The galaxy luminosity - size relation and selection biases in the Hubble Ultra Deep Field

We use the Hubble Ultra Deep Field to study the galaxy luminosity-size (M-R e) distribution. With a careful analysis of selection effects due to both detection completeness and measurement reliability we identify bias-free regions in the M-R e plane for a series of volume-limited samples. By comparison to a nearby survey also having well defined selection limits, namely the Millennium Galaxy Catalogue, we present clear evidence for evolution in surface brightness since z ∼ 0.7. Specifically, we demonstrate that the mean, rest-frame B-band µ e for galaxies in a sample spanning 8 magnitudes in luminosity between M B = −22 and −14 mag increases by ∼1.0 mag arcsec −2 from z ∼ 0.1 to z ∼ 0.7. We also highlight the importance of considering surface brightness dependent measurement biases in addition to incompleteness biases. In particular, the increasing, systematic underestimation of Kron fluxes towards low surface bright-nesses may cause diffuse, yet luminous, systems to be mistaken for faint, compact objects.