The SCUBA Local Universe Galaxy Survey -- I. First measurements of the submillimetre luminosity and dust mass functions

Abstract

This is the first of a series of papers presenting results from the SCUBA Local Universe Galaxy Survey (SLUGS), the first statistical survey of the submillimetre properties of the local Universe. As the initial part of this survey, we have used the SCUBA camera on the James Clerk Maxwell Telescope to observe 104 galaxies from the IRAS Bright Galaxy Sample. We present here the 850-μm flux measurements. The 60-, 100-, and 850-μm flux densities are well fitted by single-temperature dust spectral energy distributions, with the sample mean and standard deviation for the best-fitting temperature being Td=35.6±4.9 K and for the dust emissivity index β=1.3±0.2. The dust temperature was found to correlate with 60-μm luminosity. The low value of β may simply mean that these galaxies contain a significant amount of dust that is colder than these temperatures. We have estimated dust masses from the 850-μm fluxes and from the fitted temperature, although if a colder component at around 20 K is present (assuming a β of 2), then the estimated dust masses are a factor of 1.5–3 too low. We have made the first direct measurements of the submillimetre luminosity function (LF) and of the dust mass function. Unlike the IRAS 60-μm LF, these are well fitted by Schechter functions. The slope of the 850-μm LF at low luminosities is steeper than −2, implying that the LF must flatten at luminosities lower than we probe here. We show that extrapolating the 60-μm LF to 850 μm using a single temperature and β does not reproduce the measured submillimetre LF. A population of ‘cold’ galaxies (Td<25 K) emitting strongly at submillimetre wavelengths would have been excluded from the 60-μm-selected sample. If such galaxies do exist, then this estimate of the 850-μm flux is biased (it is underestimated). Whether such a population does exist is unknown at present. We correlate many of the global galaxy properties with the FIR/submillimetre properties. We find that there is a tendency for less luminous galaxies to contain hotter dust and to have a greater star formation efficiency (cf. Young). The average gas-to-dust ratio for the sample is 581±43 (using both the atomic and molecular hydrogen), which is significantly higher than the Galactic value of 160. We believe that this discrepancy is probably due to a ‘cold dust’ component at Td≤20 K in our galaxies. There is a surprisingly tight correlation between dust mass and the mass of molecular hydrogen, estimated from CO measurements, with an intrinsic scatter of ≃50 per cent.