Radio recombination lines (RRLs) at low frequencies (<10 GHz) can provide a multi-phase view of interstellar gas in nearby galaxies, absorption-line-systems, and AGN. Hydrogen RRLs arise in fully ionized gas and carbon RRLs trace elusive cold-HI and CO-dark molecular gas. Low frequency RRLs are typically stimulated by the radio continuum and thus may be observable within or against radio bright sources out to cosmological distances (z 6). Although long sought after, RRLs were only recently detected outside of the local universe (z 1; Emig et al., 2020, 2023). Such detections have been made possible by the advancement of wide-bandwidth spectral-line surveys on next-generation low-frequency telescopes. Precursors and pathfinders to the SKA have opened up this field of research and will make significant advancements over the next years by enabling surveys over large source samples. The SKA will provide access to the crucial frequency ranges where RRL line intensity is brightest. Furthermore, multi-band SKA measurements will fully characterize gas physical conditions. Key extragalactic science of low frequency RRLs will focus on (i) the conversion of baryonic material into stars across cosmic time, (ii) the evolution of the ISM and its physical conditions in galaxies, and (iii) how gas drives and inhibits AGN activity.

We report the results of a pilot study that searched for dual active galactic nuclei (AGN) in local ( z < 0.25) galaxies hosting double-peaked narrow emission lines in their optical spectra. We present high-resolution L -band (1.5 GHz or 18 cm) continuum images from the Very Long Baseline Array as well as WFC3/IR F160W images from the Hubble Space Telescope of two candidate dual AGN systems: J0948+6848 and J1223+5409. In both targets, we detected compact nonthermal radio emission that is approximately cospatial with the near-infrared AGN. Both systems host two high brightness temperature (>10 ^8 K) radio sources that indicate the presence of either a parsec-scale-separation dual AGN ( d _sep ∼ 90 and ∼56 pc, respectively) or a radio jet. Matched-resolution multiband radio observations are necessary to further characterize the AGN activity in these systems.

We present the discovery of seven molecular clouds in the radio galaxy B2 0902+34 at redshift z = 3.4. These clouds are detected as CO(0-1) absorption features against the bright radio continuum and spectrally resolved using the Karl G. Jansky Very Large Array. The velocity dispersion of the individual absorption components ranges from 3 to 7 km s-1, which is similar to values observed for molecular clouds in the Milky Way and nearby galaxies and implies cloud radii of R similar to 101-2 pc. The absorbing clouds are found in a region of high obscuration inside a 30 kpc wide stellar nebula, as revealed by rest-frame near-ultraviolet imaging performed with the Hubble Space Telescope. The fact that we spectrally resolve molecular clouds at the onset of Cosmic Noon opens prospects for studying cloud chemistry and physics that drive the formation of stars in the early Universe.

We report ALMA and ACA observations of atomic carbon ([C1](1-0)) and dust continuum in 10 Enormous Lyα Nebulae hosting ultra-luminous Type-I QSOs at z=2.2-2.5, as part of the SUrvey of Protocluster ELANe Revealing CO/CI in the Lyα Detected CGM (SUPERCOLD-CGM). We detect [C1](1-0) and dust in all ten QSOs and five companion galaxies. We find that the QSOs and companions have higher gas densities and more intense radiation fields than Luminous Infrared galaxies and high-z main sequence galaxies, with the highest values found in the QSOs. By comparing molecular gas masses derived from [C1](1-0), CO(4-3) and dust continuum, we find that the QSOs and companions display a similar low CO conversion factor of α_ CO ∼ 0.8 M_[ K km/s pc^2]^-1. After tapering our data to low resolution, the [C1](1-0) flux increases for nine QSOs, hinting at the possibility of [C1](1-0) in the circum-galactic medium (CGM) on a scale of 16-40 kpc. However, the [C1](1-0) sensitivity is too low to confirm this for individual targets, except for a tentative (2.7σ) CGM detection in Q0050+0051 with M_ H_2 = (1.0 - 2.8)× 10^10 M_. The 3σ mass limits of molecular CGM for the remaining QSO fields are (0.2-1.4) × 10^10 M_. This translates into a baryon fraction of <0.4-3% in the molecular CGM relative to the total baryonic halo mass. Our sample also includes a radio-detected AGN, Q1416+2649, which shows [C1](1-0) and CO(4-3) luminosities an order of magnitude fainter for its far-infrared luminosity than other QSOs in our sample, possibly due to a lower molecular gas mass.

