SCUBA SOURCES, FAILED STARS, and the DARK MATTER

SCUBA seems to be a kind of zoom facility to take us straight to the high-redshift universe - several groups have made deep SCUBA surveys and claimed identifications with very faint galaxies at z=1-5. These exciting results imply that most of the star formation in the young universe occurred in luminous dusty starbursts. However, one worry is that the IDs are usually ambiguous, because of the large submm beamwidth. In the one published example of an interferometer detection (HDF850.1 - Downes et al A&A 347, 809, 1999) the arcsecond position is completely blank even to the depth of the Hubble Deep Field. But what if SCUBA is actually detecting a new class of astronomical object, so cold that it emits ONLY in the submm? If HDF850.1 is a local object, it has a temperature of around 7K. I tested various ideas against the observational constraints. It was easy to rule out, for example, a vast population of very cold brown dwarfs, or of comets just past Neptune. But it was not so easy to rule out the possibility of small dusty gas clouds in the local interstellar medium - at a distance of around 100pc.

What we have to go on is the flux of such objects, their surface sky density, the lack of extinction holes all over the sky, and the dynamical limits imposed by local star motions on any disk dark matter. If we postulate a mass for our clouds and assume a standard gas to dust ratio and a temperature of 7K, we can get a characteristic luminosity, distance, and so space density. Such objects cannot be less than about a tenth of a Jupiter mass or they contain too much mass. On the other hand they cannot be bigger than about ten Jupiter masses or we would have noticed black spots on the sky. At around a Jupiter mass they would be tenth of an arcsecond across but cover maybe one part in ten million of the sky. However... they would be ten times as common as stars; the nearest one (somewhere on the sky!) would be just past the Oort cloud, and the local mass density would be around 0.1 solar masses per cubic parsec, still hideable in local disk dynamics, but quite close to the predicted local density of halo dark matter .. could SCUBA sources be the dark matter???

Over the last few years there has in fact been a minor industry of astronomers arguing that halo dark matter could be hidden in cold molecular material and wouldn't have been detected (eg Pfenniger, Combes and Martinet A&A 285, 79, 1994; Walker and Wardle ApJ 498, L125, 1998; Sciama 2000, MNRAS 312, 33) and indeed they have generally argued that Jupiter mass clumps are preferred. So perhaps SCUBA has now SEEN the dark matter? However... my calculations concern typical faint SCUBA sources at around 2 mJy, deduced to be at a distance of 100 pc.. if this population continued through the halo to 10kpc, the FIR background measured by COBE would be exceeded by a vast amount. In fact, faint SCUBA sources cannot extend by more than a factor of a few without exceeding the background (Hughes et al Nature 294, 241, 1998). This is consistent either with sources at z=3, or with sources at 100pc, implying a Galactic Plane population ... but not with halo dark matter. Such objects would still be of great importance however, not just because of completing the cosmic inventory, but because they may represent the end point for most collapsing clouds - cold dark clouds rather than brown dwarfs could be the true failed stars.

Well, this could all be an interesting fantasy. Obviously at least some SCUBA sources are high-z galaxies. But if even a quarter or a third are local objects it will be important for cosmology, as the z=3-5 claims will be selectively removed. The idea is definitely if not trivially testable, for example by conducting bright submm source counts, looking for black spots in HST elliptical galaxy images, or looking for rare stellar switch-off events.

A paper is in press in MNRAS describing all this in much more detail, and will be on astro-ph shortly.