Dharam Vir Lal

Associate Professor G
Email: dharam [at] ncra.tifr.res.in
Phone: +91 - 20 - 25719249
Extn: 9249
Office: F221
National Centre for Radio Astrophysics
Tata Institute of Fundamental Research
Savitribai Phule Pune University Campus,
Pune 411 007
Maharashtra, INDIA

Main Research Areas: Extragalactic Radio and X-ray Astronomy.


Dharam Vir Lal completed his B.Sc. and M.Sc. from St. John's College, Agra University in 1991 and 1993, respectively. He then joined the Joint Astronomy Program (JAP) at the Indian Institute of Science, Bangalore, and the Indian Institute of Astrophysics, Bangalore, in 1994 for his Ph.D, which he completed in 2001. He later held several postdoctoral fellowships, at : - Indian Institute of Astrophysics, Bangalore (2001-2002), - National Centre for Radio Astrophysics, Pune (2002-2005), - Institute of Astronomy and Astrophysics, Academia Sinica, Taipei (2005-2006), - SKADS Square Kilometre Array Design Studies postdoctoral at the Max Planck Institute for Radio Astronomy, Bonn (2006-2009), and - Harvard-Smithsonian Centre for Astrophysics, Cambridge (2009-2012). In February 2012, he took up the position of a Reader at the National Centre for Radio Astrophysics.

Research description:

My research interests are focussed on improving our understanding of physical conditions in extra-galactic radio sources, by modelling 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 and other ground- and space-based observatories. I am also interested in issues relating radio interferometry and the effects of the atmosphere, in particular the ionosphere, on radio astronomical observations. A brief description of these research areas is given below:
I. Inverse-Compton emission from radio lobes of powerful high-redshift Active Galactic Nuclei:
High-redshift radio galaxies (HzRGs) at redshifts higher than 2 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-z 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.
II. 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 us 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.
III. Ionosphere turbulence − A challenge for low radio frequency imaging:
At low radio frequencies, the structures and the turbulence in the ionosphere, leads to an effect similar to twinkling of stars seen in the optical regime. The terrestrial ionosphere is a dynamic and inhomogeneous medium which changes on time scales ranging from seconds to days, seasons and years, and has structures spanning a large range of scale sizes. As the incident wavefronts from distant cosmic sources propagate through the ionospheric structures, the wavefronts get distorted. These distortions translate into apparent changes in the intensity and structure of sources in radio images. For studying cosmic sources, it is therefore essential to understand and undo the distortions imposed by the ionosphere. Several techniques are currently being used for ionospheric calibration, but it is still a challenge to design and implement a generalised methodology to calibrate the ionosphere distortions satisfactorily.

Selected publications:

1:Gas Sloshing and Radio Galaxy Dynamics in the Core of the 3C449 Group ( Lal, D. V. et al. 2012, ApJ, 764, 83 )

2: Chandra X-Ray Observations of the Redshift 1.53 Radio-loud Quasar 3C 270.1 ( Wilkes, B. J. et al. 2012, ApJ, 745, 84 )

3. Seyfert Galaxies: Nuclear Radio Structure and Unification ( Lal, D. V. et al. 2011, ApJ, 731, 68 )

4: The Infrared Jet in 3C 31 ( (Lanz, L. et al. 2011, ApJ, 731, 52 )

5: The Radio Properties of Type 2 Quasars ( Lal, D.V. & Ho, L.C. 2010, AJ, 139, 1089 )

6: A Chandra Observation of 3C 288—Reheating the Cool Core of a 3 keV Cluster from a Nuclear Outburst at z = 0.246 ( Lal, D. V. et al. 2010, 722, 1735 )

7: Array configuration studies for the SKA - Implementation of figures of merit based on spatial dynamic range ( Lal, D.V. et al. 2010, arXiv:1001.1477)

8: 'Normal' Fanaroff-Riley type II radio galaxies as a probe of the nature of X-shaped radio sources ( Lal, D.V. et al. 2008, MNRAS, 390, 1105 )

9: 3C 223.1: A source with unusual spectral properties ( Lal, D.V. & Rao, A.P. 2005, MNRAS, 356, 232)

10: Milliarcsec-scale radio structure of a matched sample of Seyfert 1 and Seyfert 2 galaxies ( Lal, D.V. et al. 2004, A&A, 425, 99)