Abstract
Abstract The growth of galaxy clusters is energetic and may trigger and/or quench star formation and black hole activity. The ENISALA 4 4 The project is named as a tribute to the storied Enisala citadel (Dobrogea, Romania). Enisala (“new settlement,” in Turkish and Romanian) sits on top of a windswept hill, at the crossroads of the Danube Delta and the Pontus Euxinus sea (“hospitable sea,” Black Sea), forever shaped by forces of nature. It stands as a metaphor for the ever-evolving galaxy cluster environment and its profound influence on galaxy and black hole evolution. ENISALA can also be understood to stand for “ENvironmental Influence on Star formation and AGN through Line Astrophysics. project is a collection of multiwavelength observations aimed at understanding how large-scale structure drives galaxy and black hole evolution. Here, we introduce optical spectroscopy of over 800 Hα emission-line galaxies, selected in 14 z ∼ 0.15–0.31 galaxy clusters, spanning a range of masses and dynamical states. We investigate the nature of the emission lines in relation to the host galaxy properties, its location within the cluster, and the properties of the parent cluster. We uncover remarkable differences between mergers and relaxed clusters. The majority of Hα emission-line galaxies in merging cluster fields are located within 3 Mpc of their center. A large fraction of these line emitters in merging clusters are powered by star formation irrespective of cluster-centric radius, while the rest are powered by active galactic nuclei (AGNs). Star-forming galaxies are rare within 3 Mpc of relaxed clusters and AGNs are most abundant at their outskirts (∼1.5–3 Mpc). We discover a population of star-forming galaxies with large equivalent widths and blue UV–optical colors found exclusively in the merging clusters in our sample. The widespread emission-line activity in merging clusters is likely supported by triggered activity in recently accreted, gas-rich galaxies. By contrast, our observations for relaxed clusters match established models in which black hole activity is enhanced at the virial radius and star formation is quenched within the infall region. We conclude that emission-line galaxies experience distinct evolutionary paths in merging and relaxed clusters.
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