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We successfully measured the trigonometric parallax of Sagittarius A* (Sgr A*) to be 117 ± 17 micro-arcseconds ($\, \mu \mbox{as}$) using the VLBI Exploration of Radio Astrometry (VERA) with the newly developed broad-band signal-processing system named “OCTAVE-DAS.” The measured parallax corresponds to a Galactocentric distance at the Sun of $R_0 = 8.5^{+1.5}_{-1.1}\:$kpc. By combining the astrometric results with VERA and the Very Long Baseline Array (VLBA) over a monitoring period of 25 yr, the proper motion of Sgr A* is obtained to be (μα, μδ) = (−3.133 ± 0.003, −5.575 ± 0.005) mas yr−1in equatorial coordinates, corresponding to (μl, μb) = (−6.391 ± 0.005, −0.230 ± 0.004) mas yr−1in Galactic coordinates. This gives an angular orbital velocity of the Sun of Ω⊙= 30.30 ± 0.02 km s−1kpc−1. We find upper limits to the core wander, Δθ < 0.20 mas (1.6 au), peculiar motion, Δμ < 0.10 mas yr−1(3.7 km s−1), and acceleration, a< 2.6 $\, \mu \mbox{as} \:$yr−2(0.10 km s−1yr−1) for Sgr A*. Thus, we obtained upper mass limits of $\approx 3 \times 10^{4}\, M_{\odot }$and $\approx 3 \times 10^{3}\, M_{\odot }$for the supposed intermediate-mass black holes at 0.1 and 0.01 pc from the Galactic center, respectively.

We present observational results of water vapor maser emission with our high-sensitivity 22 GHz very long baseline interferometry (VLBI) imaging of the Seyfert galaxy NGC 1068. In this galaxy, there are the following four nuclear radio sources; NE, C, S1, and S2. Among them, the S1 component has been identified as the nucleus while the C component has been considered as attributed to the radio jet. In our VLBI observation, we find the following two types of water maser emission at the S1 component. One is a linearly aligned component that is considered as an edge-on disk with an inner radius of 0.62 pc. The dynamical mass enclosed within the inner radius was estimated to be 1.5 × 107M⊙by assuming the circular Keplerian motion. Note, however, that the best-fitting rotation curve shows a sub-Keplerian rotation (v∝ r−0.24±0.10). The other is water maser emission distributed around the rotating disk component up to 1.5 pc from the S1 component, suggesting a bipolar outflow from the S1 component. Further, we detected water maser emission in the C component for the first time with VLBI, and discovered a ring-like distribution of water maser emission. It is known that a molecular cloud is associated with the C component (both HCN and HCO+emission lines are detected by ALMA). Therefore, the ring-like maser emission can be explained by the jet collision to the molecular cloud. However, if these ring-like water masing clouds constitute a rotating ring around the C component, it is likely that the C component also has a supermassive black hole with a mass of ∼106M⊙that could be supplied from a past minor merger of a nucleated satellite galaxy.

We report first on two-epoch mapping observations of SiO $v =$3 $J =$1–0 maser emission in the semiregular variable W Hydrae using the Japanese VLBI Network. The flux density of the $v =$3 $J =$1–0 emission detected on 2009 February 28 was two orders of magnitude smaller than those of the $v =$1 and $v =$2 emissions, while a month and half later the $v =$3 flux density suddenly increased by a factor of $\sim $25. In contrast, the $v =$1 and $v =$2 flux densities decreased during this period, as expected from the optical light curve. At the first epoch, the $v =$3 maser features were located inside of a ring composed of the $v =$1 and $v =$2 features by 6 mas ($\gtrsim $0.5 AU) toward the central star. These offsets are meaningfully larger than the error of the fitted ring radius and the difference in the ring sizes of $v =$1 and $v =$2 masers ($\lesssim $0.2 AU). The present result at the first epoch suggests that $v =$3 $J =$1–0 SiO masers are predominantly excited in a pumping mechanism (e.g., collisional pumping) different from that recently proposed on the basis of the line overlap with infrared H$_{2}$O lines. Interestingly, the second-epoch observation revealed that the $v =$3 features were located on almost the same ring as the $v =$2 ring, which is consistent with what line-overlap theory suggests.

