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| Malin 1's cannibalistic nature unearthed | ||
A recent study by a team of scientists from the Inter-University Centre for Astronomy and Astrophysics reveals how Malin 1, the largest known low-surface-brightness galaxy, is quietly cannibalising smaller dwarf galaxies. Malin 1 has continued to puzzle astronomers since its discovery. Its central region, roughly the size of the Milky Way's stellar disk, resembles a lenticular galaxy, while faint star-forming spiral arms extend out to about 3,000 light-years. Until now, it was unclear how this isolated galaxy was transforming its core without any apparent signs of interaction. This study uncovers hidden evidence of subtle mergers, shedding light on the galaxy’s ongoing growth.
While observing Malin 1 with the Ultraviolet Imaging Telescope (UVIT) onboard AstroSat, the team identified a number of young star-forming clumps in its central region. One of these, known as the C1 clump, is relatively bright and massive. Since the central region otherwise resembles a quenched galaxy—meaning star formation has largely ceased the presence of this bright UV clump was surprising and prompted the team to investigate further.
Using integral-field spectroscopic observations from the Multi-Unit Spectroscopic Ex-plorer (MUSE) instrument on the Very Large Telescope (VLT) in Chile, the researchers found that the C1 clump is moving roughly 150 km/s faster than the surrounding material and exhibits high turbulence. This is unexpected, given that the central region is otherwise quenched, settled, and appears undisturbed. Moreover, the C1 clump appears only in certain velocity channel maps—strongly suggesting it has an external origin, likely a dwarf galaxy being cannibalised.
Using a detailed stellar population modelling of the MUSE spectra, the team performed a kind of archaeology of the C1 clump and the rest. Beneath the C1 clump lies an old stellar population, more than 6 billion years old, with solar-like metallicity and low alpha-element abundance. After a long quiescent phase, a burst of star formation oc-curred in the last 200 Million years, producing the UV-bright stars in the C1 clump along with extremely metal-poor young stars. This young population is highly alpha-enhanced, indicating rapid enrichment by core-collapse supernovae
The same pattern is observed in the other clumps. Together, the chemical signatures and high velocities strongly suggest that the C1 clump is the remnant of an infalling dwarf galaxy on a polar orbit. This orbit likely allowed pristine, early-universe–like gas, fuel for making such young stars, to be delivered to Malin 1. The dwarf galaxy was shredded into four parts or more, all sharing very similar ages and chemical properties. These findings are published in the Astrophysical Journal Letters in Dec. 2025.
Manish Kataria, the lead author of the paper, is a Ph.D. student under the supervision of Prof. Kanak Saha at IUCAA. Manish says, “Understanding how giant, low-surface-brightness (GLSB) galaxies like Malin 1 grow is crucial, as their enormous, faint struc-tures challenge existing galaxy formation theories. New ultraviolet imaging from As-troSat/UVIT combined with MUSE IFU spectroscopy offers a powerful method to un-cover the subtle, non-disruptive minor-mergers driving their hidden growth”
Prof. Saha, who supervised the project, recalls noticing the UV clumps several years ago while examining the UVIT data on Malin 1 and speculating that they might be of external origin. Thanks to the integral-field data from MUSE and the careful analysis by Manish, this idea was confirmed: Malin 1 is silently cannibalising smaller dwarf galax-ies, a process that is transforming its central region.
While observing Malin 1 with the Ultraviolet Imaging Telescope (UVIT) onboard AstroSat, the team identified a number of young star-forming clumps in its central region. One of these, known as the C1 clump, is relatively bright and massive. Since the central region otherwise resembles a quenched galaxy—meaning star formation has largely ceased the presence of this bright UV clump was surprising and prompted the team to investigate further.
Using integral-field spectroscopic observations from the Multi-Unit Spectroscopic Ex-plorer (MUSE) instrument on the Very Large Telescope (VLT) in Chile, the researchers found that the C1 clump is moving roughly 150 km/s faster than the surrounding material and exhibits high turbulence. This is unexpected, given that the central region is otherwise quenched, settled, and appears undisturbed. Moreover, the C1 clump appears only in certain velocity channel maps—strongly suggesting it has an external origin, likely a dwarf galaxy being cannibalised.
Using a detailed stellar population modelling of the MUSE spectra, the team performed a kind of archaeology of the C1 clump and the rest. Beneath the C1 clump lies an old stellar population, more than 6 billion years old, with solar-like metallicity and low alpha-element abundance. After a long quiescent phase, a burst of star formation oc-curred in the last 200 Million years, producing the UV-bright stars in the C1 clump along with extremely metal-poor young stars. This young population is highly alpha-enhanced, indicating rapid enrichment by core-collapse supernovae
The same pattern is observed in the other clumps. Together, the chemical signatures and high velocities strongly suggest that the C1 clump is the remnant of an infalling dwarf galaxy on a polar orbit. This orbit likely allowed pristine, early-universe–like gas, fuel for making such young stars, to be delivered to Malin 1. The dwarf galaxy was shredded into four parts or more, all sharing very similar ages and chemical properties. These findings are published in the Astrophysical Journal Letters in Dec. 2025.
Manish Kataria, the lead author of the paper, is a Ph.D. student under the supervision of Prof. Kanak Saha at IUCAA. Manish says, “Understanding how giant, low-surface-brightness (GLSB) galaxies like Malin 1 grow is crucial, as their enormous, faint struc-tures challenge existing galaxy formation theories. New ultraviolet imaging from As-troSat/UVIT combined with MUSE IFU spectroscopy offers a powerful method to un-cover the subtle, non-disruptive minor-mergers driving their hidden growth”
Prof. Saha, who supervised the project, recalls noticing the UV clumps several years ago while examining the UVIT data on Malin 1 and speculating that they might be of external origin. Thanks to the integral-field data from MUSE and the careful analysis by Manish, this idea was confirmed: Malin 1 is silently cannibalising smaller dwarf galax-ies, a process that is transforming its central region.
Figure 1. Left: A colour image of the giant galaxy Malin 1 made using optical data from the Canada–France–Hawaii Telescope (CFHT). The dashed box highlights the central region studied in this work. Top right: An ultraviolet image from AstroSat, showing bright patches of young stars in the same region, which are clear signs of recent star formation. Bottom right: Observations from the MUSE instrument on the Very Large Telescope reveal glowing gas associated with these young, star-forming regions.
Publication Details
Chemically Primitive Dwarf Accretion Reignites the Inner Disk Assembly of Malin 1
Manish Kataria, Kanak Saha
Published in: ApJL [DOI: 10.3847/2041-8213/ae200d]
Manish Kataria, Kanak Saha
Published in: ApJL [DOI: 10.3847/2041-8213/ae200d]
- arXiv: 10.48550/arXiv.2511.12177
Research contacts:
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Mr. Manish Kataria Senior Research Fellow (SRF) IUCAA, Pune E-mail: manish_at_iucaa.in |
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Prof. Kanak Saha IUCAA, Pune E-mail: kanak_at_iucaa.in |