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We report on the detection of a large, extended H i cloud complex in the Galaxy and Mass Survey G23 field, located at a redshift of z ∼ 0.03, observed as part of the MeerKAT Habitat of Galaxies Survey campaign (a pilot survey to explore the mosaicing capabilities of the MeerKAT telescope). The cloud complex, with a total mass of 1010.0M⊙, lies in proximity to a large galaxy group with Mdyn ∼ 1013.5M⊙. We identify seven H ɪ peak concentrations, interconnected as a tenuous chain structure, extending ∼400 kpc from east to west, with the largest (central) concentration containing 109.7M⊙ in H ɪ gas distributed across 50 kpc. The main source is not detected in ultraviolet, optical, or infrared imaging. The implied gas mass-to-light ratio (MH I/Lr) is extreme (>1000) even in comparison to other dark clouds. The complex has very little kinematic structure (110 km s−1), making it difficult to identify cloud rotation. Assuming pressure support, the total mass of the central concentration is > 1010.2M⊙, while a lower limit to the dynamical mass in the case of full rotational support is 1010.4M⊙. If the central concentration is a stable structure, it has to contain some amount of unseen matter, but potentially less than is observed for a typical galaxy. It is, however, not clear whether the structure has any gravitationally stable concentrations. We report a faint UV-optical-infrared source in proximity to one of the smaller concentrations in the gas complex, leading to a possible stellar association. The system nature and origins is enigmatic, potentially being the result of an interaction with or within the galaxy group it appears to be associated with. [ABSTRACT FROM AUTHOR]

We present the detections of shocked molecular hydrogen (H2) gas in near- and mid-infrared and broad CO in millimeter from the mixed-morphology supernova remnant (SNR) HB 3 (G132.7+1.3) using the Palomar Wide-field InfraRed Camera, the Spitzer GLIMPSE360 and Wide-field Infrared Survey Explorer (WISE) surveys, and the Heinrich Hertz Submillimeter Telescope. Our near-infrared narrow-band filter H2 2.12 μm images of HB 3 show that both Spitzer Infrared Array Camera and WISE 4.6 μm emission originates from shocked H2 gas. The morphology of H2 exhibits thin filamentary structures and a large scale of interaction sites between the HB 3 and nearby molecular clouds. Half of HB 3, the southern and eastern shell of the SNR, emits H2 in a shape of a butterfly or W, indicating the interaction sites between the SNR and dense molecular clouds. Interestingly, the H2 emitting region in the southeast is also co-spatial to the interacting area between HB 3 and the H ii regions of the W3 complex, where we identified star-forming activity. We further explore the interaction between HB 3 and dense molecular clouds with detections of broad CO(3-2) and CO(2-1) molecular lines from the southern and southeastern shell along the H2 emitting region. The widths of the broad lines are 8–20 km s−1; the detection of such broad lines is unambiguous, dynamic evidence of the interactions between the SNR and clouds. The CO broad lines are from two branches of the bright, southern H2 shell. We apply the Paris–Durham shock model to the CO line profiles, which infer the shock velocities of 20–40 km s−1, relatively low densities of 103–4 cm−3, and strong (>200 μG) magnetic fields. [ABSTRACT FROM AUTHOR]

We present a detailed study of emission-line systems in the Galaxy And Mass Assembly (GAMA) G23 region, making use of Wide-field Infrared Survey Explorer (WISE) photometry that includes carefully measured resolved sources. After applying several cuts to the initial catalog of ∼41,000 galaxies, we extract a sample of 9809 galaxies. We then compare the spectral diagnostic Baldwin, Philips & Terlevich (BPT) classification of 1154 emission-line galaxies (38% resolved in W1) to their location in the WISE color–color diagram, leading to the creation of a new zone for mid-infrared "warm" galaxies located 2σ above the star-forming sequence, below the standard WISE active galactic nucleus (AGN) region. We find that the BPT and WISE diagrams agree on the classification for 85% and 8% of the galaxies as non-AGN (star-forming = SF) and AGN, respectively, and disagree on ∼7% of the entire classified sample. Thirty-nine percent of the AGNs (all types) are broad-line systems for which the [N ii ] and [Hα] fluxes can barely be disentangled, giving in most cases spurious [N ii ]/[Hα] flux ratios. However, several optical AGNs appear to be completely consistent with SF in WISE. We argue that these could be low-power AGNs, or systems whose hosts dominate the IR emission. Alternatively, given the sometimes high [O iii ] luminosity in these galaxies, the emission lines may be generated by shocks coming from super-winds associated with SF rather than AGN activity. Based on our findings, we have created a new diagnostic: [W1 – W2] versus [N ii ]/[Hα], which has the virtue of separating SF from AGNs and high-excitation sources. It classifies 3 to ∼5 times more galaxies than the classic BPT. [ABSTRACT FROM AUTHOR]