We present the large-scale distribution and kinematics of cold molecular gas across the compact galaxy group Stephan's Quintet, based on CO(2-1) observations performed with the Atacama Compact Array (ACA) and CO(1-0) data from the Combined Array for Research in Millimeter-wave Astronomy (CARMA). We find coherent structures of molecular gas associated with the galaxies and intragroup medium, which follow the distribution of warm H2 previously seen with the James Webb Space Telescope (JWST). CO is associated with a ridge of shocked gas that crosses the galaxy group, and with a spiral arm of the intruding galaxy NGC 7318b, which interacts with the intragroup medium along the ridge. Although the ridge contains widespread shocks, turbulent gas, and warm H2, the CO lines are narrower than elsewhere in Stephan's Quintet (FWHM similar to 25-65 km s-1), indicative of settled cold gas. At a distinctly different velocity, CO is found in the active galaxy NGC 7319 and northern star-forming region SQ-A. A bridge of turbulent molecular gas connects NGC 7319 with the ridge, covering a gap of similar to 700 km s-1 between these structures. The gas excitation ranges from LCO(2-1 ' / LCO(1-0 ' similar to 0.3 in the bridge and SQ-A, to similar to 0.5 along the ridge, to near unity in the center of NGC 7319. We also detect either a molecular outflow or turbulent molecular gas associated with the radio source in NGC 7319. These ACA data are part of a program with the Atacama Large Millimeter/submillimeter Array and JWST to study the physics of molecular gas from the largest to the smallest scales across the intragroup medium of Stephan's Quintet.

We obtain CO(1-0) molecular gas measurements with the Australia Telescope Compact Array on a sample of 43 spectroscopically confirmed H alpha emitters in the Spiderweb protocluster at z = 2.16 and investigate the relation between their star formation activities and cold gas reservoirs as a function of environment. We achieve a CO(1-0) detection rate of similar to 23 +/- 12% with ten dual CO(1-0) and H alpha detections within our sample at 10 < log M-*/M-circle dot < 11.5. In addition, we obtain upper limits for the remaining sources. In terms of total gas fractions (F-gas), we find our sample is divided into two different regimes mediated by a steep transition at log M-*/M-circle dot approximate to 10.5. Galaxies below that threshold have gas fractions that in some cases are close to unity, indicating that their gas reservoir has been replenished by inflows from the cosmic web. However, objects at log M-*/M-circle dot > 10.5 display significantly lower gas fractions than their lower stellar mass counterparts and are dominated (12 out of 20) by objects hosting an active galactic nucleus (AGN). Stacking results yield F-gas approximate to 0.55 for massive emitters excluding AGN, and F-gas approximate to 0.35 when examining only AGN candidates. Furthermore, depletion times of our sample show that most H alpha emitters at z = 2.16 will become passive by 1 < z < 1.6, concurrently with the surge and dominance of the red sequence in the most massive clusters. Our environmental analyses suggest that galaxies residing in the outskirts of the protocluster have larger molecular-to-stellar mass ratios and lower star formation efficiencies than galaxies residing in the core. However, star formation across the protocluster structure remains consistent with the main sequence, indicating that galaxy evolution is primarily driven by the depletion of the gas reservoir towards the inner regions. We discuss the relative importance of inflow and outflow processes in regulating star formation during the early phases of cluster assembly and conclude that a combination of feedback and overconsumption may be responsible for the rapid cold gas depletion these objects endure.