We conducted radio detection observations at 8.4 GHz for 22 radio-loud broad absorption line (BAL) quasars, selected from the Sloan Digital Sky Survey Third Data Release, by a very-long-baseline interferometry (VLBI) technique. The VLBI instrument we used was developed by the Optically ConnecTed Array for VLBI Exploration project (OCTAVE), which is operated as a subarray of the Japanese VLBI Network. We aimed to select BAL quasars with nonthermal jets suitable for measuring their orientation angles and ages by subsequent detailed VLBI imaging studies to evaluate two controversial issues of whether BAL quasars are viewed nearly edge-on, and of whether BAL quasars are in a short-lived evolutionary phase of the quasar population. We detected 20 out of 22 sources using the OCTAVE baselines, implying brightness temperatures greater than 10$^{5}$K, which presumably come from nonthermal jets. Hence, BAL outflows and nonthermal jets can be generated simultaneously in these central engines. We also found four inverted-spectrum sources, which are interpreted as Doppler-beamed, pole-on-viewed relativistic jet sources, or young radio sources: single edge-on geometry cannot describe all BAL quasars. We discuss the implications of the OCTAVE observations for investigations for the orientation and evolutionary stage of BAL quasars.

We present a study of H$_{2}$O maser emission from the LINER, IC 1481, based on single-dish and highly sensitive VLBI observations. The Nobeyama 45-m telescope detected new maser features near the systemic velocity and blue-shifted by 124–205 km s$^{-1}$. In addition, the known features red-shifted by $\approx $97–155 km s$^{-1}$were monitored. VLBI observations with the High Sensitivity Array (HSA) revealed that the maser features are distributed along a line, with a velocity gradient, strongly suggesting a nearly edge-on molecular gas disk. The radius and thickness of the disk are $r$$=$2.8–14.0 pc and 2$H$$=$1.5–4.2 pc, respectively, which are the largest among the maser disks detected in active galactic nuclei. The disk is rotating at $V_{{\rm rot}}$$=$124–168 km s$^{-1}$, having a large velocity dispersion of $\triangle V$$\approx $31 km s$^{-1}$. The rotation curve of the disk is sub-Keplerian ($V_{{\rm rot}}$$\propto$$r^{-0.19 \pm 0.04}$), indicating a massive disk. The disk mass is estimated to be (4.3$ \pm $0.3) $\times$10$^{7} M_{\odot }$and the mass of a putative central black hole is $\lt $10$^{7} M_{\odot }$. The massive disk is gravitationally unstable. A radio continuum source was also detected to be offset from the maser disk by 1.6 pc.

We monitored the velocities of high-velocity features for a $\mathrm{H}_2 \mathrm{O}$megamaser, NGC 4258, using the 45-m telescope of the Nobeyama Radio Observatory. The spiral shock model of maser disks predicts that the red-shifted features of observed maser spectra decelerate and the blue-shifted features accelerate for trailing arms. To confirm this model, we measured the velocity drift rates. Using the data of 1992–2005, the drift rate was detected to be ${\overline{a} =-0.036 \pm 0.007 \,\mathrm{km} \,\mathrm{s}^{-1} \,\mathrm{yr}^{-1}}$on the average for 10 red-shifted features. The drift rate of a blue-shifted feature was $a = 0.20 \pm 0.10 \,\mathrm{km} \,\mathrm{s}^{-1} \,\mathrm{yr}^{-1}$. These results are consistent with a prediction of the spiral shock model. The pitch angles of the spirals were obtained to be $\overline{\theta_{\mathrm{p}}} = {2{}^{\mathrm {\circ }}} \pm {1{}^{\mathrm {\circ }}}$on the average for the red-shifted features and $\theta_{\mathrm{p}} = {13{}^{\mathrm {\circ }}} \pm {7{}^{\mathrm {\circ }}}$for the blue-shifted feature from the measured drift rates.

