Search

We present a detailed analysis of the kinematics of SiO maser stars around the center of the Milky Way, Sagittarius A* (Sgr A*). We used the archive data in the SiO v=1, J=2-1 emission line obtained by the Atacama Large Millimeter/Submillimeter Array (ALMA) in 2017 and 2021 (#2016.1.00940.S, PI Darling, J. and #2019.1.00292.S, PI Paine, J.). We detected 37 SiO maser stars in the channel maps and derived their angular offsets relative to Sgr A* and LSR radial velocities. We derived the proper motions of 35 stars by comparing their angular offsets in the two epochs. The proper motions of Wolf-Rayet and O star in the Nuclear Star Cluster (NSC) are reported to be rather random, except for the co-moving clusters IRS13E and IRS13N (Tsuboi et al. 2022). However, the derived proper motions of SiO maser stars do not look completely random. The proper motions of the SiO maser stars show a tendency to lie along the Galactic plane. The proper motion amplitudes of SiO maser stars are larger than the LSR velocity amplitudes. We estimated the 3D motions from the proper motions and LSR velocities. Many 3D velocities are near to or larger than the upper limit velocities for Kepler orbits around Sgr A*, whose mass is assumed to be 4x10^6 Msun. These indicate that the SiO maser stars around Sgr A* are members of the Nuclear Stellar Disk rather than the NSC., Comment: 20 pages, 10 figures, 2 tables, submitted to PASJ

We propose that the ring structure found by the Event Horizon Telescope Collaboration (EHTC) as the black hole shadow of Sgr A*is an artifact by the bumpy PSF (Point Spread Function) of the EHT2017. The imaging using sparse u-v data requires detailed scrutiny of the PSF. The estimated shadow diameter (48.7 +- 7 muas) is equal to the spacing between the main beam and the first sidelobe of the PSF (49.09 muas), which immediately suggests a potential problem in the deconvolution of the PSF. We show that the ring image can be derived from non-ring simulated datasets (noise only; point source) with a narrow Field-of-View (FOV) and an assumed self-calibration suggesting the EHT2017's u-v coverage is insufficient for reliable imaging. The EHTC analysis, based on calibrations with assumptions about the source's size and properties, selected the final image by prioritizing appearance rate of the similar structure from a large imaging parameter space over data consistency. Our independent analysis with the conventional hybrid mapping reveals an elongated east-west structure, consistent with previous observations. We believe it to be more reliable than the EHTC image, due to half the residuals in normalized visibility amplitude. The eastern half is brighter, possibly due to a Doppler boost from the rapid rotating disk. We hypothesize our image shows a portion of the accretion disk about 2 to a few Rs away from the black hole, rotating with nearly 60 % of the speed of light viewed from an angle of 40 -45 degrees., Comment: 28 pages, 19 figures

JASMINE is a Japanese planned space mission that aims to reveal the formation history of our Galaxy and discover habitable exoEarths. For these objectives, the JASMINE satellite performs high-precision astrometric observations of the Galactic bulge and high-precision transit monitoring of M-dwarfs in the near-infrared (1.0-1.6 microns in wavelength). For feasibility studies, we develop an image simulation software named JASMINE-imagesim, which produces realistic observation images. This software takes into account various factors such as the optical point spread function (PSF), telescope jitter caused by the satellite's attitude control error (ACE), detector flat patterns, exposure timing differences between detector pixels, and various noise factors. As an example, we report a simulation for the feasibility study of astrometric observations using JASMINE-imagesim. The simulation confirms that the required position measurement accuracy of 4 mas for a single exposure of 12.5-mag objects is achievable if the telescope pointing jitter uniformly dilutes the PSF across all stars in the field of view. On the other hand, the simulation also demonstrates that the combination of realistic pointing jitter and exposure timing differences in the detector can significantly degrade accuracy and prevent achieving the requirement. This means that certain countermeasures against this issue must be developed. This result implies that this kind of simulation is important for mission planning and advanced developments to realize more realistic simulations help us to identify critical issues and also devise effective solutions., Comment: 13 pages, 7 figures, 1 table

