Blank-field submm sources, failed stars and the dark matter

I discuss the possibility that a significant fraction of the extremely comm on faint submm sources found in recent surveys are not in fact high-redshif t galaxies, but actually local objects emitting only in the submm, with a t emperature around 7 K. The majority of faint SCUBA sources clearly really a re distant galaxies. However, if even a quarter or a third of the SCUBA sou rces are actually local objects, the cosmological implications are signific ant, as this would selectively remove the objects believed to be at z > 3. Two hypotheses - very cold brown dwarfs and outer Solar system bodies - are easily rejected. A third hypothesis - cold dark dusty gas clouds - is not so easily dismissed. I show that the observational constraints on such a po pulation - dynamical Limits on local missing matter, the FIR-mm background and the absence of gross high-latitude extinction features - constrain the mass of such objects to be in the mass range 0.1 to 10 Jupiter masses. On t he assumption of virial equilibrium, their sizes are in the range 1-100 au, with angular sizes around 0.1 arcsec. They would be completely opaque at v isible and IR wavelengths. The characteristics deduced are closely similar to those of the objects proposed by Walker & Wardle to explain 'extreme sca ttering events' in quasar radio light curves, and which they propose fill t he Galactic halo and explain halo dark matter. Indeed, at around 1 Jupiter mass, the local population density would be similar to that in dark halo mo dels. However, such objects, if they explain a large fraction of the SCUBA submm sources, cannot extend through the halo without greatly exceeding the FIR-mm background. Instead, I deduce the characteristic distance of the SC UBA sources to be around 100 pc, consistent with being drawn from a disc po pulation with a scaleheight of a few hundred pc. Possibly a 'Population II' dustless version of such objects could exist in the halo. Regardless of th e dark matter problem, the possible existence of such compact substellar bu t non-degenerate objects is intriguing. Such objects should collapse on a v ery short time-scale, but at such a low temperature that it is possible tha t cosmic ray heating can maintain them in equilibrium. The main theoretical objection is that such an equilibrium may be unstable on a thermal time-sc ale. If however, such objects do exist, they may be seen as 'failed stars', representing an alternative end-point to stars and brown dwarfs. It is pos sible that they greatly outnumber both stars and brown dwarfs. The nearest such object could be a fraction of a pc away. Several relatively simple obs ervations could critically test this hypothesis.