We made a survey of water-vapor maser emission for 93 AGNs with the Nobeyama 45-m and Mopra 22-m telescopes from 1999 to 2002. A megamaser was detected in a Seyfert 2 galaxy, NGC 6926, at a distance of 80 Mpc, in 2002 June. [Greenhill et al. (2003a)have also reported a detection of the megamaser at the close date.] The peak flux density was 110 mJy, and the total isotropic luminosity was 340${L}_{\odot}$. The maser shows triply peaked spectrum, suggesting an edge-on disk. A narrow-line feature of the maser components at $V_{\mathrm{LSR}} = 6001 \,\mathrm{km} \,\mathrm{s}^{-1}$was strongly variable with a time scale of a few tens of days, and the variation should be of intrinsic origin. We also showed a possibility of variability of water-vapor maser emission of a megamaser previously detected in a Seyfert/ultraluminous FIR galaxy, NGC 6240.

We observed a Seyfert 2/LINER galaxy NGC 3079 using the NRO 45-m telescope from 1992 to 2002, and the Very Long Baseline Array (VLBA) in 2001 December, at 22 GHz. The 45-m telescope detected weak red-shifted and systemic features as well as strong blue-shifted features. High spatial resolution observations with the VLBA indicated that the $\mathrm{H_2O}$maser spots are distributed along the north-south direction, showing blue-shifted velocities in the north and red-shifted in the south. We propose a nearly edge-on molecular gas disk in the nuclear region of NGC 3079, whose radius varies from 0.4 to 1.2 pc and whose thickness is 0.3 pc. The disk is rotating at $V_\mathrm{rot} = 200 \hbox{--} 100 \,\mathrm{km} \,\mathrm{s}^{-1}$, having a large velocity dispersion of $\Delta V \approx 50 \,\mathrm{km} \,\mathrm{s}^{-1}$. The binding mass within the disk is $\sim 10^6 \,{{{M}_{\odot}}}$. The disk is inclined such that the east side is nearest to us. The large luminosity of the maser emission and exceptionally strong blue-shifted features of NGC 3079 are probably the result of local shocks in the disk. Three continuum components are interpreted as jets emitted from the nucleus. The velocities of the components relative to the dynamical center of the masing disk are $(0.09 \hbox{--} 0.17) \,c, <0.005 \,c$, and $<0.05 \,c$.

We present results of the radio observations of the afterglow of GRB 030329 with the Nobeyama 45-m telescope. The observations were made at 23.5 GHz, 43 GHz, and 90 GHz. The light curves show a steep decline after a constant phase. The start time of the decline depends on the frequency. Namely, the decline started earlier at a higher frequency. The spectrum has a peak in the mm wavelength range. The peak frequency and the peak flux decreased with time. These results are consistent with the expectation from the fireball model.

We report on a flare-up of water-vapor maser emission in a prototypical ultraluminous FIR galaxy with an AGN, NGC 6240. The flux density of the detected maser line was $ \approx 0.1 \,\mathrm{Jy}$with $ \mathit{FWHM} \approx 5 \,\mathrm{km} \,\mathrm{s}^{-1}$, indicating an apparent isotropic luminosity of $\approx 100 \,{L}_{\odot}$at a distance of 100 Mpc, which is typical for water megamasers. The maser line was redshifted by $ \approx 200 \,\mathrm{km} \,\mathrm{s}^{-1}$relative to the systemic velocity of the galaxy, while no systemic-velocity feature was detected. The previous possible detection or observed upper limits were at a level lower than the flux density in the present paper, indicating that the water maser in NGC 6240 flared up by more than a factor of 5 in 2001.

