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Morphology and Models of Faint Galaxy Counts
D.C. Koo
University of California Observatories/Lick Observatory University of California, Santa Cruz
Most models of faint galaxy counts continue to rely on transforming observed distributions of
morphology of local galaxies to estimate the distribution of K corrections. Such procedures
have led many groups to conclude that the observations and the predictions from standard no­
evolution or mild luminosity evolution models are so discrepant that more exotic models are
needed. Some examples include rapid mergers, new populations of galaxies at high redshift
that disappear by today, dramatic enhancements of star­formation among low luminosity
dwarfs, and even a high cosomological constant.
We stress that the transformation from morphology to an estimate of the intrinsic color is
subject to a number of uncertainties. Some obvious ones include the observed wide spread
in colors of any given morphology type; the possibility that the luminosity function versus
morphology found in the Virgo cluster may not apply to the general field; or the danger
that important classes of objects (very compact blue galaxies, interactive systems, peculiar
galaxies, low­surface brightness galaxies, etc.) have not been properly accounted for.
We have developed a new technique to derive the luminosity functions of galaxies with
different K corrections that bypasses any of the local determinations of either morphology
or luminosity function. The method uses a non­negative least­squares fit to the observed
counts, colors, and redshift distributions from B ё 15 to 24 and fainter. Even without
any evolution, the model fits to the B; r; K counts are far superior to prior models. For
example, at B ё 24, our no­evolution models predict counts that are only 50% shy of the
observed (Koo, Gronwall, & Bruzual 1993, ApJL, 415, L21). This is an order of magnitude
improvement over that of previous models which had predicted counts low by factors of 5
to 15. New evolutionary models that included standard luminosity evolution were presented
(Gronwall & Koo 1994, ApJL, submitted). These new models further improved the fits. We
thus conclude that the need for extensive amounts (factors of 2 or more) of exotic components
to galaxy evolution models are premature.
Presented at Quantifying Galaxy Morphology at High Redshift, a workshop held at
the Space Telescope Science Institute, Baltimore MD, April 27--29 1994
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