We report on results from new high-sensitivity, high-resolution 86 GHz (3.5 mm) observations of the jet base in the nearby radio galaxy M87, obtained by the Very Long Baseline Array in conjunction with the Green Bank Telescope. The resulting image has a dynamic range exceeding 1500 to 1, the highest ever achieved for this jet at this frequency, resolving and imaging a detailed jet formation/collimation structure down to ∼10 Schwarzschild radii ( ). The obtained 86 GHz image clearly confirms some important jet features known at lower frequencies, i.e., a jet base with a wide opening angle, a limb-brightened intensity profile, a parabola-shape collimation profile and a counter jet. The limb-brightened structure is already well developed at mas ( , projected) from the core, where the corresponding apparent opening angle becomes as wide as ∼100°. The subsequent jet collimation near the black hole evolves in a complicated manner; there is a “constricted” structure at tens of from the core, where the jet cross section is locally shrinking. We suggest that external pressure support from the inner part of the radiatively inefficient accretion flow may be dynamically important in shaping/confining the footprint of the magnetized jet. We also present the first 86 GHz polarimetric experiment using very long baseline interferometry for this source, where a highly polarized (∼20%) feature is detected near the jet base, indicating the presence of a well-ordered magnetic field. As a by-product, we additionally report a 43/86 GHz polarimetric result for our calibrator 3C 273, suggesting an extreme rotation measure near the core.

We study the kinematics of the M87 jet using the first-year data of the KVN and VERA Array (KaVA) large program, which has densely monitored the jet at 22 and 43 GHz since 2016. We find that the apparent jet speeds generally increase from ≈0.3c at ≈0.5 mas from the jet base to ≈2.7c at ≈20 mas, indicating that the jet is accelerated from subluminal to superluminal speeds on these scales. We perform a complementary jet kinematic analysis by using archival Very Long Baseline Array monitoring data observed in 2005–2009 at 1.7 GHz and find that the jet is moving at relativistic speeds up to ≈5.8c at distances of 200–410 mas. We combine the two kinematic results and find that the jet is gradually accelerated over a broad distance range that coincides with the jet collimation zone, implying that conversion of Poynting flux to kinetic energy flux takes place. If the jet emission consists of a single streamline, the observed trend of jet acceleration (Γ ∝ z ^0.16±0.01) is relatively slow compared to models of a highly magnetized jet. This indicates that Poynting flux conversion through the differential collimation of poloidal magnetic fields may be less efficient than expected. However, we find a nonnegligible dispersion in the observed speeds for a given jet distance, making it difficult to describe the jet velocity field with a single power-law acceleration function. We discuss the possibility that the jet emission consists of multiple streamlines following different acceleration profiles, resulting in jet velocity stratification.

Context. Very long baseline interferometry (VLBI) observations at 86 GHz (wavelength, λ = 3 mm) reach a resolution of about 50 μas, probing the collimation and acceleration regions of relativistic outflows in active galactic nuclei (AGN). The physical conditions in these regions can be studied by performing 86 GHz VLBI surveys of representative samples of compact extragalactic radio sources. Aims. To extend the statistical studies of compact extragalactic jets, a large global 86 GHz VLBI survey of 162 compact radio sources was conducted in 2010–2011 using the Global Millimeter VLBI Array (GMVA). Methods. The survey observations were made in a snapshot mode, with up to five scans per target spread over a range of hour angles in order to optimize the visibility coverage. The survey data attained a typical baseline sensitivity of 0.1 Jy and a typical image sensitivity of 5 mJy beam⁻¹, providing successful detections and images for all of the survey targets. For 138 objects, the survey provides the first ever VLBI images made at 86 GHz. Gaussian model fitting of the visibility data was applied to represent the structure of the observed sources and to estimate the flux densities and sizes of distinct emitting regions (components) in their jets. These estimates were used for calculating the brightness temperature (T_b) at the jet base (core) and in one or more moving regions (jet components) downstream from the core. These model-fit-based estimates of T_b were compared to the estimates of brightness temperature limits made directly from the visibility data, demonstrating a good agreement between the two methods. Results. The apparent brightness temperature estimates for the jet cores in our sample range from 2.5 × 10⁹ K to 1.3 × 10¹² K, with the mean value of 1.8 × 10¹¹ K. The apparent brightness temperature estimates for the inner jet components in our sample range from 7.0 × 10⁷ K to 4.0 × 10¹¹ K. A simple population model with a single intrinsic value of brightness temperature, T₀, is applied to reproduce the observed distribution. It yields T₀ = (3.77_−0.14^+0.10) × 10¹¹ K for the jet cores, implying that the inverse Compton losses dominate the emission. In the nearest jet components, T₀ = (1.42_−0.19^+0.16) × 10¹¹ K is found, which is slightly higher than the equipartition limit of ∼5 × 10¹⁰ K expected for these jet regions. For objects with sufficient structural detail detected, the adiabatic energy losses are shown to dominate the observed changes of brightness temperature along the jet.