Using the Karl G. Jansky Very Large Array (VLA), we have detected absorption lines due to carbon-monoxide, CO(J=0-1), and the cyano radical, CN(N=0-1), associated with radio galaxy B2 0902+34 at redshift z=3.4. The detection of millimeter-band absorption observed 1.5 Gyr after the Big Bang facilitates studying molecular clouds down to gas masses inaccessible to emission-line observations. The CO absorption in B2 0902+34 has a peak optical depth of τ ≥ 8.6 redshift as previously detected 21-cm absorption of neutral hydrogen (HI) gas. Each CO component traces an integrated H_2 column density of N(H2) ≥ 3x10^20 cm^-2. CN absorption is detected for both CO components, as well as for a blueshifted component not detected in CO, with CO/CN line ratios ranging from ≤0.4 to 2.4. We discuss the scenario that the absorption components originate from collections of small and dense molecular clouds that are embedded in a region with more diffuse gas and high turbulence, possibly within the influence of the central Active Galactic Nucleus or starburst region. The degree of reddening in B2 0902+34, with a rest-frame color B-K   4.2, is lower than the very red colors (B-K > 6) found among other known redshifted CO absorption systems at z<1. Nevertheless, when including also the many non-detections from the literature, a potential correlation between the absorption-line strength and B-K color is evident, giving weight to the argument that the red colors of CO absorbers are due to a high dust content.

We present continuum observations from the Atacama Large Millimeter/submillimeter Array of 10 high-redshift (2.2 ≤ z ≤ 2.7) ultraluminous quasars (QSOs) and constrain the presence of hot, ionized, circumgalactic gas in a stacking analysis. We measure a Compton- y parameter profile with a peak value of (1.7 ± 1.1) × 10 ^−6 at a radius of ∼50 kpc. We compare our stacked observations to active galactic nucleus feedback wind models and generalized Navarro–Frenk–White pressure profile models to constrain the wind luminosity and halo mass of the stacked QSOs. Our observations constrain the observed average halo mass to M _500 < 1 × 10 ^13 M _⊙ and the average feedback wind power <1 × 10 ^12 L _⊙ , which is <1% of the bolometric luminosity of the quasar.

We report the results of the deep and wide Atacama Large Millimeter/submillimeter Array (ALMA) 1.2 mm mapping of the Spiderweb protocluster at z=2.16. The observations were divided into six contiguous fields covering a survey area of 19.3 arcmin(2). With similar to 13h of on-source time, the final maps in the six fields reach the 1 sigma rms noise in a range of 40.3-57.1 mu Jy at a spatial resolution of 0 ''.5-0 ''.9. By using different source extraction codes and careful visual inspection, we detected 47 ALMA sources at a significance higher than 4 sigma. We constructed the differential and cumulative number counts down to similar to 0.2 mJy after the correction for purity and completeness obtained from Monte Carlo simulations. The ALMA 1.2 mm number counts of dusty star-forming galaxies (DSFGs) in the Spiderweb protocluster are overall two times that of general fields, with some regions showing even higher overdensities (more than a factor of three). This is consistent with the results from previous studies over a larger scale using single-dish instruments. Comparison of the spatial distributions between different populations indicates that our ALMA sources are likely drawn from the same distribution as CO(1-0) emitters from the COALAS large program but are distinct from that of H alpha emitters. The cosmic star formation rate density of the ALMA sources is consistent with previous results (e.g., LABOCA 870 mu m observations) after accounting for the difference in volume. We show that molecular gas masses estimates from dust measurements are not consistent with the ones derived from CO(1-0) and thus have to be taken with caution. The multiplicity fraction of single-dish DSFGs is higher than that of the field. Moreover, two extreme concentrations of ALMA sources were found on the outskirts of the Spiderweb protocluster, with an excess of more than 12 times that of the general fields. These results indicate that the ALMA-detected DSFGs are supplied through gas accretion along filaments and are triggered by intense star formation by accretion shocks before falling into the cluster center. The identified two galaxy groups are likely falling into the protocluster center and will trigger new merger events eventually, as indicated in simulations.

