Herschel-ATLAS/GAMA : the environmental density of far-infrared bright galaxies at z < 0.5

Abstract

We compare the environmental and star formation properties of far-infrared detected and non-far-infrared detected galaxies out to z similar to 0.5. Using optical spectroscopy and photometry from the Galaxy And Mass Assembly (GAMA) and Sloan Digital Sky Survey, with far-infrared observations from the Herschel Astrophysical Terahertz Large Area Survey (ATLAS) Science Demonstration Phase, we apply the technique of Voronoi tessellations to analyse the environmental densities of individual galaxies. Applying statistical analyses to colour, r-band magnitude and redshift-matched samples, we show that there is a significant difference at the 3.5 Sigma level between the normalized environmental densities of these two populations. This is such that infrared emission (a tracer of star formation activity) favours underdense regions compared to those inhabited by exclusively optically observed galaxies selected to be of the same r-band magnitude, colour and redshift. Thus, more highly star-forming galaxies are found to reside in the most underdense environments, confirming previous studies that have proposed such a correlation. However, the degeneracy between redshift and far-infrared luminosity in our flux-density-limited sample means that we are unable to make a stronger statement in this respect. We then apply our method to synthetic light cones generated from semi-analytic models, finding that over the whole redshift distribution the same correlations between star formation rate and environmental density are found.