ODIN: Using multiplicity of Lyman-alpha emitters to assess star formation activity in dark matter halos

Aims. We investigate if systems of multiple Lyman-alpha emitters (LAEs) can serve as a proxy for dark matter halo mass, assess how their radiative properties relate to the underlying halo conditions, and explore the physics of star formation activity in LAEs and its relation to possible physically related companions. Methods. We used data from the One-hundred-deg2 DECam Imaging in Narrowbands (ODIN) survey, which targets LAEs in three narrow redshift slices. We identified physically associated LAE multiples in the COSMOS field at z = 2.4, z = 3.1, and z = 4.5, and we used a mock catalog from the IllustrisTNG100 simulation to assess the completeness and contamination affecting the resulting sample of LAE multiples. We then studied their statistical and radiative properties as a function of multiplicity, for which we adopted the term “multiplicity” to refer to the number of physically associated LAEs. Results. We find a strong correlation between LAE multiplicity and host halo mass in the mocks, with higher multiplicity systems preferentially occupying more massive halos. In both the ODIN and the mock sample, we find indications that the mean Lyα luminosity and UV magnitude of LAEs in multiples increase with multiplicity. The halo-wide LAE surface brightness densities in Lyα and UV increase with multiplicity, reflecting more compact and actively star-forming environments. The close agreement between the model and ODIN-COSMOS observations supports the validity of the Lyα emission model in capturing key physical processes in LAE environments. Finally, a subhalo-based perturbation-induced star formation model reproduces the minimum subhalo mass distribution in simulations at z = 2.4, suggesting that local perturbations–rather than the presence of LAE companions–drive star formation activity in these systems. For the higher redshift samples, neighbor perturbations do not seem to be the main driver that triggers star formation.