Indian astronomers discover a rare and farthest gamma-ray emitting galaxy

Indian astronomers discover a rare and farthest gamma-ray emitting galaxy

Hummingbird News Desk

NEW DELHI, 13 APRIL: Indian astronomers along with researchers from other institutions 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.

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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.

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.

What are Active Galactic Nuclei (AGN)?

Most of the galaxies are powered by a supermassive black hole at their center. Normal black holes that form from the death of a single star are ten times the mass of the Sun, but the supermassive black holes are millions to billions of times the mass of the Sun.

These supermassive black holes found at the center of the galaxies are the reason that the compact region at the center of a galaxy has an extremely high luminosity. The bright central regions of these galaxies are called Active Galactic Nuclei (AGN). The galaxies hosting Active Galactic Nuclei (AGN) are called active galaxies.

Compared to normal galaxies, these active galaxies can emit up to thousands of times of energy. The emissions of active galaxies are non-stellar (non-thermal) that are observed in the form of microwave, radio, infrared, optical, X-ray, ultra-violet, and gamma-ray wavebands.

The gamma-ray emitting Narrow-Line Seyfert 1 (NSL1) galaxies

The Narrow-Line Seyfert 1 (NSL1) galaxies are classified as AGN. The type 1 Seyfert galaxies have narrow emission lines in their optical spectrum.

Earlier, the NSL1 galaxies were thought to be powered by low mass black holes and therefore were deemed very unlikely to produce relativistic jets. This was proved wrong when gamma-ray emissions were detected in them. Gamma-ray emissions areindicative of the presence of relativistic jets.

So far, dozens of NSL1 galaxies with redshifts lesser than one have been identified with gamma-ray emissions. But there was no method in place till now to find NSL1 galaxies with redshifts larger than one.

This changed when scientists from the Aryabhatta Research Institute of observational sciencES (ARIES), Nainital, an autonomous institute of the Department of Science & Technology (DST),found a genuine high-gamma ray emitting NLS1 galaxy at a high redshift of 1.34 (larger than 1).

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.

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.

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