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 7 K. 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 1–100 au, with angular sizes around 0.1 arcsec. 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 100 pc, 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.