Jayaram N. Chengalur

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.


Dwarf galaxies:

The dwarf irregular galaxies that our group studies 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 21cm emission from a large sample (~ 75 objects) of nearby, extremely faint, dwarf irregular galaxies, viz. the Faint Irregular Galaxies GMRT Survey (FIGGS).


Neutral hydrogen content of the 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.


HI-21cm absorption studies:

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.


Neutral hydrogen on large scales:

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 the Raman Research Institute, the Indian Institute for Science Education and Research, Mohali, and the Indian Institute of Technology, Kharagpur.


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