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We report sub-parsec-scale observations of the 321 GHz H2O emission line in the radio galaxy NGC 1052. The H2O line emitter size is constrained in <0.6 mas distributed on the continuum core component. The brightness temperature exceeding 106 K and the intensity variation indicate certain evidence for maser emission. The maser spectrum consists of redshifted and blueshifted velocity components spanning ∼400 km s−1, separated by a local minimum around the systemic velocity of the galaxy. The spatial distribution of maser components shows a velocity gradient along the jet direction, implying that the population-inverted gas is driven by the jets interacting with the molecular torus. We identified a significant change of the maser spectra between two sessions separated by 14 days. The maser profile showed a radial velocity drift of 127 ± 13 km s−1 yr−1 implying inward gravitational acceleration at 5000 Schwarzschild radii. The results demonstrate the feasibility of future very long baseline interferometry observations to resolve the jet–torus interacting region. [ABSTRACT FROM AUTHOR]

We report dual-frequency very long baseline interferometry (VLBI) observations at 22 and 43 GHz toward the nucleus of a nearby radio galaxy NGC 4261. In particular, we present a VLBI image of the 22 GHz H2O maser line and its location in the circumnuclear region of NGC 4261. H2O maser emission is marginally detected above the three times the image rms level at channels within a velocity range of approximately 2250–2450 km s−1, slightly red-shifted with respect to the systemic velocity. H2O maser emission is located approximately 1 milliarcsecond east of the brightest continuum component at 22 GHz, where the continuum spectrum is optically thick, that is, at the free–free absorbed receding jet by ionized gas. A positional coincidence between H2O maser emission and an ionized gas disk implies that the H2O maser emission arises from the near side of the disk, amplifying continuum emission from the background receding jet. If the disk axis is oriented 64° relative to the line of sight, the H2O maser emission is expected to be at a mean radius of 0.3 pc in the disk. The broad line width of the H2O maser emission can be attributed to complex kinematics in the immediate vicinity of the supermassive black hole (SMBH), including ongoing gas infall on to the SMBH, turbulence, and outflow. This is analogous to the multi-phase circumnuclear torus model in the nearest radio-loud H2O megamaser source NGC 1052. An alternative explanation for H2O maser association is the shock region between the jet and ambient molecular clouds. However, this explanation fails to describe the explicit association of H2O maser emission only with the free–free absorbed receding jet. [ABSTRACT FROM AUTHOR]

The Atacama Large Millimeter/submillimeter Array (ALMA) serendipitously detected H2O |$J_{K_{\rm a}, K_{\rm c}} = 10_{2,9}$| –93, 6 emission at 321 GHz in NGC 1052. This is the first submillimeter maser detection in a radio galaxy and the most luminous 321 GHz H2O maser known to-date with the isotropic luminosity of |$1090\, L_{\odot }$|⁠. The line profile consists of a broad velocity component with FWHM = 208 ± 12 km s−1 straddling the systemic velocity and a narrow component with FWHM = 44 ± 3 km s−1 blueshifted by 160 km s−1. The profile is significantly different from the known 22 GHz 61, 6–52, 3 maser which shows a broad profile redshifted by 193 km s−1. The submillimeter maser is spatially unresolved with a synthesized beam of |${0{^{\prime \prime}_{.}}68} \times {0{^{\prime \prime}_{.}}56}$| and coincides with the continuum core position within 12 pc. These results indicate amplification of the continuum emission through high-temperature (>1000 K) and dense [ n (H2O) > 104 cm−3] molecular gas in front of the core. [ABSTRACT FROM AUTHOR]

A radio core represents the peak of intensity in VLBI images and is located at the base of jets. It appears at different positions depending on frequencies. This is known as “core shift”, caused by absorption of the core emission. The position of the central engine in an AGN can be estimated accurately by measuring the core shift with multifrequency and phase-referencing observations. We observed NGC 4261 using the VLBA at seven frequencies. This source is a nearby FR–I type radio galaxy at the distance of 30 Mpc and has prominent two-sided jets. We measured the core shifts in not only approaching side but also counter side of the jets. The positions of core at infinity of frequency in both side indicated to come close asymptotically to the same position, which was separated by 82±16 μas from 43 GHz core position, corresponding to 310±60 Rs (Schwarzschild radius). This source also has another feature that there is a region affected by free–free absorption (FFA) in the vicinity of the core and toward the counter jet. Moreover, we also found the same feature in other three sources, 3C 84, Cen A and Cyg A, which are also nearby galaxies with two-sided jets and with an indication of the FFA regions. We will measure the core shifts in these sources by using same technique as NGC 4261 in order to study the structure of circumnuclear plasma, to determine the position of the central engine and to test core shifts due to FFA.

An investigation of the SiO masers in W Hydrae and VY Canis Majoris reveals that the positionally coincident maser emission from the J rotational transitional level associated with the first two levels in the SiO maser is pumped collisionally in distinct regions. This indicates that the maser is not radiatively pumped. The SiO maser has a high brightness temperature which enables the investigation to be performed with very-long-baseline interferometry (VLBI). The VLBI studies also provide information on the stellar winds generated by these masers.

We report the first clear detection of the Zeeman splitting of a CCS emission line at 45 GHz toward the nearby pre-stellar dense filament, Taurus Molecular Cloud 1 (TMC-1). We observed HC |$_3$| N non-Zeeman lines simultaneously with the CCS line, and did not detect any significant splitting of the HC |$_3$| N lines. Thus, we conclude that our detection of CCS Zeeman splitting is robust. The derived line-of-sight magnetic field strength is about |$117 \pm 21 \, \mu$| G, which corresponds to a normalized mass-to-magnetic flux ratio of 2.2 if we adopt an inclination angle of 45 |$^\circ$|⁠. Thus, we conclude that the TMC-1 filament is magnetically supercritical. Recent radiative transfer calculations of the CCS and HC |$_3$| N lines along the line of sight suggest that the filament is collapsing with a speed of |$\sim$| 0.6 km s |$^{-1}$|⁠ , which is comparable to three times the isothermal sound speed. This infall velocity appears to be consistent with the evolution of a gravitationally infalling core. [ABSTRACT FROM AUTHOR]

We have carried out multi-epoch VLBI observations of the H2O maser sources associated with young stellar objects (YSOs) in nearby molecular clouds with VERA (VLBI Exploration of Radio Astrometry), which is a newly constructed VLBI network in Japan (Kobayashi et al. 2003). The main goal of our study is to measure the absolute proper motions and distances to nearby molecular clouds within 1 kpc from the Sun, to reveal their 3-dimensional structures and dynamical properties. Using the VERA dual-beam receiving system (Honma et al. 2003), we have carried out phase-referencing VLBI observations and measured annual parallaxes and absolute proper motions of the H2O maser features with respect to the extragalactic radio sources. We have successfully detected the annual parallax of one of the H2O maser features in Orion KL to be 2.29±0.10 mas, corresponding to the distance of 437±19 pc from the Sun (Hirota et al. 2007). In addition, the annual parallax of SVS13 in NGC 1333 is also determined to be4.10±0.17 mas, corresponding to the distance of 244±10 pc from the Sun, although the life time of the maser features are only 6 months. The absolute proper motions of the H2O maser features associated with Orion KL and NGC 1333 are derived, possibly indicating the outflow motions from the YSOs as well as the systemic motions of the powering sources. [ABSTRACT FROM PUBLISHER]