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

I discuss the possibility that a significant fraction of the extremely common faint submm sources found in recent surveys are not in fact high-redshift galaxies, but actually local objects emitting only in the submm, with a temperature around 7K. The majority of faint SCUBA sources clearly really are distant galaxies. However, if even a quarter or a third of the SCUBA sources are actually local objects, the cosmological implications are significant, 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 population - 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 the assumption of virial equilibrium, their sizes are in the range 1-100au, with angular sizes around 0.1arcsec. They would be completely opaque at visible and IR wavelengths. The characteristics deduced are closely similar to those of the objects proposed by Walker & Wardle to explain `extreme scattering events' in quasar radio light curves, and which they propose fill the 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 models. 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 SCUBA sources to be around 100pc, consistent with being drawn from a disc population with a scaleheight of a few hundred pc. Possibly a `Population II' dustless version of such objects could exist in the halo. Regardless of the dark matter problem, the possible existence of such compact substellar but non-degenerate objects is intriguing. Such objects should collapse on a very short time-scale, but at such a low temperature that it is possible that cosmic ray heating can maintain them in equilibrium. The main theoretical objection is that such an equilibrium may be unstable on a thermal time-scale. 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 possible that they greatly outnumber both stars and brown dwarfs. The nearest such object could be a fraction of a pc away. Several relatively simple observations could critically test this hypothesis.