The bright radio source, GLEAM J091734-001243 (hereafter GLEAM J0917-0012), was previously selected as a candidate ultra-high redshift (z>5) radio galaxy due to its compact radio size and faint magnitude (K(AB)=22.7). Its redshift was not conclusively determined from follow-up millimetre and near-infrared spectroscopy. Here we present new HST WFC3 G141 grism observations which reveal several emission lines including [NeIII]3867, [NeV]3426 and an extended (~4.8 kpc), [OII]3727 line which confirm a redshift of 3.004+/-0.001. The extended component of the [OII]3727 line is co-spatial with one of two components seen at 2.276 GHz in high resolution (60x20 mas) Long Baseline Array data, reminiscent of the alignments seen in local compact radio galaxies. The BEAGLE stellar mass (~2x10^11 Msun) and radio luminosity (L_500MHz}~10^28 W Hz^-1) put GLEAM J0917-0012 within the distribution of the brightest high-redshift radio galaxies at similar redshifts. However, it is more compact than all of them. Modelling of the radio jet demonstrates that this is a young, ~50 kyr old, but powerful, 10^39 W, compact steep spectrum radio source. The weak constraint on the active galactic nucleus bolometric luminosity from the [NeV]3426 line combined with the modelled jet power tentatively implies a large black hole mass, >10^9 Msun, and a low, advection-dominated accretion rate, an Eddington ratio <0.03. The [NeV]3426/[NeIII]3867 vs [OII]3727/[NeIII]3867 line ratios are most easily explained by radiative shock models with precursor photoionisation. Hence, we infer that the line emission is directly caused by the shocks from the jet and that this radio source is one of the youngest and most powerful known at cosmic noon. We speculate that the star-formation in GLEAM J0917-0012 could be on its way to becoming quenched by the jet.

Our knowledge of galaxy formation and evolution has incredibly progressed through multi-wavelength observational constraints of the interstellar medium (ISM) of galaxies at all cosmic epochs. However, little is known about the physical properties of the more diffuse and lower surface brightness reservoir of gas and dust that extends beyond ISM scales and fills dark matter haloes of galaxies up to their virial radii, the circumgalactic medium (CGM). New theoretical studies increasingly stress the relevance of the latter for understanding the feedback and feeding mechanisms that shape galaxies across cosmic times, whose cumulative effects leave clear imprints into the CGM. Recent studies are showing that a – so far unconstrained – fraction of the CGM mass may reside in the cold (T < 104 K) molecular and atomic phase, especially in high-redshift dense environments. These gas phases, together with the warmer ionised phase, can be studied in galaxies from z ∼ 0 to z ∼ 10 through bright far-infrared and sub-millimeter emission lines such as [C ii] 158µm, [O iii] 88 µm, [C I] 609µm, [C i] 370µm, and the rotational transitions of CO. Imaging such hidden cold CGM can lead to a breakthrough in galaxy evolution studies but requires a new facility with the specifications of the proposed Atacama Large Aperture Submillimeter Telescope (AtLAST). In this paper, we use theoretical and empirical arguments to motivate future ambitious CGM observations with AtLAST and describe the technical requirements needed for the telescope and its instrumentation to perform such science.

We report Atacama Large Millimeter/submillimeter Array and Atacama Compact Array observations of atomic carbon ([C I](1-0)) and dust continuum in 10 enormous Ly alpha nebulae hosting ultraluminous Type-I QSOs at z = 2.2-2.5, as part of the Survey of Protocluster ELANe Revealing CO/C I in the Ly alpha Detected CGM. We detect [C I](1-0) and dust in all 10 QSOs and five companion galaxies. We find that the QSOs and companions have higher gas densities and more intense radiation fields than Luminous Infrared galaxies and high-z main sequence galaxies, with the highest values found in the QSOs. By comparing molecular gas masses derived from [C I](1-0), CO(4-3), and dust continuum, we find that the QSOs and companions display a similar low CO conversion factor of alpha(CO) similar to 0.8 M-circle dot[Kkms-1pc(2)](-1). After tapering our data to low resolution, the [C I](1-0) flux increases for nine QSOs, hinting at the possibility of [C I](1-0) in the circumgalactic medium (CGM) on a scale of 16-40 kpc. However, the [C I](1-0) sensitivity is too low to confirm this for individual targets, except for a tentative (2.7 sigma) CGM detection in Q0050+0051 with M-H2 = (1.0-2.8) x 10(10)M(circle dot). The 3 sigma mass limits of molecular CGM for the remaining QSO fields are (0.2-1.4) x 10(10)M(circle dot). This translates into a baryon fraction of <0.4%-3% in the molecular CGM relative to the total baryonic halo mass. Our sample also includes a radio-detected active galactic nuclei, Q1416+2649, which shows [C I](1-0) and CO(4-3) luminosities an order of magnitude fainter for its far-infrared luminosity than other QSOs in our sample, possibly due to a lower molecular gas mass.

