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

Baugh, C. and Bower, R. and Cole, Shaun and Eke, V. and Baldry, I. K. and Balogh, M. and Bland-Hawthorn, J. and Bridges, T. and Cannon, R. and Colless, M. and Collins, C. and Couch, W. and Cross, N. and Dalton, G. de and Driver, S. P. and Efstathiou, G. and Ellis, R. S. and Frenk, C. S. and Glazebrook, K. and Gomez, P. and Gray, A. and Hawkins, E. and Jackson, C. and Lahav, O. and Lewis, I. and Lumsden, S. and Maddox, S. and Madgwick, D. and Miller, C. and Nichol, R. and Norberg, P. and Peacock, J. A. and Percival, W. and Peterson, B. A. and de Propris, R. and Sutherland, W. and Taylor, K. (2004) 'Galaxy ecology : groups and low-density environments in the SDSS and 2dFGRS.', Monthly notices of the Royal Astronomical Society., 348 (4). pp. 1355-1372.


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) Σ5, 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.

Item Type:Article
Keywords:Glaxies, Clusters, Evolution, Interactions.
Full text:(NA) Not Applicable
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Date of first online publication:March 2004
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