Hubble Space Telescope observations of vibrationally excited molecular hydrogen in cluster cooling flow nebulae

We report the results of Hubble Space Telescope near-infrared camera and mu ltiobject spectrometer and Wide Field Planetary Camera 2 imaging of emissio n-line nebulae in the central galaxies of three clusters of galaxies purpor ted to host massive cooling flows: Perseus (NGC 1275), Abell 2597, and PKS 0745-191. The spectral signature of vibrationally excited molecular hydroge n has been seen in every galaxy searched thus far that is central to a clus ter cooling flow with an optical emission-line nebula. With the exquisite s patial resolution available to us with the Hubble Space Telescope, we have discovered that the vibrationally excited molecular hydrogen gas extends se veral kiloparsecs from the centers of Abell 2597 and PKS 0745-191, while th e vibrationally excited molecular hydrogen in NGC 1275 appears to be mostly confined to its nucleus, with some extended emission less than 1 kpc from the center. The molecular hydrogen in Abell 2597 and PKS 0745-191 seems to be nearly cospatial with the optical emission-line filaments in those syste ms. There may be a tiny jet visible in the 1.6 mum image of PKS 0745-191. W e also find significant dust absorption features in the 1.6 mum images of a ll three systems. The dust lanes are not strictly cospatial with the emissi on-line filaments, but are aligned with and perhaps intermingled with them. The morphology of the emission-line systems suggests that the presence of vibrationally excited molecular hydrogen is not purely an active galactic n ucleus-related property of cluster "cooling flow" nebulae, and that the opt ical and infrared emission-line gas, that is, the ionized and vibrationally excited molecular gas, have similar origins, if not also similar energy so urces. The infrared molecular hydrogen lines are much too bright to be gene rated by gas simply cooling from a cooling flow; furthermore, the gas, beca use it is dusty, likely did not condense from the hot intracluster medium ( ICM). We examine some candidates for heating the nebulae, including X-ray i rradiation by the ICM, UV fluorescence by young stars, and shocks. UV heati ng by young stars provides the most satisfactory explanation for the H-2 em ission in A2597; X-ray irradiation is energetically unlikely and strong sho cks (v greater than or similar to km s(-1)) are ruled out by the high H-2/H -alpha ratios. If UV heating is the main energy input, a few billion solar masses of molecular gas are present in A2597 and PKS 0745-191. UV irradiati on models predict a significant amount of 1.0-2.0 mum emission line from hi gher excitation H-2 transitions and moderate far-infrared luminosities (sim ilar to 10(44) h(-2) ergs s(-1)) for A2597 and PKS 0745-191. Even in the co ntext of UV fluorescence models, the total amount of H-2 gas and star forma tion inferred from these observations is too small to account for the cooli ng flow rates and longevities inferred from X-ray observations. We note an interesting new constraint on cooling flow models: the radio sources do not provide a significant amount of shock heating, and therefore they cannot c ounterbalance the cooling of the X-ray gas in the cores of these clusters..