2022ApJS..260...54W

The Subaru Hyper Suprime-Cam (HSC) Strategic Survey is the latest-generation multiband optical imaging survey for galaxy evolution and structure formation. The "Ultra-deep" component of the HSC survey provides grizy broadband images over ∼3.4 deg² to detection limits of ∼26-28 AB, along with narrowband images, in the COSMOS and SXDS fields. These images provide an unprecedented combination of depths and area coverage, for studies of galaxies up to z ∼ 7. However, the lack of coverage at <4000 Å implies an incomplete sampling of the rest-frame UV at z <= 3, which is critically needed for understanding the buildup of stellar mass in later cosmic time. We conducted a multiyear CFHT u*-band imaging campaign in the two HSC Ultra-deep fields with CFHT MegaCam. By including shallower archival data, we reached 5σ depths of u* = 28.1 and 28.4 (AB) at the centers of the COSMOS and SXDS fields, respectively, and u* = 27.7 and 27.8 in the central 1 deg² fields. The image quality is >= 0.''90, fairly good for the u* band. Both the photometric and astrometric quality of our data are excellent. We show that the combination of our u*-band and HSC data can lead to high-quality photometric redshifts at z = 0-3, and robust measurements of rest-frame UV on galaxies at 0.4 < z < 0.6 for distinguishing green-valley galaxies from star-forming and quiescent galaxies. We publicly release our reduced u*-band images and reference catalogs, which can be used readily for scientific studies.