Two critical aspects of radio interferometric imaging analysis are data calibration and deconvolution of the point spread function (PSF) structure. Both of these are particularly important for high-frequency observations using a VLBI network consisting of a small number of stations, such as those conducted by the Event Horizon Telescope (EHT). The Event Horizon Telescope Collaboration (EHTC) has presented images of ring-shaped black holes from observations of M 87 (d = 42 +- 3 muas)(EHTC2019a) and the Galactic Center (d = 51.8 +- 2.3 muas)(EHTC2022a). The ring structures seen in the EHTC images are consistent with the estimated shadow diameter of the black hole based on its mass and distance. However, these black hole ring sizes are also the same with the typical up and down spacings (e.g., the intervals between the main beam and nearby 1st-sidelobes) seen in the point spread function (PSF; dirty beam) for each observation. These facts suggest that the EHTC ring structures are artifacts derived from the shape of the PSFs rather than the intrinsic structure of the SMBHs in M 87 and the Galactic Center. The EHTC utilizes novel imaging techniques in addition to the standard CLEAN algorithm. The CLEAN method was designed for PSF shape deconvolution in mind, yet in practice, it may not always be able to completely remove the PSF shape. In the imaging analysis of data from interferometers with a small number of antennas like the EHT, it is crucial to assess the PSF shape and compare it with the imaging results. The novel imaging methods employed by the EHTC have not yet been fully evaluated for PSF deconvolution performance, and it is highly recommended that their performance in this regard be thoroughly examined. It is also important to investigate the data calibration capability, i.e., the ability to separate error noise from the observed data., Comment: 5 pages, 4 figures, Manuscript for the Proceedings of the 13th IVS General Meeting & 25th Anniversary held at Tsukuba, Japan, March 2024

M 87 is the best target for studying black hole accretion and jet formation. Reanalysis of the EHT public data at 230 GHz shows a core-knots structure at the center and jet features (Miyoshi et al. 2022a). We here compare this with the new results of GMVA at 86 GHz showing a spatially resolved central core (Lu et al. 2023a). There are similarities and differences between the two. At 86 GHz,"two bright regions" are seen on the ring in the core."Core-Knot-Westknot", triple structure in the 230 GHz image shows apparent appearance of two peaks similar to the "two bright regions" when convolved with the GMVA beam. This similarity suggests that both frequencies reveal the same objects in the core area. Protrusions are observed on both the south and north sides of the core at both frequencies, becoming prominent and wing-like at 230 GHz. The 86 GHz image shows a triple ridge jet structure, while the 230 GHz image shows only a bright central ridge with two roots. Both frequencies show a shade between the core and the central ridge. To detect the faint features from the EHT2017 data, we found that the use of all baseline data is essential. Using all including the ultrashort baseline data, revealed the jet and faint structures. Without the ultrashort baselines, these structures were not detectable. The lack of detection of any faint structures other than the ring in the M 87 data by the EHTC is presumably due to the exclusion of ultrashort baselines from their analysis., Comment: 17 pages, 4 figures, ApJL, Accepted 27, January, 2024

By connecting two antennas, Kashima 34~m and Nobeyama 45~m, an east-west baseline of 200~km is formed. At that time, because Nobeyama 45~m had the world's number one sensitivity in the 43~GHz band, and also Kashima 34~m was the world's third-largest one, the Kashima-Nobeyama baseline provided the highest sensitivity at 43~GHz VLBI (Figure 1). The construction of the Kashima 34~m antenna began in 1988, also almost at the same time, a domestic project of mm-VLBI (KNIFE, Kashima Nobeyama INterFrermeter) started. Nobeyama Radio Observatory provided the first cooled-HEMT 43~GHz receiver in the world to the Kashima 34~m. In October 1989, the first fringe at 43~GHz was detected. We here review the achievements of the KNIFE at that time., Comment: 5 pages, 5 figures, manuscript in Proceedings of the 18th NICT TDC Symposium (Kashima, October 1, 2020)

On June 14, 2022, the EHT collaboration (hereafter EHTC) made the web page (https://eventhorizontelescope.org/blog/imaging-reanalyses-eht-data) with the title "Imaging Reanalyses of EHT Data," in which they made comments on our recent Miyoshi et al .2022 published in the Astrophysical Journal. We investigated the EHTC comments and found that all of the five points raised by the EHTC are subjective and unsubstantiated claims. Thus they do not prove the correctness of the result of EHTC. Sincerely we hope that the EHTC will publish, not a collection of unsubstantiated claims, but a discussion based on scientific arguments., Comment: Comments on the EHTC description on their web page

