SENSITIVE LIMITS ON THE MOLECULAR GAS CONTENT OF CLUSTER COOLING FLOWS

We have searched for molecular gas toward six cluster cooling flows in the CO(2-1) line using the James Clerk Maxwell Telescope. The sample includes clusters with estimated total cooling rates Of m(CF) approximately 10-600M. yr-1, at redshifts between z approximately 0.01-0.06. None were detected either in emission or absorption. Our molecular mass limits in the inner approximately 25 kpc are typically M(H2) less than or similar to 4 10(9) M.. Our limit for the nearest cluster, A 1060, is an order of magnitude lower. In the aggregate, these are the most sensitive limits available for cluster cooling flows. Using model spectra we find that molecular cloud populations as massive as few 10(10) M. with velocity dispersions less than or similar to 500 km s-1 are usually excluded, unless the material is colder and/or and has a significantly lower metal abundance compared to average Galactic clouds. If the steady state cooling flow model is correct, the cooling material must be converted efficiently to a non-gaseous state on a timescale of approximately 10(7-8) yr, or the putative cold clouds must be accelerated to a velocity dispersion greater than or similar to 700 km s-1 on a similar timescale. Future observations with broader bandwidths should yield improved limits. Failure to detect CO at levels significantly lower than these may be difficult to reconcile with standard cooling flow models