Hiroyuki Kaneko

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.

The outer Galaxy (Galactocentric distance ≳13.5 kpc) serves as an excellent laboratory for investigating the chemical complexity in low-metallicity environments. Here we present the chemical analyses for the outer Galactic hot core Sh 2-283-1a SMM1 ( D _GC = 15.7 kpc and Z ∼ 0.3 Z _⊙ ), recently detected by T. Ikeda et al. using the Atacama Large Millimeter/submillimeter Array. Toward this source, a variety of molecular species, including complex organic molecules (COMs; CH ₃ OH, ¹³ CH ₃ OH, CH ₂ DOH, and CH ₃ OCH ₃ ), are detected. The molecular abundances relative to CH ₃ OH are similar to those of another outer Galactic hot core, demonstrating that chemically rich hot cores exist in different regions of the outer Galaxy. We also compared molecular abundances among hot cores in the inner Galaxy, outer Galaxy, and Magellanic Clouds. This comparison revealed that the metallicity-corrected N (SO ₂ )/ N (H ₂ ) ratios of outer Galactic hot cores are significantly lower than those of the inner Galactic ones, while their N (CH ₃ OH)/ N (H ₂ ) ratios are similar. The Magellanic hot cores show different trends despite having metallicities similar to those of the outer Galaxy, indicating that the chemical complexity of hot cores is governed by environmental conditions (e.g., cosmic-ray intensity and dust temperature) rather than simple metallicity scaling. These environmental differences would also affect the production efficiency of COMs derived from CH ₃ OH, as the N (CH ₃ OCH ₃ )/ N (CH ₃ OH) and N (C ₂ H ₅ OH)/ N (CH ₃ OH) ratios in the outer Galactic sources are moderately lower than those of inner Galactic sources. The N (CH ₂ DOH)/ N (CH ₃ OH) ratio of Sh 2-283-1a SMM1 is 1.5 − 1.2 + 3.9 %, comparable to that of inner Galactic high-mass sources.

We present Atacama Large Millimeter/submillimeter Array Total Power CO(1–0) mapping of Stephan’s Quintet (SQ), a prototypical compact galaxy group, with a uniform noise level at a spatial scale of ∼25 kpc. These observations provide the first complete view of molecular gas across the whole system. Molecular gas is found to spread over a wide area (∼120 × 80 kpc), mainly over the two main member galaxies (NGC 7318B and 7319), but also in the shocked ridges between these galaxies, the tidal tail, and also in intergalactic regions north of the tail. The total CO(1–0) luminosity is (2.47 ± 0.12) × 10⁹ K km s⁻¹ pc², corresponding to a molecular gas mass of (1.07 ± 0.05) × 10¹⁰ M _⊙ assuming the Galactic CO-to-H₂ conversion factor. The global star formation efficiency of SQ is estimated at 0.29–0.70 Gyr⁻¹, comparable to or lower than that of nearby star-forming galaxies. Molecular gas spans a velocity range of ∼1300 km s⁻¹, which can be divided into three components (low, mid, and high). The low- and mid-velocity components, linked to NGC 7318B and the ridge, show relatively active star formation, whereas the high-velocity component, associated with NGC 7319, shows suppressed star formation. Our mapping reveals molecular gas extending ∼100 kpc in projection along the inner tail and north of it, containing (1.64 ± 0.08) × 10⁹ M _⊙ (15% of total) with low velocity dispersion (∼20 km s⁻¹) and ongoing star formation. While previous studies suggested in situ molecular gas formation in the tail, our data suggest an additional contribution from gas stripped from NGC 7319.

We present the first detection of spatially resolved protostellar outflows and jets in the outer Galaxy. We observed five star-forming regions in the outer Galaxy (Sh 2-283 and NOMF05-16/19/23/63; galactocentric distance = 15.7–17.4 kpc) with the Atacama Large Millimeter/submillimeter Array. Toward Sh 2-283, we have detected distinct outflow (∼5–50 km s⁻¹) and jet components (∼50–100 km s⁻¹) associated with the protostar in CO(3–2) emission. The outflows and jets are well collimated, with the jets exhibiting multiple bullet structures. The position–velocity diagram along the CO flow axis shows two characteristic structures: (a) the flow velocity, which linearly increases with the position offset from the core center (the Hubble-like flow); and (b) the continuous velocity components of the periodical flows (spine-like structures), which may indicate episodic mass ejection events. The time intervals of the mass ejection events are estimated to be 900–4000 yr, based on the slopes of these spine-like structures. These characteristics align with those of nearby protostellar systems, indicating that early star formation in low-metallicity environments, such as the outer Galaxy, resembles that in the inner Galaxy. In contrast to the physical similarities, the N(SiO)/N(CO) ratio in the jet bullet appears to be lower than that measured in the low-mass protostellar sources in the inner Galaxy. This may indicate a different shock chemistry or different dust composition in the outer Galaxy source, although non–local thermodynamic equilibrium effects could also affect the observed low N(SiO)/N(CO) ratio. We also report the new detection of four other outflow sources in the outer Galaxy.

