Galactic Center
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The Galactic Centre

Moving stars

The central region of our Milky Way is an extremly 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.

Since 1992 we have observed the central parsec of the galactic center in near infrared wavelengths. Until 2002 we mainly used our Speckle-Camera SHARP I on the NTT located at La Silla, Chile, for K band observations; since 2002 our main instrument is the adaptive optics assisted NIR-camera NAOS/CONICA at the VLT on Cerro Paranal, Chile, performing H, K and L imaging. Both facilities are operated by the European Southern Observatory (ESO).


WARNING - The movie to the right is quite big (roughly 6MB). If you wish, you can stop the download and download a lightweight MPEG-version.

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

So far, we have shown that at least two periods of star formation have occured. There are three sorts of stars in the galactic center:

  • Stars with He I - emission
  • Stars which show CO - absorption
  • Embedded stars with featureless spectra
The Galactic Centre

A HKL colour composite of the Galactic Centre region. The central black hole is located in the centre of the box which marks the area shown in the images above and below.


Stellar dynamics in the innermost region

Time resolved astrometry over a time span of now already 12 years 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 distance between earth and Sgr A* and the mass of Sgr A* could be calculated.

Radio and X-ray emission from Sgr A*

The supermassive black hole in the centre of the Milky Way was discovered as a bright non-thermal radio source in the 1970s and termed Sagittarius A* (Sgr A*). Potential X-ray radiation by Sgr A* was detected with the X-ray observatory Rosat in the 1990s. A reliable identification of X-rays from Sgr A* was finally possible with the new X-ray satellites Chandra and XMM, with their high spatial resolution and sensitivity. The radio emission of SgrA* only varies slowly on time scales of several days to a few hundred days and generally with an amplitude <10%. However, in the X-ray regime, SgrA* was found to exhibit two different states. On the one hand, in the quiescent state, weak X-ray emission appears to come from a slightly extended area around the black hole that appears to be evidence of hot accreting gas in the environment of SgrA*. On the other hand, SgrA* shows X-ray flares with a period of about one per day. During these flares, the emission rises by factors up to 100 during several tens of minutes and a distinctive point source becomes visible at the location of SgrA*. The short rise-and-decay times of the flares suggest that the radiation must origin from a region within less than 10 Schwarzschild radii of a 3.6 million solar mass black hole.

Near-infrared flares from the black hole Infrared flare

Near-infrared high-resolution observations of the galactic centre (GC) became possible since the beginning of the 1990s. Since then, the GC stellar was regularly monitored by high-resolution NIR imaging. 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. During subsequent observations in 2003 and 2004, we could observe more flares from Sgr A* in the H, K and L-bands (1.7, 2.2 and 3.8 microns) and also quiescent emission from a source at this location. With hindsight, we could also detect a flaring source in older, longer wavelength data from 2002. Independently, flaring and variability of SgrA* in the L-band was also observed at the Keck telescope by researchers from the University of California, LA, in June 2003.

The quiescent and flaring NIR emission from Sgr A* fills an important gap in our knowledge of the spectrum of this source and will allow to constrain the existing models on how the radiation is produced. While the quiescent emission appears to be largely consistent with an origin in the high-energy tail of a synchrotron spectrum, the mechanism of the NIR flares is uncertain and its explanation one of our main goals. Simultaneous, multi-wavelength NIR and X-ray observations of the GC were executed in 2004. Additionally we observed Sgr A* with the AO-assisted near infrared integral field spectrometer SINFONI in July 2004 and were able to detect and measure a weak flare, providing the first spectrum of a Sgr A* flare ever obtained.

Observation of a Sgr A* flare with SINFONI

A weak flare as seen by SINFONI on July 15, 2004. The time in minutes is shown in the images.

The chances are high that these observations will provide the required data to constrain the models and to establish (or exclude) a relation between the X-ray and NIR variability.

A spin measurement of the black hole?

Light curves of Sgr A* NIR flares

Light curves of the Sgr A* NIR flares in 2002 and 2003, observed with NACO/VLT. The L'-band flare on August 30, 2002, was only partially covered by observations. Gaps in the time series of the H-band flare on May 10, 2003, and of the KS-band flare on June 15, 2003, are due to sky observations and instrument failure, respectively. For comparison, the emission of the steady emission of the star S1 near Sgr A* is shown in all the plots (light grey data points). Arrows in the plots of the two KS-band flares indicate substructure peaks of the flares. Both KS-band flares show very similar quasi-periodicity, although the second flare was observed more than 24 h after the first one and must thus have been an unrelated event. The upper right panel shows the normalised power spectrum of the two KS-band flares. Both of them show a significant peak at a frequency corresponding to time scales of 16.8±2.0 min. In both cases, the power spectrum of S1 does not show such a frequency.

The two K-band flares observed on the 15th and 16th of June 2003 are the flares that were completely covered by observations. Although they happened more than 24 hours apart and thus appear to be unrelated events, they both show a striking quasi-periodicity of the flare with a period of about 17 min. Of all possible periodic processes near a black hole (acoustic modes of a thin disk, Lense-Thirring precession, precession of orbital nodes, orbital motion), the period of matter circling the black hole near the last stable orbit is the shortest one. The observed period of 17 min is so short, however, that the only reasonable explanation is that the oscillations are produced by Doppler boosting of hot gas near the last stable orbit of a spinning (Kerr) black hole. The spin of the black hole will allow for a last stable orbit closer to the event horizon and thus with a shorter orbital frequency. From the observed 17 min period we estimate that the supermassive black hole Sgr A* has a spin that is half as big as the maximum possible spin of such an object. Additional observations of flares and their quasi-periodicity will be needed in order to confirm this result. Should the quasi-periodicity indeed be an intrinsic feature of the flares then this will mean that the era of black hole physics has begun with the properties of black holes accessible to direct measurements!


© Infrared and Submillimeter Astronomy Group at MPE
last update: 24/08/2006, editor of this page: Thomas Ott


                                                                                                                                                                                                                                                                                                                                                                                                               

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