ar X iv : a st ro - p h / 06 07 07 2 v 1 5 J ul 2 00 6 Sub - milliarcsecond Imaging of SgrA * and M 87

We present and discuss new result from mm-VLBI observations of M 87 and Sgr A*. The imaging of these sources with a spatial resolutions of a few to a few ten Schwarzschild radii offers new possibilities to study the immediate environment of super-massive black holes. 1. Introduction In astronomy, the highest angular resolution is obtained with Very Long Baseline Interferometry (VLBI). VLBI observations at short millimeter wavelengths (mm-VLBI) provide angular resolutions in the ten micro-arcsecond range (i.e. 14 µas for a 10, 000 km baseline at λ = 1.3 mm (ν = 230 GHz)). While VLBI observations at 2 mm and 1 mm wavelength are still in a stage of technical development, 3 mm VLBI experiments are performed on a regular basis. The Global mm-VLBI Array (GMVA 1) is operational since early 2000. At 86 GHz (λ = 3.5 mm), it combines the European antennas (Effelsberg 100 m, Pico Veleta 30 m, the phased Plateau de Bure interferometer 6x15 m, Onsala 20 m, Metsähovi 14 m) with the VLBA. With the participation of the two sensitive IRAM telescopes and the 100 m Effelsberg telescope, the array sensitivity is improved by a factor of 3 − 4, when compared to the VLBA alone. For compact galactic and extragalactic radio sources, the GMVA provides VLBI images with an angular resolution of down to 40 µas. In the following we present and discuss results from mm-VLBI observations of the nuclei of M 87 and of Sgr A*. When expressed in terms of Schwarzschild radii (R S = 2GM BH /c 2), the spatial resolution obtained for M 87 and Sgr A* become very comparable. M 87 is about ∼ 10 3 times more distant than Sgr A*, but has a putative black hole at its center, which is also ∼ 10 3 times more massive than that of Sgr A*. Present day 3 mm-VLBI therefore allows to image both sources with a spatial resolution of only 10 − 20R S. In view of the morphological difference between Sgr A* and M 87 (the latter exhibits a long relativistic jet), a comparison of the sub-milliarcsecond structure and kinematics of both objects may lead to a better understanding of the astro-physical processes acting in the vicinity of such super-massive black holes.