ODIN: A New Lyα Blob Selection Method, Sample, and Statistical Analysis at z ∼ 3.

Lyα blobs (LABs) are large, spatially extended Lyα-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-deg² 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 deg² E-COSMOS field. We begin with the conventional LAB selection approach, crossmatching Lyα-emitting galaxies with extended Lyα 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 Lyα 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 Lyα 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 Lyα 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.