MS. RAJESHWARI DUTTA

Understanding the physical conditions and volume filling factor of the cold neutral medium (CNM) phase is essential to draw a holistic picture of the interstellar medium and star formation history of galaxies. Our Cold HI 21-cm Absorption Line Survey (CHITALS) is aimed at understanding the cold gas phase of galaxies through HI 21-cm absorption. This will allow us to probe the thermal state of the HI gas as well as the parsec-scale structure in the gas and relate them to the in-situ star formation. Further, the results from our survey would be extremely relevant for interpreting on-going and future blind 21-cm absorption searches, and thus determine the CNM fraction in the galactic disks and halos at all redshifts. Using 250 hrs of GMRT and VLA time, we are conducting a systematic HI 21-cm absorption survey of radio-loud quasars whose sightlines happen to pass through the gas disks/halos of nearby galaxies (referred as quasar-galaxy pairs or QGPs). Our sample of 46 QGPs (z<0.4) spans a wide range of impact parameters (<40 kpc), galaxy luminosity, morphology and environment. Optical spectra of these QGPs from SDSS and SALT are being utilised to measure the redshifts for conducting radio spectral line observations, and to derive the galaxy properties for correlating with the 21-cm absorption optical depth. From our ongoing survey, 21-cm absorption has been detected from eight QGPs till now. In this talk, the speaker will describe the survey and present its preliminary results.