The jets of radio AGN provide one of the most important forms of active galactic nuclei (AGN) feedback, yet considerable uncertainties remain about how they are triggered. Since the molecular gas reservoirs of the host galaxies can supply key information about the dominant triggering mechanism(s), here we present Atacama Large Millimeter/sub-millimeter Array CO(1-0) observations of a complete sample of 29 powerful radio AGN (P-1.4GHz>10(25) W Hz(-1) and 0.05 < z < 0.3) with an angular resolution of about 2-3 arcsec (corresponding to 2-8 kpc). We detect molecular gas with masses in the range 10(8.9)<M-H2<10(10.2) M circle dot in the early-type host galaxies of ten targets, while for the other 19 sources, we derive upper limits. The detection rate of objects with such large molecular masses - 34 +/- 9 per cent - is higher than in the general population of non-active early-type galaxies (ETGs: <10 per cent). The kinematics of the molecular gas are dominated in most cases by rotating disc-like structures, with diameters up to 25 kpc. Compared with the results for samples of quiescent ETG in the literature, we find a larger fraction of more massive, more extended and less settled molecular gas structures. In most of the CO-detected sources, the results are consistent with triggering of the AGN as the gas settles following a merger or close encounter with a gas-rich companion. However, in a minority of objects at the centres of rich clusters of galaxies, the accretion of gas cooling from the hot X-ray haloes is a plausible alternative to galaxy interactions as a triggering mechanism.

Submillimeter and millimeter wavelengths provide a unique view of the Universe, from the gas and dust that fills and surrounds galaxies to the chromosphere of our own Sun. Current single-dish facilities have presented a tantalising view of the brightest (sub-)mm sources, and interferometers have provided the exquisite resolution necessary to analyse the details in small fields, but there are still many open questions that cannot be answered with current facilities. In this report we summarise the science that is guiding the design of the Atacama Large Aperture Submillimeter Telescope (AtLAST). We demonstrate how tranformational advances in topics including star formation in high redshift galaxies, the diffuse circumgalactic medium, Galactic ecology, cometary compositions and solar flares motivate the need for a 50m, single-dish telescope with a 1-2 degree field of view and a new generation of highly multiplexed continuum and spectral cameras. AtLAST will have the resolution to drastically lower the confusion limit compared to current single-dish facilities, whilst also being able to rapidly map large areas of the sky and detect extended, diffuse structures. Its high sensitivity and large field of view will open up the field of submillimeter transient science by increasing the probability of serendipitous detections. Finally, the science cases listed here motivate the need for a highly flexible operations model capable of short observations of individual targets, large surveys, monitoring programmes, target of opportunity observations and coordinated observations with other observatories. AtLAST aims to be a sustainable, upgradeable, multipurpose facility that will deliver orders of magnitude increases in sensitivity and mapping speeds over current and planned submillimeter observatories.

