Pune: Scientists at the Inter-University Centre for Astronomy and Astrophysics (IUCAA) finally solved the mystery of how Malin 1, the largest spiral galaxy ever discovered, is growing in size. Instead of violently colliding with other large galaxies, as is the standard process for giant galaxies to develop, Malin 1 is gradually absorbing much smaller dwarf galaxies.This silent cannibalism is helping Malin 1 grow without damaging its thin, faint spiral shape, as would be the case after violent collisions. The research team, led by Manish Kataria and Kanak Saha, used India’s AstroSat satellite and the MUSE instrument on the Very Large Telescope to look into the heart of the galaxy and found a group of young stars (200 million to 500 million years old) mixing with ancient ones (6 billion years old), transforming its central region quietly.
The paper was published in the Astrophysical Journal Letters in Dec 2025. If the Milky Way is a well-lit small town, Malin 1 would be a vast, dimly lit megacity seen from afar, Kataria said, to give an idea of how different the two galaxies are. “Malin 1 is extremely faint, massive, and, in terms of size, this is the largest disk galaxy in the local universe. Its central region, or the heart itself, is as big as our Milky Way’s stellar disk.” Despite its size, Malin 1 remained a puzzle for 40 years since its discovery. “To build such a huge disk of stars and gas, a galaxy must retain high angular momentum. In the early universe, a galaxy lost angular momentum via mergers and interactions. But Malin 1 somehow managed to keep high angular momentum. In other words, Malin 1 violates the typical angular momentum retention pathways,” said Kataria, adding that, naively speaking, such a large disk would fragment into pieces. Now it is very common for galaxies to merge. For example, the Milky Way will eventually merge with Andromeda, but Kataria said Malin 1 is doing so in an unusually gentle way. “Identifying past minor mergers is much like doing archaeology: it’s possible for nearby galaxies like the Milky Way and Andromeda, but in Malin 1, which is over 1.2 billion light-years away, it is extremely challenging,” said Kataria. The researchers think that a small dwarf galaxy fell into the central region of Malin 1 on a polar orbit, which allowed the dwarf to be absorbed into the core without disturbing the fragile outer disk. “In addition, the orbit brought in pristine, metal-poor gas that fuelled the formation of young, chemically distinct stars in the central region,” said Saha. In the core, scientists were surprised to observe a strange clump, which they called C1, which led them to their discovery. “Clump C1 is the brightest of all and was first identified in a far-UV image taken by UVIT on AstroSat. The clump appeared to host young stars in an otherwise largely dormant galaxy. Normally, material in a galaxy moves roughly with the local rotational speed, but C1 was found to be moving about 150 km/s faster than its surroundings. This distinct velocity indicates that the clump’s bulk material is kinematically decoupled from the rest of the galaxy, suggesting an external origin,” explained Saha. What the researchers expected, based on predictions until now for Malin 1, which already has gas with roughly Sun-like metallicity, is that if it formed new stars using its own gas, those stars should be metal-rich (like the sun) and not alpha-enriched (with oxygen, magnesium, silicon, etc). But what they saw in C1 was young stars that are metal-poor and alpha-enriched, which indicates the gas likely came from outside, probably from a small, metal-poor dwarf galaxy that merged with Malin 1.
