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.
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.”
BCDs and star formation
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.”
Missing reservoirs of neutral gas
Only the most recent, outermost star-forming regions emit energy in far-ultraviolet.
“These extended FUV disks are tell-tale signatures of galaxy disks that are accreting neutral hygrogen gas from the surrounding environment, referred to as the intergalactic medium,” explained Borgohain.
Another phenomenon that could likely occur here is passively existing reservoirs of neutral gas, according to Borgohain. Over billions of years, instabilities in the outer parts of this gas disk lead to localised collapse of gas, igniting the region to form new stars.
But these large reservoirs of neutral gas have not been detected around galaxies, especially with modern-day powerful telescopes capable of detecting them. It is likely that present-day dwarf galaxies around us have evolved enough to have exhausted the gas reservoirs, or that interactions with nearby galaxies would have destroyed it, speculated Borgohain.
The clumpy structure of this less dense region also indicates that it is not gravitationally stable or uniform, and is getting swept up in the rotating galaxy’s forces and sucked towards its centre.
The findings provide a key step in understanding the evolution of BCDs, and the next step will be to understand how low-density star formation regions occur around these dwarf galaxies and what drives them.
“We don’t know whether our findings are special cases or a common phenomena,” said Borgohain. “We are planning a survey for these kinds of galaxies to build their statistical significance and also to find if we could have missed out on something.” (The Print)
