LABOCA mapping of the infrared dark cloud MSXDC G304.74+01.32

Context. Infrared dark clouds (IRDCs) likely represent very early stages of high-mass star/star cluster formation. Aims. The aim is to determine the physical properties and spatial distribution of dense clumps in the IRDC MSXDC G304.74+01.32 (G304.74), and bring these characteristics into relation to theories concerning the origin of IRDCs and their fragmentation into clumps and star-forming cores. Methods. G304.74 was mapped in the 870 mu m dust continuum with the LABOCA bolometer on APEX. The 870 mu m map was compared with the 1.2 mm continuum map of the cloud by Beltran et al. (2006, A&A, 447, 221). Archival MSX and IRAS infrared data were used to study the nature and properties of the submillimetre clumps within the cloud. The H-2 column densities were estimated using both the 870 mu m dust emission and the MSX 8 mu m extinction data. The obtained values were compared with near-infrared extinction which could be estimated along a few lines of sight. We compared the clump masses and their spatial distribution in G304.74 with those in several other recently studied IRDCs. Results. Twelve clumps were identified from the 870 mu m dust continuum map. Three of them are associated with the MSX and IRAS point sources. Moreover, one of the clumps (SMM 6) is associated with two MSX 8 mu m point-like sources. Thus, there are 8 clumps within G304.74 which are not associated with mid-infrared (MIR) emission. The H-2 column densities derived from the dust continuum and extinction data are similar. The comparison suggests that the dust temperature may be elevated (20-30 K) near the southern end of the cloud, whereas the starless clumps in the centre and in the north are cool (T-d similar to 15 K). There is a high likelihood that the clump mass distributions in G304.74 and in several other IRDCs represent the samples of the same parent distribution. In most cases the spatial distributions of clumps in IRDCs do not deviate significantly from random distributions. Conclusions. G304.74 contains several massive clumps that are not associated with MIR emission. On statistical grounds it is likely that some of them are or harbour high-mass starless cores (HMSCs). The fact that the clump mass distributions (resembling the high-mass stellar IMF), and in some cases also the random-like spatial distributions, seem to be comparable between different IRDCs, is consistent with the idea that the origin of IRDCs, and their further sub-fragmentation down to scales of clumps is caused by supersonic turbulence in accordance with results from giant molecular clouds.