Herschel-ATLAS/GAMA: how does the far-IR luminosity function depend on galaxy group properties?

We use the Herschel Astrophysical Terahertz Large Area Survey (H-ATLAS) Phase I data to study the conditional luminosity function of far-IR (250 μm) selected galaxies in optically selected galaxy groups from the Galaxy And Mass Assembly (GAMA) spectroscopic survey, as well as environmental effects on the far-IR-to-optical colour. We applied two methods, which gave consistent results for the far-IR conditional luminosity functions. The direct matching method matches H-ATLAS sources to GAMA/SDSS (Sloan Digital Sky Survey) galaxies, then links the optical counterparts to GAMA groups. The stacking method counts the number of far-IR sources within the projected radii of GAMA groups, subtracting the local background. We investigated the dependence of the far-IR (250 μm) luminosity function on group mass in the range 10¹² < M_h < 10¹⁴ h⁻¹ M_⊙ and on redshift in the range 0 < z < 0.4, using a sample of 3000 groups containing H-ATLAS sources with GAMA redshifts over an area of 126 deg². We find that the characteristic 250 μm luminosity, L*(250), increases with group mass up to M_h ∼ 10¹³ h⁻¹ M_⊙, but is roughly constant above this, while it increases with redshift at high group masses, but less so at low masses. We also find that the group far-IR luminosity-to-mass ratio L(250)/M_h increases with redshift and is higher in low-mass groups. We estimate that around 70 per cent of the 250 μm luminosity density in the local Universe is contributed by groups with M_h > 10¹²h⁻¹ M_⊙. We also find that the far-IR-to-optical colours of H-ATLAS galaxies are independent of group mass over the range 10¹² < M_h < 10¹⁴ h⁻¹ M_⊙ in the local Universe. We also compare our observational results with recent semi-analytical models, and find that none of these galaxy formation models can reproduce the conditional far-IR luminosity functions of galaxy groups.