A team of astronomers from India, the US, and France, led by Pune’s Inter-University Centre for Astronomy and Astrophysics (IUCAA), has observed the formation of new stars in the outer regions of distant dwarf galaxies. These galaxies, known as ‘Blue Compact Dwarf’ (BCD) galaxies, have also exhibited evidence of the new stars migrating inwards towards its centre, adding to the galaxies’ mass and volume, Trend reports citing The Print.
The team detected these star formation regions in eleven BCDs using the Ultra-Violet Imaging Telescope of AstroSat, India’s first dedicated multi-wavelength space telescope aimed at studying celestial sources. Their findings, which have been peer-reviewed, were published in the scientific journal Nature last week.
The findings indicate “extended star formation” in the material that’s coming together to form a dwarf galaxy. This is significant, considering it is otherwise very hard to observe the formation of these early BCD galaxies as they are too small and faint and distant.
AstroSat was able to observe these galaxies in both visible and ultraviolet (UV) light. This is the first time extended far-ultraviolet (FUV) disks have been observed in distant dwarf galaxies, according to the study.
“We are now able to observe how dwarf galaxies in the comparatively early universe are acquiring their stellar content and are on their way to evolve into present-day dwarf galaxies,” said Anshuman Borgohain, astronomer from Tezpur University, Assam, and lead author of the paper, in an email to ThePrint. “This will help bridge the gap of understanding in the diverse dwarf galaxy population that we see around us in the present day.”
AstroSat observes distant objects on the other end of the electromagnetic spectrum compared to the other space telescope that’s making news, NASA’s James Webb Space Telescope. While Webb studies faraway galaxies in various parts of the infrared wavelength, AstroSat observes objects in ultraviolet (UV), X-ray, and visible light. NASA’s Hubble space telescope also uses UV, visible as well as near-infrared regions.
The light from distant objects is ‘red-shifted’ because of the expansion of the universe — which means that the further away an object moves, the more the visible light from it shifts towards the red end of the spectrum.
In contrast, young stars that are just forming emit a majority of their energy in UV. Thus, galaxies that contain large clusters of young, hot, and massive stars, appear to be blue in colour, and are thus named Blue Compact Dwarfs.
BCDs have been hard to study and understand as they are extremely compact and also very faint.
Just like Webb, AstroSat is also capable of peering into the past; that is, observing light sources as they were billions of years ago when the light was emitted. The galaxies that were observed in this study have a ‘lookback time’ of 1.3–2.8 billion years.
The team also noticed that the UV disk extending from the centre of the galaxy exceeded that of the optically visible disk of accreting material.
“Galaxies grow progressively faint as we go from inner to outer regions. So, to be able to detect emission in such regions, we require long observation hours,” explained lead author Borgohain. Even Hubble’s deep observations failed to yield data on star-forming regions in visible wavelengths.
“The detection of these star forming regions in UV suggest they contain young stars because such stars emit predominantly in the UV wavelength regime. Also, since we don’t see anything in the optical, it means that there are no old stars in the outer regions,” Borgohain said, adding: “Star formation seen in outer low-density environments is puzzling because the gas in these regions is inefficient to form stars.”