Makoto Miyoshi

A geometric distance to the galaxy NGC4258 from orbital motions in a nuclear gas disk

The accurate measurement of extragalactic distances is a central challenge of modern astronomy, being required for any realistic description of the age, geometry and fate of the Universe. The measurement of relative extragalactic distances has become fairly routine, but estimates of absolute distances are rare. In the vicinity of the Sun, direct geometric techniques for obtaining absolute distances, such as orbital parallax, are feasible, but such techniques have hitherto been difficult to apply to other galaxies. As a result, uncertainties in the expansion rate and age of the Universe are dominated by uncertainties in the absolute calibration of the extragalactic distance ladder. Here we report a geometric distance to the galaxy NGC4258, which we infer from the direct measurement of orbital motions in a disk of gas surrounding the nucleus of this galaxy. The distance so determined—7.2 ± 0.3 Mpc—is the most precise absolute extragalactic distance yet measured, and is likely to play an important role in future distance-scale calibrations.

Discovery of a Subparsec Jet 4000 Radii Away from the Central Schwarzschild Engine of NGC 4258

We report the VLBI detection of compact continuum emission associated with the maser disk in NGC 4258. The strongest emission is located about 0.5 mas (0.015 pc) north of the dynamical center of the disk. It has an average flux density of about 3 mJy and varies by ~100% on timescales of weeks. We postulate that we have detected nonthermal synchrotron emission in the base of the northern jet, which is seen on parsec-to-kiloparsec scales in radio to X-ray emission. We also report a detection of emission from the southern jet which may be attenuated by thermal absorption in a layer of ionized gas above the molecular disk. The average flux density of the maser emission in the systemic velocity range is correlated with the flux density of the northern continuum emission. Together with the geometry of the disk, these data suggest that the masers amplify the southern continuum emission and that the southern and northern jet emission are correlated in strength. We discuss the results in the context of jet emission models and properties of other compact radio continuum sources.

Kinematics and Distance of Water Masers in W3 IRS 5

We have made multiepoch VLBA observations of water masers in the massive-star forming region W3 IRS 5. We measured radial velocities and proper motions of 108 water maser features during three observing sessions. The masers are clustered in two groups, which are associated with at least two different outflows. Positions of the outflow origins are close to the hypercompact radio continuum sources, which are clustered within a scale of 6000 AU. The continuum sources are probably driving sources of the outflows. We performed a three-dimensional kinematic model analysis of the maser motions in one of the two outflows, assuming a spherically symmetric expanding flow. We obtained a distance to the W3 IRS 5 region as 1.83 ? 0.14 kpc. The directions of the two outflows are roughly in the north-south direction, which is not significantly different from the direction of the global outflow seen in CO (J = 2-1) emission. This suggests that the massive-star cluster in the W3 IRS 5 region was created during formation of hierarchical structure in a parent molecular-cloud core. Such a structure is expected to preserve the angular momentum vector during star formation process, which may be along the directions of the present outflows.

THE ORIGIN OF LARGE PECULIAR MOTIONS OF STAR-FORMING REGIONS AND SPIRAL STRUCTURES OF OUR GALAXY

Recent Very Long Baseline Interferometer (VLBI) 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–1 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 large systematic errors of ~2-3 kpc in the distance from the Sun.

VLBA Continuum Observations of NGC 4258: Constraints onan Advection-dominated Accretion Flow

We report a 3 sigma upper limit of 220 microJanskys on any 22-GHz continuum emission coincident with the central engine in NGC 4258. If NGC 4258 is powered by an advection-dominated accretion flow, this radio upper limit implies that the inner advection-dominated flow cannot extend significantly beyond about 100 Schwarzschild radii.

Flaring up of the compact cloud G2 during the close encounter with Sgr A

A compact gas cloud G2 (Gillessen+2012) is predicted to reach the pericentre of its orbit around the super massive black hole (SMBH) of our galaxy, Sagittarius (Sgr A*), by summer 2013. 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 pericentre passage. The strong tidal force by the SMBH stretches the cloud along its orbit, and compress 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 100

Measuring spin of a supermassive black hole at the Galactic centre — implications for a unique spin

L_{\odot}

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

by July 2013. This flare should be easily observed in the near infrared.

Location of H2O maser in the double-nuclei system of NGC 6240

We determine the spin of a supermassive black hole in the context of discseismology by comparing newly detected quasi-periodic oscillations (QPOs) of radio emission in the Galactic centre, Sagittarius A* (Sgr A*), as well as infrared and X-ray emissions with those of the Galactic black holes. We find that the spin parameters of black holes in Sgr A* and in Galactic X-ray sources have a unique value of � 0:44 which is smaller than the generally accepted value for supermassive black holes, suggesting evidence for the angular momentum extraction of black holes during the growth of supermassive black holes. Our results demonstrate that the spin parameter approaches the equilibrium value where spin-up via accretion is balanced by spin-down via the Blandford-Znajek mechanism regardless of its initial spin. We anticipate that measuring the spin of black holes by using QPOs will open a new window for exploring the evolution of black holes in the Universe.

A search for extragalactic H

Water vapor maser emission in the 616-523 transition towards the narrow-line type 1 Seyfert (NLS1) galaxy NGC 4051 was 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 sym. 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 L.sun.) is not typical for masers that coincide with the radio continuum nucleus and appear assocd. 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. [on SciFinder (R)]