We report the detection of the CO(2–1) emission line with a spatial resolution of 0.″9 (3.5 kpc) from the host galaxy of the fast radio burst (FRB), FRB 20191001A at z = 0.2340, using the Atacama Large Millimeter/submillimeter Array. This is the first detection of spatially resolved CO emission from the host galaxy of an FRB at a cosmological distance. The inferred molecular gas mass of the host galaxy is (2.3 ± 0.4) × 10¹⁰ M _⊙, indicating that it is gas-rich, as evidenced by the measured molecular gas fraction μ _gas = 0.50 ± 0.22. This molecular gas mass and the star formation rate of the host, SFR = (8.06 ± 2.42) M _⊙ yr⁻¹, differ from those observed in the other FRB host galaxies with the average M _gas = 9.6 × 10⁸ M _⊙ and SFR = 0.90M _⊙ yr⁻¹. This lends further credibility to the hypothesis that FRBs may originate from single or multiple progenitors across a diverse range of galaxy environments. Based on the observed velocity field modeling, we find that the molecular gas disk is dominated by an ordered circular rotation, despite the fact that the host galaxy has a gas-rich companion galaxy with a projected separation of ∼25 kpc. The formation of the FRB’s progenitor might not have been triggered by this interaction. We derive the 3σ upper limit of the molecular gas column density at the FRB detection site to be 21 cm⁻² with a 3σ upper limit.

Radio-loud active galactic nuclei (RLAGNs) are a unique AGN population and were thought to be preferentially associated with supermassive black holes (SMBHs) at low accretion rates. They could impact the host galaxy evolution by expelling cold gas through the jet-mode feedback. In this work, we studied CO(6-5) line emission and continuum emission in a high-redshift radio galaxy, MRC 0152-209, at z = 1.92 using ALMA up to a 0.024″-resolution (corresponding to ∼200 pc at z = 1.92). This system is a starburst major merger comprising two galaxies: the northwest (NW) galaxy hosting the RLAGN with jet kinetic power L_jet ≳ 2 × 10⁴⁶ erg s⁻¹ and the other galaxy to the southeast (SE). Based on the SED fitting for the entire system (NW+SE galaxies), we find an AGN bolometric luminosity L_{AGN, bol} ∼ 3 × 10⁴⁶ erg s⁻¹ with a lower limit of ∼0.9 × 10⁴⁶ erg s⁻¹ for the RLAGN. We estimate the black hole mass through M_BH − M_⋆ scaling relations and find an Eddington ratio of λ_Edd ∼ 0.07 − 4 conservatively by adopting the lower limit of L_{AGN, bol} and considering the dispersion of the scaling relation. These results suggest that the RLAGN is radiatively efficient and the powerful jets could be launched from a super-Eddington accretion disc. ALMA Cycle 6 observations further reveal a massive (⁠|${M}_\mathrm{H_2}=(1.1-2.3)\times 10^9\ \rm M_\odot$|⁠), compact (∼500 pc), and monopolar molecular outflow perpendicular to the jet axis. The corresponding mass outflow rate (⁠|$1200^{+300}_{-300}-2600^{+600}_{-600}\ \mathrm{M_\odot }\ \rm yr^{-1}$|⁠) is comparable with the star formation rate of at least |$\sim 2100\ \mathrm{M_\odot }\ \rm yr^{-1}$|⁠. Depending on the outflowing molecular gas mass, the outflow kinetic power/L_{AGN, bol} ratio of ∼0.008 − 0.02, and momentum boost factor of ∼3 − 24 agree with a radiative-mode AGN feedback scenario. On the other hand, the jets can also drive the molecular outflow within its lifetime of ∼2 × 10⁵ yr without additional energy supply from AGN radiation. The jet-mode feedback is then capable of removing all cold gas from the host galaxy through the long-term, episodic launching of jets. Our study reveals a unique object where starburst activity, powerful jets, and rapid BH growth co-exist, which may represent a fundamental stage of AGN-host galaxy co-evolution.

The tip of the tidal tail, resulting from an encounter between galaxies, features gas concentrations and some star-forming regions, such as tidal dwarf galaxies (TDGs). This region provides a unique laboratory for examining the star formation process in a dynamical environment distinct from that of disk galaxies. Using the Nobeyama 45 m telescope, we conducted ¹²CO(1−0) position-switching observations at the tips of the southern tidal tail in the Antennae galaxies. We detected CO emission not only from the two star-forming TDG candidates but also in regions with no significant star formation. Adopting a Galactic CO-to-H₂ conversion factor without helium correction, the H₂ gas surface density is ∼5–12 M _⊙ pc⁻². In most regions, the molecular-to-atomic gas ratio is around unity (0.6–1.9), but we find a region with a high ratio with a 3σ lower limit of >7.2. The star formation efficiency (SFE) of molecular gas is notably low (−1), indicating less active star formation than in both nearby disk galaxies (∼0.5–1.0 Gyr⁻¹) and other TDGs previously observed. Including previous observations, the molecular gas SFEs vary widely among TDGs/tidal tails, from 10⁻² to 10 Gyr⁻¹, demonstrating significant variations in star formation activity. Potential factors contributing to the low SFE in the Antennae tail tips include extensive tides and/or the young age of the tail.