The Position of Sagittarius A*: Accurate Alignment of the Radio and Infrared Reference Frames at the Galactic Center

We present a novel approach to the long-standing problem of locating the position of the compact nonthermal radio source Sgr A* on infrared images of the Galactic center region. Using the Very Large Array, we have detected SiO and H2O maser emission toward several sources within the central parsec of our Galaxy. These masers arise from the innermost parts of circumstellar envelopes of giant and supergiant stars that are members of the nuclear star cluster and appear as compact infrared sources in a diffraction-limited 2.2 μm infrared image. One of the SiO masers is associated with the M-type supergiant IRS 7, the most prominent 2.2 μm point source in the Galactic center region. The radio data allow measurements of the maser positions relative to the compact nonthermal radio continuum source Sgr A* with milliarcsecond accuracy. Because stellar SiO masers near the Galactic center trace their host stars to within a few milliarcseconds, these relative positions can be used to calibrate the plate scale and rotation of the infrared image. Our method allows registration of the radio relative to the infrared reference frame with an estimated accuracy of 0.″03. Using the improved position accuracy we put a stringent upper limit on the 2.2 μm flux density of Sgr A* that is significantly lower than values predicted by recent theoretical model calculations.

[1]  D. Backer,et al.  Proper Motion of the Compact, Nonthermal Radio Source in the Galactic Center, Sagittarius A* , 1999, astro-ph/9906048.

[2]  Alfred Krabbe,et al.  The Dark Mass Concentration in the Central Parsec of the Milky Way , 1996 .

[3]  A. Eckart,et al.  Observations of stellar proper motions near the Galactic Centre , 1996, Nature.

[4]  G. Rieke,et al.  Stellar Kinematics and the Black Hole in the Galactic Center , 1996 .

[5]  W. Goss,et al.  VLA OH Observations of High Negative Velocity Gas toward Sagittarius A West: A High-Velocity Cloud Interacting with the Galactic Center , 1995 .

[6]  J. Moran,et al.  Interferometric Observations of the SiO Masers and Dust Shell of VX Sagittarii , 1995 .

[7]  I. McLean,et al.  A Circumstellar H2O Maser Associated with the Circumnuclear Molecular Disk at the Galactic Center , 1995 .

[8]  Alfred Krabbe,et al.  The Nuclear Cluster of the Milky Way: Star Formation and Velocity Dispersion in the Central 0.5 Parsec , 1995 .

[9]  Reinhard Genzel,et al.  High Angular Resolution Spectroscopic and Polarimetric Imaging of the Galactic Center in the Near-Infrared , 1995 .

[10]  Ramesh Narayan,et al.  Explaining the spectrum of Sagittarius A* with a model of an accreting black hole , 1995, Nature.

[11]  S. Padin,et al.  Small-scale structure and position of Sagittarius A(*) from VLBI at 3 millimeter wavelength , 1994 .

[12]  S. Kameno,et al.  Collisional pumping of SiO masers in evolved stars , 1994, Nature.

[13]  A. Kemball,et al.  Observation of a Ring Structure in SiO Maser Emission from Late-Type Stars , 1994 .

[14]  Fulvio Melia,et al.  An accreting black hole model for Sagittarius A(*). 2: A detailed study , 1994 .

[15]  Reinhard Genzel,et al.  The nucleus of our Galaxy , 1994 .

[16]  P. Bowers,et al.  VLA positions and distributions of H2O masers associated with 15 Mira and semiregular variables , 1994 .

[17]  L. Tacconi-Garman,et al.  Near-infrared 0.15 arcsec resolution imaging of the Galactic center , 1993 .

[18]  P. Bowers,et al.  Positions and distributions of water masers around late-type stars , 1993 .

[19]  Fulvio Melia,et al.  The bow shock structure of IRS 7 - Wind-wind collision near the Galactic center , 1992 .

[20]  M. Morris,et al.  A windswept cometary tail on the Galactic center supergiant IRS 7 , 1991 .

[21]  K. Menten,et al.  A subarcsecond H2O maser shell surrounding a variable star , 1990 .

[22]  Donald N. B. Hall,et al.  Velocity dispersion and the stellar population in the central 1.2 parsecs of the Galaxy , 1990 .

[23]  E. Tollestrup,et al.  High-resolution IRCCD images of the Galactic center at 2. 2 and 3. 5 microns , 1989 .

[24]  G. Rieke,et al.  Stellar velocities and the mass distribution in the galactic center , 1988 .

[25]  K. Sellgren,et al.  Radial velocities of late-type stars in the galactic center , 1987 .

[26]  M. Reid,et al.  The distance to the center of the Galaxy , 1987 .

[27]  J. Pipher,et al.  Sagittarius A(asterisk) and the positions of infrared sources in the galactic center , 1986 .

[28]  E. Serabyn,et al.  Forbidden Ne II observations of the galactic center - Evidence for a massive black hole , 1985 .

[29]  A. Winnberg,et al.  OH/IR stars within 50 parsecs of the galactic center. , 1985 .

[30]  G. Neugebauer,et al.  High-resolution maps of the galactic center at 2.2 and 10 microns , 1975 .

[31]  R. L. Brown,et al.  Intense sub-arcsecond structure in the galactic center , 1974 .

[32]  M. Rees,et al.  On Quasars, Dust and the Galactic Centre , 1971 .