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H 2 pure-rotational emission lines are detected from warm (100-1500 K) molecular gas in 17/55 (31% of) radio galaxies at redshift z < 0.22 observed with the Spitzer IR Spectrograph. The summed H 2 0-038 -2 × 10 42 erg s −1 , yielding warm H 2 masses up to 2 × 10 10 M . These radio galaxies, of both FR radio morphological types, help to firmly establish the new class of radio-selected molecular hydrogen emission galaxies (radio MOHEGs). MOHEGs have extremely large H 2 to 7.7 μm polycyclic aromatic hydrocarbon (PAH) emission ratios: L(H 2 )/L(PAH7.7) = 0.04-4, up to a factor 300 greater than the median value for normal star-forming galaxies. In spite of large H 2 masses, MOHEGs appear to be inefficient at forming stars, perhaps because the molecular gas is kinematically unsettled and turbulent. Low-luminosity mid-IR continuum emission together with low-ionization emission line spectra indicates low-luminosity active galactic nuclei (AGNs) in all but three radio MOHEGs. The AGN X-ray emission measured with Chandra is not luminous enough to power the H 2 emission from MOHEGs. Nearly all radio MOHEGs belong to clusters or close pairs, including four cool-core clusters (Perseus, Hydra, A2052, and A2199). We suggest that the H 2 in radio MOHEGs is delivered in galaxy collisions or cooling flows, then heated by radio-jet feedback in the form of kinetic energy dissipation by shocks or cosmic rays.

The edge-on starburst galaxy NGC 253 has been imaged with the VLA at wavelengths ranging from 1.3 to 20 cm, with resolutions between 1 and 15 pc. These images reveal a large number of compact radio sources embedded within the di †use radio structure in the inner 200 pc of the galaxy. We have identiÐed approximately 64 individual compact radio sources in the galaxy. Of the strongest 17 sources, for which the Ñux densities are high enough to measure spectral indices with moderately low errors, about half have Ñat spectra and half have steep spectra ; this indicates that perhaps half of the individual radio sources are dominated by thermal radio emission from H II regions. Over an 8 yr timescale, few, if any, of the strongest sources have varied in Ñux density, with limits of 1%È2% yr~1 on the rate of decrease. At the highest resolution of 1 pc, a number of radio sources are beginning to be resolved, and at least 15 di †erent sources can be identiÐed within the innermost 20 pc of the galaxy.Individual radio sources have been explored in more detail. The strongest source, 5.79[39.0 (TH2), assumed to be at the nucleus of the galaxy, has a brightness temperature greater than 20,000 K at 22 GHz and greater than 40,000 K at 15 GHz. It is unresolved at the VLA and may be either an active galactic nucleus or a very compact (nonvariable) supernova remnant. A resolved Ñat-spectrum source, 5.72[40.1 (TH6), is located D20 pc to the southwest. It has an apparent size of 2.4 ] 1.2 pc and appears to be an H II region similar to the inner part of 30 Doradus, containing approximately 105 M _ in stars, as well as D600 of ionized gas. Source counts, the lack of variability, and the lack of new M _ sources imply that the radio supernova rate is no more than 0.3 yr~1, consistent with estimates made in other wave bands.

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