The nature of Hα star-forming galaxies at z ~ 0.4 in and around Cl 0939+4713: The environment matters*

Cluster star-forming galaxies are found to have an excess of far-infrared emission relative to Hα, when compared to those in the field, which could be caused by intense active galactic nuclei (AGN) activity, dust and/or declining star formation histories. Here we present spectroscopic observations of Hα emitters in the Cl 0939+4713 (Abell 851) super-cluster at z = 0.41, using AF2+ WYFFOS on the William Herschel Telescope. We measure [O II], Hβ, [O III], Hα and [N II] for a sample of 119 Hα emitters in and around the cluster. We find that 17 ± 5 per cent of the Hα emitters are AGN, irrespective of environment. For star-forming galaxies, we obtain Balmer decrements, metallicities and ionization parameters with different methods, individually and by stacking. We find a strong mass-metallicity relation at all environments, with no significant dependence on environment. The ionization parameter declines with increasing stellar mass for low-mass galaxies. Hα emitters residing in intermediate environments show the highest ionization parameters (along with high [O III]/Hα and high [O III]/[O II] line ratios, typically twice as large as in the highest and lowest densities), which decline with increasing environmental density. Dust extinction (A_Hα) correlates strongly with stellar mass, but also with environmental density. Star-forming galaxies in the densest environments are found to be significantly dustier (A_Hα ≈ 1.5 - 1.6) than those residing in the lowest density environments (A_Hα ≈ 0.6), deviating significantly from what would be predicted given their stellar masses.