Abstract
Abstract Motivated by the apparently conflicting results reported in the literature on the effect of environment on nuclear activity, we have carried out a new analysis by comparing the fraction of galaxies hosting active galactic nuclei (AGNs) in the most overdense regions (rich galaxy clusters) and the most underdense ones (voids) in the local universe. Exploiting the classical BPT diagnostics, we have extracted volume-limited samples of star-forming and AGN galaxies. We find that, at variance with star-forming galaxies, AGN galaxies have similar distributions of specific star formation rates and of galactic ages (as indicated by the D n 4000 parameter) both in clusters and in voids. In both environments galaxies hosting AGNs are generally old, with low star formation activity. The AGN fraction increases faster with stellar mass in clusters than in voids, especially above 1010.2 M ⊙. Our results indicate that, in the local universe, the nuclear activity correlates with stellar mass and galaxy morphology and is weakly, if at all, affected by the local galaxy density.
Highlights
It has long been known that, in the nearby universe, galaxy properties such as star formation rate (SFR), morphology and stellar mass are strongly correlated with the surrounding galaxy density (Oemler 1974; Dressler 1980; Hashimoto et al 1998; Kauffmann et al 2004; Baldry et al 2006)
The general conclusion of these investigations is that interactions/mergers substantially enhance the specific SFR, but the effect on active galactic nuclei (AGN) activity depends on the AGN type
To this end to compare the properties of star-forming galaxies (SFGs) and AGNs in clusters and in voids in a way as homogeneous as possible, AGN and SFG galaxies in both environments were randomly drawn from the parent samples in such a way to have the same number of objects in each stellar mass bin so that the stellar mass distribution is the same for both populations (Fig. 2)
Summary
It has long been known that, in the nearby universe, galaxy properties such as star formation rate (SFR), morphology and stellar mass are strongly correlated with the surrounding galaxy density (Oemler 1974; Dressler 1980; Hashimoto et al 1998; Kauffmann et al 2004; Baldry et al 2006). Kauffmann et al (2004) reported a strong anti-correlation of nuclear activity on local density at fixed stellar mass for powerful AGNs (L([OIII]) > 107 L ), generally residing in massive galaxies with significant star formation activity. The general conclusion of these investigations is that interactions/mergers substantially enhance the specific SFR, but the effect on AGN activity depends on the AGN type. The AGN fraction was found to be substantially higher in close galaxy pairs (Ellison et al 2011, 2015; Satyapal et al 2014), implying that mergers can trigger nuclear activity and enhance the accretion rate. Scott et al (2014) reported a modest decrease of the Seyfert fraction in close pair galaxies compared to isolated galaxies, suggesting that either mergers may not trigger AGN activity at the close pair stage or may trigger a different AGN type.
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