A VLA search for neutral hydrogen in cooling flow clusters

Abstract

We present a Very Large Array (VLA) search for neutral hydrogen in three clusters, which are considered to be prime examples of so-called cooling flow clusters: Virgo (Virgo A), Abell 2199 (3C 338), and Abell 780 (Hydra A). We looked for H I in absorption against the central radio sources with a velocity coverage approximately 2600 km/s and with a velocity resolution approximately 87 km/s. We do not detect a spatially extended distribution of cold clouds. Our 3 sigma optical depth limits are 0.0005 over a typical velocity range of 500 km/s. Assuming the currently popular cooling flow scenarios in which the distribution of the cold gas can be characterized by a core radius of approximately 100 kpc, our results place an upper limit of 1.2 x 1010 solar mass to the mass of neutral hydrogen in such a flow for an assumed spin temperature of 100 K. This limit is a factor of 30 below what is expected from the Einstein Solid State Spectrometer observations of cooling flow clusters and is 1-2 orders of magnitude below what is expected from the cooling flow models. The expected large amount of H I is unlikely to exist in these clusters even as optically thick clouds unless their spin temperature is less than 10 K and/or their covering factor is much less than 1. We discuss some physical conditions in which the expected large amounts of H I could have escaped detection. We do, however, find H I absorption against the core of Hydra A. This line is narrow (approximately 50 kms) and is not seen against the extended radio lobes. The properties of this absorption system are similar to those seen in some other cooling flow clusters and in isolated radio galaxies. We also detect a spiral galaxy in A 2199 with a total mass of 1.8 x 109 solar mass. Despite this detection of what might be a small H I cloud near the systemic velocity of the core of Hydra A, we do place stringent limits on the existence of such clouds anywhere within the primary beam. The upper limit to H I emission from such clouds ranges from 107 to 109 solar mass.