Nissim Kanekar

Associate Professor G
Email: nkanekar [at] ncra.tifr.res.in
Phone: +91 - 20 - 25719246
Extn: 9246
Office: F219
National Centre for Radio Astrophysics
Tata Institute of Fundamental Research
Savitribai Phule Pune University Campus,
Pune 411 007
Maharashtra, INDIA


Main Research Areas: Fundamental constant evolution, galaxy formation and evolution, the interstellar medium, high-redshift galaxies, atomic and molecular spectroscopy.

Biography:

Nissim Kanekar obtained his B.Sc. from Mumbai University in 1993, his M.Sc. from Pune University in 1995, and his Ph.D. from Pune University in 2000, with the doctoral research carried out at the National Centre for Radio Astrophysics. After a NOVA Fellowship at the Kapteyn Institut, Groningen, The Netherlands, and a Jansky Fellowship and a Max-Planck Fellowship, both at the National Radio Astronomy Observatory, Socorro, USA, he joined the National Centre for Radio Astrophysics in September 2009. He is now an Associate Professor and a DST Swarnajayanti Fellow.

Research description:

My main research interests lie in the following broad areas: (1) Astronomical studies of fundamental constant evolution, (2) Galaxy evolution, and (3) Understanding conditions in the Interstellar Medium (ISM) of the Milky Way.
Fundamental constant evolution:
A generic prediction of higher-dimensional theories that attempt to unify the standard model of particle physics and general relativity is that low energy fundamental constants such as the fine structure constant and the ratios of particle masses should evolve with time. Astronomical studies allow one to test for such evolution over cosmological timescales. My research in this field involves using spectroscopic techniques (using combinations of different spectral lines detected in high-redshift galaxies) to probe fundamental constant evolution, as well as devising new techniques for this purpose. Recent results include the strongest present constraints on changes in the proton-electron mass ratio, using inversion and rotational lines, and, independently, methanol lines, over lookback times of 7 - 8 billion years, and stringent constraints on changes in a combination of the fine structure constant and the proton-electron mass ratio over the last 3 Gyrs, using hydroxyl lines. I am currently using significantly deeper methanol and ammonia spectra from the Very Large Array and the Green Bank Telescope to significantly improve our sensitivity to changes in the proton-electron mass ratio.
Galaxy evolution:
In galaxy evolution, much of my research has focussed on understanding the nature of and evolution of ``damped Lyman-alpha absorbers'' (DLAs), gas-rich galaxies that are detected due to their strong absorption of the light of background quasars. Over the last decade, we have used radio absorption studies of such galaxies to study the evolution of physical conditions in their interstellar media. We have shown that atomic gas in these galaxies is predominantly warm at high redshifts, apparently due to their low metallicity and a paucity of cooling routes, and that their cold gas content increases with decreasing redshift. We have also come up with a new method to image such galaxies, based on quasar sightlines with two DLAs, and using the higher-redshift absorber as a ''blocking filter'' to blank out the background quasar at certain wavelengths, so that one can then image the lower redshift galaxy. Our imaging studies of a large sample of such galaxies with the Keck Telescope and the Hubble Space Telescope (HST) showed that typical DLAs have very low star formation rates at high redshifts. We have recently used the Atacama Large Millimeter/submillimeter Array (ALMA) to detect carbon monoxide (CO) and ionized carbon emission from the hosts of DLAs over a wide range of redshifts, finding that high-metallicity DLAs appear to arise in massive galaxies. We are now using ALMA, HST and Very Large Array imaging studies of high-redshift DLAs to determine their size, total gas content, dynamical mass, etc.
The Interstellar Medium:
My work on the ISM of the Milky Way has aimed to understand the distribution of gas between different temperature phases, and specifically whether gas exists in the ``unstable'' temperature phase, ~ 1000 K. Our studies have found evidence for significant amounts of gas in this unstable phase, which is not expected in standard three-phase models of the ISM. We have also found evidence that the presence of cold gas depends on the total hydrogen column density, with very low cold gas fractions below a threshold column density that is likely to be set by self-shielding against ultraviolet photons. We are currently attempting to carry out a self-consistent modelling of hydrogen absorption and emission spectra, to obtain a better understanding of physical conditions in the ISM.

Selected publications:

1. Stringent Constraints on Fundamental Constant Evolution Using Conjugate 18 cm Satellite OH Lines (N. Kanekar et al. 2018, Phys. Rev. Lett., 120, 061302)

2. Massive, Absorption-selected Galaxies at Intermediate Redshifts (N. Kanekar et al. 2018, ApJL, 856, L23)

3. The gas and stellar mass of low-redshift damped Lyman-α absorbers (N. Kanekar et al. 2018, MNRAS, 473, L54)

4. [C II] 158-μm emission from the host galaxies of damped Lyman-alpha systems (M. Neeleman, N. Kanekar, et al. 2017, Science, 355, 1285)

5. The Gas Mass of Star-forming Galaxies at z ~ 1.3 (N. Kanekar et al. 2016, ApJL, 818, L28)

6. Constraints on changes in the proton-electron mass ratio using methanol lines (N. Kanekar et al. 2015, MNRAS, 448, L104)

7. On Detecting Millisecond Pulsars at the Galactic Center (J-P. Macquart & N. Kanekar 2015, ApJ, 805, 172)

8. The spin temperature of high-redshift damped Lyman α systems (N. Kanekar et al. 2014, MNRAS, 438, 2131)

9. An HI Column Density Threshold for Cold Gas Formation in the Galaxy. (N. Kanekar et al. 2011, ApJL, 737, L33)

10. Constraining Changes in the Proton-Electron Mass Ratio with Inversion and Rotational Lines. (N. Kanekar 2011, ApJL, 728, L12)




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