  The growth of galaxy clusters is energetic and may trigger and/or quench star
formation and black hole activity. The ENISALA 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 H$\alpha$ 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 H$\alpha$ 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.
Star-forming galaxies are rare within 3 Mpc of relaxed clusters and active
galactic nuclei 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.
