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 ``no-evolution'' fields of faint galaxy images, demonstrating the need for significant evolution in the 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 assuming 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, where the product of disk area and space density is conserved with redshift. Here the increased surface brightness generated by stellar evolution is accomodated by the reduced average galaxy size, for a wide range of geometries. These models are useful for assessing the limitations of the HDF images, by calculating their rates of incompleteness and the degree of over-counting. Finally we demonstrate the potential for improvement in quantifying evolution at fainter magnitudes using the HST Advanced Camera, with its superior UV and optical performance.