Combining high-fidelity group characterization from the Galaxy and Mass Assembly survey and source-tailored z < 0.1 photometry from the Wide-Field Infrared Survey Explorer (WISE) survey, we present a comprehensive study of the properties of ungrouped galaxies, compared to 497 galaxy groups (4 ≤ NFoF ≤ 20) as a function of stellar and halo mass. Ungrouped galaxies are largely unimodal in WISE color, the result of being dominated by star-forming, late-type galaxies. Grouped galaxies, however, show a clear bimodality in WISE color, which correlates strongly with stellar mass and morphology. We find evidence for an increasing early-type fraction, in stellar mass bins between 1010 M⊙ ≲ Mstellar ≲ 1011M⊙, with increasing halo mass. Using ungrouped, late-type galaxies with star-forming colors (W2−W3 > 3), we define a star-forming main sequence (SFMS), which we use to delineate systems that have moved below the sequence ("quenched" for the purposes of this work). We find that with increasing halo mass, the relative number of late-type systems on the SFMS decreases, with a corresponding increase in early-type, quenched systems at high stellar mass (Mstellar > 1010.5M⊙), consistent with mass quenching. Group galaxies with masses Mstellar < 1010.5M⊙ show evidence of quenching consistent with environmentally driven processes. The stellar mass distribution of late-type, quenched galaxies suggests that it may be an intermediate population as systems transition from being star-forming and late-type to the "red sequence." Finally, we use the projected area of groups on the sky to extract groups that are (relatively) compact for their halo mass. Although these show a marginal increase in their proportion of high-mass and early-type galaxies compared to nominal groups, a clear increase in quenched fraction is not evident. [ABSTRACT FROM AUTHOR]

The Galaxy And Mass Assembly (GAMA) survey furnishes a deep redshift catalog that, when combined with the Wide-field Infrared Survey Explorer (WISE), allows us to explore for the first time the mid-infrared properties of >110, 000 galaxies over 120 deg2 to z ≃ 0.5. In this paper we detail the procedure for producing the matched GAMA-WISE catalog for the G12 and G15 fields, in particular characterizing and measuring resolved sources; the complete catalogs for all three GAMA equatorial fields will be made available through the GAMA public releases. The wealth of multiwavelength photometry and optical spectroscopy allows us to explore empirical relations between optically determined stellar mass (derived from synthetic stellar population models) and 3.4 μm and 4.6 μm WISE measurements. Similarly dust-corrected Hα-derived star formation rates can be compared to 12 μm and 22 μm luminosities to quantify correlations that can be applied to large samples to z < 0.5. To illustrate the applications of these relations, we use the 12 μm star formation prescription to investigate the behavior of specific star formation within the GAMA-WISE sample and underscore the ability of WISE to detect star-forming systems at z ∼ 0.5. Within galaxy groups (determined by a sophisticated friends-of-friends scheme), results suggest that galaxies with a neighbor within 100 h–1 kpc have, on average, lower specific star formation rates than typical GAMA galaxies with the same stellar mass. [ABSTRACT FROM AUTHOR]

We present radio continuum mapping, optical imaging, and spectroscopy of the newly discovered double-peaked, broad-lined active galactic nucleus (AGN) WISE J233237.05–505643.5 at redshift z = 0.3447. This source exhibits an FR-I and FR-II hybrid morphology, characterized by a bright core, jet, and Doppler-boosted lobe structures in Australian Telescope Compact Array continuum maps at 1.5, 5.6, and 9 GHz. Unlike most FR-II objects, W2332–5056 is hosted by a disk-like galaxy. The core has a projected 5″ linear radio feature that is perpendicular to the curved primary jet, hinting at unusual and complex activity within the inner 25 kpc. The multi-epoch, optical-near-IR photometric measurements indicate significant variability over a 3-20 yr baseline from the AGN component. Gemini South optical data show unusual double-peaked emission-line features: the centroids of the broad-lined components of Hα and Hβ are blueshifted with respect to the narrow lines and host galaxy by ∼3800 km s–1. We examine possible cases that involve single or double supermassive black holes in the system and discuss the required future investigations to disentangle the mysterious nature of this system. [ABSTRACT FROM AUTHOR]

