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Session 30 - Galaxy Evolution I.
Oral session, Monday, January 15
1st Floor, La Villita Assembly Building

[30.04] Constraints on the Evolution of Faint Field Galaxies

C. Gronwall (UCO/Lick and UCSC)

The nature of faint field galaxy evolution remains controversial. The discovery that the redshift distributions of faint field galaxies match those of standard no-evolution models coupled with the excess of observed galaxy counts above those same models has led to the introduction of many exotic theories such as rapid merging, disappearing populations of dwarf galaxies, and the adoption of a cosmological constant. Another viewpoint is that there are significant uncertainties in our knowledge of the local population of galaxies which hinder our interpretation of distant galaxy samples.

We present the results of our attempts to model the evolution of faint field galaxies. Our models differ from previous work in that we assume that the local luminosity function and galaxy mix are not well-defined. Instead, we use a non-negative least squares fitting technique to derive a set best-fitting local luminosity functions for different galaxy spectral types. By only including traditional luminosity evolution (i.e., the photometric evolution of stars in a galaxy over time given reasonable assumptions of the form of the star-formation history for various spectral types), plus the addition of simple galaxy reddening, we are able to fit the observed optical and near-IR counts, B - R colors, and redshifts of faint field galaxies extremely well. We conclude that uncertainties in our understanding of the local galaxy population may account for much of the discrepancies between previous luminosity evolution models and the observations.

We then use these models to explore other constraints on the evolution of faint field galaxies. There is a wealth of available data which provide additional information of the nature of field galaxy evolution. These include the measurement of the galaxy luminosity function vs redshift from faint redshift surveys, and morphological and structural information from HST images. These data suggest that additional physical processes may be required to explain faint field galaxy evolution such as a mild amount of merging and/or a starbursting component of galaxies.

Program listing for Monday