We report our independent image reconstruction of the M 87 from the public data of the Event Horizon Telescope Collaborators (EHTC). Our result is different from the image published by the EHTC. Our analysis shows that (a) the structure at 230 GHz is consistent with those of lower frequency VLBI observations, (b) the jet structure is evident at 230 GHz extending from the core to a few mas, though the intensity rapidly decreases along the axis, and (c) the unresolved core is resolved into bright three features presumably showing an initial jet with a wide opening angle of about 70 deg. The ring-like structures of the EHTC can be created not only from the public data, but also from the simulated data of a point image. Also, the rings are very sensitive to the FOV size. The u-v coverage of EHT lack about 40 micro-asec fringe spacings. Combining with a very narrow FOV, it created the 40 micro-asec ring structure. We conclude that the absence of the jet and the presence of the ring in the EHTC result are both artifacts owing to the narrow FOV setting and the u-v data sampling bias effect of the EHT array. Because the EHTC's simulations only take into account the reproduction of the input image models, and not those of the input noise models, their optimal parameters can enhance the effects of sampling bias and produce artifacts such as the 40 micro-asec ring structure, rather than reproducing the correct image., Comment: 68 pages, 34 figures, ApJ accepted 5th May 2022

Atacama Large Millimeter/Submillimeter Array (ALMA) is promising to be a powerful tool for precision astrometry of the area around Sagittarius A$^\ast$ (Sgr A$^\ast$) because it has the high angular resolution, high sensitivity, and wide field of view. We have observed the area including the Nuclear Star Cluster at 230 GHz with ALMA in October 2017. The angular resolution is ~0.03". We determined the relative positions to Sgr A$^\ast$ of 65 compact objects in the area with the accuracy of 0.001". We also analyzed the similar ALMA archival data obtained in June 2019 and determined the 64 relative positions in these objects. We derived the proper motions relative to Sgr A$^\ast$ by comparing these positions. The derived proper motions are roughly described with both clockwise and counterclockwise rotations around Sgr A$^\ast$. The rotation velocities are reproduced by Kepler orbits bounded around Sgr A$^\ast$. Moreover, the proper motions include co-moving clusters for example IRS13E and IRS13N. The positions and proper motions are almost consistent with those by previous infrared observations. Therefore the observational demonstrations would prove that ALMA is a powerful tool for precision astrometry of the region., Comment: 29 pages, 10 figures, 3 tables; Accepted for publication in PASJ

We performed a search of cloud-cloud collision (CCC) sites in the Sagittarius A molecular cloud (SgrAMC) based on the survey observations using the Nobeyama 45-m telescope in the C$^{32}$S $J=1-0$ and SiO $v=0~J=2-1$ emission lines. We found candidates being abundant in shocked molecular gas in the Galactic Center Arc (GCA). One of them, M0.014-0.054, is located in the mapping area of our previous ALMA mosaic observation. We explored the structure and kinematics of M0.014-0.054 in the C$^{32}$S $J=2-1$, C$^{34}$S $J=2-1$, SiO $v=0~J=2-1$, H$^{13}$CO$^+ J=1-0$, and SO $N,J=2,2-1,1$ emission lines and fainter emission lines. M0.014-0.054 is likely formed by the CCC between the vertical molecular filaments (VP) of the GCA, and other molecular filaments along Galactic longitude. The bridging features between these colliding filaments on the PV diagram are found, which are the characteristics expected in CCC sites. We also found continuum compact objects in M0.014-0.054, which have no counterpart in the H42$\alpha$ recombination line. They are detected in the SO emission line, and would be "Hot Molecular Core (HMC)"s. Because the LTE mass of one HMC is larger than the virial mass, it is bound gravitationally. This is also detected in the CCS emission line. The embedded star would be too young to ionize the surrounding molecular cloud. The VP is traced by poloidal magnetic field. Because the strength of the magnetic field is estimated to be $\sim m$Gauss using the CF method, the VP is supported against fragmentation. The star formation in the HMC of M0.014-0.054 is likely induced by the CCC between the stable filaments, which may be a common mechanism in the SgrAMC., Comment: 40 pages, 14 figures, 2 tables, accepted for publication in PASJ

The Galactic Center Lobe (GCL) is a peculiar object widely protruding from the Galactic plane toward the positive Galactic latitude, which had been found toward the Galactic Center (GC) in the early days of the radio observation. The peculiar shape has suggested any relation with historical events, star burst, large explosion and so on in the GC. However, the issue whether the GCL is a single large structure located in the GC region is not yet settled conclusively. In the previous observations, the silhouette against the low frequency emission was found in the western part of the GCL (WPGCL), This suggests that the part is located in front of the GC region. On the other hand, the LSR velocity of the radio recombination line toward it was found to be as low as 0 kms$^{-1}$. However, these observations cannot determine the exact position on the line-of-sight. There is still another possibility that it is in the near side area of the GC region. In this analysis, we compare these results with the visual extinction map toward the GC. We found that the distribution of the visual extinction larger than 4 mag. clearly corresponds to the silhouette of the WPGCL. The WPGCL must be located at most within a few kpc from us and not in the GC region. This would be a giant HII region in the Galactic disk., Comment: 10 pages, 3 figures, Accepted for publication in PASJ

