Kouichiro Nakanishi

We present Atacama Large Millimeter/submillimeter Array (ALMA) CO( J = 1−0) observations of a nearby radio galaxy, NGC 1316, at 100 pc resolution to investigate the impact of active galactic nucleus (AGN) jets on the molecular gas. The molecular gas exhibits complex spatial and kinematic distributions, with broad CO line widths (>50 km s ⁻¹ ) observed in several regions. The interferometric CO flux is only 34%–38% compared to single-dish data, indicating a large fraction of spatially extended molecular gas, especially in the central regions. We identified 24 giant molecular cloud associations (GMAs) primarily within the “NW Shell” and the “SE Blob”; these GMAs show velocity dispersions approximately twice as high as those in typical star-forming galaxies for their sizes. Analysis of archival ALMA CO( J = 2−1) and CO( J = 3−2) data reveals elevated line ratios ( R ₂₁ ∼ 1 and R ₃₁ ∼ 1) in gas near the jet but, away from the jet, typical values ( R ₂₁ ∼ 0.7, R ₃₁ ∼ 0.3). A multiwavelength comparison reveals a ∼5 kpc warm ionized gas shell that encompasses the molecular NW Shell. The observed energetics and bubble morphology are consistent with an expanding bubble model driven by the jet assuming a jet power of 1.6 × 10 ⁴³ erg s ⁻¹ . We propose that the high extended gas fraction results from the destruction of molecular clouds due to interactions with the jet plasma. NGC 1316 may be a good example of jet-induced negative feedback through the ablation, dispersal, and rarification of dense molecular clouds through jet–interstellar medium interactions.

We present ¹² CO(1–0) mapping across the entire system of Stephan’s Quintet, a well-known compact galaxy group, observed by Atacama Compact Array (7 m array + Total Power) of the Atacama Large Millimeter/submillimeter Array. These observations provide the first large-scale (137 kpc × 119 kpc), spatially resolved (∼5.5 kpc) molecular gas map of a compact group. Our CO map revealed that most of the molecular gas resides in the disk of the member galaxy NGC 7319 and in the intergalactic regions, including components along the shocked filament and the optically identified tidal tail extending from NGC 7319. Along the tidal tail and its surroundings, we found not only an extended molecular gas component but also four discrete CO clumps, with velocity dispersions of ∼10–30 km s ⁻¹ and molecular gas masses of order 10 ⁷ –10 ⁸ M _⊙ . Three of these clumps spatially overlap with H i , whereas the remaining clump shows no associated H i or counterparts at optical and infrared wavelengths. Using star formation rates derived from H α luminosities of H ii regions, we found that star formation efficiencies (SFEs) span ∼2.2 dex (∼0.02–4 Gyr ⁻¹ ) and negatively correlate with CO velocity dispersion. While regions with small velocity dispersion exhibit SFEs comparable to those of nearby disk galaxies, those with large velocity dispersion (∼50–150 km s ⁻¹ ) around the shocked filament show strongly suppressed star formation. These results suggest that turbulence plays a significant role in regulating star formation in interacting systems.

Time variability is a strong probe of energetic phenomena which occur at small spatial scales, like in active galactic nuclei (AGN). We use Atacama Large Millimeter/submillimeter Array observations at 100 GHz executed over a period of ∼2.5 months to look for time variability in the Antennae galaxies, a prototypical early stage merger galaxy pair, for which there are no previous signatures of an AGN in the optical, IR, or X-rays. Most 100 GHz detections of the Antennae are spatially extended and associated with star-forming regions, but two sources in the southern galaxy NGC 4039 are compact. One of these compact sources, S3, is offset by ∼1″ in the northeast direction from the stellar peak of NGC 4039, and marginally resolved at 10 pc resolution. The other source, S4, is cospatial with the stellar peak of NGC 4039 and unresolved even at a resolution of ∼4 pc. We examine the time variability of these two sources using their power spectra. We find that S4 varies with a characteristic timescale of 13 ± 3 days, indicating that the phenomenon responsible for the 100 GHz emission is smaller than 0.01 pc. By comparing the observed fluxes of the two sources with various candidate origins, we show that while S3 can be explained either by a young massive stellar cluster or an AGN, S4 is likely to be an AGN that is possibly Compton thick.

