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

Abstract

I discuss the possibility that a significant fraction (possibly a third) of the faint SCUBA sources are not in fact high redshift galaxies, but actually local cold dark dusty gas clouds emitting only in the submm, with a temperature around 7K. I show that the observational constraints on such a population - dynamical limits on missing matter, the FIR-mm background, and the absence of gross high-latitude extinction features - constrains the mass of such objects to be in the range 0.1 - 10 Jupiter masses. The characteristics deduced are closely similar to those of the objects proposed by Walker and Wardle (1998) to explain halo dark matter. However, such objects, if they explain a large fraction of the SCUBA 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 disk population with a scale height of few hundred parsecs. Regardless of the dark matter problem, the possible existence of such compact sub-stellar but non-degenerate objects is intriguing. They may be seen as failed stars, representing an alternative end-point to brown dwarfs. It is possible that they greatly outnumber both stars and brown dwarfs. The nearest such object could be a fraction of a parsec away. Several relatively simple observations could critically test this hypothesis.