We present the first astrometry catalog from the Japanese VLBI (very long baseline interferometer) project VERA (VLBI Exploration of Radio Astrometry). We have compiled all the astrometry results from VERA, providing accurate trigonometric-annual-parallax and proper-motion measurements. In total, 99 maser sources are listed in the VERA catalog. Among them, 21 maser sources are newly reported, while the rest of the 78 sources are referred to in previously published results or those in preparation for forthcoming papers. The accuracy in the VERA astrometry is revisited and compared with that from the other VLBI astrometry projects such as BeSSeL (The Bar and Spiral Structure Legacy) Survey and GOBELINS (the Gould’s Belt Distances Survey) with the VLBA (Very Long Baseline Array). We have confirmed that most of the astrometry results are consistent with each other, and the largest error sources are due to source structure of the maser features and their rapid variation, along with the systematic calibration errors and different analysis methods. Combined with the BeSSeL results, we estimate the up-to-date fundamental Galactic parameters of $R_{0}=7.92\pm 0.16_{\rm {stat.}}\pm 0.3_{\rm {sys.}}\:$kpc and $\Omega _{\odot }=30.17\pm 0.27_{\rm {stat.}}\pm 0.3_{\rm {sys.}}\:$km$\:$s$^{-1}\:$kpc$^{-1}$, where $R_{0}$and $\Omega _{\odot }$are the distance from the Sun to the Galactic center and the Sun’s angular velocity of the Galactic circular rotation, respectively.

A new method to discover obscured active galactic nuclei (AGNs) by utilizing X-ray and infrared data has recently been developed. We carried out a survey of H2O maser emission toward 10 obscured AGNs with the Nobeyama 45 m telescope. We newly detected the maser emission with a signal-noise-ratio (SNR) of above 4 from two AGNs; NGC 1402 and NGC 7738. We also found a tentative detection with SNR> 3 in NGC 5037. The detection rate of 20% is higher than those of previous surveys (usually several percent).

We present survey data of six NH3(J, K) = (1, 1), (2, 2), (3, 3), (4, 4), (5, 5), and (6, 6) lines, simultaneously observed with the Tsukuba 32 m telescope, in the main part of the central molecular zone of the Galaxy. The total number of on-source positions was 2655. The three lower transitions were detected with S/N> 3 at 2323 positions (93% of all the on-source positions). Among 2323, the S/N's of (J, K) = (4, 4), (5, 5), and (6, 6) exceeded 3.0 at 1426(54%), 1150(43%), and 1359(51%) positions, respectively. Simultaneous observations of the lines enabled us to accurately derive intensity ratios with less systematic errors. Boltzmann plots indicate that there are two temperature components: cold (∼20 K) and warm (∼100 K). Typical intensity ratios of Tmb(2, 2)/Tmb(1, 1), Tmb(4, 4)/Tmb(2, 2), Tmb(5, 5)/Tmb(4, 4), and Tmb(6, 6)/Tmb(3, 3) were 0.71, 0.45, 0.65, and 0.17, respectively. These line ratios correspond to the diversity of the rotational temperature, which results from mixing of the two temperature components.

We conducted a high-sensitivity radio detection survey for 40 narrow-line Seyfert 1 (NLS1) galaxies using a very long baseline interferometry (VLBI) technique at 22 GHz through phase-referencing long-time integration and using a newly developing recorder with a data rate of 8 Gbps, which is a candidate of the next generation VLBI data recording systems of the Japanese VLBI Network. The baseline sensitivity was typically a few mJy. The observations resulted in a detection rate of 12/40 for our radio-selected NLS1 sample: 11 out of the 12 detected NLS1s showed inverted radio spectra between 1.4 and 22 GHz on the basis of the Very Large Array flux densities and the VLBI detections. These high fractions suggest that a compact radio core with a high brightness temperature is frequently associated with NLS1 nuclei. On the other hand, at least half of the sample indicated apparently steep spectra even with the limited VLBI sensitivity. Both the inverted and the steep spectrum radio sources are included in the NLS1 population.

