[en] 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 × 10⁸ M_☉ 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 × 10⁶ M_☉ 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.

[en] HIZOA J0836-43 is the most H I-massive (M_HI = 7.5 x 10¹⁰ M_sun) galaxy detected in the HIPASS volume (δ = -90⁰ to + 25⁰, v<12,700 km s^-1) and lies optically hidden behind the Milky Way. Markedly different from other extreme H I disks in the local universe, it is a luminous infrared galaxy (LIRG) with an actively star-forming disk (>50 kpc), central to its ∼130 kpc gas disk, with a total star formation rate (SFR) of ∼20.5 M_sun yr^-1. Spitzer spectroscopy reveals an unusual combination of powerful polycyclic aromatic hydrocarbon (PAH) emission coupled to a relatively weak warm dust continuum, suggesting photodissociation-region-dominated emission. Compared to a typical LIRG with similar total infrared luminosity (L_TIR = 10¹¹ L_sun), the PAHs in HIZOA J0836-43 are more than twice as strong, whereas the warm dust continuum (λ>20 μm) is best fit by a star-forming galaxy with L_TIR = 10¹⁰ L_sun. Mopra CO observations suggest an extended molecular gas component (H₂ + He>3.7 x 10⁹ M_sun) and a lower limit of ∼64% for the gas-mass fraction; this is above average compared to local disk systems, but similar to that of z ∼ 1.5 BzK galaxies (∼57%). However, the star formation efficiency (SFE = L_IR/L'_CO) for HIZOA J0836-43 of 140 L_sun (K km s^-1 pc²)^-1 is similar to that of local spirals and other disk galaxies at high redshift, in strong contrast to the increased SFE seen in merging and strongly interacting systems. HIZOA J0836-43 is actively forming stars and building a massive stellar disk. Its evolutionary phase of star formation (M_stellar, SFR, and gas fraction) compared to more distant systems suggests that it would be considered typical at redshift z ∼ 1. This galaxy provides a rare opportunity in the nearby universe for studying (at z ∼ 0.036) how disks were building and galaxies evolving at z ∼ 1, when similarly large gas fractions were likely more common.

[en] The Stephan's Quintet (hereafter SQ) is a template source to study the impact of galaxies interaction on the physical state and energetics of their gas. We report on IRAM single-dish CO observations of the SQ compact group of galaxies. These observations follow up the Spitzer discovery of bright mid-IR H₂ rotational line emission (L(H₂) ≈ 10³⁵ W) from warm (10^2–3 K) molecular gas, associated with a 30 kpc long shock between a galaxy, NGC 7318b, and NGC 7319's tidal arm. We detect CO(1-0), (2-1) and (3-2) line emission in the inter-galactic medium (IGM) with complex profiles, spanning a velocity range of ≈1000 km s^–1. The spectra exhibit the pre-shock recession velocities of the two colliding gas systems (5700 and 6700 km s^–1), but also intermediate velocities. This shows that much of the molecular gas has formed out of diffuse gas accelerated by the galaxy-tidal arm collision. CO emission is also detected in a bridge feature that connects the shock to the Seyfert member of the group, NGC 7319, and in the northern star forming region, SQ-A, where a new velocity component is identified at 6900 km s^–1, in addition to the two velocity components already known. Assuming a Galactic CO(1-0) emission to H₂ mass conversion factor, a total H₂ mass of ≈5 × 10⁹ M_☉ is detected in the shock. The ratio between the warm H₂ mass derived from Spitzer spectroscopy, and the H₂ mass derived from CO fluxes is ≈0.3 in the IGM of SQ, which is 10--100 times higher than in star-forming galaxies. The molecular gas carries a large fraction of the gas kinetic energy involved in the collision, meaning that this energy has not been thermalized yet. The kinetic energy of the H₂ gas derived from CO observations is comparable to that of the warm H₂ gas from Spitzer spectroscopy, and a factor ≈5 greater than the thermal energy of the hot plasma heated by the collision. In the shock and bridge regions, the ratio of the PAH-to-CO surface luminosities, commonly used to measure the star formation efficiency of the H₂ gas, is lower (up to a factor 75) than the observed values in star-forming galaxies. We suggest that turbulence fed by the galaxy-tidal arm collision maintains a high heating rate within the H₂ gas. This interpretation implies that the velocity dispersion on the scale of giant molecular clouds in SQ is one order of magnitude larger than the Galactic value. The high amplitude of turbulence may explain why this gas is not forming stars efficiently.

