Faint 6.7 Micron Galaxies and Their Contributions to the Stellar Mass Density in the Universe

Abstract

We discuss the nature of faint 6.7 μm galaxies detected with the mid-infrared camera ISOCAM on board the Infrared Space Observatory (ISO). The 23 hr integration on the Hawaii Deep Field SSA13 has provided a sample of 65 sources down to 6 μJy at 6.7 μm. For 57 sources, optical or near-infrared counterparts were found using a statistical method. All four Chandra sources, three SCUBA sources, and one VLA/FIRST source in this field were detected at 6.7 μm with high significance. Using their optical to mid-infrared colors, we divided the 6.7 μm 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 spectral energy distributions (SEDs). They show a high-redshift tail in their distribution with faint (<30 μJy) galaxies at z>1. The 6.7 μm galaxies tend to have brighter K magnitudes and redder I-K colors than the blue dwarf population at intermediate redshifts. Stellar masses of the 6.7 μm galaxies were estimated from their rest-frame, near-infrared luminosities. Massive galaxies (M_star~10¹¹ M_solar) 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 6.7 μm 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.

Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands, and the United Kingdom) and with the participation of ISAS and NASA.