The structures of distant galaxies - I. Galaxy structures and the merger rate to z ~ 3 in the Hubble Ultra-Deep Field

Abstract

This paper begins a series in which we examine the structures of distant galaxies to directly determine the history of their formation modes. We start this series by examining the structures of z_F850LP < 27 galaxies in the Hubble Ultra-Deep Field (UDF), the deepest high-resolution optical image taken to date. We investigate a few basic features of galaxy structure using this image. These include: (1) the agreement of visual eye-ball classifications and non-parametric quantitative (CAS, Gini/M₂₀) methods; (2) how distant galaxy quantitative structures can vary as a function of rest-frame wavelength; and (3) the evolution of distant galaxy structures up to z ~ 3. One of our major conclusions is that the majority of galaxies with z₈₅₀ < 27 are peculiar in appearance, and that galaxy assembly is rapidly occurring at these magnitudes, even up to the present time. We find a general agreement between galaxy classification by eye and through quantitative methods, as well as a general agreement between the CAS and the Gini/M₂₀ parameters. We find that the Gini/M₂₀ method appears to find a larger number of galaxy mergers than the CAS system, but contains a larger contamination from non-mergers. We furthermore calculate the merger rate of galaxies in the UDF up to z ~ 3, finding an increase with redshift as well as stellar mass, confirming previous work in the Hubble Deep Field. We find that massive galaxies with M_* > 10¹⁰M_solar undergo 4.3^-0.8_+0.8 major galaxy mergers at z < 3, with all of this merging occurring at z > 1.