A radio source at the Galactic center Sgr A* is a prime supermassive black hole candidate and therefore key to developing our understanding of them. Time variations in the 230 GHz band flux of Sgr A* have been found with the Atacama Large Millimeter/submillimeter Array (ALMA) Cycle 5 observations. Measuring the flux density of Sgr A* in 1 min snapshots at 217.5, 219.5, and 234.0 GHz, we obtained light curves for ten 70 min periods. The light curves show variations at a few tens of minutes and hourly scales. The shorter timescale is similar to the orbital period of the innermost stable circular orbit around a $4\times 10^{6}$ $M_{\odot}$ black hole, suggesting that the variation originates from the immediate vicinity of Sgr A*. We also detected no time lag between 217.5 and 234.0 GHz and a dependence of the spectral index on the flux density., Comment: 8 pages, 3 figures, 1 table, accepted for publication in ApJL

The Galactic Center IRS 13E cluster is a very intriguing IR object located at $\sim0.13$ pc from Sagittarius A$^\ast$ (Sgr A$^\ast$) in projection distance. There are both arguments for and against the hypothesis that a dark mass like an intermediate mass black hole (IMBH) exists in the cluster. Recently we have detected the rotating ionized gas ring around IRS 13E3, which belongs to the cluster, in the H30$\alpha$ recombination line using ALMA. The enclosed mass is derived to be $M_{\mathrm{encl.}}\simeq2\times10^4$ $M_\odot$, which agrees with an IMBH and is barely less than the astrometric upper limit mass of the IMBH around Sgr A$^\ast$. Because the limit mass depends on the true three-deminsional (3D) distance from Sgr A$^\ast$, it is very important to determine it observationally. However, the 3D distance is indefinite because it is hard to determine the line-of-sight (LOS) distance by usual methods. We would attempt to estimate the LOS distance by spectroscopic informations. The CH$_3$OH molecule is easily destroyed by cosmic ray around Sgr A$^{\ast}$. However, we detected a highly excited CH$_3$OH emission line in the ionized gas stream associated with IRS 13E3. This indicates that IRS 13E3 is located at $r\gtrsim 0.4$ pc from Sgr A$^{\ast}$., Comment: 9 pages, 2 figures, 1 table, Accepted for publication in PASJ

IRS 7 is an M red supergiant star which is located at $5".5$ north of Sagittarius A$^\ast$. We detected firstly the continuum emission at 340 GHz of IRS 7 using ALMA. The total flux density of IRS 7 is $S_\nu=448\pm45 \mu$Jy. The flux density indicates that IRS 7 has a photosphere radius of $R=1170\pm60 ~R_\odot$, which is roughly consistent with the previous VLTI measurement. We also detected a shell like feature with north extension in the H30$\alpha$ recombination line by ALMA. The electron temperature and electron density of the shell like structure are estimated to be $\bar{T}^\ast_{\mathrm e}=4650\pm500$ K and $\bar{n}_{\mathrm e}=(6.1\pm0.6)\times10^4$ cm$^{-3}$, respectively. The mass loss rate is estimated to be $\dot{m} \sim 1\times 10^{-4} M_\odot$ yr$^{-1}$, which is consistent with a typical mass loss rate of a pulsating red supergiant star with $M=20-25 M_\odot$. The kinematics of the ionized gas would support the hypothesis that the shell like structure made by the mass loss of IRS 7 is supersonically traveling in the ambient matter toward the south. The brightened southern half of the structure and the north extension would be a bow shock and a cometary-like tail structure, respectively., Comment: 15 pages, 5 figures Accepted for publication in PASJ

We have observed the compact HII region complex nearest to the dynamical center of the Galaxy, G-0.02-0.07, using ALMA in the H42a recombination line, CS J=2-1, H13CO+ J=1-0, and SiO v=0, J=2-1 emission lines, and 86 GHz continuum emission. The HII regions HII-A to HII-C in the cluster are clearly resolved into a shell-like feature with a bright-half and a dark-half in the recombination line and continuum emission. The absorption features in the molecular emission lines show that HII-A, B and C are located on the near side of the 50 km/s Molecular Cloud (50MC) but HII-D is located on the far side. The electron temperatures and densities range Te=5150-5920 K and ne=950-2340 cm-3, respectively. The electron temperatures on the bright-half are slightly lower than those on the dark-half, while the electron densities on the bright-half are slightly higher than those on the dark-half. The HII regions are located on the molecular filaments in the 50MC. They have already broken through the filaments and are growing in the surrounding molecular gas. There are some shocked molecular gas components around the HII regions. From line width of the H42a recombination line, the expansion velocities from HII-A to HII-D are estimated to be Vexp=16.7, 11.6, 11.1, and 12.1 km/s, respectively. The expansion timescales from HII-A to HII-D are estimated to be Tage~1.4x0^4, 1.7x10^4, 2.0x10^4, and 0.7x10^4 years, respectively. The spectral types of the central stars from HII-A to HII-D are estimated to be O8V, O9.5V, O9V, and B0V, respectively. The positional relation among the HII regions, the SiO molecule enhancement area, and Class-I maser spots suggest that the shock wave caused by a cloud-cloud collision propagated along the line from HII-C to HII-A in the 50MC. The shock wave would trigger the massive star formation., Comment: 33 pages, 13 figures, 3 tables. Accepted for publication in PASJ

