Cosmic hydrogen - fuel for star formation and tracer of baryon flow
Date
2015
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Rhee, Jonghwan
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A new window to explore the intermediate redshift regime 0.1< z <0.4 in the 21-cm emission line of atomic hydrogen has been opened by a new observational approach called HI spectral stacking. Through this window, a way to measure HI gas content of galaxies beyond the local universe is found, and the cosmic density of neutral hydrogen in galaxies across the redshift range 0.1< z <0.4 has been quantified. To investigate the evolution of hydrogen gas - an important ingredient for star formation - over cosmic time, three different galaxy fields at cosmic epoch between z=0.1 and 0.4 have been observed with two radio telescopes: the Giant Metrewave Radio Telescope (GMRT) and the Westerbork Synthesis Radio Telescope (WSRT). The HI spectral stacking technique, using measured optical redshifts and celestial coordinates, has been applied to measure very weak radio emission in the HI 21-cm line, leading to determination of the cosmic HI mass density and the ratio of HI mass to optical luminosity over the last 4 Gyr of cosmic time. In this technique, HI emission spectra from many galaxies with measured optical redshifts in the target field are co-added to measure the average HI mass of the catalogued galaxies in the field. This thesis reports observations made with radio telescopes equipped to study galaxies in 4 redshift bins: z{u0303}0.1, 0.2, 0.32 and 0.37. From the HI mass measurement, the cosmic HI gas densities at those redshifts are derived and compared to other measurements at different redshifts compiled from the literature to assess the global trend of neutral hydrogen gas evolution in cosmic history. From measurements from z=0 to 0.37, it is found that there is no significant variation of HI density during the last 4 Gyr, and all measurements of HI density using 21-cm emission at z<0.4 are consistent with one another. The observationally challenging redshift ranges of z>0.3 are explored with the GMRT, one of the most sensitive radio telescopes capable of observation at redshifted HI frequencies up to z{u0303}0.4. The VVDS 14h field at z{u0303}0.32 and zCOSMOS field at z{u0303}0.37 were observed using the GMRT with long integration time of 136 h and 134 h, respectively. 165 galaxies in VVDS 14h field are used for HI stacking to obtain high signal-to-noise ratio of HI signal. For zCOSMOS field, HI spectra of 474 field galaxies are co-added. This thesis successfully demonstrates the viability of the HI spectral stacking technique at 0.1< z <0.4 with current radio telescopes and proves that this is a promising technique to push the sensitivity limit of future radio facilities still further.
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