Figure 1. Measured (dots) and calculated (line) orbital position of S0-2 based on 16 years of observations. The orbit matches Kepler’s law perfectly. From Ghez, et.al., Astrophysical Journal, Aug 21, 2008.
Peer toward the Milky Way’s center in visible light, and you see clouds of dust and gas obscuring all the stars. But, switch to infra red, near 2.2 microns wavelength, and 1 million stars within 1 square arc second of the sky, come clear.
This region has the highest observed stellar density of the known universe. Using a distance to the galactic center of 7.6 kpc (kilo parsecs), this translates to about 1 million solar masses per cubic parsec within 1 parsec of the black hole, hidden in the middle of this forest. Our nearby space has only about 1 solar mass per cubic parsec.
Two independent teams have been peering into the center of our galaxy, within a tiny square, 1 arc second on a side, and reported their latest results this year.
One team, led by Genzel and Eckart from the Max Planck Institute for Extraterrestrial Physics used the 3.5 m NTT telescope in La Silla, Chile. Another group, lead by Andrea Ghez from UCLA, used the 10 m Keck telescope on Mauna Kea, Hawaii.
Both teams observed the motions of the 15 stars orbiting within 1 arc second of the galactic center with a spatial resolution of less than 0.0003 arc seconds. This is equivalent to seeing a basketball on the moon.
One star in particular, designated as S0-2, has been observed through one complete orbit in both position and velocity. Astonishingly, it obeys a perfect Keplerian orbit, with a period of 15.8 years. From both position and velocity, all the orbital parameters, including the mass of the object it orbits, can be extracted. The mass is 4.53 +/- 0.34 million solar masses.
At its closest approach (periapse), S0-2 travels at 4,000 km/sec, faster than 1% the speed of light and gets within 0.57 +/- 0.037 mpc (milli parsec), or about 100 AU of the object it orbits. How do you fit 4 million solar masses in a distance less than three times the diameter of Pluto’s orbit? The most likely candidate is a black hole.
Recent, very long baseline interferometry measurements of this region by a group at MIT using simultaneous measurements from around the globe, pinned down the spatial size of the radio source at the galactic center to be less than 1/3 of an AU, or 30 million miles. This is consistent with the 16 million mile event horizon for a 4 million solar mass black hole.
The incredible astrometry measurements of S0-2 that enabled its orbital parameters to be measured with such precision are due in part to advances in Laser Guide Star Adaptive Optics (LGSAO), implemented within the last 4 years.
Measured (dots) and calculated (line) orbital position of S0-2 based on 16 years of observations. The orbit matches Kepler’s law perfectly. From Ghez, et.al., Astrophysical Journal, Aug 21, 2008
In the upper part of the Earth’s mesosphere (90 km altitude), there is a 5-10 km thick layer rich in Na atoms, deposited by the ablation of micrometeorites. These atoms can be excited and then re-radiate by projecting a 10 Watt pulsed laser tuned to the Na D atomic transition (589 nm). This creates an artificial near point source with an apparent magnitude of about 10. Using this reference source, the deformable mirror of the Keck telescope can significantly correct for short term atmospheric turbulence.
Ghez and her team leveraged many techniques familiar to all amateur astrophotographers. In addition to LGSAO, to achieve 0.3 milli arc-sec of spatial resolution, she used background sky-subtraction, flat field scaling, bad-pixel correction, distortion correction, multiple frame sorting, aligning and stacking, dithering and re-sampling‘
Figure 2. The center of the Milky Way, with normal resolution from the Keck telescope (left) and the same view, with adaptive optics turned on (right). Sgr A* is at the center. From http://www.keckobservatory.org/support/magazine/2007/dec/index.htm#one
Without the LGSAO for atmospheric distortion correction, 10,000 frames, each of 0.1 sec exposure, were taken. Bad frames, with smeared out point sources, were discarded and the good frames aligned and stacked. Of more than 1 millon frames taken, 10,000 made it through the final cut, resulting in a spatial positioning error of about 1 milli arc-sec.
With LGSAO turned on, 10 integrations, each about 2.8 second long, were taken as one frame and the telescope position randomly dithered by 0.7 arc-sec. Again, the good frames were aligned and stacked. Of the approximately 1,000 frames taken, approximately 500 frames had sharp enough resolution. The typical positional accuracy was less than 0.2 milli arc-sec. This is compared with the pixel resolution of the camera of about 10 milli arc-sec.
Sixteen years of patient, high precision observing has resulted into the clearest picture to date of our very own super massive black hole living at the center our galaxy.
For see an astonishing movie of the measured orbital motions of the stars orbiting this black hole, be sure to check out: http://keckobservatory.org/cosmicmatters/animation/orbits_animation/
Published in Jan 2009 Cosmic Messenger
