The SCUBA Local Universe Galaxy Survey II . 450 μ m data – evidence for cold dust in bright

This is the second in a series of papers presenting results from the SCUBA Local Universe Galaxy Survey. In our first paper we provided 850μm flux densities for 104 galaxies selected from the IRAS Bright Galaxy Sample and we found that the 60, 100μm (IRAS) and 850μm (SCUBA) fluxes could be adequately fitted by emission from dust at a single temperature. In this paper we present 450μm data for the galaxies. With the new data, the spectral energy distributions of the galaxies can no longer be fitted with an isothermal dust model – two temperature components are now required. Using our 450μm data and fluxes from the literature, we find that the 450μm/850μm flux ratio for the galaxies is remarkably constant and this holds from objects in which the star formation rate is similar to our own Galaxy, to ultraluminous infrared galaxies (ULIRGS) such as Arp 220. The only possible explanation for this is if the dust emissivity index for all of the galaxies is ∼ 2 and the cold dust component has a similar temperature in all galaxies (Tc ∼ 20 − 21 K). The 60μm luminosities of the galaxies were found to depend on both the dust mass and the relative amount of energy in the warm component, with a tendency for the temperature effects to dominate at the highest L60. The dust masses estimated using the new temperatures are higher by a factor ∼ 2 than those determined previously using a single temperature. This brings the gas-to-dust ratios of the IRAS galaxies into agreement with those of the Milky Way and other spiral galaxies which have been intensively studied in the submm.