Oral Presentation

Tracing Transition Galaxies in a z = 1.5 Cluster: Star Formation and Morphology in the Euclid Era

Author(s): Ko Ishida (Tohoku University) Tadayuki Kodama(Tohoku University) Yusei Koyama(NAOJ) Ichi Tanaka (NAOJ)

Presenter: Ko Ishida (Tohoku University)

We present early results on the environmental effects in a z = 1.5 galaxy cluster, combining Euclid near-infrared photometry, ground-based narrowband Hα imaging, and spectroscopic data. Color–magnitude diagrams (CMDs) using Euclid Y–H and J–H colors reveal a distinct separation between star-forming galaxies (HAEs) and red-sequence candidates. Among them, we identify a subset of “red HAEs”—massive galaxies that exhibit both Hα emission and red Euclid NIR colors. These galaxies typically show disk-like morphologies, and one object in particular shows a high Sérsic index and lies near the cluster core, possibly indicating a transition toward quenching. These findings suggest a diversity of star-forming main-sequence populations at high redshift. Further analysis will constrain their spatial and structural properties, offering insight into early environmental quenching. As Euclid identifies more distant clusters, combining rest-frame color, morphology, and Hα emission will enable the systematic selection of such transition galaxies and allow statistical studies of their evolution in dense environments.