We detected a compact ionized gas associated physically with IRS13E3, an Intermediate Mass Black Hole (IMBH) candidate in the Galactic Center, in the continuum emission at 232 GHz and H30$\alpha$ recombination line using ALMA Cy.5 observation (2017.1.00503.S, P.I. M.Tsuboi). The continuum emission image shows that IRS13E3 is surrounded by an oval-like structure. The angular size is $0".093\pm0".006\times 0".061\pm0".004$ ( $1.14\times10^{16}$ cm $\times 0.74\times10^{16}$ cm). The structure is also identified in the H30$\alpha$ recombination line. This is seen as an inclined linear feature in the position-velocity diagram, which is usually a defining characteristic of a rotating gas ring around a large mass. The gas ring has a rotating velocity of $V_\mathrm{rot}\simeq230$ km s$^{-1}$ and an orbit radius of $r\simeq6\times10^{15}$ cm. From these orbit parameters, the enclosed mass is estimated to be $M_{\mathrm{IMBH}}\simeq2.4\times10^4$ $M_\odot$. The mass is within the astrometric upper limit mass of the object adjacent to Sgr A$^{\ast}$. Considering IRS13E3 has an X-ray counterpart, the large enclosed mass would be supporting evidence that IRS13E3 is an IMBH. Even if a dense cluster corresponds to IRS13E3, the cluster would collapse into an IMBH within $\tau<10^7$ years due to the very high mass density of $\rho \gtrsim8\times10^{11} M_\odot pc^{-3}$. Because the orbital period is estimated to be as short as $T=2\pi r/V_\mathrm{rot}\sim 50-100$ yr, the morphology of the observed ionized gas ring is expected to be changed in the next several decades. The mean electron temperature and density of the ionized gas are $\bar{T}_{\mathrm e}=6800\pm700$ K and $\bar{n}_{\mathrm e}=6\times10^5$ cm$^{-3}$, respectively. Then the mass of the ionized gas is estimated to be $M_{\mathrm{gas}}=4\times10^{-4} M_\odot$., Comment: accepted for publication in PASJ. 13 pages, 4 figures

We present the high angular resolution and high sensitivity images of the "Circum-Nuclear Disk (CND)" and its surrounding region of Milky Way Galaxy in the CS J=2-1, SiO v=0 J=2-1, H^13CO^+ J=1-0, C^34S J=2-1, and CH_3OH J_{K_a, K_c}=2_{1,1}-1_{1,0}A_{--} emission lines using ALMA. The CND is recognized as a torus-like molecular gas with gaps in these emission lines except for the CH_3OH emission line. The inner and outer radii of the CND are estimated to be R_in~1.5 and R_out~2 pc, respectively. The velocities of the rotation and radial motion are estimated to be V_rot~115 km s^-1 and V_rad~23 km s^-1, respectively. The LTE molecular gas mass is estimated to be M_LTE~3x10^4 Mo. We also found some anomalous molecular clouds in the surrounding region. One of the molecular clouds is positionally connected to a part of the CND adjacent to the "Western Arc". However, the cloud is seen to rotate in the opposite direction to the CND. The molecular cloud would be falling currently from the outer region to the CND and being disrupted by the tidal shear of Sagittarius A*(Sgr A*) because the velocity is not yet assimilated into that of the CND. Another molecular cloud is continuously connected to the tip of the "Eastern Arm (EA)". The velocity of this cloud is consistent with that of the ionized gas in the EA. These facts suggest that the molecular cloud is falling from the outer region to the vicinity of Sgr A*, being disrupted by the tidal shear, and ionized by strong UV emission from the Central Cluster because the impact parameter of the cloud is smaller than the first cloud. These falling clouds would play an important role in transferring material from the outer region to the CND and/or the vicinity of Sgr A*., Comment: 31 pages, 12 figures, Accepted for publication in PASJ