We report on a distance measurement of 22 GHz H2O maser features associated with an ultra-compact H iiregion G7.47+0.06 using VERA (VLBI Exploration of Radio Astrometry). Since the source is located farther away than 10 kpc, it turned out to be difficult to derive the distance from annual parallax measurement. Meanwhile, we clearly detected the source's proper motion parallel to the Galactic plane. The proper motion is μ = −5.03 ± 0.07 mas yr−1and is approaching the Galactic center. We applied a new method to determine the source distance based on absolute proper motions. Considering uncertainties of the Galactic rotation curve and the solar peculiar motion, the detected proper motion leads to a source distance of D= 20 ± 2 kpc, demonstrating that astrometric observation can provide an accurate distance measurement at a 10% level even for sources too distant to measure the annual parallax. Lastly, we scale the physical parameters of the H iiregion estimated in a previous paper to be 20 kpc, and show that the H2O maser features are associated with a massive star-forming region corresponding to the spectral type of O5.5.

We present the results of ammonia observations toward the center of NGC 3079. The NH3(J, K) = (1, 1) and (2, 2) inversion lines were detected in absorption with the Tsukuba 32 m telescope, and the NH3(1, 1) through (6, 6) lines with the VLA, although the profile of NH3(3, 3) was in emission in contrast to the other transitions. The background continuum source, whose flux density was ∼ 50 mJy, could not be resolved with the VLA beam of ≲ 0${^{\prime\prime}_{.}}$09 × 0${^{\prime\prime}_{.}}$08. All ammonia absorption lines have two distinct velocity components: one is at the systemic velocity and the other is blueshifted, and both components are aligned along the nuclear jets. For the systemic components, the relatively low temperature gas is extended more than the high temperature gas. The blueshifted NH3(3, 3) emission can be regarded as ammonia masers associated with shocks by strong winds probably from newly formed massive stars or supernova explosions in dense clouds in the nuclear megamaser disk. Using para-NH3(1, 1), (2, 2), (4, 4), and (5, 5) lines with VLA, we derived the rotational temperature Trot= 120 ± 12 K and 157 ± 19 K for the systemic and blueshifted components, respectively. The total column densities of NH3(0, 0)–(6, 6), assuming Tex≈ Trot, were (8.85 ± 0.70) × 1016cm−2and (4.47 ± 0.78) × 1016cm−2for the systemic and blueshifted components, respectively. The fractional abundance of NH3relative to molecular hydrogen H2for the systemic and blueshifted was [NH3]/[H2] = 1.3 × 10−7and 6.5 × 10−8, respectively. We also found the F= 4–4 and F= 5–5 doublet lines of OH 2Π3/2J= 9/2 in absorption, which could be fitted by two velocity components, systemic and redshifted components. The rotational temperature of OH was estimated to be Trot, OH≥ 175 K, tracing hot gas associated with the interaction of the fast nuclear outflow with dense molecular material around the nucleus.

We present large-field (15.7 $ \times$16.9 arcmin$ ^2$) CO ($ J$$ =$1$ \rightarrow$0) observations of the starburst galaxy M 82, at an angular resolution of 22$ ''$with the NRO 45-m telescope. The CO emission was detected in the galactic disk, outflow (driven by the galactic wind) up to $ \sim$2 kpc above the galactic plane in the halo, and in tidal streams. The kinematics of the outflow (including CO line splitting) suggests that it has the shape of a cylinder that is diverging outwards. The mass and kinetic energy of the molecular gas outflow are estimated to be (0.26–1.0) $ \times$10$ ^{9}\ M_{\odot}$and (1–4) $ \times$10$ ^{56}$erg. A clump of CO gas was discovered 3.5 kpc above the galactic plane; it coincides with a dark lane previously found in X-ray observations, and a peak in H Iemission. A comparison with H I, hot molecular hydrogen and dust suggests that the molecular gas shows signatures of warm and cool components in the outflow and tidal streams, respectively.