We present the first Herschel spectroscopic detections of the [O I] 63 μm and [C II] 158 μm fine-structure transitions, and a single para-H2O line from the 35 × 15 kpc2 shocked intergalactic filament in Stephan's Quintet. The filament is believed to have been formed when a high-speed intruder to the group collided with a clumpy intergroup gas. Observations with the PACS spectrometer provide evidence for broad (>1000 km s–1) luminous [C II] line profiles, as well as fainter [O I] 63 μm emission. SPIRE FTS observations reveal water emission from the p-H2O (111-000) transition at several positions in the filament, but no other molecular lines. The H2O line is narrow and may be associated with denser intermediate-velocity gas experiencing the strongest shock-heating. The [C II]/PAHtot and [C II]/FIR ratios are too large to be explained by normal photo-electric heating in photodissociation regions. H II region excitation or X-ray/cosmic-ray heating can also be ruled out. The observations lead to the conclusion that a large fraction the molecular gas is diffuse and warm. We propose that the [C II], [O I], and warm H2 line emission is powered by a turbulent cascade in which kinetic energy from the galaxy collision with the intergalactic medium is dissipated to small scales and low velocities, via shocks and turbulent eddies. Low-velocity magnetic shocks can help explain both the [C II]/[O I] ratio, and the relatively high [C II]/H2 ratios observed. The discovery that [C II] emission can be enhanced, in large-scale turbulent regions in collisional environments, has implications for the interpretation of [C II] emission in high-z galaxies. [ABSTRACT FROM AUTHOR]

Stern et al. presented a study of Wide-field Infrared Survey Explorer (WISE) selection of active galactic nuclei (AGNs) in the 2 deg² COSMOS field, finding that a simple criterion W1-W2 ⩾ 0.8 provides a highly reliable and complete AGN sample for W2 < 15.05, where the W1 and W2 passbands are centered at 3.4μm and 4.6μm, respectively. Here we extend this study using the larger 9 deg² NOAO Deep Wide-Field Survey Boötes field which also has considerably deeper WISE observations than the COSMOS field, and find that this simple color cut significantly loses reliability at fainter fluxes. We define a modified selection criterion combining the W1-W2 color and the W2 magnitude to provide highly reliable or highly complete AGN samples for fainter WISE sources. In particular, we define a color-magnitude cut that finds 130 ± 4 deg-2 AGN candidates for W2 < 17.11 with 90% reliability. Using the extensive UV through mid-IR broadband photometry available in this field, we study the spectral energy distributions of WISE AGN candidates. We find that, as expected, the WISE AGN selection can identify highly obscured AGNs, but that it is biased toward objects where the AGN dominates the bolometric luminosity output. We study the distribution of reddening in the AGN sample and discuss a formalism to account for sample incompleteness based on the step-wise maximum-likelihood method of Efstathiou et al. The resulting dust obscuration distributions depend strongly on AGN luminosity, consistent with the trend expected for a receding torus. At LAGN ~ 3 × 1044 erg s-1, 29% ± 7% of AGNs are observed as Type 1, while at ~4 × 1045 erg s-1 the fraction is 64% ± 13%. The distribution of obscuration values suggests that dust in the torus is present as both a diffuse medium and in optically thick clouds. [ABSTRACT FROM AUTHOR]

We present results from a Spitzer mid-infrared spectroscopy study of a sample of 74 galaxies located in 23 Hickson Compact Groups (HCGs), chosen to be at a dynamically active stage of Hi depletion.We find evidence for enhanced warm H2 emission (i.e., above that associated with UV excitation in star-forming regions) in 14 galaxies (~20%), with 8 galaxies having extreme values of L(H2 S(0)-S(3))/L(7.7μm polycyclic aromatic hydrocarbon), in excess of 0.07. Such emission has been seen previously in the compact group HCG 92 (Stephan's Quintet), and was shown to be associated with the dissipation of mechanical energy associated with a large-scale shock caused when one group member collided, at high velocity, with tidal debris in the intragroup medium. Similarly, shock excitation or turbulent heating is likely responsible for the enhanced H2 emission in the compact group galaxies, since other sources of heating (UV or X-ray excitation from star formation or active galactic nuclei) are insufficient to account for the observed emission. The group galaxies fall predominantly in a region of mid-infrared color-color space identified by previous studies as being connected to rapid transformations in HCG galaxy evolution. Furthermore, the majority of H2-enhanced galaxies lie in the optical "green valley" between the blue cloud and red sequence, and are primarily early-type disk systems. We suggest that H2-enhanced systems may represent a specific phase in the evolution of galaxies in dense environments and provide new insight into mechanisms which transform galaxies onto the optical red sequence. [ABSTRACT FROM AUTHOR]

