Milky Way Galaxy

The Sagittarius A Complex

Among the radio sources strung across the galactic equator in Sagittarius, the most important is the Sagittarius A (Sgr A) complex, which sits at the center of our Galaxy. It is in the deepest part of the Galaxy's gravitational potential well, and it comprises the greatest density of stars within the Galaxy. Most importantly, at its center sits a massive black hole, Sagittarius A^*, that controls the motions of the stars in the innermost parsec of the Galaxy.

East Meets West

A picture of Sgr A at radio wavelengths shows a somewhat oblong region. At radio frequencies, the long axis, which lies along the Galactic equator, is 3^? on the sky, while the short axis is 2^?. For a distance from Earth of 7.62 kpc, Sgr A is 7 parsecs by 5 parsecs. In the first radio maps created for this region, the Western half, which is called Sgr A West, distinguished itself from the Eastern half, which is called Sgr A East, by the character of the radio emission. The radio emission from the Western half is thermal, meaning that the radio waves are emitted by electrons that have come into thermal equilibrium with each other. The Eastern half, on the other hand, is characterized by non-thermal emission, which drew immediate comparisons to supernova remnants.^[1]

Today this division of Sgr A into West and East does not stand up so well, because better observations show that Sgr A East encloses Sgr A West, with the latter offset to the western side of Sgr A East. On the other hand, the theory that Sgr A East is a supernova remnant has stood up to better observations. As with the remnants of many type II supernovae, Sgr A East is an x-ray emitter. Elements such as iron are found in high concentrations at the center of Sgr A East, which is another characteristic of type II supernovae. There is a suggestion that a high-velocity neutron star created in the stellar core collapse that created Sgr A East is visible as a point x-ray source to the north of the remnant, just outside of the supernova remnant.^[2] Supernova are rare, occurring at a rate of around once every 50 years in the Milky Way Galaxy, so finding one at the center of the Galaxy, despite the high number of stars in the region, is a surprise. With the number of stars in the Galactic center, one expects a supernova once every 250,000 years, but Sgr A East is probably no more than 10,000 years old.^[3] Is this unlikely occurrence of a supernova remnant in the Galactic center a consequence of some physics unique to the Galactic center, or is it simply luck?

Sgr A West is a much more complex region than Sgr A East. In radio, it is compact, extending 1^? (3 parsecs) along the Galactic equator and ^? (1 parsecs) perpendicular to the Galactic equator. Sgr A West has a complex shape, appearing as a three-armed spiral of gas in the 2 cm radio wave band.^[3] The center of this spiral coincides with the maximum stellar density in the Galactic center and with the point radio source Sgr A^*. The interpretation of these features is obvious: The stars and gas of Sgr A West are in orbit around a massive black hole that sits at the bottom of the Galaxy's gravitational potential well.

Sagittarius A^*

Sgr A^* was first seen as a non-thermal radio source. It remains most easily seen as a radio emitter. But as befits a black hole candidate, it is also an x-ray emitter, easily seen by current x-ray observatories as a variable x-ray source. But in the infrared band, the band used to study the orbits of nearby stars around the black hole, Sgr A^* is seen with great difficulty. This black hole is simply not a big energy emitter, so it is quite dim. This low power generation makes Sgr A^* very different from the presumed black holes powering the active galactic nuclei (AGNs), which is bad for those hoping to understand AGNs by studying our own Galaxy, but good for those wanting to study the physics of black holes without having the observational effects of an accretion disk and a plasma jet mixing in with the observational effects of general relativity. The properties of Sgr A^* as a black hole are discussed elsewhere.

The Stars of Sgr A

The stars of Sgr A West constitute the most interesting component of the complex. Nowhere in the Galaxy do we find such a high density of stars. Within the central 10 parsecs of the Galaxy, the density of stars is of order 10,000 solar masses per cubic parsec (locally the density is much less than 1 solar mass per cubic parsec). The density rises as one moves towards Sgr A^*, reach a density of order 1 million solar masses per cubic parsec within 1 parsec of the black hole.

The way a star orbits Sgr A^* is set by the distance of the star from the black hole. Beyond about 1 parsec, the mass in stars exceeds the 3.6 million solar masses of the black hole, and the stars beyond this radius move on orbits that reflect the distribution of stars in the Galactic center. Inside 1 parsec, the mass in stars is much less than the mass in the black hole, so each star orbits as if it and Sgr A^* were the only two objects in the system.^[4] Effectively, the black hole generates most of the gravitational field within Sgr A West. These nearby stars move on Keplerian orbits that depend only on the mass of the black hole and two characteristics of the star's orbit: the semimajor axis and the eccentricity. We know the mass of Sgr A^*, because we know the orbits of these stars.

The surprising characteristic of the stars close to Sgr A^* is that many of them are very young stars. This is particularly true of the stars used to derive the mass of Sgr A^*. These stars have semimajor axes of between 0.12? and 0.41?, which corresponds to a size of 0.004 to 0.015 parsecs (910 to 3100 AU), with some of these stars approaching within 100 AU of the black hole.^[5] These stars are normal B stars, which are large, short-lived, main-sequence stars. The presence of these massive stars was not expected by astrophysicists, because it was believed that the gravitational gradient around Sgr A^* is too steep for star formation to occur, and it was believed that the time for stars to sink deep into the gravitational potential well is much longer than the age of these young stars. This conundrum remains unresolved.