We present analyses to determine the fundamental parameters of the Galaxy based on VLBI astrometry of 52 Galactic maser sources obtained with VERA, VLBA, and EVN. We model the Galaxy's structure with a set of parameters, including the Galaxy center distance $ R_0$, the angular rotation velocity at the LSR $ \Omega_0$, the mean peculiar motion of the sources with respect to Galactic rotation ($ U_{\rm src}$, $ V_{\rm src}$, $ W_{\rm src}$), the rotation-curve shape index, and the $ V$component of the Solar peculiar motions, $ V_\odot $. Based on a Markov chain Monte-Carlo method, we find that the Galaxy center distance is constrained at a 5% level to be $ R_0$$ =$8.05 $ \pm$0.45 kpc, where the error bar includes both statistical and systematic errors. We also find that the two components of the source peculiar motion $ U_{\rm src}$and $ W_{\rm src}$are fairly small compared to the Galactic rotation velocity, being $ U_{\rm src}$$ =$1.0 $ \pm$1.5 km s$ ^{-1}$and $ W_{\rm src}$$ =$$-$1.4 $ \pm$1.2 km s$ ^{-1}$. Also, the rotation curve shape is found to be basically flat between Galacto-centric radii of 4 and 13 kpc. On the other hand, we find a linear relation between $ V_{\rm src}$and $ V_\odot $as $ V_{\rm src}$$ =$$ V_\odot $$-$19 ($ \pm$2) km s$ ^{-1}$, suggesting that the value of $ V_{\rm src}$is fully dependent on the adopted value of $ V_\odot $. Regarding the rotation speed in the vicinity of the Sun, we also find a strong correlation between $ \Omega_0$and $ V_\odot $. We find that the angular velocity of the Sun, $ \Omega_{\odot}$, which is defined as $ \Omega_\odot$$ \equiv$$ \Omega_0$$ +$$ V_\odot/R_0$, can be well constrained with the best estimate of $ \Omega_\odot$$ =$31.09 $ \pm$0.78 km s$ ^{-1}$kpc$ ^{-1}$. This corresponds to $ \Theta_0$$ =$238 $ \pm$14 km s$ ^{-1}$if one adopts the above value of $ R_0$and recent determination of $ V_\odot $$ \sim$12 km s$ ^{-1}$.

We present the results of single-dish and VLBI observations for the water-vapor masers at the nucleus of a Seyfert 2, IC 2560. We monitored the velocities of the maser features with the 45-m telescope of the Nobeyama Radio Observatory. Using data form 1995–2006, the velocity drift rate was detected to be $ \bar{a}$$ =$$ +$2.57 $ \pm$0.04 km s$ ^{-1}$yr$ ^{-1}$on the average for 6 systemic features. The Very Long Baseline Array (VLBA) with the Very Large Array (VLA) firstly detected red-shifted and blue-shifted maser features of IC 2560, in addition to systemic maser features and a continuum component. We propose a maser disk in the nuclear region. The systemic and red-shifted features are emitted from a nearly edge-on disk with a position angle of PA$ =$$-$46$ ^{\circ}$, which is almost perpendicular to the galactic disk. Assuming Keplerian rotation, the radii of the maser disk are $ r$$ =$0.087–0.335 pc, and the thickness is 2$ H$$ \leq$0.025 pc. The binding mass is 3.5 $ \times$10$ ^{6}$$ M\odot$at a distance of $ D$$ =$26 Mpc, and the mean volume density within the inner radius is 1.3 $ \times$10$ ^{9}M\odot$pc$ ^{-3}$, strongly suggesting a massive black hole at the center. A continuum component was detected at 0.2 pc southwest of the disk center, and is considered to be a jet ejected from the nucleus, with an angle of 70$ ^{\circ}$from the disk. The blue-shifted maser feature is located on the continuum component, being interpreted to be a ``jet maser.'' The distance to IC 2560 was estimated to be $ D$$ =$31$ ^{+12}_{-14}$Mpc from the geometry of the maser disk and the velocity drift rate.