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(Jayaram N. Chengalur, Nissim Kanekar, Yogesh Wadadekar, Archishman Khasnovis, Rashi Jain, Pralay Biswas, Former members: D. J. Saikia, J. N. H. S. Aditya, O. S. Bait, P. K. Mishra, A. Bera, A. Chowdhury, S. Purkayastha, B. Kaur)

Understanding the nature of high-redshift galaxies, as well as galaxy evolution, is an important research area at NCRA-TIFR. Astronomers here use diverse methods, in both emission and absorption, and over a wide range of observing frequencies, to probe physical conditions in high-redshift galaxies. The techniques used include neutral hydrogen (HI) 21cm absorption studies, HI 21cm emission studies of individual galaxies, "stacking" of HI 21cm emission, millimetre-wave carbon monoxide (CO) emission and absorption studies, ionized carbon (C+) emission studies, hydroxyl (OH) 18cm absorption studies, radio continuum studies, optical and ultraviolet imaging and spectroscopy, etc.. Some of the questions being addressed by NCRA-TIFR astronomers are discussed in more detail below.

Damped Lyman-alpha Absorbers (DLAs)

Damped Lyman-alpha absorbers (DLAs) are absorption-selected galaxies, identified by the presence of strong Lyman-alpha absorption in quasar spectra. Since these galaxies are detected by their absorption signatures, DLA samples contain no bias towards brighter galaxies (unlike emission-selected galaxy samples); understanding the nature of DLAs has hence long been considered an important issue in galaxy evolution.

For DLAs towards radio-loud quasars, HI 21cm absorption studies allow one to measure the spin temperature of the absorbing gas and the cold gas content of the absorber. NCRA-TIFR astronomers have long used low-frequency radio telescopes like the Giant Metrewave Radio Telescope (GMRT) and the Green Bank Telescope (GBT) to carry out deep searches for HI 21cm absorption in DLAs, obtaining the majority of the detections of HI 21cm absorption in high-z DLAs as well as lower limits to the spin temperature in a large number of these galaxies. They have used optical spectroscopy to measure the metallicity of the absorbers, and have connected the high spin temperatures in DLAs to the paucity of metals in these galaxies. They have also extended HI 21cm absorption studies to lower redshifts, targetting strong MgII absorption systems, allowing the identification of DLAs via their HI 21cm absorption and providing the first understanding of the redshift evolution of the cold gas fraction in normal galaxies.

HI 21cm emission studies of galaxies allow one to directly measure their atomic gas mass, a critical input to describing galaxies and understanding their evolution. Follow-up interferometric mapping studies allow one to measure the velocity field of the galaxies, and infer their dynamical mass. Unfortunately, the weakness of the HI 21cm line has meant that such studies have only been possible at very low redshifts. NCRA-TIFR astronomers have led searches for HI 21cm emission in low-redshift DLAs, finding HI 21cm emission in a number of DLAs at z<0.1. They are now using the GMRT to map the HI 21cm emission in some of these galaxies.

CO emission studies of galaxies allow one to estimate their molecular gas mass, and, with follow-up mapping studies, the velocity field. NCRA-TIFR astronomers have been using such CO emission studies of high-z DLAs with the Atacama Large Millimeter/submillimeter Array (ALMA), the Very Large Array (VLA) and the NOrth European Millimetre Array (NOEMA) to measure the molecular gas masses of the DLA host galaxies. Remarkably, the ALMA and VLA studies have yielded extremely high molecular gas masses for a number of high-metallicity DLAs at both intermediate and high redshifts, raising questions on our understanding of the absorber host galaxies.

Identifying the host galaxies of high-redshift DLAs has been an important open problem in galaxy evolution for thirty years. The difficulty lies in the fact that one has to detect a faint foreground object (the absorber host) in the presence of a far brighter object (the background quasar). NCRA-TIFR astronomers have used ALMA studies in the redshifted ionized carbon (C+) 158-micron line and the dust continuum as a new tool to identify and study the absorber host galaxies of DLAs at z~4. Initial ALMA studies have shown that the DLA hosts have relatively high star formation rates and C+ line luminosities, and large sizes.

Stacking HI 21cm emission from high-redshift galaxies

Measuring the atomic gas mass of high-z galaxies requires the detection of their HI 21cm line emission. Unfortunately, the weakness of the HI 21cm line has meant that HI 21cm emission has so far only been detected out to relatively low redshifts, z~0.38. NCRA-TIFR astronomers use "stacking" of HI 21cm emission from galaxies with known optical redshifts to extend such HI 21cm emission searches to much higher redshifts, to measure the average gas mass of galaxy samples, as well as the cosmological gas mass density in such galaxies. This is an exciting research area today at NCRA-TIFR, due to the new wideband receivers, new correlator and high sensitivity of the upgraded GMRT

Gas in AGN environments

HI 21cm absorption studies of radio-loud active galactic nuclei (AGNs) allow one to probe the presence of, and physical conditions in, neutral hydrogen in AGN environments. The detection of HI 21cm absorption from such "associated" gas allows one to measure the velocity of the gas relative to the AGN. Such studies thus also provide information on whether the gas is predominantly falling into (and thus, fuelling) the AGN, or is being driven out into the intergalactic medium by the AGN. The wide-band receivers of the upgraded GMRT will allow such associated HI 21cm absorption studies to track the redshift evolution of gas conditions in the AGN environments over a wide variety of AGN types.

Molecules in high-redshift galaxies

NCRA-TIFR astronomers use both molecular absorption and emission studies of high-redshift galaxies to obtain information on physical conditions in these objects. Such studies include searches for OH 18cm megamasers in ultra-luminous infra-red galaxies, blind surveys for CO and HCO+ absorption at high redshifts, searches for OH absorption in red quasars, searches for CO emission in Lyman-alpha emitters and Lyman-break galaxies, searches for molecular oxygen absorption in high-z galaxies, etc.