Yoshiaki Hagiwara

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.

We report multifrequency Very Long Baseline Array (VLBA) observations at 2.3 and 8.4 GHz of three nearby ultraluminous infrared galaxies, identified via mid-infrared spectroscopic analyses as hosting deeply embedded active galactic nuclei (AGNs). Milliarcsecond-scale observations at both frequencies reveal compact continuum emission in IRAS F00188−0856 and IRAS F01298−0744, accounting for ∼10% of the flux density measured on arcsecond scales. The nondetection in IRAS F00091−0738 and the lower limit on the intrinsic 8.4 GHz brightness temperature of 10^6.1 K in IRAS F01298−0744 yield no conclusive evidence of AGN-driven radio emission, whereas the measurement of 10^7.8 K in IRAS F00188−0856 confirms an AGN origin. Thus, the mid-infrared AGN classification remains robust, with at least one object exhibiting compact radio emission indicative of AGN activity. We further investigate the high-frequency spectral steepening observed in all three galaxies. In each case, this steepening arises from spectral aging in diffuse kiloparsec-scale emission, which is resolved out by the VLBA observations. One possible explanation for the steepening of the sample is merger-induced particle acceleration. IRAS F00188−0856 exhibits a peaked radio spectrum, characteristic of a young radio source, with the high-frequency steepening attributable to this AGN activity. Consequently, the spectral steepening at high frequencies arises from particles accelerated by merger dynamics or AGN activity.

The nearby ultraluminous infrared galaxy (ULIRG) IRAS F01004−2237 exhibits 100 kpc scale continuum emission at radio wavelengths. The absence of extended X-ray emission in IRAS F01004−2237 has suggested an active galactic nucleus (AGN) origin for the extended radio emission, whose properties and role in merging systems still need to be better understood. We present the results of multifrequency observations of IRAS F01004−2237 conducted by the Very Long Baseline Array at 2.3 and 8.4 GHz. Compact 8.4 GHz continuum emission was detected on a 1 pc scale in the nuclear region with an intrinsic brightness temperature of 10^8.1 K suggesting that the radio source originated from an AGN, potentially driving the extended emission. In contrast, no significant emission was observed at 2.3 GHz, indicating the presence of low-frequency absorption. This absorption cannot be attributed solely to synchrotron self-absorption; alternatively, free–free absorption due to thermal plasma is mainly at work in the spectrum. From combined perspectives, including mid-infrared and X-ray data, the AGN is obscured in a dense environment. The kinetic power of the nonthermal jet, as inferred from the extended emission, can play a more important role in dispersing the surrounding medium than the thermal outflow in IRAS F01004−2237. These findings hint that jet activities in ULIRGs may contribute to AGN feedback during galaxy evolution induced by merger events.

The results of high-resolution spectral-line observations of dense molecular gas are presented towards the nuclear region of the type 2 Seyfert galaxy NGC 1068. MERLIN observations of the 22 GHz H₂O maser were made for imaging the known off-nuclear maser emission at radio jet component located about 0.3” north-east of the radio nucleus in the galaxy. High angular resolution ALMA observations have spatially resolved the molecular gas emissions of HCN and HCO⁺ in this region. The off-nuclear maser spots are found to nearly overlap with a ring-like molecular gas structure and are tracing an evolving shock-like structure, which appears to be energized by interaction between the radio jet and circumnuclear medium. The scenario of the dynamic jet-ISM interaction is further supported by a systematic shift of the centroid velocities of the off-nuclear maser features over a period of 35 years. The enhanced integrated flux ratios of the HCN to HCO⁺ line emission features at component C suggest a kinetic temperature T_k|$\gtrsim$|300K and an H₂ density of |$\gtrsim$|10⁶ cm⁻³, which are conditions where water masers may be formed. The diagnostics of the masering action in this jet-ISM interaction region is exemplary for galaxies hosting off-nuclear H₂O maser emission.

The nearby radio galaxy M87 offers a unique opportunity to explore the connections between the central supermassive black hole and relativistic jets. Previous studies of the inner region of M87 revealed a wide opening angle for the jet originating near the black hole^1,2,3,4. The Event Horizon Telescope resolved the central radio source and found an asymmetric ring structure consistent with expectations from general relativity⁵. With a baseline of 17 years of observations, there was a shift in the jet’s transverse position, possibly arising from an 8- to 10-year quasi-periodicity³. However, the origin of this sideways shift remains unclear. Here we report an analysis of radio observations over 22 years that suggests a period of about 11 years for the variation in the position angle of the jet. We infer that we are seeing a spinning black hole that induces the Lense–Thirring precession of a misaligned accretion disk. Similar jet precession may commonly occur in other active galactic nuclei but has been challenging to detect owing to the small magnitude and long period of the variation.

We report on recently conducted technical developments of front- and back-end for four 20-m radio telescopes of VLBI Exploration of Radio Astrometry (VERA). In this short article, a brief overview of the developments of a dual circular polarization receivers and ultra-wide band (16 \gbps) recording systems that were installed to each of the four telescopes of VERA is presented. With the new technical developments that enable wider band VLBI polarization observations at 22 and 43 GHz bands, it is expected that VERA will reveal magnetic fields in plasma jets of active galactic nuclei, masers in star-forming cites, and late-type stars at unprecedented sensitivity. The wide band calibrated spectra, delay solutions, and the polarization intensity maps of extragalactic sources at 22 and 43 GHz that obtained from the most recent test observations are presented to show the current capabilities of the VERA polarimetry performance.

We explored the parsec-scale nuclear regions of a sample of radio-loud narrow-line Seyfert 1 galaxies (NLSy1s) using the VLBI Exploration of Radio Astrometry wideband (at a recording rate of 16 Gbps) polarimetry at 22 and 43 GHz. Our targets include 1H 0323+342, SBS 0846+513, PMN J0948+0022, 1219+044, PKS 1502+036, and TXS 2116-077, which are all known to exhibit γ-ray emission indicative of possessing highly beamed jets similar to blazars. For the first time, we unambiguously detected Faraday rotation toward the parsec-scale radio core of NLSy1s, with a median observed core rotation measure (RM) of 2.7 × 10³ rad m⁻² (or 6.3 × 10³ rad m⁻² for redshift-corrected). This level of RM magnitude is significantly larger than those seen in the core of BL Lacertae objects (BLOs; a dominant subclass of blazars), suggesting that the nuclear environment of NLSy1s is more gas-rich than that in BLOs. Interestingly, the observed parsec-scale polarimetric properties of NLSy1s (low core fractional polarization, large core RM and jet–EVPA misalignment) are rather similar to those of flat-spectrum radio quasars (FSRQs). Our results are in accordance with the scenario that NLSy1s are in an early stage of active galactic nucleus evolution with their central black hole masses being smaller than those of more evolved FSRQs.