The Galactic Center is the nuclear region of the nearest spiral galaxy, Milky Way, and contains the supermassive black hole with M~4x10^6 Msun, Sagittarius A* (Sgr A*). One of basic questions about the Galactic Center is whether Sgr A* alone exists as a "massive" black hole in the region or not. The IRS13E complex is a very intriguing IR object which contains a large dark mass comparable to the mass of an intermediate mass black hole (IMBH) from the proper motions of the main member stars. However, the existence of the IMBH remains controversial. There are some objections to accepting the existence of the IMBH. In this study, we detected ionized gas with a very large velocity width (Delta v_{FWZI} ~ 650 km/s) and a very compact size (~400 AU) in the complex using ALMA. We also found an extended component connecting with the compact ionized gas. The properties suggest that this would be an ionized gas flow on the Keplerian orbit with high eccentricity. The enclosed mass is estimated to be 10^4 Msun by the analysis of the orbit. The mass does not conflict with the upper limit mass of the IMBH around Sgr A* which is derived by the long-term astrometry with VLBA. In addition, the object probably has an X-ray counterpart. Consequently, a very fascinated possibility is that the detected ionized gas is rotating around an IMBH embedded in the IRS13E complex., Comment: 19 pages, 4 figures, 1 table, accepted for publication in APJL

We have performed the observation of the "Galactic Center Mini-spiral(GCMS)" in H42alpha recombination line as a part of the first large-scale mosaic observation in the Sagittarius A complex using Atacama Millimeter/sub-millimeter Array (ALMA). We revealed the kinematics of the ionized gas streamers of the GCMS. We found that the ionized gas streamers of the Northern Arm(NA) and Eastern Arm(EA) in their outer regions somewhat deviate from the Keplerian orbits which were derived previously from the trajectories in the inner regions. In addition, we found that the streamer corresponding to the Bar of the GCMS has a Keplerian orbit with an eccentricity of e~0.8, which is independent from the Keplerian orbits of the other streamers of the GCMS. We estimated the LTE electron temperature and electron density in the ionized gas streamers. We confirmed the previously claimed tendency that the electron temperatures increase toward Sgr A*. We found that the electron density in the NA and EA also increases with approaching Sgr A* without the lateral expansion of the gas streamers. This suggests that there is some external pressure around the GCMS. The ambient ionized gas may cause the confinement and/or the perturbation for the orbits. There is a good positional correlation between the protostar candidates detected by JVLA at 34 GHz and the ionized gas streamer, Northeastern Arm, newly found by our H42alpha recombination line observation. This suggests that the candidates had formed in the streamer and they were brought to near Sgr A* as the streamer falls., Comment: 35 pages, 12 figures, Accepted for publication in ApJ

We present further results of a search for extragalactic submillimeter H2O masers using the Atacama Large Millimeter/Submillimeter Array (ALMA). The detection of a 321 GHz H2O maser in the nearby Type 2 Seyfert galaxy, the Circinus galaxy, has previously been reported, and here the spectral analysis of four other galaxies is described. We have discovered H2O maser emission at 321 GHz toward the center of NGC 4945, a nearby Type 2 Seyfert. The maser emission shows Doppler-shifted velocity features with velocity ranges similar to those of the previously reported 22 GHz H2O masers, however the non-contemporaneous observations also show differences in velocity offsets. The sub-parsec-scale distribution of the 22 GHz H2O masers revealed by earlier VLBI (Very Long Baseline Interferometry) observations suggests that the submillimeter masers could arise in an edge-on rotating disk. The maser features remain unresolved by the synthesized beam of ~ 0.54 (~30 pc) and are located toward the 321 GHz continuum peak within errors. A marginally detected (3 sigma) high-velocity feature is redshifted by 579 km/s with respect to the systemic velocity of the galaxy. Assuming that this feature is real and arises from a Keplerian rotating disk in this galaxy, it is located at a radius of ~0.020 pc (~1.5 x 10^5 Schwarzschild radii), which would enable molecular material closer to the central engine to be probed than the 22 GHz H2O masers. This detection confirms that submillimeter H2O masers are a potential tracer of the circumnuclear regions of active galaxies, which will benefit from higher angular resolution studies with ALMA., Comment: 9 pages, 3 figures, accepted for ApJ (June 15, 2016)

