Nissim Kanekar

My main research interests are in the areas of galaxy formation and evolution, tests of fundamental physics, and the interstellar medium of galaxies. Individual areas are described in more detail below:


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: Absorption-selected galaxies.

In galaxy evolution, my recent research has focussed on two broad areas: (1) 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, and (2) tracing the gas content of star-forming galaxies at high redshifts. Over the last decade, we have used radio absorption studies of DLAs in the hyperfine 21cm line of neutral atomic hydrogen (HI) to study the evolution of physical conditions in their interstellar media. We have shown that atomic gas in the DLA 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 identify the galaxies associated with a number of DLAs over a wide range of redshifts, out to z~4.5, via their carbon monoxide (CO) and ionized carbon (CII-158 micron) emission. We have shown that the higher-metallicity DLAs appear to arise in massive galaxies, and that DLA galaxies follow a mass-metallicity relation similar to emission-selected galaxies at similar redshifts. We have used ALMA to map the CII-158 micron emission in two DLA galaxies at z~4, finding the first clear evidence for the presence of a massive rotating disk galaxy at these redshifts. 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.

Galaxy evolution: Star-forming galaxies.

In parallel, we have used HI 21cm emission studies of samples of star-forming galaxies out to z~1.5 to try to measure the average atomic gas properties of the population. Unfortunately, the weakness of the hyperfine HI 21cm line, the main tracer of the neutral atomic hydrogen content of galaxies, means that it is not possible to detect this line from high-redshift galaxies with today's telescopes. We have hence used the upgraded Giant Metrewave Radio Telescope (GMRT) to target regions with a large number of galaxies with measured spectroscopic redshifts lying within the GMRT primary beam, and have ``stacked'' the HI 21cm emission signals of the sample of galaxies to measure their average gas properties. This approach has yielded the first estimate of the average HI mass of galaxies at z~1, during the epoch of galaxy assembly, measurements of the cosmological gas mass density of neutral atomic hydrogen at z~0.34 and z~1, and estimates of the HI-to-stars mass ratio and the HI depletion time in galaxies at z~0.34 and z~1.0. We find that the average HI-to-stars mass ratio increases with increasing redshift, while the HI depletion time is significantly lower at z~1 than in the local Universe. This finding can explain the observed decline in the cosmic star formation rate density at z<1, if gas accretion on to galaxies does not proceed at a rate sufficient to replenish the atomic hydrogen. We are now carrying out a detailed characterization of the dependence of the HI properties of galaxies at z~1 on their other properties, and also pushing such HI 21cm studies to even higher redshifts, z~3.

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.


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