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

David Sobral, Andra Stroe, Yusei Koyama, Behnam Darvish, João Calhau, Ana Afonso, Tadayuki Kodama, Fumiaki Nakata

Research output: Contribution to journalArticlepeer-review

35 Citations (Scopus)


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) correlates strongly with stellar mass, but also with environmental density. Star-forming galaxies in the densest environments are found to be significantly dustier (A ≈ 1.5 - 1.6) than those residing in the lowest density environments (A ≈ 0.6), deviating significantly from what would be predicted given their stellar masses.

Original languageEnglish
Pages (from-to)3443-3454
Number of pages12
JournalMonthly Notices of the Royal Astronomical Society
Issue number4
Publication statusPublished - 2016 Mar 17
Externally publishedYes


  • Cosmology: Observations
  • Galaxies: Clusters: General
  • Galaxies: Evolution
  • Intergalactic medium
  • Large-scale structure of Universe

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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