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.

Context. The Spitzer Space Telescope has detected a powerful (L H 2 ∼ 10 41 erg s −1 ) mid-infrared H 2 emission towards the galaxywide collision in the Stephan's Quintet (henceforth SQ) galaxy group. This discovery was followed by the detection of more distant H 2 -luminous extragalactic sources, with almost no spectroscopic signatures of star formation. These observations place molecular gas in a new context where one has to describe its role as a cooling agent of energetic phases of galaxy evolution. Aims. The SQ postshock medium is observed to be multiphase, with H 2 gas coexisting with a hot (∼5 × 10 6 K), X-ray emitting plasma. The surface brightness of H 2 lines exceeds that of the X-rays and the 0−0 S(1) H 2 linewidth is ∼900 km s −1 , of the order of the collision velocity. These observations raise three questions we propose to answer: (i) why is H 2 present in the postshock gas? (ii) How can we account for the H 2 excitation? (iii) Why is H 2 a dominant coolant? Methods. We consider the collision of two flows of multiphase dusty gas. Our model quantifies the gas cooling, dust destruction, H 2 formation and excitation in the postshock medium. Results. (i)The shock velocity, the post-shock temperature and the gas cooling timescale depend on the preshock gas density. The collision velocity is the shock velocity in the low density volume-filling intercloud gas. This produces a ∼5 × 10 6 K, dust-free, X-ray emitting plasma. The shock velocity is lower in clouds. We show that gas heated to temperatures of less than 10 6 K cools, keeps its dust content and becomes H 2 within the SQ collision age (∼5 × 10 6 years).(ii) Since the bulk kinetic energy of the H 2 gas is the dominant energy reservoir, we consider that the H 2 emission is powered by the dissipation of kinetic turbulent energy. We model this dissipation with non-dissociative MHD shocks and show that the H 2 excitation can be reproduced by a combination of low velocities shocks (5−20 km s −1 ) within dense (n H > 10 3 cm −3 ) H 2 gas. (iii) An efficient transfer of the bulk kinetic energy to turbulent motion of much lower velocities within molecular gas is required to make H 2 a dominant coolant of the postshock gas. We argue that this transfer is mediated by the dynamic interaction between gas phases and the thermal instability of the cooling gas. We quantify the mass and energy cycling between gas phases required to balance the dissipation of energy through the H 2 emission lines. Conclusions. This study provides a physical framework to interpret H 2 emission from H 2 -luminous galaxies. It highlights the role that H 2 formation and cooling play in dissipating mechanical energy released in galaxy collisions. This physical framework is of general relevance for the interpretation of observational signatures, in particular H 2 emission, of mechanical energy dissipation in multiphase gas.

We present a detailed analysis of the gas conditions in the H 2 luminous radio galaxy 3C 326 N at z ∼ 0.1, which has a low starformation rate (SFR ∼ 0.07 M yr −1 ) in spite of a gas surface density similar to those in starburst galaxies. Its star-formation efficiency is likely a factor ∼10−50 lower than those of ordinary star-forming galaxies. Combining new IRAM CO emission-line interferometry with existing Spitzer mid-infrared spectroscopy, we find that the luminosity ratio of CO and pure rotational H 2 line emission is factors 10−100 lower than what is usually found. This suggests that most of the molecular gas is warm. The Na D absorption-line profile of 3C 326 N in the optical suggests an outflow with a terminal velocity of ∼−1800 km s −1 and a mass outflow rate of 30−40 M yr −1 , which cannot be explained by star formation. The mechanical power implied by the wind, of order 10 43 erg s −1 , is comparable to the bolometric luminosity of the emission lines of ionized and molecular gas. To explain these observations, we propose a scenario where a small fraction of the mechanical energy of the radio jet is deposited in the interstellar medium of 3C 326 N, which powers the outflow, and the line emission through a mass, momentum and energy exchange between the different gas phases of the ISM. Dissipation times are of order 10 7−8 yrs, similar or greater than the typical jet lifetime. Small ratios of CO and PAH surface brightnesses in another 7 H 2 luminous radio galaxies suggest that a similar form of AGN feedback could be lowering star-formation efficiencies in these galaxies in a similar way. The local demographics of radio-loud AGN suggests that secular gas cooling in massive early-type galaxies of ≥10 11 M could generally be regulated through a fundamentally similar form of "maintenance-phase" AGN feedback.

We present results from the mid-infrared spectral mapping of Stephan's Quintet using the Spitzer Space Telescope 10 . A 1000 km s −1 collision (t col = 5 × 10 6 yr) has produced a group-wide shock and for the first time the large-scale distribution of warm molecular hydrogen emission is revealed, as well as its close association with known shock structures. In the main shock region alone we find 5.0 ×10 8 M ⊙ of warm H 2 spread over ∼ 480 kpc 2 and additionally report the discovery of a second major shock-excited H 2 feature, likely a remnant of previous tidal interactions. This brings the total H 2 line luminosity of the group in excess of 10 42 erg s −1 . In the main shock, the H 2 line luminosity exceeds, by a factor of three, the X-ray luminosity from the hot shocked gas, confirming that the H 2 -cooling pathway dominates over the X-ray. [Si ii]34.82µm emission, detected at a luminosity of 1/10th of that of the H 2 , appears to trace the group-wide shock closely and in addition, we detect weak [Fe ii]25.99µm emission from the most X-ray luminous part of the shock. Comparison with shock models reveals that this emission is consistent with regions of fast shocks (100 < V s < 300 km s −1 ) experiencing depletion of iron and silicon onto dust grains.Star formation in the shock (as traced via ionic lines, PAH and dust emission) appears in the intruder galaxy, but most strikingly at either end of the radio shock. The shock ridge itself shows little star formation, consistent with a model in which the tremendous H 2 power is driven by turbulent energy transfer from motions in a post-shocked layer which suppresses star formation. The significance of the molecular hydrogen lines over other measured sources of cooling in fast galaxy-scale shocks may have crucial implications for the cooling of gas in the assembly of the first galaxies.