We cross-matched Wide-field Infrared Survey Explorer sources brighter than 1 mJy at 12 µm with the Sloan Digital Sky Survey galaxy spectroscopic catalog to produce a sample of ~105 galaxies at (z) = 0.08, the largest of its kind. This sample is dominated (70%) by star-forming (SF) galaxies from the blue sequence, with total IR luminosities in the range ~108-1012 Lʘ. We identify which stellar populations are responsible for most of the 12 µm emission. We find that most (~80%) of the 12 µm emission in SF galaxies is produced by stellar populations younger than 0.6 Gyr. In contrast, the 12 µm emission in weak active galactic nuclei (AGNs; L[OIII] < 1O7Lʘ) is produced by older stars, with ages of ~l-3 Gyr. We find that L12/im linearly correlates with stellar mass for SF galaxies. At fixed 12 µm luminosity, weak AGNs deviate toward higher masses since they tend to be hosted by massive, early-type galaxies with older stellar populations. SF galaxies and weak AGNs follow different L12µm-SFR (star formation rate) relations, with weak AGNs showing excess 12 µm emission at low SFR (0.02-1 Mʘ yr-1). This is likely due to dust grains heated by older stars. While the specific star formation rate (SSFR) of SF galaxies is nearly constant, the SSFR of weak AGNs decreases by ~3 orders of magnitude, reflecting the very different star formation efficiencies between SF galaxies and massive, early-type galaxies. Stronger type II AGNs in our sample (L[OIII] > 107 Lʘ), act as an extension of massive SF galaxies, connecting the SF and weak AGN sequences. This suggests a picture where galaxies form stars normally until an AGN (possibly after a starburst episode) starts to gradually quench the SF activity. We also find that 4.6-12 µm color is a useful first-order indicator of SF activity in a galaxy when no other data are available. [ABSTRACT FROM AUTHOR]

We have carried out a survey of the north and south ecliptic poles, EP-N and EP-S, respectively, with the Spitzer Space Telescope and the Wide-field Infrared Survey Explorer (WISE). The primary objective was to cross-calibrate WISE with the Spitzer and Midcourse Space Experiment (MSX) photometric systems by developing a set of calibration stars that are common to these infrared missions. The ecliptic poles were continuous viewing zones for WISE due to its polar-crossing orbit, making these areas ideal for both absolute and internal calibrations. The Spitzer IRAC and MIPS imaging survey covers a complete area of 0.40 deg² for the EP-N and 1.28 deg² for the EP-S. WISE observed the whole sky in four mid-infrared bands, 3.4, 4.6, 12, and 22 μm, during its eight-month cryogenic mission, including several hundred ecliptic polar passages; here we report on the highest coverage depths achieved by WISE, an area of ~1.5 deg² for both poles. Located close to the center of the EP-N, the Sy-2 galaxy NGC 6552 conveniently functions as a standard calibrator to measure the red response of the 22 μm channel of WISE. Observations from Spitzer-IRAC/MIPS/IRS-LL and WISE show that the galaxy has a strong red color in the mid-infrared due to star-formation and the presence of an active galactic nucleus (AGN), while over a baseline >1 year the mid-IR photometry of NGC 6552 is shown to vary at a level less than 2%. Combining NGC 6552 with the standard calibrator stars, the achieved photometric accuracy of the WISE calibration, relative to the Spitzer and MSX systems, is 2.4%, 2.8%, 4.5%, and 5.7% for W1 (3.4 μm), W2 (4.6 μm), W3 (12 μm), and W4 (22 μm), respectively. The WISE photometry is internally stable to better than 0.1% over the cryogenic lifetime of the mission. The secondary objective of the Spitzer-WISE Survey was to explore the poles at greater flux-level depths, exploiting the higher angular resolution Spitzer observations and the exceptionally deep (in total coverage) WISE observations that potentially reach down to the confusion limit of the survey. The rich Spitzer and WISE data sets were used to study the Galactic and extragalactic populations through source counts, color-magnitude and color-color diagrams. As an example of what the data sets facilitate, we have separated stars from galaxies, delineated normal galaxies from power-law-dominated AGNs, and reported on the different fractions of extragalactic populations. In the EP-N, we find an AGN source density of ~260 deg-2 to a 12 μm depth of 115 μJy, representing 15% of the total extragalactic population to this depth, similar to what has been observed for low-luminosity AGNs in other fields. [ABSTRACT FROM AUTHOR]