Circumgalactic Ly alpha nebulae are gaseous halos around galaxies exhibiting luminous extended Ly alpha emission. This work investigates Ly alpha nebulae from deep imaging of similar to 12 deg(2) sky, targeted by the MAMMOTH-Subaru survey. Utilizing the wide-field capability of Hyper Suprime-Cam, we present one of the largest blind Ly alpha nebula selections, including QSO nebulae, Ly alpha blobs, and radio galaxy nebulae down to the typical 2 sigma Ly alpha surface brightness of ( 5 - 10 ) x 10 (- 18) erg s( -) (1) cm (- 2) arcsec (- 2) . The sample contains 117 nebulae with Ly alpha sizes of 40-400 kpc, and the most gigantic one spans about 365 kpc, and is referred to as the Ivory Nebula. Combining multiwavelength data, we investigate diverse nebula populations and associated galaxies. We find a small fraction of Ly alpha nebulae have QSOs (similar to 7%), luminous infrared galaxies (LIRGs; similar to 1%), and radio galaxies (similar to 2%). Remarkably, among the 28 enormous Ly alpha nebulae (ELANe) exceeding 100 kpc, about 80% are associated with UV-faint galaxies (M-UV > -22), and are categorized as Type II ELANe. We underscore that Type II ELANe constitute the majority but remain largely hidden in current galaxy and QSO surveys. Dusty starburst and obscured AGN activity are proposed to explain the nature of Type II ELANe. The spectral energy distribution of stacking all Ly alpha nebulae also reveals signs of massive dusty star-forming galaxies with obscured AGNs. We propose a model to explain the dusty nature where the diverse populations of Ly alpha nebulae capture massive galaxies at different evolutionary stages undergoing violent assembly. Ly alpha nebulae provide critical insights into the formation and evolution of today's massive cluster galaxies at cosmic noon.

Using the Karl G. Jansky Very Large Array, we have detected absorption lines due to carbon monoxide, CO(J = 0 -> 1), and the cyano radical, CN(N = 0 -> 1), associated with radio galaxy B2 0902+34 at redshift z = 3.4. The detection of millimeter-band absorption observed 1.5 Gyr after the Big Bang facilitates studying molecular clouds down to gas masses inaccessible to emission-line observations. The CO absorption in B2 0902+34 has a peak optical depth of tau >= 8.6% and consists of two components, one of which has the same redshift as previously detected 21 cm absorption of neutral hydrogen (H I) gas. Each CO component traces an integrated H-2 column density of N-H2 greater than or similar to 3 x 10(20) cm(-2). CN absorption is detected for both CO components, as well as for a blueshifted component not detected in CO, with CO/CN line ratios ranging from less than or similar to 0.4 to 2.4. We discuss the scenario that the absorption components originate from collections of small and dense molecular clouds that are embedded in a region with more diffuse gas and high turbulence, possibly within the influence of the central active galactic nucleus or a starburst region. The degree of reddening in B2 0902+34, with rest-frame color B - K similar to 4.2, is lower than the very red colors (B - K > 6) found among other known redshifted CO absorption systems at z < 1. Nevertheless, when including the many nondetections from the literature, a potential correlation between the absorption-line strength and B - K color is evident, giving weight to the argument that the red colors of CO absorbers are due to a high dust content.

The growth of galaxies in the early Universe is driven by accretion of circum- and intergalactic gas. Simulations have predicted that steady streams of cold gas penetrate the dark matter halos of galaxies and provide the raw material necessary to sustain star formation. We report a filamentary stream of gas that extends for 100 kiloparsecs and connects to the massive radio galaxy 4C 41.17. We detected the stream using submillimeter observations of the 3P1 to 3P0 emission from the [C i] line of atomic carbon, a tracer of neutral atomic or molecular hydrogen gas. The galaxy contains a central gas reservoir that is fueling a vigorous starburst. Our results show that the raw material for star formation can be present in cosmic streams outside galaxies.

