SITELLE Hα Imaging Spectroscopy of z ∼ 0.25 Clusters: Emission-line Galaxy Detection and Ionized Gas Offset in Abell 2390 and Abell 2465

Environmental effects are crucial to the understanding of the evolution of galaxies in dense environments, such as galaxy clusters. Using the large field of view of SITELLE, the unique imaging Fourier transform spectrograph at the Canada-France-Hawaii Telescope, we are able to obtain 2D spectral information for a large and complete sample of cluster galaxies out to the infall region. We describe a pipeline developed to identify emission-line galaxies (ELGs) from the datacube using cross-correlation techniques. We present results based on the spatial offsets between the emission-line regions and stellar continua in ELGs from two z ∼ 0.25 galaxy clusters, Abell 2390 and A2465. We find a preference for the offsets to be pointed away from the cluster center. Combining the two clusters, there is a 3σ excess for high-velocity galaxies within the virial radius to have the offsets pointed away from the cluster center. Assuming the offset is a proxy for the velocity vector of a galaxy, as expected from ram pressure stripping, this excess indicates that ram pressure stripping occurs most effectively during the first passage of an infalling galaxy, leading to the quenching of its star formation. We also find that, outside the virial region, the continuum-normalized Hα line flux for infalling galaxies with large offsets is on average lower than those with small or no measurable offset, further supporting ram pressure as a dominant quenching mechanism during the initial infall stages.