AN OBSERVATIONAL TEST OF DARK MATTER AS COLD FRACTAL CLOUDS

摘要

Using the Very Large Array (VLA), we have performed the first observational test of dark matter in the form of cold, primordial fractal clouds, as envisioned by Pfenniger, Combes, & Martinet and Pfenni- ge r & Combes. We show that, after a Hubble time, primordial fractal clouds will convert most of their H l to H,, but a small fraction of H I remains which is optically thick. This opens up a new window for detecting dark matter which may exist in this form. The detectability of such gas depends on its filling factor and temperature and therefore should be observable in absorption against a background source with observations of sufficient sensitivity and resolution. The current VLA observations have made a first step toward this goal by taking advantage of a fortuitous alignment between the extension of the H l disk of the nearby galaxy, NGC 3079, and a background quasar, Q0957+561. Our observations probe 28 independent beams against the quasar and all of velocity space between the extension of a flat rotation curve and a Keplerian decline for the halo region of NGC 3079. We do not detect any absorp- tion features and investigate, in detail, the implication of this result for the hypothesis that dark matter is in the form of fractal clouds. In particular, we calculate the probability that our observations would have detected such clouds as a function of the model parameters. The chance of detection is significant for an interesting region (fractal dimension 1.7 ≤D ≤ 2 and cloud radius 30 pc < Rc < 3 kpc) of parameter space and rises above 95/100 for a small region of parameter space. While our analysis does not rule out fractal clouds as dark matter, it does lay the groundwork for future, more sensitive observations, and we consider what form these might take to probe the range of possible cloud properties more deeply. It is interesting that the observations can rule out cold, optically thin H I gas, if it exists, to a limit of 0.001/100 of the dark matter. In contrast, the existence of cold H I in a fractal hierarchy would be an efficient way of hiding dark matter.