[en] We present results from the mid-infrared spectral mapping of Stephan's Quintet using the Spitzer Space Telescope. A 1000 km s^-1 collision (t_col = 5 x 10⁶ 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 x 10⁸ M_sun of warm H₂ spread over ∼480 kpc² and additionally report the discovery of a second major shock-excited H₂ feature, likely a remnant of previous tidal interactions. This brings the total H₂ line luminosity of the group in excess of 10⁴² erg s^-1. In the main shock, the H₂ line luminosity exceeds, by a factor of 3, the X-ray luminosity from the hot shocked gas, confirming that the H₂-cooling pathway dominates over the X-ray. [Si II]34.82 μm emission, detected at a luminosity of 1/10th of that of the H₂, appears to trace the group-wide shock closely, and in addition, we detect weak [Fe II]25.99 μm 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 km s^-1 < 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, polycyclic aromatic hydrocarbon 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 H₂ 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.

[en] 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 ≤ N _FoF ≤ 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 _⊙ ≲ M _stellar ≲ 10¹¹ M _⊙, 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 (M _stellar > 10^10.5 M _⊙), consistent with mass quenching. Group galaxies with masses M _stellar < 10^10.5 M _⊙ 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.

[en] We present a Gemini-GMOS spectroscopic study of Hubble Space Telescope (HST)-selected Hα-emitting regions in Stephan's Quintet (HCG 92), a nearby compact galaxy group, with the aim of disentangling the processes of shock-induced heating and star formation in its intra-group medium. The ≈40 sources are distributed across the system, but most densely concentrated in the ∼kiloparsec-long shock region. Their spectra neatly divide them into narrow- and broad-line emitters, and we decompose the latter into three or more emission peaks corresponding to spatial elements discernible in HST imaging. The emission-line ratios of the two populations of Hα-emitters confirm their nature as H II regions (90% of the sample) or molecular gas heated by a shock front propagating at ≲300 km s^–1. Their redshift distribution reveals interesting three-dimensional structure with respect to gas-phase baryons, with no H II regions associated with shocked gas, no shocked regions in the intruder galaxy NGC 7318B, and a sharp boundary between shocks and star formation. We conclude that star formation is inhibited substantially, if not entirely, in the shock region. Attributing those H II regions projected against the shock to the intruder, we find a lopsided distribution of star formation in this galaxy, reminiscent of pileup regions in models of interacting galaxies. The Hα luminosities imply mass outputs, star formation rates, and efficiencies similar to nearby star-forming regions. Two large knots are an exception to this, being comparable in stellar output to the prolific 30 Doradus region. We also examine Stephan's Quintet in the context of compact galaxy group evolution, as a paradigm for intermittent star formation histories in the presence of a rich, X-ray-emitting intra-group medium. All spectra are provided as supplemental materials.

[en] We present a combined optical and X-ray analysis of the rich cluster ABELL 1882 (A1882) with the aim of identifying merging substructure and understanding the recent assembly history of this system. Our optical data consist of spectra drawn from the Galaxy and Mass Assembly survey, which lends itself to this kind of detailed study thanks to its depth and high spectroscopic completeness. We use 283 spectroscopically confirmed cluster members to detect and characterize substructure. We complement the optical data with X-ray data taken with both Chandra and XMM. Our analysis reveals that A1882 harbors two main components, A1882A and A1882B, which have a projected separation of ∼2 Mpc and a line of sight velocity difference of v_los∼-428⁺¹⁸⁷_-139 km s^–1. The primary system, A1882A, has velocity dispersion σ_v=500_-26⁺²³ km s^–1 and Chandra (XMM) temperature kT = 3.57 ± 0.17 keV (3.31^+0.28_-0.27 keV) while the secondary, A1882B, has σ_v=457⁺¹⁰⁸_-101 km s^–1 and Chandra (XMM) temperature kT = 2.39 ± 0.28 keV (2.12 ± 0.20 keV). The optical and X-ray estimates for the masses of the two systems are consistent within the uncertainties and indicate that there is twice as much mass in A1882A (M₅₀₀ = 1.5-1.9 × 10¹⁴ M_☉) when compared with A1882B (M₅₀₀ = 0.8-1.0 × 10¹⁴ M_☉). We interpret the A1882A/A1882B system as being observed prior to a core passage. Supporting this interpretation is the large projected separation of A1882A and A1882B and the dearth of evidence for a recent (<2 Gyr) major interaction in the X-ray data. Two-body analyses indicate that A1882A and A1882B form a bound system with bound incoming solutions strongly favored. We compute blue fractions of f_b = 0.28 ± 0.09 and 0.18 ± 0.07 for the spectroscopically confirmed member galaxies within r₅₀₀ of the centers of A1882A and A1882B, respectively. These blue fractions do not differ significantly from the blue fraction measured from an ensemble of 20 clusters with similar mass and redshift

[en] 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]/PAH_tot 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

[en] 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.

[en] 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 H I 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(7.7 μm 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.