What do we learn from IRAC observations of galaxies at 2<z<3.5?

We analyze very deep HST, VLT, andSpitzer photometry of galaxies at 2 < z < 3: 5 in the HubbleDeep Field-South. The sample is selected from the deepest public K- band imaging currently available. We show that the rest- frame U - V versus V - J color-color diagram is a powerful diagnostic of the stellar populations of distant galaxies. Galaxies with red rest- frame U - V colors are generally red in rest- frame V - J as well. However, at a given U - V color a range in V - J colors exists, and we show that this allows us to distinguish young, dusty galaxies from old, passively evolving galaxies. We quantify the effects of IRAC photometry on estimates of masses, ages, and the dust content of z > 2 galaxies. The estimated distributions of these properties do not change significantly when IRAC data are added to the UBVIJHK photometry. However, for individual galaxies the addition of IRAC can improve the constraints on the stellar populations, especially for red galaxies: uncertainties in stellar mass decrease by a factor of 2.7 for red [ (U - V)(rest) > 1] galaxies, but only by a factor of 1.3 for blue [(U-V)(rest) < 1] galaxies. We find a similar color dependence of the improvement for estimates of age and dust extinction. In addition, the improvement from adding IRAC depends on the availability of full NIR JHK coverage; if only K band were available, the mass uncertainties of blue galaxies would decrease by a more substantial factor of 1.9. Finally, we find that a trend of galaxy color with stellar mass is already present at z > 2. The most massive galaxies at high redshift have red rest- frame U - V colors compared to lower mass galaxies, even when allowing for complex star formation histories.