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Galactic Center

The references on the Galactic center are generally recent, reflecting the recent advances in infrared telescopes and instrumentation.  The Galactic center is very complex, with a tremendous concentration of stars and gas swirling around what appears to be a massive black hole.  The black hole candidate is at the center  of a complex of gas and molecular clouds called Saggitarius A (Sgr A).  The black hole candidate is a point radio source called Saggitarius A* (Sgr A*); its mass is measured at 3.61±0.32 million solar masses.

Two review articles provide a helpful introduction to the objects sitting at the center of the Galaxy. Because of the speed at which advances are occurring, neither of these review articles are current, despite the most recent being only five years old.  The earlier review is that of Genzel and Townes (1987).  This review discusses the region within 5° of the galactic center as observed in the radio and microwave bands.  The review describes the distribution of stars and gas at the galactic center, with much of the review devoted to the extended region of gas called Sgr A.  The later review by Melia and Falcke (2001) updates what is known about Sgr A*.

An early paper that presents observations of the Galactic center in the microwave is that of Becklin (1978).  A more recent radio map of the Galactic center is presented in LaRosa et al. (2000); this map cover a region of 4° by 5° on the sky, which corresponds at a distance of 7.6 kpc to a region  of 530 pc by 660 pc.  Sgr A is at the center of this map.

Early papers that present observations of Sgr A are those of Ekers et al. (1975) and Ekers et al. (1983).  All of these papers are radio observations of the Sgr A complex.  A higher-resolution radio map of the Galactic center at the 330 MHz frequency is presented by Nord et al. (2004).  This map shows the details of the Sgr A region.  Sgr A* is a bright point on each of these maps.

Infrared observations of Sgr A* are presented by Genzel et al. (2003), Ghez et al. ( 2004), and Eisenhauer et al. (2005).  The appearance of Sgr A* at x-ray energies is presented in the paper by Baganoff et al. (2003).

The mass and distance of Sgr A* can be derived by observing the motions of stars orbiting Sgr A*.  This method is discussed by  Salim and Gould (1999).  Applying this method, Eisenhauer et al. (2003) derive a distance to the central black hole of 7.94±0.42 kpc and a mass of  3.59±0.59 million solar masses.  An improved distance of 7.62±0.32 kpc and mass of 3.61±0.326 million solar masses is presented by Eisenhauer et al. (2005).  In Ghez et al. (2005), a mass of 3.7±0.2 (R/8kpc)3 million solar masses is derived, which gives a mass of 3.2 million solar masses for a distance of 7.6 kpc.

A great mystery surrounds how Sgr A* acquired the numerous stars orbiting nearyby  These stars are very young, but they are unlikely to have been born close to the central black hole.  Many theories have been developed to explain this observations; among them are those presented by Morris (1993), Gerhard (2001), Hansen and Milosavljević (2003), and Tal and Livio (2004).

References

Alexander, Tal, and Livio, Mario. “Orbital Capture of Stars by a Massive Black Hole Via Exchanges with Compact Remants.” The Astrophysical Journal Letters 606 (1 May 2004): L21–L24.

Baganoff, F. K., et al. “Chandra X-ray Spectroscopic Imaging of Sagittarius A* and the Central Parsec of the Galaxy.” The Astrophysical Journal 591 (10 July 2003): 891–915.

Becklin, E. E., Matthews, K., Neugebauer, G., and Willner, S. P.  “Infrared observations of the galactic center.  I–Nature of the compact sources.”  The Astrophysical Journal 219 (1978): 121–128.

Eisenhauer, F., Schödel, Genzel, R., Ott, T., Tecza, M., Abuter, R., Eckart, A., and Alexander, T.  “A Geometric Determination of the Distance to the Galactic Center.”  The Astrophysical Journal Letters 597 (10 November 2003): L121–L124.

Eisenhauer, F., et al. “Sinfoni in the Galactic Center: Young Stars and Infrared Flares in the Central Light-Month.” The Astrophysical Journal 628 (20 July 2005): 246–259.

Ekers, R.D., Goss, W.M., Schwarz, U.J., Downes, D., and Rogstad, D.H.  “A Full Synthesis Map of Sgr A at 5 GHz.”  Astronomy and Astrophysics 43 (1975): 159–166.

Ekers, R.D., van Gorkom, J.H., Schwarz, U.J., and Goss, W.M. “The Radio Structure of Sgr A.” Astronomy and Astrophysics 122 (1983) 143–150.

Genzel, R., and Townes, C. H. “Physical Conditions, Dynamics, and Mass Distribution in the Center of the Galaxy,” in Annual Review of Astronomy and Astrophysics, vol. 25. Palo Alto: Annual Reviews, 1987: 377–423.

Genzel, R., Schödel, R., Ott, T., Eckart, A., Alexander, T., Lacombe, F., Rouan, D., and Aschenbach, B.  “Near-infrared flares from accreting gas around the supermassive black hole at the Galactic Centre,”  Nature 425 (30 October 2003): 934–937.

Gerhard, Ortwin. “The Galactic Center He I Stars: Remains of a Dissolved Young Cluster?” The Astrophysical Journal Letters 546 (1 January 2001): L39–L42.

Ghez, A.M., Wright, S.A., Matthews, K., Thompson, D., Le Mignant, D., Tanner, A., Hornstein, S.D., Morris, M., Becklin, E.E., and Soifer, B.T.  “Infrared Emission from the Supermassive Black Hole at the Center of the Milky Way.”  The Astrophysical Journal Letters 601 (1 February 2004): L159–L162.

Ghez, A.M., Hornstein, S.D., Tanner, A., Lu, J.R., Morris, M., Becklin, E.E., Duchêne, G.  “Stellar Orbits Around the Galactic Center Black Hole.”  The Astrophysical Journal 620 (20 February 2005): 744–757.

Hansen, Brad M. S., and Milosavljević, Miloš. “A Second Black Hole at the Galactic Center.” The Astrophysical Journal Letters 593 (20 August 2003): L77–L80.

LaRosa, T.N., Kassim, Namir E., Lazio, T. Joseph W., and Hyman, S. D. “A Wide-Field 90 Centimeter VLA Image of the Galactic Center Region.” The Astronomical Journal 119 (January 2000): 207–240.

Melia, Fulvio, and Falcke, Heino. “The Supermassive Black Hole at the Galactic Center.” In Annual Reviews of Astronomy and Astrophysics, edited by Geoffrey Burbidge, Allan Sandage, and Frank H. Shu, vol. 39. Palo Alto, California: Annual Reviews, 2001.

Morris, Mark. “Massive Star Formation Near the Galactic Center and the Fate of Stellar Remnants.” The Astrophysical Journal 408 (10 May 1993): 496–506.

Nord, Michael E., Lazio, Joseph W., Kassim, Namir E., Hyman, S.D., LaRosa, T.N., Brogan, C.L., and Duric, N. “High-Resolution, Wide-Fieeld Imaging of the Galactic Center Region at 330 MHz.” The Astronomical Journal 128 (October 2004): 1646–1670.

Salim, Samir, and Gould, Andrew. “Sagittarius A* ‘Visual Binaries’: A Direct Measurement of the Galactocenric Distance.” The Astrophysical Journal 523 (1 October 1999): 633—641.

Schödel, R., et al. “Stellar Dynamics in the Central Arcsecond of Our Galaxy.” The Astrophysical Journal 596 (20 October 2003): 1015–1034.

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