Galaxy ecology: Groups and low-density environments in the SDSS and 2dFGRS

Michael Balogh, Vince Eke, Chris Miller, Ian Lewis, Richard Bower, Warrick Couch, Robert Nichol, Joss Bland-Hawthorn, Ivan K. Baldry, Carlton Baugh, Terry Bridges, Russell Cannon, Shaun Cole, Matthew Colless, Chris Collins, Nicholas Cross, Gavin Dalton, Roberto De Propris, Simon P. Driver, George EfstathiouRichard S. Ellis, Carlos S. Frenk, Karl Glazebrook, Percy Gomez, Alex Gray, Edward Hawkins, Carole Jackson, Ofer Lahav, Stuart Lumsden, Steve Maddox, Darren Madgwick, Peder Norberg, John A. Peacock, Will Percival, Bruce A. Peterson, Will Sutherland, Keith Taylor

Research output: Contribution to journalReview articlepeer-review

442 Citations (Scopus)


We analyse the observed correlation between galaxy environment and Hα emission-line strength, using volume-limited samples and group catalogues of 24 968 galaxies at 0.05 < z < 0.095, drawn from the 2dF Galaxy Redshift Survey (MbJ < -19.5) and the Sloan Digital Sky Survey (Mr < -20.6). We characterize the environment by: (1) ∑5, the surface number density of galaxies determined by the projected distance to the fifth nearest neighbour; and (2) ρ1.1 and ρ5.5, three-dimensional density estimates obtained by convolving the galaxy distribution with Gaussian kernels of dispersion 1.1 and 5.5 Mpc, respectively. We find that star-forming and quiescent galaxies form two distinct populations, as characterized by their Hα equivalent width, W0(Hα). The relative numbers of star-forming and quiescent galaxies vary strongly and continuously with local density. However, the distribution of W0(Hα) amongst the star-forming population is independent of environment. The fraction of star-forming galaxies shows strong sensitivity to the density on large scales, ρ5.5, which is likely independent of the trend with local density, ρ1.1. We use two differently selected group catalogues to demonstrate that the correlation with galaxy density is approximately independent of group velocity dispersion, for σ = 200-1000 km s-1. Even in the lowest-density environments, no more than ∼70 per cent of galaxies show significant Hα emission. Based on these results, we conclude that the present-day correlation between star formation rate and environment is a result of short-time-scale mechanisms that take place preferentially at high redshift, such as starbursts induced by galaxy-galaxy interactions.

Original languageEnglish
Pages (from-to)1355-1372
Number of pages18
JournalMonthly Notices of the Royal Astronomical Society
Issue number4
Publication statusPublished - 11 Mar 2004
Externally publishedYes


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