Sagittarius A* (Sgr A*), the Galactic Center supermassive black hole (SMBH), is one of the best targets in which to resolve the innermost region of an SMBH with very long baseline interferometry (VLBI). In this study, we have carried out observations toward Sgr A* at 1.349 cm (22.223 GHz) and 6.950 mm (43.135 GHz) with the East Asian VLBI Network, as a part of the multiwavelength campaign of the Event Horizon Telescope (EHT) in 2017 April. To mitigate scattering effects, the physically motivated scattering kernel model from Psaltis et al. (2018) and the scattering parameters from Johnson et al. (2018) have been applied. As a result, a single, symmetric Gaussian model well describes the intrinsic structure of Sgr A* at both wavelengths. From closure amplitudes, the major-axis sizes are ∼704 ± 102 μas (axial ratio ∼ 1.19 − 0.19 + 0.24 ) and ∼300 ± 25 μas (axial ratio ∼1.28 ± 0.2) at 1.349 cm and 6.95 mm, respectively. Together with a quasi-simultaneous observation at 3.5 mm (86 GHz) by Issaoun et al. (2019), we show that the intrinsic size scales with observing wavelength as a power law, with an index ∼1.2 ± 0.2. Our results also provide estimates of the size and compact flux density at 1.3 mm, which can be incorporated into the analysis of the EHT observations. In terms of the origin of radio emission, we have compared the intrinsic structures with the accretion flow scenario, especially the radiatively inefficient accretion flow based on the Keplerian shell model. With this, we show that a nonthermal electron population is necessary to reproduce the source sizes.

We report the detection of two CH₃OH lines (J _K = 2 _K –1 _K and 3 _K –2 _K ) between the progenitor’s disks (“Overlap”) of the mid-stage merging galaxy VV 114 obtained using the Atacama Large Millimeter/submillimeter Array (ALMA) Band 3 and Band 4. The detected CH₃OH emission shows an extended filamentary structure (∼3 kpc) across the progenitor’s disks with relatively large velocity width (FWZI ∼ 150 km s⁻¹). The emission is only significant in the “overlap” and not detected in the two merging nuclei. Assuming optically thin emission and local thermodynamic equilibrium, we found the CH₃OH column density relative to H₂ ( ) peaks at the “Overlap” (∼8 × 10⁻⁹), which is almost an order of magnitude larger than that at the eastern nucleus. We suggest that kpc-scale shocks driven by galaxy–galaxy collision may play an important role to enhance the CH₃OH abundance at the “Overlap.” This scenario is consistent with shock-induced large velocity dispersion components of ionized gas that have been detected in optical wavelength at the same region. Conversely, low at the nuclear regions might be attributed to the strong photodissociation by nuclear starbursts and/or a putative active galactic nucleus, or inefficient production of CH₃OH on dust grains due to initial high-temperature conditions (i.e., desorption of the precursor molecule, CO, into gas phase before forming CH₃OH on dust grains). These ALMA observations demonstrate that CH₃OH is a unique tool to address kpc-scale shock-induced gas dynamics and star formation in merging galaxies.