An observational test of dark matter as cold fractal clouds

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 env isioned by Pfenniger, Combes, & Martinet and Pfenniger & Combes. We show th at, after a Hubble time, primordial fractal clouds will convert most of the ir H I to H-2. 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 t his form. The detectability of such gas depends on its filling factor and t emperature and therefore should be observable in absorption against a backg round source with observations of sufficient sensitivity and resolution. Th e current VLA observations have made a first step toward this goal by takin g advantage of a fortuitous alignment between the extension of the H I disk of the nearby galaxy, NGC 3079, and a background quasar, Q0957+561. Our ob servations probe 28 independent beams against the quasar and all of velocit y space between the extension of a flat rotation curve and a Keplerian decl ine for the halo region of NGC 3079. We do not detect any absorption featur es and investigate, in detail, the implication of this result for the hypot hesis that dark matter is in the form of fractal clouds. In particular, we calculate the probability that our observations would have detected such cl ouds as a function of the model parameters. The chance of detection is sign ificant for an interesting region (fractal dimension 1.7 less than or simil ar to D less than or similar to 2 and cloud radius 30 pc < R-c 3 kpc) of pa rameter space and rises above 95% for a small region of parameter space. Wh ile our analysis does not rule out fractal clouds as dark matter, it does l ay 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, opt ically thin H I gas, if it exists, to a limit of 0.001% 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.