Submillimetre point sources from the Archeops experiment:: very cold clumps in the Galactic plane

Aims. Archeops is a balloon-borne experiment, mainly designed to measure the Cosmic Microwave Background (CMB) temperature anisotropies at high angular resolution (similar to 12 arcmin). By-products of the mission are shallow sensitivity maps over a large fraction of the sky (about 30%) in the millimetre and submillimetre range at 143, 217, 353 and 545 GHz. From these maps, we produce a catalog of bright submillimetre point sources. Methods. We present in this paper the processing and analysis of the Archeops point sources. Redundancy across detectors is the key factor allowing us to distinguish glitches from genuine point sources in the 20 independent maps. Results. We look at the properties of the most reliable point sources, totalling 304. Fluxes range from 1 to 10 000 Jy (at the frequencies covering 143 to 545 GHz). All sources are either planets (2) or of galactic origin. The longitude range is from 75 to 198 degrees. Some of the sources are associated with the well-known Lynds Nebulae and HII compact regions in the galactic plane. A large fraction of the sources have an IRAS counterpart. Except for Jupiter, Saturn, the Crab and Cas A, all sources show a dust-emission-like modified blackbody emission spectrum. Temperatures cover a range from 7 to 27 K. For the coldest sources (T < 10 K), a steep nu(beta) emissivity law is found with a surprising beta similar to 3 to 4. An inverse relationship between T and beta is observed. The number density of sources at 353 GHz with flux brighter than 100 Jy is of the order of 1 per degree of Galactic longitude. These sources will provide a strong check for the calibration of the Planck HFI focal plane geometry as a complement to planets. These very cold sources observed by Archeops should be prime targets for mapping observations by the Akari and Herschel space missions and ground-based observatories.