We present Atacama Large Millimeter/submillimeter Array (ALMA) 870 μm (345 GHz) data for 49 high-redshift (0.47 < z < 2.85), luminous (11.7 log 14. < < ( ) L L bol  2) radio-powerful active galactic nuclei (AGNs), obtained to constrain cool dust emission from starbursts concurrent with highly obscured radiative-mode black hole (BH) accretion in massive galaxies that possess a small radio jet. The sample was selected from the Wide-field Infrared Survey Explorer with extremely steep (red) mid-infrared colors and with compact radio emission from NVSS/FIRST. Twenty-six sources are detected at 870 μm, and we find that the sample has large mid- to farinfrared luminosity ratios, consistent with a dominant and highly obscured quasar. The rest-frame 3 GHz radio powers are 24.7 log W Hz 27.3 P3.0 GHz , 1 < < ( ) - and all sources are radio-intermediate or radio-loud. BH mass estimates are 7.7 < log(MBH/Me) < 10.2. The rest-frame 1–5 μm spectral energy distributions are very similar to the “Hot DOGs” (hot dust-obscured galaxies), and steeper (redder) than almost any other known extragalactic sources. ISM masses estimated for the ALMA-detected sources are 9.9 < log (MISM/Me) < 11.75 assuming a dust temperature of 30 K. The cool dust emission is consistent with star formation rates reaching several thousand Me yr−1, depending on the assumed dust temperature, but we cannot rule out the alternative that the AGN powers all the emission in some cases. Our best constrained source has radiative transfer solutions with approximately equal contributions from an obscured AGN and a young (10–15 Myr) compact starburst., Astronomy

We present [C II] and [O I] observations from Herschel and CO(1-0) maps from the Combined Array for Research in Millimeter Astronomy (CARMA) of the Hickson compact group HCG 57, focusing on the galaxies HCG 57a and HCG 57d. HCG 57a has been previously shown to contain enhanced quantities of warm molecular hydrogen consistent with shock or turbulent heating. Our observations show that HCG 57d has strong [C II] emission compared to LFIR and weak CO(1-0), while in HCG 57a, both the [C II] and CO(1-0) are strong. HCG 57a lies at the upper end of the normal distribution of the [C II]/CO and [C II]/FIR ratios, and its far-infrared (FIR) cooling supports a low-density, warm, diffuse gas that falls close to the boundary of acceptable models of a photon-dominated region. However, the power radiated in the [C II] and warm H2 emissions have similar magnitudes, as seen in other shock-dominated systems and predicted by recent models. We suggest that shock heating of the [C II] is a viable alternative to photoelectric heating in violently disturbed, diffuse gas. The existence of shocks is also consistent with the peculiar CO kinematics in the galaxy, indicating that highly noncircular motions are present. These kinematically disturbed CO regions also show evidence of suppressed star formation, falling a factor of 10-30 below normal galaxies on the Kennicutt-Schmidt relation. We suggest that the peculiar properties of both galaxies are consistent with a highly dissipative, off-center collisional encounter between HCG 57d and 57a, creating ring-like morphologies in both systems. Highly dissipative gas-on-gas collisions may be more common in dense groups because of the likelihood of repeated multiple encounters. The possibility of shock-induced star-formation suppression may explain why a subset of these HCG galaxies has been found previously to fall in the mid-infrared green valley. [ABSTRACT FROM AUTHOR]

In order to explore local large-scale structures and velocity fields, accurate galaxy distance measures are needed. We now extend the well-tested recipe for calibrating the correlation between galaxy rotation rates and luminosities—capable of providing such distance measures—to the all-sky, space-based imaging data from the Wide-field Infrared Survey Explorer (WISE) W1 (3.4 μm) and W2 (4.6 μm) filters. We find a correlation of line width to absolute magnitude (known as the Tully-Fisher relation, TFR) of (0.54 mag rms) and (0.56 mag rms) from 310 galaxies in 13 clusters. We update the I-band TFR using a sample 9% larger than in Tully & Courtois. We derive (0.46 mag rms). The WISE TFRs show evidence of curvature. Quadratic fits give (0.52 mag rms) and (0.55 mag rms). We apply an I-band –WISE color correction to lower the scatter and derive and (both 0.46 mag rms). Using our three independent TFRs (W1 curved, W2 curved, and I band), we calibrate the UNION2 Type Ia supernova sample distance scale and derive H0 = 74.4 ± 1.4(stat) ± 2.4(sys) km s–1 Mpc–1 with 4% total error. [ABSTRACT FROM AUTHOR]

