2003ApJ...587...90X

Phenomenological models for evolution of dusty and E/S0 galaxies, respectively, are developed to address two major questions concerning galaxy populations in deep infrared (IR) surveys: (1) Do normal late-type galaxies or starburst galaxies (including galaxies with obscured active galactic nuclei) dominate among sources in deep IR surveys? (2) How much do E/S0 galaxies contribute to the counts in deep mid-infrared (MIR: 3-20 µm) surveys? Among three new models for evolution of dusty galaxies, it is assumed in model S1 that starburst galaxies are the dominant population and in model S2 that normal galaxies dominate. Model S3 is an intermediate model. Comparing the model predictions with a wide range of observational data collected from the literature, we find that none of these models can be ruled out, given the uncertainties of the data. We show that the most direct method to distinguish these models is to compare the predicted color distributions of IR galaxies with observations, which will soon be available from the SIRTF Wide-Area Infrared Extragalactic (SWIRE) survey. The models for E/S0 galaxies follow a simple passive evolution approach. Among the three E/S0 models (E1, E2, and E3) investigated in this paper, model E2, which is specified by a peak formation redshift z_peak=2 and an e-folding formation timescale ω=2 Gyr, fits the data best. This suggests a synchronization between the evolution of E/S0 galaxies and that of starburst galaxies, in the sense that the peak of the formation function of E/S0 galaxies (z_peak=2) is close to the peak of the evolution functions of starburst galaxies (z_peak=1.4). We find that E/S0 galaxies contribute about 10%-30% of the counts in the MIR bands of less than 10 µm and up to 30%-50% of the optical/near-IR counts in the bright end. Their contributions to counts in the UV (2000 Å) and in the longer wavelength IR (≥12 µm) bands are negligible. Taking into account this contribution, new predictions for counts and confusion limits in the SIRTF bands are presented.