Jayaram N. Chengalur

Senior Professor I
Email: chengalur [at] ncra.tifr.res.in
Phone: +91 - 20 - 25719248
Extn: 9248
Office: F201
National Centre for Radio Astrophysics
Tata Institute of Fundamental Research
Savitribai Phule Pune University Campus,
Pune 411 007
Maharashtra, INDIA


Main Research Areas: Extragalactic astronomy; Cosmology; The interstellar medium; Fundamental constant evolution.

Biography:

Jayaram N. Chengalur obtained his B.Tech. in Electrical Engineering from IIT-Kanpur in 1987. He then moved to Cornell University for his doctoral studies, completing his Ph.D. in 1994. Following this, he worked as a post-doctoral fellow at the Netherlands Institute for Radio Astronomy (ASTRON) in the Netherlands, before joining the National Centre for Radio Astrophysics in 1996.

Research description:

My main research focus is extragalactic astronomy, particularly studies of nearby dwarf irregular galaxies, neutral hydrogen (HI) absorption in very high redshift galaxies (the so called "damped Lyman alpha" systems), and in general studies of the evolution of the neutral hydrogen content of the universe.

The dwarf irregular galaxies that we study are 1000 to 10000 times less massive than our own galaxy, the Milky Way. These galaxies are interesting in not only in their own right, but also in the context of hierarchical galaxy formation models in which large galaxies like our own form by the hierarchical merger of smaller progenitors. In this picture, the very small "dwarf" galaxies in the local universe are the survivors of this merger process, and are possible analogs of the galaxies in the early universe. Detailed studies of these galaxies hence provide insights and constraints on galaxy formation models. Particular topics that our group has been investigating include the distribution and total dark matter content of, as well as the processes that govern the conversion of gas into stars in, some of the smallest known galaxies. Most of this work is in the context of a survey of HI emission from a large sample (~ 75 objects) of nearby, extremely faint, dwarf irregular galaxies, viz. the Faint Irregular Galaxies GMRT Survey (FIGGS). The FIGGS data substantially extend the baseline in mass and luminosity space for a comparative study of galaxy properties. FIGGS data have been used to study the shapes of the gas disks of these galaxies, the content and distribution of dark matter, the Tully-Fisher, Baryonic Tully-Fisher and Radio-FIR relations, etc. There have also been several detailed studies on the relationship between gas and star formation in these galaxies, as well as a comparison of the HI column density distribution observed in these galaxies and the distribution observed in Damped Lyman-alpha systems in the early Universe.

The redshift evolution of the gas content of galaxies is being studied using deep HI 21cm emission observations of field and galaxy clusters at redshifts between ~0.2 and ~0.4. These observations constrain the evolution of the HI content of galaxies as well as the effect of the cluster environment on the gas content. At these redshifts, the emission from the individual galaxies is too faint to detect; instead, the average emission is measured by stacking together the spectra of all the known galaxies in the observed field. The detection of emission at a redshift of ~ 0.4 represents the highest redshift at which there is a direct constraint on the gas associated with star-forming galaxies.

At still higher redshifts, observations of HI 21cm absorption raising in damped-Lyman alpha systems help us understand the physical conditions in the gas in these systems. Our observations indicate that the spin temperature of the hydrogen in damped Lyman-alpha systems is significantly larger than that typical of large spiral galaxies like the Milky Way. Comparisons between the observed redshifts of different spectral lines in these systems also allows us to constrain the variation of fundamental constants like the fine structure constant and the ratio of the proton mass to the electron mass with cosmological time.

A new experiment is also being planned to study the large scale neutral hydrogen correlation function at a redshift of ~ 3 using the upgraded Ooty Radio Telescope. This is a large collaboration involving astronomers from RRI, IISER-Mohali, and IIT-Kharagpur.

Selected publications:

1. Phased array observations with infield phasing (S. Kudale & J. N. Chengalur 2017, Experimental Astronomy, 44, 97)

2. Giant Metrewave Radio Telescope observations of neutral atomic hydrogen gas in the COSMOS field at z ~ 0.37(J. Rhee, P. Lah, J. N. Chengalur, F. H. Briggs, M. Colless 2016, 460, 2675)

3. Non-linear redundancy calibration (V. R. Marthi & J. N. Chengalur 2014, MNRAS, 437, 524)

4. The temperature of the diffuse HI in the Milky Way - II. Gaussian decomposition of the H I-21 cm absorption spectra (N. Roy, N. Kanekar, J. N. Chengalur 2013, MNRAS, 436, 2366)

5. FLAGCAL: a flagging and calibration package for radio interferometric data (J. Prasad & J. N. Chengalur 2012, Experimental Astronomy, 33, 157)

6. When are extremely metal-deficient galaxies extremely metal deficient? (B. Ekta & J. N. Chengalur 2010, 406, 1238)

7. Star formation in extremely faint dwarf galaxies (S. Roychowdhury, J. N. Chengalur, A. Begum, I. D. Karachentsev 2009, MNRAS, 397, 1435)

8. FIGGS: Faint Irregular Galaxies GMRT Survey - overview, observations and first results (Begum A., Chengalur J. N., et al. 2008, MNRAS, 386, 1667)

9. Foregrounds for redshifted 21-cm studies of reionization: Giant Metrewave Radio Telescope 153-MHz observations (S. S. Ali, S. Bharadwaj, J. N. Chengalur 2008, MNRAS, 385, 2166)

10. Constraining the Variation of Fundamental Constants using 18cm OH Lines (J. N. Chengalur & N. Kanekar 2003, PhRvL, 91, 241302)




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