We present 33 GHz photometry of 103 galaxy nuclei and extranuclear star-forming complexes taken with the Green Bank Telescope as part of the Star Formation in Radio Survey. Among the sources without evidence for an active galactic nucleus, and also having lower frequency radio data, we find a median thermal fraction at 33 GHz of ≈76% with a dispersion of ≈24%. For all sources resolved on scales ≲0.5 kpc, the thermal fraction is even larger, being ≳90%. This suggests that the rest-frame 33 GHz emission provides a sensitive measure of the ionizing photon rate from young star-forming regions, thus making it a robust star formation rate (SFR) indicator. Taking the 33 GHz SFRs as a reference, we investigate other empirical calibrations relying on different combinations of warm 24 μm dust, total infrared (IR; 8-1000 μm), Hα line, and far-UV continuum emission. The recipes derived here generally agree with others found in the literature, albeit with a large dispersion that most likely stems from a combination of effects. Comparing the 33 GHz to total IR flux ratios as a function of the radio spectral index, measured between 1.7 and 33 GHz, we find that the ratio increases as the radio spectral index flattens which does not appear to be a distance effect. Consequently, the ratio of non-thermal to total IR emission appears relatively constant, suggesting only moderate variations in the cosmic-ray electron injection spectrum and ratio of synchrotron to total cooling processes among star-forming complexes. Assuming that this trend solely arises from an increase in the thermal fraction sets a maximum on the scatter of the non-thermal spectral indices among the star-forming regions of . [ABSTRACT FROM AUTHOR]

The Wide-field Infrared Survey Explorer (WISE) is an extremely capable and efficient black hole finder. We present a simple mid-infrared color criterion, W1 – W2 ⩾ 0.8 (i.e., [3.4]–[4.6] ⩾0.8, Vega), which identifies 61.9 ± 5.4 active galactic nucleus (AGN) candidates per deg2 to a depth of W2 ∼ 15.0. This implies a much larger census of luminous AGNs than found by typical wide-area surveys, attributable to the fact that mid-infrared selection identifies both unobscured (type 1) and obscured (type 2) AGNs. Optical and soft X-ray surveys alone are highly biased toward only unobscured AGNs, while this simple WISE selection likely identifies even heavily obscured, Compton-thick AGNs. Using deep, public data in the COSMOS field, we explore the properties of WISE-selected AGN candidates. At the mid-infrared depth considered, 160 μJy at 4.6 μm, this simple criterion identifies 78% of Spitzer mid-infrared AGN candidates according to the criteria of Stern et al. and the reliability is 95%. We explore the demographics, multiwavelength properties and redshift distribution of WISE-selected AGN candidates in the COSMOS field. [ABSTRACT FROM AUTHOR]

We report the detection of strong, resolved emission from warm H2 in the Taffy galaxies and bridge. Relative to the continuum and faint polyclic aromatic hydrocarbon (PAH) emission, the H2 emission is the strongest in the connecting bridge, approaching L(H2)/L(PAH 8 μm) = 0.1 between the two galaxies, where the purely rotational lines of H2 dominate the mid-infrared spectrum in a way very reminiscent of the group-wide shock in the interacting group Stephan's Quintet (SQ). The surface brightness in the 0-0 S(0) and S(1) H2 lines in the bridge is more than twice that observed at the center of the SQ shock. We observe a warm H2 mass of 4.2 × 108M☼ in the bridge, but taking into account the unobserved bridge area, the total warm mass is likely to be twice this value. We use excitation diagrams to characterize the warm molecular gas, finding an average surface mass of ∼5 × 106M☼ kpc–2 and typical excitation temperatures of 150-175 K. H2 emission is also seen in the galaxy disks, although there the emission is more consistent with normal star-forming galaxies. We investigate several possible heating mechanisms for the bridge gas but favor the conversion of kinetic energy from the head-on collision via turbulence and shocks as the main heating source. Since the cooling time for the warm H2 is short (∼5000 yr), shocks must be permeating the molecular gas in the bridge region in order to continue heating the H2. [ABSTRACT FROM AUTHOR]