Deep Mid-Infrared Observations of Lyman Break Galaxies

As part of the in-orbit checkout activities for the Spitzer Space Telescope, the Infrared Array Camera (IRAC) team carried out a deep observation (average integration time ∼8 hr) of a field surrounding the bright QSO HS 1700+6416. This field contains several hundred z ∼ 3 Lyman break galaxy (LBG) candidates, and we report here on their mid-infrared properties, including the IRAC detection rate, flux densities, and colors, and the results of fitting population synthesis models to the optical, near-infrared, and IRAC magnitudes. The results of the model fitting show that previous optical/near-infrared studies of LBGs were not missing large, hidden old stellar populations. The LBG candidates' properties are consistent with those of massive star-forming galaxies at z ∼ 3. Other IRAC sources in the same field have similar properties, so IRAC selection may prove to be a promising method of finding additional high-redshift galaxies.