Dharam Vir Lal

My research interests are focussed on improving our understanding of physical conditions in extragalactic radio sources, by modeling the gaseous environments of radio galaxies in both the early and the late Universe. This work is based on data from the Chandra Space Observatory, the Giant Metrewave Radio Telescope, and other ground- and space-based observatories. We are also interested in issues relating radio interferometry and the effects of the atmosphere, specifically, the ionosphere, on radio observations. A brief description of two of these research areas is given below:


Inverse-Compton emission from high-redshift Active Galactic Nuclei:

High-redshift radio galaxies (HzRGs) are excellent beacons for pin-pointing the most massive objects in the early universe, including galaxies, super-massive black holes (SMBHs), or galaxy clusters. The strongest constraint on the high-redshift evolution of SMBHs comes from the observation of powerful HzRGs. The luminosities of these sources imply that SMBHs of mass comparable to a billion solar masses were already in place when the universe was only 1–3 Gyr old. To grow from seed fluctuations up to such a high mass requires an almost continuous accretion of gas. Therefore, to understand the evolution of the first SMBHs in the first pre-galactic radio sources and their impact on the reionization of the universe, it is important to understand the balance in the energy budget between mechanical and radiative power at these high redshifts.


Morphology of (head-tail) radio galaxies as tracers of cluster potential:

Head-tail sources are characterized by a head identified with the optical galaxy and two trails of radio emission sweeping back from the head. The long tails of these galaxies carry the imprint of relative motion between the non-thermal plasma and the ambient hot gas. Fortunately, the jets survive the encounter with the ICM, with possible shocks leading to the formation of the long tails, and specifically, they seem to be devoid of the growth of Kelvin-Helmholtz instabilities. Hence, in the parlance of the field, they reflect the weather conditions of the ICM, which allows one to make quantitative statements about their dynamics and energetics. Such observations can potentially reveal details of cluster mergers such as subsonic/transonic bulk flows, shocks and turbulence.


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