The evolution of galaxies in the Hubble Deep Fields




Gwyn, Stephen Donald Jermy

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This thesis is a study of several aspects of the evolution of galaxies using photometric redshifts in the Hubble Deep Fields (HDF's). The photometric redshift method is used in the HDF's down to a magnitude limit of I = 28. The large sample and the unprecedented depth of the Hubble Deep Fields allow one to trace the evolution of several properties of galaxies from z = 5 to the present in a statistical manner. This thesis studies four such aspects: (1) The clustering of galaxies is examined. When the redshift distributions of the HDF-North and the HDF-South are compared, one finds a significantly greater number of galaxies around z = 0.5. This suggests the presence of a structure (a very weak cluster or a very strong group) in the HDF-North. (2) The star formation rate density (SFRD) is determined by measuring the UV-luminosity density. After correcting for dust extinction, the star formation rate is found to decrease exponentially with time with an e-folding period of about 4 Gyr. (3) The difference between the rate of declines of the B band galaxy number density and the luminosity densities are used to examine the merging history of the Universe. While the total B band luminosity density of the Universe decreases only slightly with time since z = 5, the number density of galaxies drops considerably more. On average, a present day galaxy is the product of ∼3 progenitors. (4) The morphology of galaxies is quantified using a “lumpiness” parameter, L, which measures the number of local maxima in the image of a galaxy. Rest-frame B band images are made of both HDF's by k-correcting each pixel of each galaxy in the frames using the photometric redshifts of the parent galaxies. It is found that L increases with increasing absolute brightness and increasing redshift, albeit only slightly.



Photometric redshifts, Galaxies, Pure sciences