FAINT 6.7μm GALAXIES AND THEIR CONTRIBUTIONS TO THE STELLAR MASS DENSITY IN THE UNIVERSE

We discuss the nature of faint 6.7μm galaxies detected with the mid-infrared camera ISOCAM on board the Infrared Space Observatory (ISO). The 23hour integration on the Hawaii Deep Field SSA13 has provided a sample of 65 sources down to 6μJy at 6.7μm. For 57 sources, optical or near-infrared counterparts were found with a statistical method. All four Chandra sources, three SCUBA sources, and one VLA/FIRST source in this field were detected at 6.7μm with high significance. Using their optical to mid-infrared colors, we divided the 6.7μm sample into three categories: low redshift galaxies with past histories of rapid star formation, high redshift ancestors of these, and other star forming galaxies. Rapidly star forming systems at high redshifts dominate the faintest end. Spectroscopically calibrated photometric redshifts were derived from fits to a limited set of template SEDs. They show a high redshift tail in their distribution with faint (< 30μJy) galaxies at z > 1. The 6.7μm galaxies tend to have brighter K magnitudes and redder I − K colors than the blue dwarf population at intermediate redshifts. Stellar masses of the 6.7μm galaxies were estimated from their rest-frame near-infrared luminosities. Massive galaxies (Mstar ∼ 10 M⊙) were found in the redshift range of z = 0.2–3. Epoch dependent stellar mass functions indicate a decline of massive galaxies’ comoving space densities with redshift. Even with such a decrease, the contributions of the 6.7μm galaxies to the stellar mass density in the universe are found to be comparable to those expected from UV bright galaxies detected in deep optical surveys. Subject headings: cosmology: observations — galaxies: evolution — galaxies: luminosity function, mass function — galaxies: stellar content — infrared: galaxies — surveys