  We present the unWISE Catalog, containing the positions and fluxes of roughly
two billion objects observed by the Wide-field Infrared Survey Explorer (WISE)
over the full sky. The unWISE Catalog has two advantages over the existing WISE
catalog (AllWISE): first, it is based on significantly deeper imaging, and
second, it features improved modeling of crowded regions. The deeper imaging
used in the unWISE Catalog comes from the coaddition of all publicly available
3$-$5 micron WISE imaging, including that from the ongoing NEOWISE-Reactivation
mission, thereby increasing the total exposure time by a factor of 5 relative
to AllWISE. At these depths, even at high Galactic latitudes many sources are
blended with their neighbors; accordingly, the unWISE analysis simultaneously
fits thousands of sources to obtain accurate photometry. Our new catalog
detects sources at 5-sigma roughly 0.7 magnitudes fainter than the AllWISE
catalog and more accurately models millions of faint sources in the Galactic
plane, enabling a wealth of Galactic and extragalactic science. In particular,
relative to AllWISE, unWISE doubles the number of galaxies detected between
redshifts 0 and 1 and triples the number between redshifts 1 and 2, cataloging
more than half a billion galaxies over the whole sky.