We present continuum images of the "Galactic Center Mini-spiral" of 100, 250, and 340 GHz bands with the analysis of the Cy.0 data acquired from the Atacama Large Millimeter/submillimeter Array (ALMA) archive. Pretty good UV coverage of the data and the "self-calibration" method give us an opportunity to obtain dynamic ranges of over 2x10^4 in the resultant maps of the 250 and 340 GHz bands. In particular the image of the 340 GHz band has high dynamic ranges unprecedented in sub-millimeter wave. The angular resolutions attain to 1.57"x1.33" in the 100 GHz band, 0.63"x0.53" in the 250 GHz band, and 0.44"x0.38" in the 340 GHz band, respectively. The continuum images clearly depict the "Mini-spiral", which is an ionized gas stream in the vicinity of Sgr A*. We found the tight correlation between the dust emission peaks and the OB/WR stars in the Northern-arm of the "Mini-spiral". The core mass function of the dust core identified by the clumpfind algorithm would obey the flat power-law dN/dM=aM^-1.5+/-0.4 on the high-mass side. These support the scenario that the star forming cloud has fallen into the immediate vicinity of Sgr A* for the origin of the Central cluster., Comment: 11 pages, 4 figures, accepted for publication in PASJ, Letter

In order to explore any change caused by the G2 cloud approaching, we have monitored the flux density of Sgr A* at 22 GHz from Feb. 2013 to Aug. 2014 with a sub-array of Japanese VLBI Network . The observation period included the expected periastron dates. The number of observation epochs was 283 days. We have observed no significant microwave enhancement of Sgr A* in the whole observation period. The average flux density in the period is $S=1.23+/-0.33$ Jy. The average is consistent with the usually observed flux density range of Sgr A* at 22 GHz., Comment: Accepted for publication in Astrophysical Journal Letters, 14 pages, 1 figure

We present the first detection of extragalactic submillimeter H2O maser in the 321 GHz transition toward the center of Circinus galaxy, the nearby Type 2 Seyfert using the Atacama Large Millimeter/Submillimeter Array. We find that Doppler features of the detected 321 GHz H2O maser straddle the systemic velocity of the galaxy as seen in the spectrum of the known 22 GHz H2O maser in the galaxy. By comparing the velocities of the maser features in both transitions, it can be deduced that the 321 GHz maser occurs in a region similar to that of the 22 GHz maser, where the sub-parsec-scale distribution of the 22 GHz maser was revealed by earlier very long baseline interferometry observations. The detected maser features remain unresolved at the synthesized beam of ~0".66 (~15 pc) and coincide with the 321 GHz continuum peak within small uncertainties. We also present a tentative detection of the highest velocity feature (redshifts up to ~635 km/s) in the galaxy. If this high-velocity feature arises from a Keplerian rotating disk well established in this galaxy, it is located at a radius of ~0.018 pc (~1.2 x 10^5 Schwarzschild radii), which might probe molecular material closest to the central engine., Comment: 6 pages, 4 figures

A compact gas cloud G2 is predicted to reach the pericenter of its orbit around the super massive black hole (SMBH) of our galaxy, Sagittarius A* (Sgr A*). This event will give us a rare opportunity to observe the interaction between SMBH and gas around it. We report the result of the fully three-dimensional simulation of the evolution of G2 during the first pericenter passage. The strong tidal force by the SMBH stretches the cloud along its orbit, and compresses it strongly in the vertical direction, resulting in the heating up and flaring up of the cloud. The bolometric luminosity will reach the maximum of $\sim100 L_{\odot}$. This flare should be easily observed in the near infrared., Comment: 8 pages, 4 figures. Accepted for publication in PASJ. Animation movie and stereograms are available at http://v1.jmlab.jp/~saitoh/G2/

Astrometry using H2O maser sources in star forming regions is expected to be a powerful tool to study the structures and dynamics of our Galaxy. Honma et al. (2007) (hereafter H2007) claimed that the annual parallax of S269 is determined within an error of 0.008 milliarcsec (mas), concluding that S269 is located at 5.3 kpc +- 0.2 kpc from the sun, and R= 13.1 kpc. They claimed that the rotational velocity of S269 is equal to that of the sun within a 3% error. This small error, however, is hardly understood when taking into account the results of other observations and theoretical studies of galactic dynamics. We here reanalyzed the VERA archival data using the self-calibration method (hybrid mapping), and found that clusters of maser features of S269 are distributed in much wider area than that investigated in H2007. We confirmed that, if we make a narrow region image without considering the presence of multiple maser spots, and only the phase calibration is applied, we can reproduce the same maser structures in H2007. The distribution extent of maser spots in the feature differs 0.2 mas from east to west between our results and H2007. Moreover, we found that change of relative positions of maser spots in the cluster reaches 0.1 mas or larger between observational epochs. This suggests that if one simply assumes the time-dependent, widely distributed maser sources as a stable single point source, it could cause errors of up to 0.1 mas in the annual parallax of S269. Taking into account the internal motions of maser spot clusters, the proper motion of S269 cannot be determined precisely. We estimated that the peculiar motion of S269 with respect to a Galactic circular rotation is ~20 km/s. These results imply that the observed kinematics of maser emissions in S269 cannot give a strong constraint on dynamics of the outer part of the Galaxy, in contrast to the claim by H2007., Comment: 33 pages, 11 figures, to be published in New Astronomy

