Where is the Galactic Center?

The Galactic Center is hard to observe at optical wavelengths because of extinction by interstellar dust (typically 30 magnitudes). However, in the infrared at a wavelength of 2 microns the extinction is reduced by about an order of magnitude and one can see through the dust to the Center. The (negative) infrared image shown below shows numerous stars in the inner few arcseconds of the Galactic Center. But, which of these stars, if any, corresponds to the bright radio source Sgr A* which is probably a super-massive black hole? This problem vexed astronomers for over two decades.

The problem of identifying the infrared counterpart of Sgr A* existed because it was not possible to register the infrared and radio reference frames to the sub-tenth arcsecond level needed to sort out the complex field of stars seen in the infrared image. Recently, Karl Menten and Mark Reid teamed up with Andreas Eckart and Reinhard Genzel to precisely register radio and infrared images in the Galactic Center. Their novel technique employed stars that are detected both in the radio and infrared to determine the infrared image scale and rotation. Then the two images were slid along the two axes until the proper registration was obtained. The stars used to do this are red giants which are very bright in the infrared and are surrounded by strong molecular maser emission (eg, from water and SiO molecules) at radio wavelengths.

Since all of the stars used to align the reference frames move around significantly, this required measuring positions and proper motions of the stellar masers at radio wavelengths to milli-arcsecond levels. This has now been done for 15 stars with the Very Long Baseline Array and VLA. The infrared image at the left shows the inner 40 arcsecond (see scale bar on right side) region of the Galactic Center. The small circle near the center of the image contains Sgr A*, The nine stars with stellar masers that fall within this region are located with circles and arrows. The arrows indicate the direction and speed (see scale arrow on right side) of the proper motions of the stars relative to Sgr A*. Some stars are moving faster than 200 km/s. Combining these radio positions and proper motions with improved infrared images yields a registration accuracy of about 0.01 arcseconds (about 50 times smaller than the circle around Sgr A*).

At the left is artificially colored blow-up of an infrared image of the inner 2 arcseconds of Galactic Center from 1995. The position of stars called S1, S2 and S3 are indicated, along with a circle showing the location of Sgr A*. Surprisingly, none of the infrared stars corresponds to Sgr A*! While this solves a long-standing problem in astronomy, the result raises perplexing problems, challenging astronomers to explain how a super-massive black hole in the dense environment of the Galactic Center can be so dim.

The very small uncertainty in the position of Sgr A* is crucial to help confirm that Sgr A* is a super-massive black hole and to better understand how the regions around a black hole radiate. Infrared astronomers have been measuring the locations of the stars near Sgr A* with milli-arcsecond precision for nearly 15 years. They have detected very large proper motions (>1000 km/s) and nearly complete orbits have been observed for many stars. Stellar orbits are a dramatic confirmation of the large central mass of Sgr A*, requiring about 4 million times the mass of the Sun within a region smaller than about 100 AU (comparable to our extended Solar system). Also, the stellar orbits have a focal position that agrees precisely with the position of Sgr A*, thus linking the gravitational source with the radiative source for this super-massive black hole candidate.


Menten, Reid, Eckart & Genzel 1997, ApJ, 475, L111; Schoedel et al, 2002, Nature, 419, 694; Ghez et al, 2005, ApJ, 620, 744; Reid et al. 2003, ApJ, 587, 208; Reid et al. 2007, to appear in ApJ