2MASS Extended Sources in the Zone of Avoidance

A new high-resolution near-infrared mapping effort, the Two Micron All Sky Survey (2MASS), is now underway and will provide a complete census of galaxies as faint as 13.5 mag (3 mJy) at 2.2 μm for most of the sky, and ∼12.1 mag (10 mJy) for regions veiled by the Milky Way. This census has already discovered nearby galaxies previously hidden behind our Galaxy and will allow delineation of large-scale structures in the distribution of galaxies across the whole sky. Here we report the detection and discovery of new extended sources from this survey for fields incorporating the Galactic plane at longitudes between 40° and 70°. Follow-up H I 21 cm and optical spectroscopy observations provide positive confirmation for 14 of the new 2MASS galaxies. We perform an internal completeness and reliability analysis for the sample, consisting of 7000 sources in ∼1000 deg2 of area, including galaxies and Galactic nebulae from the W51 giant molecular cloud. The area-normalized detection rate is about one to two galaxies per deg2 brighter than 12.1 mag (10 mJy), roughly constant with Galactic latitude throughout the "Zone of Avoidance," of which 85%–95% are newly discovered sources. In conjunction with the deep H I surveys, 2MASS will greatly increase the current census of galaxies hidden behind the Milky Way. Moreover, owing to its sensitivity to elliptical and other gas-poor galaxies, 2MASS will provide a key complementary data set to that of the gas-rich–sensitive H I surveys of the Milky Way galaxy, potentially uncovering nearby galaxies critical to the local gravity and mass density fields.