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Astronomers have discovered a new active galaxy identified as the farthest gamma-ray emitting galaxy that has so far been stumbled upon. This active galaxy called the Narrow-Line Seyfert 1 (NLS1) galaxy, which is about 31 billion light-years away, opens up avenues to explore more such gamma-ray emitting galaxies that wait to meet us.

Ever since 1929, when Edwin Hubble discovered that the Universe is expanding, it has been known that most other galaxies are moving away from us. Light from these galaxies is shifted to longer (and this means redder) wavelengths - in other words, it is red-shifted. Scientists have been trying to trace such red-shifted galaxies to understand the early Universe.

Scientists from ARIES, an autonomous institute of the Department of Science & Technology (DST), Government of India, in collaboration with researchers from other institutions, studied around 25,000 luminous Active galactic nuclei (AGN) from the Sloan Digital Sky Survey (SDSS), a major optical imaging and spectroscopic survey of astronomical objects in-operation for the last 20 years and found a unique object that emits high-energy gamma rays located at a high redshift (more than 1). They identified it as a gamma-ray emitting NLS1 galaxy, which is a rare entity in space.

Powerful relativistic jets, or sources of particles in the Universe traveling nearly at speed to light, are usually produced by AGN powered by large black holes and hosted in a giant elliptical galaxy. However, detection of gamma-ray emission from NLS1 challenges the idea of how relativistic jets are formed because NLS1s are a unique class of AGN that are powered by black hole of low mass and hosted in spiral galaxy. As of today, gamma-ray emission has been detected in about a dozen NLS1 galaxies, which are a separate class of AGN identified four decades ago. All of them are at redshifts lesser than one, and no method was present till date to find NLS1 at redshifts larger than one. This discovery opens up a new way to find gamma-ray emitting NLS1 galaxies in the early Universe.

For the research, the scientists used one of the largest ground-based telescopes in the world, the 8.2 m Subaru Telescope located at Hawaii, USA. They helped establish a new method to find high redshift NLS1 galaxies that were not known previously by comparing different emission lines in their spectra. The new gamma-ray emitting NLS1 was formed when the Universe was only about 4.7 billion years old as compared to its current age of about 13.8 billion years.

The research led by Dr. Suvendu Rakshit, Scientist, ARIES, in collaboration with various scientists Malte Schramm (Japan), C. S. Stalin (IIA, India), I. Tanaka (USA), Vaidehi S. Paliya (ARIES), Indrani Pal (IIA, India), Jari Kotilainen (Finland) and Jaejin Shin (South Korea) has recently been accepted for publication in the journal Monthly Notices of Royal Astronomical Society. Motivated by this finding, Dr. Rakshit and his collaborators are keen to exploit the capabilities offered by the TIFR-ARIES Near-Infrared Spectrometer on the recently commissioned 3.6 m Devasthal Optical Telescope (DOT) at ARIES to find more such gamma-ray emitting NLS1 galaxies at much larger redshifts.

For more details, Dr. Suvendu Rakshit (suvendu[at]aries[dot]res[dot]in) can be contacted.

Publication links:

DOI: https://doi.org/10.1093/mnrasl/slab031
arXiv: https://arxiv.org/abs/2103.16521

The vast majority of Sun like stars are expected to destroy lithium (Li) over the course of their lives. An international team led by Dr. Bharat Kumar Yerra (currently a post-doctoral fellow at NAOC, Beijing) along with Prof. Eswar Reddy (IIA) carried out a large-scale investigation into the Li content of stars in the red clump phase of stellar evolution. They found that all these stars have very high levels of Li for their evolutionary stage, with an increase of a factor of around 40 over the end of the red giant branch phase (the previous stage of their evolution). This suggests that all low-mass stars undergo an Li production phase between the red giant and the red clump stages, when they undergo the helium flash. The new finding is not predicted by the existing theories of stellar evolution, revealing a stark tension between observations and models.

An extreme helium star or EHe is a low-mass supergiant that is almost devoid of hydrogen, the most common chemical element of the universe. There are 21 of them detected so far in our galaxy. The origin and evolution of these Hydrogen deficient objects have been shrouded in mystery. Their severe chemical peculiarities challenge the theory of well-accepted stellar evolution as the observed chemical composition of these stars do not match with that predicted for low mass evolved stars.

A study by the Indian Institute of Astrophysics (IIA) an autonomous institute of Department of Science and Technology which detected the presence of singly ionised fluorine for the first time in the atmospheres of hot Extreme Helium Stars makes a strong case that the main formation of these objects involves a merger of a carbon-oxygen (CO) and a Helium (He) white dwarf.

The research published in the Astrophysical Journal, led by Anirban Bhowmick (Ph.D. student, IIA, Bengaluru), Prof. Gajendra Pandey (IIA) and Prof. David Lambert (University of Texas at Texas-Austin), which showed fluorine abundances determined from singly ionized fluorine (F II) lines suggest a very high enrichment of fluorine, about a factor of 100 to 10000 times higher than normal stars.

Clues to evolution of extreme helium stars require accurate determinations of their chemical composition, and the peculiarities, if any, become very important. Fluorine plays a very crucial role in this regard to determine the actual evolutionary sequence of these hydrogen deficient objects. Severe fluorine enrichment w.r.t normal stars (of the order of 800 − 8000) was observed in the cool EHes along-with the cooler classical hydrogen deficient stars, the RCB variables (R Coronae Borealis Stars) hinting at close evolutionary connection between them. The scientists explored the relationship of hot EHes (EHes having effective temperature ≥ 14000K), with the cooler EHes, based on their fluorine abundance and spotted it in the former, thus establishing an evolutionary connection across a wide range of effective temperature.

High-resolution echelle spectra of 10 hot EHes were obtained from Hanle Echelle Spectrograph (HESP) mounted on the 2-m Himalayan Chandra Telescope at the Indian Astronomical Observatory (IAO) in Hanle, Ladakh, (remotely operated by IIA) including data from McDonald Observatory, USA, and ESO archives.

By comparing the observed fluorine abundances with other abundances of the key elements, the scientists could determine the formation channels responsible for fluorine enrichment. The varied range of observed fluorine abundance across stars having similar atmospheric parameters points out the difference in the individual star’s evolution and the ensuing nucleosynthesis. Particularly, the enrichment of fluorine in the atmospheres of carbon-rich EHes and absence of the same in carbon-poor EHes suggest that fluorine is profusely produced during the merger of a He-CO WD resulting in a carbon-rich EHe, whereas He-He WD merger that results in carbon-poor EHes does not account for fluorine overabundance.

The detection of enhanced fluorine abundances in the atmospheres of hot EHes solves a decade-old mystery about their formation. It firmly places hot EHes in an evolutionary sequence with cool EHes and other hydrogen-deficient stars and zeros in on the evolutionary scenario, which involves the merger of two double degenerate white dwarfs (WDs).

Publication details: The Astrophysical Journal, Volume 891, Issue 1, id.40 https://iopscience.iop.org/article/10.3847/1538-4357/ab6e6d