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Abstract We present a large sample of H - and K -band spectra of 32 optically line-luminous central cluster galaxies. We find significant rovibrational H 2 emission in 23 of these galaxies as well as H recombination and/or [Fe ii] emission in another five. This represents a fourfold increase in the number of molecular line detections known. A number of the detections are of extended emission (5–20 kpc). In several objects we find significant [Si vi] emission that appears to correlate with the strength of high ionization lines in the optical (e.g. [O iii]). This comprehensive sample builds on previous work and confirms that warm (1000–2500 K) molecular hydrogen is present wherever there is ionized material in the cores of cooling flows and in most cases it also coincides with CO molecular line emission. [ABSTRACT FROM AUTHOR]

The origin of rovibrational H[sub 2] emission in the central galaxies of cooling flow clusters is poorly understood. Here we address this issue using data from our near-infrared spectroscopic survey of 32 of the most line-luminous such systems, presented in the companion paper by Edge et al. We consider excitation by X-rays from the surrounding intracluster medium (ICM), ultraviolet (UV) radiation from young stars, and shocks. The v = 1-0 K-band lines with upper levels within 10[sup 4] K of the ground state appear to be mostly thermalized (implying gas densities ... 10[sup 5] cm[sup -3]), with the excitation temperature typically exceeding 2000 K, as found earlier by Jaffe, Bremer & van der Werf. Together with the lack of strong v = 2-0 lines in the H-band, this rules out UV radiative fluorescence. Using the CLOUDY photoionization code, we deduce that the H[sub 2] lines can originate in a population of dense clouds, exposed to the same hot (T ∼ 50000 K) stellar continuum as the lower density gas which produces the bulk of the forbidden optical line emission in the Hα-luminous systems. This dense gas may be in the form of self-gravitating clouds deposited directly by the cooling flow, or may instead be produced in the high-pressure zones behind strong shocks. Furthermore, the shocked gas is likely to be gravitationally unstable, so collisions between the larger clouds may lead to the formation of globular clusters. [ABSTRACT FROM AUTHOR]