ODIN: A New Ly α Blob Selection Method, Sample, and Statistical Analysis at z ∼ 3.1

Abstract Ly α blobs (LABs) are large, spatially extended Ly α -emitting objects whose nature remains unclear. Their statistical properties, such as number densities and luminosity functions, are still uncertain because of small sample sizes and large cosmic variance. The One-hundred-deg 2 DECam Imaging in Narrowbands (ODIN) survey, with its large volume, offers an opportunity to overcome these limitations. We describe our LAB selection method and present 112 new LABs in the 9 deg 2 E-COSMOS field. We begin with the conventional LAB selection approach, crossmatching Ly α -emitting galaxies with extended Ly α sources, yielding 89 LAB candidates. To obtain a more complete LAB sample, we introduce a new selection pipeline that models all galaxies detected in deep broadband imaging, subtracts them from the narrowband image, and then directly detects extended Ly α emission. This method successfully identifies 23 additional low-SB LABs, which could otherwise be missed by the conventional method. The number density of ODIN LABs near an ODIN protocluster ( n = 7.5 × 10 −5 cMpc −3 ) is comparable to that found in the SSA22 protocluster and is 4 times higher than the average across the field. The cumulative Ly α luminosity function within the protocluster regions is similar to that measured for the LABs in the SSA22 protocluster, suggesting a large excess of luminous LABs relative to the average field. These findings suggest the Ly α luminosities and number densities of LABs are environment-dependent. ODIN will provide an expansive LAB and protocluster samples across six additional fields and two more redshifts, allowing us to investigate the nature of LABs in relation to their environments.