We present the results of our investigations of spectrally and spatially resolved (sub)millimeter HCN-to-HCO ⁺ flux ratios at J = 2–1, J = 3–2, and/or J = 4–3 in 18 nearby ( z < 0.15) ultraluminous infrared galaxies (ULIRGs), using ALMA ≲ 0 . ″ 2 (≲ 500 pc) resolution data. The geometry of elevated HCN-to-HCO ⁺ flux ratios (with >3 σ detections for both molecular lines) in position-position-velocity (PPV) space is visually classified into (i) a spherical shell (spectrally and spatially distinct), (ii) spectrally distinct and spatially compact, and (iii) filled (spectrally filled and spatially compact). These can naturally be explained by the elevation of the flux ratio due to (i) a spatially resolved outflow, (ii) an active galactic nucleus (AGN) and/or a spatially unresolved outflow with blueshifted and redshifted emission components, and (iii) an AGN and/or a spatially confined outflow with not clearly separated blueshifted and redshifted velocity components, respectively. Signatures of elevated HCN-to-HCO ⁺ flux ratios originated from (a) spatially resolved outflow and (b) AGN and/or spatially unresolved outflow are seen in seven and nine ULIRGs, respectively. In the former spatially resolved outflow-origin case, modest-velocity components relative to the maximum outflow velocity tend to be probed by spaxels with elevated HCN-to-HCO ⁺ flux ratios. The spectrally and spatially resolved HCN-to-HCO ⁺ flux ratios can provide additional information on the physical origin of the elevated flux ratios in nearby ULIRG nuclei compared to previously conducted spatially integrated and/or velocity-integrated analyses.

We present a study of the multiphase gas structure and kinematics of the circumnuclear disk (CND) of NGC 5506, a nearby edge-on Seyfert galaxy, at a spatial resolution of ∼20 pc. Observations of [C i ](1–0), CO(3–2), and HCO ⁺ (4–3) obtained with the Atacama Large Millimeter/submillimeter Array reveal the CND dominated by rotational motion on scales of several hundred parsecs. No significant differences in geometrical thickness or velocity structure are found between [C i ](1–0) and CO(3–2) across the CND, whereas HCO ⁺ (4–3) emission is more concentrated toward the disk plane. The ratio of velocity dispersion to rotational velocity, a proxy for disk scale height-to-radius ratio, is high (≳0.9) in the central region (≲30 pc) for both [C i ](1–0) and CO(3–2), indicating geometrically thick structures in both tracers. Regions where the [C i ](1–0)/CO(3–2) ratio exceeds the CND average are spatially correlated with the [O iii ] λ 5007 bicone observed with the Hubble Space Telescope, suggesting that CO is preferentially dissociated by the AGN-driven biconical ionized outflow. The observed CND scale height and velocity dispersions traced by [C i ](1–0) and CO(3–2) are consistent with a model in which supernova-driven turbulence provides the vertical support for the CND.

The formation of super star clusters (SSCs) in galaxies remains a fundamental yet unresolved problem. Among the proposed mechanisms, cloud–cloud collisions (CCCs) have been suggested as a potential trigger, although observational validation has been limited. Here we present high-resolution ( 0 . ″ 12 , ∼14 pc) Atacama Large Millimeter/submillimeter Array observations of CO ( J = 1–0) emission toward a super giant molecular cloud (SGMC) in the overlap region of the Antennae galaxies. The data resolve the SGMC into two distinct velocity components separated by ∼50 km s ⁻¹ . One component exhibits a “U”-shaped structure within a large filament likely shaped by ram pressure, while the other shows hub-filament morphology. Such a morphology is naturally interpreted as a CCC scenario. The 108 GHz continuum emission detected at the apparent collision interface is dominated by free–free radiation, with an ionizing photon rate consistent with the stellar mass and age of the optically identified SSCs. Supplementary infrared imaging with JWST reveals emission spatially coincident with the inferred collision interface, further supporting the CCC scenario. These results provide compelling, multiwavelength evidence that CCCs play a key role in triggering SSC formation in merging galaxies.

