How do small galaxies – the building blocks of the universe – gather masses?
The misunderstood cosmic process may now become clearer to scientists thanks to a group of Indian astronomers who have provided a crucial link to paint the big picture of galaxy growth and evolution.
Using India’s first space observatory Astrosat, astrophysicists from the Inter-University Center for Astronomy and Astrophysics (IUCAA) in Pune and their collaborators in the United States and France have shown how star-forming features on the outskirts of a dwarf galaxy migrate to the central region of the galaxy, contributing to its growth in mass and luminosity.
Thanks to high-quality telescopes, scientists’ understanding of large galaxies like the Milky Way or Andromeda has improved over the past two decades. The Milky Way, for example, is known as one of the giant galaxies with billions of stars, although very little star formation is currently occurring.
But giant galaxies are surrounded by dozens of dwarf galaxies – irregularly shaped, often forming stars. Their mass could be 50 times less than that of the Milky Way. How these dwarf and giant galaxies assemble their stars and evolve remains one of the main puzzles.
IUCAA scientist Kanak Saha, his PhD student Anshuman Borgohain and their partners from Paris Observatory, IBM Watson Research Division and Tezpur University along with a senior Indian astronomer decided to probe the problem with 17 hours of observational data captured by Astrosat’s Ultraviolet Imaging Telescope.
“We have evidence that these galaxies are accumulating matter from the outside. We have witnessed the ‘live’ formation of these distant dwarf galaxies!” Saha told DH.
The team examined 11 blue dwarf galaxies located between 1.3 and 2.8 billion light-years away. The main challenge was to detect such faint, extremely blue star-forming clusters, too distant to see, despite containing a million solar masses of matter.
“The resolving power of the ultraviolet imaging telescope and deep-field imaging techniques have been key to spotting very young, large star-forming clusters. They form at the periphery and then fly off into the galaxy within a specific time frame, adding to the growth of the galaxy,” Saha said.
Much of the research work involves meticulously calculating the time it takes for forming star clusters to migrate through the interior of the galaxy. The time span, according to Saha, could be a tenth of a galaxy’s lifespan or a billion years.
“How some small galaxies can have such active star formation is a mystery,” said team member Bruce Elmegreen from the IBM Watson Research Division in the United States, adding that such observations suggest that gas accretion in the outermost parts may be forced to move towards the center due to the attraction exerted by gas giants and star complexes.
The results appeared last week in Nature.