Milky Way Galaxy

The Galactic Center

The direction to the Galactic center, towards the constellation Sagittarius, is not apparent to the eye. The Milky Way appears as two bands in the direction of Sagittarius. This division of the Galactic disk is created by a lane of dust in the Galactic plane that extends 70 across the sky. Our view of the Galaxy in visible light is limited to stars and nebulae that are nearby, within about 1kpc of Earth. The Galactic center, at 7.6 kpc from Earth, is hidden behind the dust lane and is separated from us by three distinct spiral arms.^[1] Compared to the size of the Sun or the Moon, the Galactic center is very large on the sky, fitting in a box that is 5 along the Galactic equator and half this width perpendicular to the equator, but it is completely covered by the dust lane slicing the Milky Way, and it is much smaller than the 20 width of the Milky Way in Sagittarius. We do not see the Galactic center rising above the stars of the Galactic disk like a mountain rises above the treetops of a surrounding forest. The only visual clue to the Galactic center's direction is provided by the globular clusters we see above and below the Galactic plane?the globular clusters are distributed within a large sphere that has the the Galactic center as its own center.

To see the Galactic center, we must take advantage of windows in the electromagnetic spectrum that give us direct views into the Galactic center. Below about 2 m, the dust in the Galactic plane is transparent, so we can see directly into the Galactic center at radio and infrared frequencies.^[2] The gas and dust in the galaxy is also transparent at x-ray and gamma-ray frequencies. Through these windows we see a massive, dense, and violent cluster of stars and gas orbiting what appears to be a black hole that is millions of times more massive than the Sun.

At the 90 cm radio band, which is sensitive to the radio emission of ionized hydrogen, the Galactic center appears as a collection of extended objects lying along the plane of the Galaxy.^[3] This complex extends about 3 along the Galactic equator, extending from the north-east to the south-west on the sky. The width of the radio emission perpendicular to the Galactic equator is about . At a distance of 7.6 kpc, these angles corresponds to a length of 400 parsecs and a width of 70 parsecs.

Many of the objects in this Galactic center complex are supernova remnants and bubbles of ionized hydrogen (HII). In particular, two supernova shells and an HII shell lie at the north-eastern edge of the complex. Others are threads of radio emission created by high-velocity electrons passing through large-scale magnetic fields extending perpendicular above and below the Galactic plane. Most of these threads are short, extending about 10 arc minutes (20 pc) or less on the sky. The most prominent of these structures, which is just north of the complex's center, is dubbed the Arc; it extends all the way through the complex, giving it a length of about 1 on the sky (170 pc). These threads appear to be regions where gas is being expelled perpendicular to the Galactic plane from the Galactic center; this outflow resembles the much-more intense expulsion of gas from galaxies with active nuclei. At the very center of the complex of radio emission is a particularly bright region of synchrotron emission (the radiation from electrons in magnetic fields). This region is called Sagittarius A (Sgr A), and it is the center of our Galaxy, because here the stars in the Galactic center are most dense. At the center of this clustering of stars is a bright non-thermal radio source called Sgr A^*, which is a massive black hole candidate.

The stars at the Galactic center are seen at infrared wavelengths. Within a parsec of the black hole, their density is of order 300,000 solar masses per cubic parsec. Moving out 10 parsecs from the black hole, one finds a lower, but still extreme, density of about 3,000 solar masses per cubic parsec. Reflecting this decline in stellar mass density, the brightness per unit area on the sky of these stars falls slowly as one moves away from the black hole, falling as distance to the ?0.75 power out to a distance of 6 (800 parsecs) from Sgr A^*; beyond this distance, the brightness falls more rapidly, falling as the inverse-square of the distance. The brightness distribution of the stars on the sky is an ellipsoid, with the short axis of the distribution perpendicular to the Galactic plane. At a distance of several hundred parsecs, the ratio of height above the plane to the length within the plane for the brightness distribution is about 0.75. On a smaller scale of tens of parsecs, this flattening is more pronounced, with the ratio of length perpendicular to length parallel to the plane being 0.4. But inside about 2 parsecs from Sgr A^*, which is well inside Sgr A, the distribution of stars is spherical.

As with the black hole candidates of x-ray binaries, the environment immediately surrounding Sgr A^* best reveals itself in its x-ray and gamma-ray emissions. For such a large object, Sgr A^* is a dim x-ray source, generating about the same amount of power in x-rays as the Sun does over the full electromagnetic spectrum.^[4] The central black hole is a giant, but it is asleep.