Cloning Hubble Deep Fields. II. Models for Evolution by Bright Galaxy Image Transformation

Abstract

In a companion paper, we outlined a methodology for generating parameter-free, model-independent fields of faint galaxy images, demonstrating the need for significant evolution in the Hubble Deep Field (HDF) at faint magnitudes. Here we incorporate evolution into our procedure, by transforming the input bright galaxy images with redshift, for comparison with the HDF at faint magnitudes. Pure luminosity evolution is explored with the assumption that galaxy surface brightness evolves uniformly, at a rate chosen to reproduce the I-band counts. This form of evolution exacerbates the size discrepancy identified by our no-evolution simulations by increasing the area of a galaxy visible to a fixed isophote. Reasonable dwarf-augmented models are unable to generate the count excess invoking moderate rates of stellar evolution. A plausible fit to the counts and sizes is provided by mass-conserving density-evolution, consistent with small-scale hierarchical growth, in which the product of disk area and space density is conserved with redshift. Here the increased surface brightness generated by stellar evolution is accommodated by the reduced average galaxy size, for a wide range of geometries. These models are useful for calculating the rates of incompleteness and the degree of overcounting. Finally we demonstrate the potential for improvement in quantifying evolution at fainter magnitudes using the Hubble Space Telescope Advanced Camera, with its superior UV and optical performance.