arXiv subjects

Makoto Miyoshi

Publications and source records attributed to Makoto Miyoshi.

35 records · Page 2Linked to original sources

arXiv2017

The Second Galactic Center Black Hole?; A Possible Detection of Ionized Gas Orbiting around an IMBH embedded in the Galactic Center IRS13E complex

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.

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arXiv2017

ALMA View of the Galactic Center Mini-spiral: Ionized Gas Flows around Sagittarius A*

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.

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arXiv2016

Search for sub-millimeter H2O masers in active galaxies - the detection of a 321 GHz H2O maser in NGC4945

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.

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arXiv2016

Galactic Center Mini-spiral by ALMA - Possible Origin of the Central Cluster

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.

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arXiv2014

No Microwave Flare of Sagittarius A* around the G2 Periastron Passing

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.

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arXiv2013

Flaring up of the Compact Cloud G2 during the Close Encounter with Sgr A*

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.

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arXiv2013

Submillimeter H2O Maser in Circinus Galaxy - A New Probe for the Circumnuclear Region of Active Galactic Nuclei

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.

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arXiv2012

Analysis of H2O Masers in Sharpless 269 using VERA Archival data --- Effect of maser structures on astrometric accuracy

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.

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arXiv2009

The origin of large peculiar motions of star-forming regions and spiral structures of our Galaxy

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.

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arXiv2009

The variability of Sagittarius A* at 3 millimeter

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*.

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arXiv2008

Re-analysis of the First Fringe with 2-Beam in the VERA System from Archive Data

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.

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arXiv2008

An Approach Detecting the Event Horizon of Sgr A*

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

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arXiv2005

Flares of Sagittarius a* at Millimeter Wavelengths

We have performed monitoring observations of the flux density toward the Galactic center compact radio source, Sagittarius A* (Sgr A*), which is a supermassive black hole, from 1996 to 2005 using the Nobeyama Millimeter Array of the Nobeyama Radio Observatory, Japan. These monitoring observations of Sgr A* were carried out in the 3- and 2-mm (100 and 140 GHz) bands, and we have detected several flares of Sgr A*. We found intraday variation of Sgr A* in the 2000 March flare. The twofold increase timescale is estimated to be about 1.5 hr at 140 GHz. This intraday variability suggests that the physical size of the flare-emitting region is compact on a scale at or below about 12 AU (~150 Rs; Schwarzschild radius). On the other hand, clear evidence of long-term periodic variability was not found from a periodicity analysis of our current millimeter data set.

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arXiv2003

A search for extragalactic H2O maser emission towards IRAS galaxies II - Discovery of an H2O maser in the Seyfert 1 galaxy NGC 4051

Water vapor maser emission in the 6_{16}-5_{23} transition towards the narrow-line Seyfert 1 (NLS1) galaxy NGC 4051 has been discovered during an ongoing single-dish extragalactic water maser survey. The Doppler-shifted maser components appear to bracket maser components lying near the systemic velocity of the galaxy symmetrically. The tentative result of a Very Large Array (VLA) snapshot observation is that the masers are confined within 0.1 arcsec (5 pc at a distance D = 9.7 Mpc) of the radio continuum peak seen at 8.4 GHz. The low luminosity of the maser (~2 Lsun) is not typical for masers that coincide with the radio continuum nucleus and appear associated with Active Galactic Nucleus (AGN) activity. A low-luminosity maser in a Type 1 Seyfert nucleus could be explained by a low maser gain resulting from the lower inclination of an obscuring disk around an active nucleus.

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arXiv2003

Location of H2O maser in the double-nuclei system of NGC6240

We performed VLA observations of the 22 GHz H2O maser emission in the merging galaxy NGC6240, which hosts the well-known double active nuclei. In a previous paper, we reported on the first solid detection of the H2O maser emission in 2001. After two abortive attempts due to the weakness and probable variability of the emission, the maser was detected with the VLA in June 2002. The emission is unresolved at ~ 0.3 arcsecond and coincides with the southern 22 GHz continuum peak to ~ 0.007 arcsecond (~ 3 pc: D = 97 Mpc). The detection of the maser in the southern nucleus indicates that nuclear activity of the galaxy,which is significant in X-ray and far-infrared (FIR) bands, lies mainly in the southern nucleus, and the nucleus without a high brightness peak could be explained by thick dust emitting FIR radiation. We favour the idea that the maser in NGC6240 is associated with the AGN-activity.

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arXiv2003

The 3-D kinematics of water masers around the semiregular variable RT Virginis

We report observations of water masers around the semiregular variable RT Virginis (RT Vir), which have been made with the Very Long Baseline Array (VLBA) of the National Radio Astronomy Observatory (NRAO) at five epochs, each separated by three weeks of time. We detected about 60 maser features at each epoch. Overall, 61 features, detected at least twice, were tracked by their radial velocities and proper motions. The 3-D maser kinematics exhibited a circumstellar envelope that is expanding roughly spherically with a velocity of about 8 km/s. Asymmetries in both the spatial and velocity distributions of the maser features were found in the envelope, but less significant than that found in other semiregular variables. Systematic radial-velocity drifts of individual maser features were found with amplitudes of <= 2 km/s/yr. For one maser feature, we found a quadratic position shift with time along a straight line on the sky. This apparent motion indicates an acceleration with an amplitude of 33 km/s/yr, implying the passage of a shock wave driven by the stellar pulsation of RT Vir. The acceleration motion is likely seen only on the sky plane because of a large velocity gradient formed in the accelerating maser region. We estimated the distance to RT Vir to be about 220 pc on the basis of both the statistical parallax and model-fitting methods for the maser kinematics.

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arXiv2002

Observations of SiO Maser Sources within a Few Parsec from the Galactic Center

Mapping and monitoring observations of SiO maser sources near the Galactic center were made with the Nobeyama 45-m telescope at 43 GHz. Rectangular mapping an area of approximately 200'' x 100''$ in a 30'' grid, and triangular mapping in a 20'' grid toward the Galactic center, resulted in 15 detections of SiO sources; the positions of the sources were obtained with errors of 5--10'', except for a few weak sources. Three-year monitoring observations found that the component at V_lsr=-27 km/s of IRS 10 EE flared to about 1.5 Jy during 2000 March--May, which was a factor of more than 5 brighter than its normal intensity. Using the radial velocities and positions of the SiO sources, we identified 5 which are counterparts of previously observed OH 1612 MHz sources. The other 10 SiO sources have no OH counterparts, but two were previously detected with VLA, and four are located close to the positions of large-amplitude variables observed at near-infrared wavelengths. A least-squares fit to a plot of velocities versus Galactic longitudes gives a rather high speed for the rotation of the star cluster around the Galactic center. The observed radial-velocity dispersion is roughly consistent with a value obtained before. It was found that all of the SiO sources with OH 1612 MHz counterparts have periods of light variation longer than 450 days, while SiO sources without OH masers often have periods shorter than 450 days. This fact suggests that lower-mass AGB stars are more often detected in SiO masers than in the OH 1612 MHz line.

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