We combine James Webb Space Telescope (JWST) and Hubble Space Telescope imaging with Atacama Large Millimeter Array CO(2–1) spectroscopy to study the highly turbulent multiphase intergalactic medium (IGM) in Stephan’s Quintet on 25–150 pc scales. Previous Spitzer observations revealed luminous H 2 line cooling across a 45 kpc-long filament, created by a giant shock wave, following the collision with an intruder galaxy, NGC 7318b. We demonstrate that the Mid-Infrared Instrument/F1000W/F770W filters are dominated by 0–0 S(3) H 2 and a combination of polycyclic aromatic hydrocarbon and 0–0 S(5) H 2 emission. These observations reveal the dissipation of kinetic energy as massive clouds experience collisions, interactions, and likely destruction/recycling within different phases of the IGM. In 1 kpc-scaled structure, warm H 2 was seen to form a triangular-shaped head and tail of compressed and stripped gas behind a narrow shell of cold H 2 . In another region, two cold molecular clumps with very different velocities are connected by an arrow-shaped stream of warm, probably shocked, H 2 suggesting a cloud–cloud collision is occurring. In both regions, a high warm-to-cold molecular gas fraction indicates that the cold clouds are being disrupted and converted into warm gas. We also map gas associated with an apparently forming dwarf galaxy. We suggest that the primary mechanism for exciting strong mid-IR H 2 lines throughout Stephan’s Quintet is through a fog of warm gas created by the shattering of denser cold molecular clouds and mixing/recycling in the post-shocked gas. A full picture of the diverse kinematics and excitation of the warm H 2 will require future JWST mid-IR spectroscopy. The current observations reveal the rich variety of ways that different gas phases can interact with one another.

We combine JWST and HST imaging with ALMA~CO(2-1) spectroscopy to study the highly turbulent multi-phase intergalactic medium (IGM) in Stephan's Quintet on 25-150 pc scales. Previous Spitzer observations revealed luminous H$_2$ line cooling across a 45 kpc-long filament, created by a giant shock-wave, following the collision with an intruder galaxy NGC~7318b. We demonstrate that the MIRI/F1000W/F770W filters are dominated by 0-0~S(3)~H$_2$ and a combination of PAH and 0-0~S(5)~H$_2$ emission. They reveal the dissipation of kinetic energy as massive clouds experience collisions, interactions and likely destruction/re-cycling within different phases of the IGM. In one kpc-scaled structure, warm H$_2$ formed a triangular-shaped head and tail of compressed and stripped gas behind a narrow shell of cold H$_2$. In another region, two cold molecular clumps with very different velocities are connected by an arrow-shaped stream of warm, probably shocked, H$_2$ suggesting a cloud-cloud collision is occurring. In both regions, a high warm-to-cold molecular gas fraction indicates that the cold clouds are being disrupted and converted into warm gas. We also map gas associated with an apparently forming dwarf galaxy. We suggest that the primary mechanism for exciting strong mid-IR H$_2$ lines throughout Stephan's Quintet is through a fog of warm gas created by the shattering of denser cold molecular clouds and mixing/recycling in the post-shocked gas. A full picture of the diverse kinematics and excitation of the warm H$_2$ will require future JWST mid-IR spectroscopy. The current observations reveal the rich variety of ways that different gas phases can interact with one another.

The Dragonfly galaxy (MRC 0152-209), the most infrared-luminous radio galaxy at redshift z ∼ 2, is a merger system containing a powerful radio source and large displacements of gas. We present kiloparsec-resolution data from the Atacama Large Millimeter/submillimeter Array and the Very Large Array of carbon monoxide (6−5), dust, and synchrotron continuum, combined with Keck integral field spectroscopy. We find that the Dragonfly consists of two galaxies with rotating disks that are in the early phase of merging. The radio jet originates from the northern galaxy and brightens when it hits the disk of the southern galaxy. The Dragonfly galaxy therefore likely appears as a powerful radio galaxy because its flux is boosted into the regime of high- z radio galaxies by the jet–disk interaction. We also find a molecular outflow of (1100 ± 550) M ⊙ yr −1 associated with the radio host galaxy, but not with the radio hot spot or southern galaxy, which is the galaxy that hosts the bulk of the star formation. Gravitational effects of the merger drive a slower and longer-lived mass displacement at a rate of (170 ± 40) M ⊙ yr −1 , but this tidal debris contains at least as much molecular gas mass as the much faster outflow, namely M H2 = (3 ± 1) × 10 9 ( α CO /0.8) M ⊙ . This suggests that both the active-galactic-nucleus-driven outflow and mass transfer due to tidal effects are important in the evolution of the Dragonfly system. The Keck data show Ly α emission spread across 100 kpc, and C iv and He ii emission across 35 kpc, confirming the presence of a metal-rich and extended circumgalactic medium previously detected in CO(1–0).