Hubble Space TelescopeObservations of Vibrationally Excited Molecular Hydrogen in Cluster Cooling Flow Nebulae

Abstract

We report the results of Hubble Space Telescope near-infrared camera and multiobject spectrometer and Wide Field Planetary Camera 2 imaging of emission-line nebulae in the central galaxies of three clusters of galaxies purported to host massive cooling flows: Perseus (NGC 1275), Abell 2597, and PKS 0745-191. The spectral signature of vibrationally excited molecular hydrogen has been seen in every galaxy searched thus far that is central to a cluster cooling flow with an optical emission-line nebula. With the exquisite spatial resolution available to us with the Hubble Space Telescope, we have discovered that the vibrationally excited molecular hydrogen gas extends several kiloparsecs from the centers of Abell 2597 and PKS 0745-191, while the 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 systems. There may be a tiny jet visible in the 1.6 μm image of PKS 0745-191. We also find significant dust absorption features in the 1.6 μm images of all three systems. The dust lanes are not strictly cospatial with the emission-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 nucleus-related property of cluster "cooling flow" nebulae, and that the optical and infrared emission-line gas, that is, the ionized and vibrationally excited molecular gas, have similar origins, if not also similar energy sources. The infrared molecular hydrogen lines are much too bright to be generated by gas simply cooling from a cooling flow; furthermore, the gas, because it is dusty, likely did not condense from the hot intracluster medium (ICM). We examine some candidates for heating the nebulae, including X-ray irradiation by the ICM, UV fluorescence by young stars, and shocks. UV heating by young stars provides the most satisfactory explanation for the H₂ emission in A2597; X-ray irradiation is energetically unlikely and strong shocks (v 40 km s^-1) are ruled out by the high H₂/Hα 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 irradiation models predict a significant amount of 1.0-2.0 μm emission line from higher excitation H₂ transitions and moderate far-infrared luminosities (~10⁴⁴ h^-2 ergs s^-1) for A2597 and PKS 0745-191. Even in the context of UV fluorescence models, the total amount of H₂ gas and star formation inferred from these observations is too small to account for the cooling 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 counterbalance the cooling of the X-ray gas in the cores of these clusters.