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The Environments of Local Luminous Infrared Galaxies: Star Formation Rates increase with Density

This work studies the environments and star formation relationships of local luminous infrared galaxies (LIRG) in comparison to other types of local and distant (z~1) galaxies. The infrared (IR) galaxies are drawn from the IRAS sample. The density of the environment is quantified using 6dF and Point Source Catalogue redshift survey (PSCz) galaxies in a cylinder of 2h^-1 Mpc radius and 10h^-1 Mpc length. Our most important result shows the existence of a dramatic density difference between local LIRGs and local non-LIRG IR galaxies. LIRGs live in denser environments than non-LIRG IR galaxies implying that L_IR=10^11 h^-2 L_sun marks an important transition point among IR-selected local galaxies. We also find that there is a strong correlation between the densities around LIRGs and their L_IR luminosity, while the IR-activity of non-LIRG IR galaxies does not show any dependence on environment. This trend is independent of mass-bin selection. The SF-density trend in local LIRGs is similar to that found in some studies of blue cloud galaxies at z~1 which show a correlation between star formation and local density (the reversal of the relation seen for local galaxies). This, together with the rapid decline of the number count of LIRGs since z~1, could mean that local LIRGs are survivors of whatever process transformed blue cloud galaxies at z~1 to the present day or local LIRGs came into existence by similar process than high redshift LIRGs but at later stage.