The central region of our Milky Way is an extremely interesting and fascinating field of research. Within few light years we find here ten thousands of stars forming a dense cluster, and the geometric centre of our Galaxy harbours a supermassive black hole with around 3.6 million solar masses. Due to its relative proximity of around 8 kiloparsecs, the Galactic Centre is a perfect laboratory to examine the physical processes in a galactic nucleus.
Our primary goals are to uncover
- the physical properties of the central supermassive black hole
- when and where the stars were born
- the nature of these stars
- the dynamics in this region
Time resolved astrometry over a time span of now more than 2 decades allows a description of the proper motions of the Galactic Centre stars. The observations clearly show, that some stars in the immediate vicinity of Sgr A* - i.e. in distances up to around 30 light days - move on Keplerian orbits around the central mass. From the shape of these orbits, the mass of Sgr A* and its distance from the Earth can be calculated.
In order to achieve this, we have observed the central parsec of the galactic center in near infrared wavelengths since 1992. The main instruments are:
- the adaptive optics assisted NIR-camera NAOS/CONICA at the VLT on Cerro Paranal, Chile, performing H, K and L imaging
- the imaging spectrograph SINFONI, also located at the VLT on Cerro Paranal, Chile
- the Laser Guide Star Facility at the VLT
- until 2002 we used the Speckle-Camera SHARP I on the NTT located at La Silla, Chile
All facilities are operated by the European Southern Observatory (ESO).
In 2011, we discovered a dense gas cloud ("G2") approximately three times the mass of Earth that is falling into the accretion zone of Sgr A*. Our observations tightly constrain G2's orbit to be highly eccentric (e = 0.966), with a pericenter distance of only 2,200 Schwarzschild radii that will be reached in late 2013. Over the past four years G2 has begun to disrupt through tidal shearing arising from the black hole’s gravitational force. Ultimately, the gas might feed the black hole, leading to an overall, broad-band increased emission of Sgr A*.
Since the first near-infrared high-resolution observations of the galactic centre in the beginning of the 1990s, the GC was regularly monitored. However, in spite of all efforts, no unambiguous NIR counterpart of SgrA* could be detected up to 2003. On the 9th of May, during routine observations of the GC star cluster at 1.7 microns with NAOS/CONICA at the VLT, we witnessed a powerful flare at the location of the black hole. Within a few minutes, the flux of a faint source increased by a factor of 5-6 and fainted again after about 30 min. The flare was found to have happened within a few milli-arcseconds of the position of Sgr A*. The short rise-and-decay times told us that the source of the flare was located within less than 10 Schwarzschild radii of the black hole.
The Laser Guide Star of the VLT pointing at the Galactic Centre. Image courtesy Yuri Beletsky.