The Hubble Deep Field and the disappearing dwarf galaxies

Several independent lines of reasoning, both theoretical and observational, suggest that the very faint (B≳24) galaxies seen in deep images of the sky are small low-mass galaxies that have experienced a short epoch of star formation at redshifts 0.5≲z≲1 and have since faded into low-luminosity, low-surface-brightness objects. Such a scenario, which arises naturally if star formation in dwarf galaxies is delayed by photoionizaton resulting from the metagalactic UV radiation field, provides an attractive way to reconcile the Einstein–de Sitter (Ω=1; Λ=0) cosmological model with the steeply rising galaxy counts observed at blue wavelengths. Babul & Ferguson constructed a specific realization of this model, deriving the dwarf galaxy mass function from the cold dark matter (CDM) power spectrum, and arguing that the gas in dwarf galaxy haloes will recombine at z∼1. The Hubble Deep Field (HDF) images provide a stringent test of this model. We compare the model with the data by constructing simulated images that reproduce the spatial resolution and noise properties of the real data, and by carrying out source detection and photometry for the simulations in the same way they were carried out for the real data. The selection biases and systematic errors that are inevitable in dealing with faint galaxies are thus incorporated directly into the model. We compare the model predictions for the counts, sizes, and colours of galaxies observed in the HDF to observations and to the predictions from a low-q0 pure-luminosity-evolution (PLE) model. Both models fail to reproduce the observations. The low-q0 model predicts far more Lyman-break ‘dropouts’ than are seen in the data. The fading dwarf model predicts too many remnants: faded dwarf galaxies in the redshift range 0.2<z<0.5 that should be detectable in the HDF as low-surface-brightness red objects but are not seen. If fading dwarf galaxies are to reconcile the Einstein–de Sitter geometry with the counts, then the dwarf population must (i) form earlier than z∼1, with a higher initial luminosity; (ii) have an initial-mass function more heavily weighted toward massive stars than the Salpeter IMF; or (iii) expand much more than assumed during the supernova wind phase.