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Extragalactic Astronomy

Galaxies are the building blocks of the universe. Recent years have seen rapid advances in our understanding of the nature of matter (seen and unseen) of our own Milky Way and of galaxies in local universe and beyond. Investigating formation of Milky Way as well as other galaxies and galaxy clusters forms one of the thrust research areas of astronomy division at ARIES.

The astrophysical problems under this category include : How does a galaxy form ? What is the nature of dark matter in the galaxy ? Why some galaxies are active ? What is the role of black holes in formation of galaxies ? What is the origin of highly energetic extra-galactic objects such as gamma rays burst and supernovae ?


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For about a decade it has been well established that supermassive black holes (SMBHs, with masses between 10^6 – 10^10 M ⊙) are present in the nuclei of all galaxies with stellar bulges. At any given time a few percent of these SMBHs are fed a sufficient amount of gas that they will possess significant accretion disks. These disks can emit more radiation than all of the stars in the entire host galaxy because of the general relativistic effects that yield a very high efficiency for the conversion of matter into radiation as it spirals into a BH. This is the fundamental mechanism underlying Active Galactic Nuclei (AGN).

It has long been known that there are two major classes of luminous AGN (i.e., quasars). Roughly 85−90 % of these have very little radio emission F 5GHz /F B ≤ 10, here F 5GHz = flux at radio 5 GHz and F B = flux at optical B band 4400 ̊ A) and are therefore called radio-quiet quasars (RQQSOs). The remaining ∼ 10–15 % of quasars are radio-loud quasars (RLQSOs).

A small subset of RLQSOs shows rapid flux variability at almost all wavelengths of the electromagnetic (EM) spectrum and also have strongly polarized emission. Such flat spectrum radio quasars (FSRQs) are now usually clubbed together with the intrinsically weaker, but highly variable, BL Lacertae (BL Lac) objects and are collectively known as blazars. BL Lac objects show featureless (no prominent emission or absorption lines) optical continuum while FSRQs show prominent emission lines in their optical spectra. Blazars have spectral energy distributions (SED) that show two peaks and this leads to two subclasses of blazars: LBL (red or low energy or radio selected) and HBL (blue or high energy or X-ray selected). The lower frequency SED component peaks at IR/optical in LBLs and at UV/X-ray in HBLs. The second component extends up to gamma-rays, usually peaking at GeV in LBLs and at TeV in HBLs. Blazar radiation at all wavelengths is predominantly nonthermal. The EM emission is dominated by a synchrotron component at low-energy and at high energy probably by an inverse Compton component. Blazar emission is Doppler boosted jet emission, and blazars (and other radio loud active galaxies) eject relativistic jets in opposite directions (perpendicular to the accretion disk and/or aligned with the BH spin axis) that can grow into the largest physically connected objects in the Universe.

The research group working on AGNs focus on multi-wavelength variability of blazars, intra-night optical variability of radio-quiet and radio-loud AGNs, and AGN properties based on optical spectral lines. For doing these projects, multi-wavelength data of various public archive as well new observations with ARIES telescopes as well as other national and international facilities are taken.

  • Group Members
    Faculties : A. C. Gupta
    Postdocs : Haritma Gaur, Pankaj Kushwaha
    Research Scholars : Vinit Dhiman
  • Selected publications



Group Members :

Faculties : A. C. Gupta, Amitesh Omar, Brijesh Kumar, Kuntal Misra, S. B. Pandey
Postdocs : Ashwini Pandey, Haritma Gaur, Pankaj Kushwaha
Research Scholars : Ankur Ghosh, Raya Dastidar, Vinit Dhiman

Recent Publications :