India’s AstroSat witnessing the ‘live’ formation of dwarf galaxies

Shedding light on how dwarf galaxies assemble their stars.


Dwarf galaxies are small galaxies composed of a few billion stars. They are challenging to detect due to their low luminosity, low mass, and small size. However, it remains elusive how these dwarf and giant galaxies assemble their stars and evolve into modern-day galaxies.

India’s first dedicated multi-wavelength space observatory, AstroSat, cracked this mystery. A team of scientists using AstroSat shows how the star-forming complexes in the outskirts of a dwarf galaxy migrate towards the central region and contribute to its growth in mass and luminosity.

The team includes astronomers from India, the USA, and France. Professor Kanak Saha at the Inter-University Centre for Astronomy and Astrophysics (IUCAA), Pune, conceived this study. Mr. Anshuman Borgohain is the lead author of the paper.

According to scientists, “This process is important in understanding the bigger picture of galaxy growth and evolution.”

The team analyzed 17 hours of observational data captured by Astrosat’s Ultraviolet Imaging Telescope. They found evidence that these dwarf galaxies are an accreting matter from outside. They also witnessed it live.

Prof. Saha said, “We are witnessing the ‘live’ formation of these far-way dwarf galaxies! UVIT’s resolving power, and deep field imaging techniques have been the key to spotting some very young, large star-forming clumps. These form on the periphery and then spiral into the visible (optical) boundary of their galaxy within a billion years, thus adding to the growth of the galaxy. Most of our research meticulously calculates the time required for the clumps to migrate inside the galaxy.”

The team looked at 11 blue dwarf galaxies that are 1.3-2.8 billion light years away.

Prof. Saha emphasized that “the key challenge has been to firmly establish the detection of these faint, extremely blue, star-forming clumps which are very far away to see although they have a million solar masses of material within them.”

“At slightly larger distances, the UVIT would not resolve these galaxies, and we do not have an example of an extended disk seen in UV in any present-day dwarf galaxies. The redshift of these dwarfs has been optimal to probe these blue clumpy structures in their outskirts.”

Another coauthor, Prof. Francoise Combes of Observatoire de Paris, France, further added that “the discovery teaches us how surprisingly stars can form in metal-poor gas disk. Normally these dwarf galaxies are dominated by dark matter, and the gas disk would not be unstable. But our discovery is evidence that even such a gas disk fragments.”

Prof. Bruce Elmegreen of IBM Watson Research Division, USA, who contributed to the study, said, “It has been a mystery how some small galaxies like these can have such active star formation. These observations suggest that accreting gas in the far outer parts can be forced to move towards the center because of the inward torques exerted by giant gas and stellar complexes. This migration builds up the central density over the galaxy’s lifetime.”

Journal Reference:

  1. Borgohain, A., Saha, K., Elmegreen, B. et al. Extended far-ultraviolet emission in distant dwarf galaxies. Nature 607, 459–462 (2022). DOI: 10.1038/s41586-022-04905-9


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