the universe was a fifth of its size now. Its stars appear to be only a few hundred million years old. Its spectrum reveals the presence of a moderate amount of dust as well, which is natural, since dust grains are both by-products of star formation and necessary ingredients for forging stars.
All said and done, the numbers of the mass and age of stars should be taken with a pinch of salt, because they depend on a few theoretical assumptions. At the same time, the rate at which ancient star-opulent galaxies are being discovered is certainly making theorists think afresh about how galaxies form.
The writer is an astronomer, science writer and novelist. He is currently a visiting professor, IISER Mohali
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