We present a morphological analysis of Atacama Large Millimeter/submillimeter Array (ALMA) and James Webb Space Telescope (JWST) NIRCam images of nine dusty star-forming galaxies (DSFGs) at z _spec ≈ 3.09, all embedded within the cosmic web filaments at the SSA22 protocluster core. The ALMA 870 μ m and 1.1 mm images are obtained at spatial resolutions ranging from 0.5″ to 0.05″ (350 pc at z = 3.09). The high-resolution images enable us to resolve inner structures traced by dust continuum, identifying compact dusty cores, clumps, and offset ridges within bars. Sérsic profile fitting was performed for both ALMA 870 μ m and NIRCam F444W images at comparable resolutions (∼0.15″). The Sérsic index measured for 870 μ m, masking bright regions, indicates values close to unity, suggesting that dust emission arises from disks with superimposed compact core components. For the JWST F444W images (restframe ∼1 μ m), the Sérsic indices range between n _F444W ∼ 1–3, pointing to the coexistence of bulges and stellar disks in these DSFGs. A comparison of dust mass surface density, n _F444W , and F200W–F444W color (restframe ∼0.5–1 μ m) reveals diversity among the DSFGs, likely reflecting different evolutionary stages, including some DSFGs with red cores, indicating ongoing rapid bulge growth phases heavily obscured by dust. The predominantly disklike morphologies observed in most DSFGs in the protocluster core contrast sharply with early-type morphologies that dominate the highest density environment in the local Universe. This suggests that we are witnessing the early formation of the morphology-density relation, as massive galaxies undergo rapid growth as late-type galaxies fueled by cosmic web gas filaments.

We present an analysis of rest-frame optical and far-infrared continuum emission in three luminous submillimeter galaxies (SMGs) at 3.0 ≲ z ≲ 4.5. The SMGs are spatially resolved down to 400–500 pc ( ∼ 0 . ″ 05 ) resolution by James Webb Space Telescope and Atacama Large Millimeter/submillimeter Array observations. Despite similarities in their observed far-infrared properties (flux density, infrared luminosity, and effective radius), the three SMGs exhibit heterogeneous morphologies both across wavelengths and among the sources themselves. While two of them (AzTEC-4 and AzTEC-8) show a disk-like structure in the optical continuum, AzTEC-1 is dominated by a highly concentrated component with a Sérsic index of n = 5.4, where its far-infrared continuum emission is clumpy and less concentrated. AzTEC-4, which is confirmed to be at z = 4.198, shows a two-arm spiral of dust, but not in the stellar distribution. These three SMGs exemplify that multiple physical mechanisms exist in triggering starbursts in luminous SMGs at high redshift: secular instability in gas disks (AzTEC-4) in addition to possible minor mergers (AzTEC-8), and a combination of the efficient gas supply to the central core induced by a gas-rich major merger and the re-formation of a cold gas disk (AzTEC-1).

We present the results of our ALMA observations of the dense molecular HCN J = 4–3 and HCO ⁺ J = 4–3 lines at ≲1 pc (≲14 mas) resolution in the nuclear region of the nearby (∼14 Mpc) well-studied AGN NGC 1068. Both emission lines are clearly detected around the AGN along an almost east–west direction, which we ascribe to the dusty molecular torus. The HCN J = 4–3 emission is brighter than the HCO ⁺ J = 4–3 emission in the compact (≲3–5 pc) torus region. Apparent counterrotation between the inner (≲2 pc) and outer (≳2 pc) parts of the western torus, previously seen in ∼1.5 pc-resolution HCN J = 3–2 and HCO ⁺ J = 3–2 data, is also confirmed in our new ≲1 pc-resolution HCN J = 4–3 and HCO ⁺ J = 4–3 data. We apply a physically counterrotating torus model, in which a compact dense gas clump collided with the western side of the existing rotating torus from the opposite direction, and we find that this model largely reproduces the observed properties of the combined new ≲1 pc-resolution HCN J = 4–3 and HCO ⁺ J = 4–3 data, and the previously obtained ≲1.5 pc-resolution HCN J = 3–2 and HCO ⁺ J = 3–2 data.