The inferred redshift distribution of the faint blue excess

Abstract

We infer the redshift distribution of the faint blue galaxy excess (FBE) at B=23.5 by subtracting the predicted distribution of giant/normal galaxies from the observed N(z) distribution for all types. This is possible because of the recent deep {\it Hubble Space Telescope} (HST) WFPC2 morphological number counts which have convincingly demonstrated that little evolution of the giant population is seen to B=26.0. The mean redshift of the FBE at B=23.5 is found to be <z>_{FBE}=0.40 +/- 0.07 with upper and lower quartiles defined by z_{0.75}=0.58 +/- 0.05 and z_{0.25}=0.28 +/- 0.05, respectively. We compare this inferred FBE N(z) distribution to the predictions from three generic faint galaxy models: dwarf dominated (no evolution), pure luminosity evolution, and evolving dwarfs. The inferred FBE N(z) distribution strongly supports a hybrid evolving dwarf--rich model wherein a large population of dwarfs present at z=0.5 has subsequently faded to obscurity. The total integrated number density of dwarfs (down to M_{B}=-11) is estimated to be a factor of 20 times greater than that of E---Sc galaxies and the estimated fading to be 1.0 < Δm < 1.4 mags. Thus, the dwarf population is estimated to be responsible for ~30% of the luminosity density locally, rising to ~57% at z=0.5.

To appear in The Astrophysical Journal Letters (July 20th). 12 pages including 1 figure (uuencoded postscript)