First Data Release of the Merian Survey: A Wide-field Imaging Survey of Dwarf Galaxies at z ∼ 0.06-0.10

We present an overview of the Merian Survey and its forthcoming first data release (DR1), an optical imaging survey optimized for studying bright-star-forming dwarf galaxies. Merian uses two medium-band filters (N708 and N540, centered at 708 and 540 nm), custom-built for the Dark Energy Camera (DECam) on the Blanco telescope. Merian covers ∼750 deg2 of equatorial fields, overlapping with the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) wide, deep, and ultra-deep fields. Combined with the HSC-SSP imaging data (grizy), Merian DECam medium-band imaging allows for photometric redshift measurements via detection of Hα and [O III] line emission flux excess in the N708 and N540 filters, respectively, at 0.06 < z < 0.10. We present the survey design, observations taken to date, data reduction using the Legacy Survey of Space and Time Science Pipelines, aperture-matched photometry for accurate galaxy colors, and the contents of DR1. The key science goals of Merian include probing the dark matter halos of dwarf galaxies out to their virial radii using high signalto- noise weak lensing profile measurements, decoupling the effects of baryonic processes from dark matter, and understanding the role of black holes in dwarf galaxy evolution. This rich data set will also offer unique opportunities for studying extremely metal-poor galaxies via their strong [O III] emission and Hα lines, as well as [O II] emitters at z ∼ 0.4, and Lyα emitters at z ∼ 3.3 and z ∼ 4.8. Merian showcases the power of utilizing narrow and medium-band filters alongside broad-band filters for sky imaging, demonstrating their synergistic capacity to unveil astrophysical insights across diverse astrophysical phenomena.