We have performed monitoring observations of the 3-mm flux density toward the Galactic Center compact radio source Sgr A* with the Australia Telescope Compact Array since 2005 October. Careful calibrations of both elevation-dependent and time-dependent gains have enabled us to establish the variability behavior of Sgr A*. Sgr A* appeared to undergo a high and stable state in 2006 June session, and a low and variable state in 2006 August session. We report the results, with emphasis on two detected intra-day variation events during its low states. One is on 2006 August 12 when Sgr A* exhibited a 33% fractional variation in about 2.5 hr. The other is on 2006 August 13 when two peaks separated by about 4 hr, with a maximum variation of 21% within 2 hr, were seen. The observed short timescale variations are discussed in light of two possible scenarios, i.e., the expanding plasmon model and the sub-Keplerian orbiting hot spot model. The fitting results indicate that for the adiabatically expanding plasmon model, the synchrotron cooling can not be ignored, and a minimum mass-loss rate of 9.7*10^{-10}M_sun /yr is obtained based on parameters derived for this modified expanding plasmon model. Simultaneous multi-wavelength observation is crucial to our understanding the physical origin of rapid radio variability in Sgr A*., Comment: 25 pages, 6 figures, accepted by ApJ

Recent VLBI (Very Long Baseline Interferometer) observations determined the distances and proper motions of star-forming regions in spiral arms directly. They showed that star-forming regions and young stars have large peculiar motions, as large as 30 km/s with complex structures. Such a large peculiar motion is incompatible with the prediction of the standard theory of quasi-stationary spiral arms. We use a high-resolution, self-consistent N-body+hydrodynamical simulation to explore how the spiral arms are formed and maintained, and how star-forming regions move. We found that arms are not quasi-stationary but transient and recurrent, as suggested in alternative theories of spiral structures. Because of this transient nature of the spiral arms, star-forming regions exhibit a trend of large and complex non-circular motions, which is qualitatively consistent with the VLBI observations. Owing to this large non-circular motion, a kinematically estimated gas map of our Galaxy has a large systematic errors of ~2-3 kpc in the distance from the Sun., Comment: 11 pages, 11 figures. Accepted to ApJ

We report results from re-analysis of the visibility data of the first 2-beam observations with VERA (VLBI Exploration of Radio Astrometry), previously reported by Honma et al., 2003 (hereafter A2003). Independently we checked the archival data and found the features noted in A2003 were not from the effect of phase referencing by simultaneous differential VLBI but mainly from a removal of large phase change by subtracting an arbitrary fitted curve to the phase variations. The differential phase of the observed H2O masers between W49 North (W49N) and OH~43.8-0.1 did not show a sinusoidal variation with a period of one sidereal day due to a positional offset from the real celestial positions. We therefore could not reproduce the results in A2003 by a normal positional correction estimated from all time data, but could reproduce almost the same phases only for the first hour by adjusting parameters. Using the parameters, we could not suppress the large amount of phase variations for the successive time data that A2003 did not show in their paper. It is appropriate to regard the analysis in A2003 as not being proper for showing the performance of the instrument for phase referencing, which should be demonstrated by other experiments observing several pairs of continuum sources., Comment: 7 pages, 6 figures, accepted to PNAOJ(2008.12.16)

Imaging the vicinity of a black hole is one of the ultimate goals of VLBI astronomy. The closest massive black hole, Sgr A*, located at the Galactic center, is the leading candidate for such observations. Combined with recent VLBI recording technique and submillimeter radio engineering, we now have sufficient sensitivity for the observations. Here we show performance simulations of submillimeter VLBI arrays for imaging Sgr A*. Good images are obtained from submillimeter VLBI arrays in the southern hemisphere composed of more than 10 stations. We also note that even with a small array, we can estimate the shadow size and then the mass of the black hole from visibility analysis. Now, all we need is to construct a submillimeter VLBI array in the southern hemisphere if we wish to unveil the black hole environment of Sgr A*. Key words: black hole, event horizon, Sgr A*, submillimeter, Comment: 9 pages, 5 figures