The structure of the nuclear stellar cluster of the Milky Way

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Scientific paper

We used AO assisted high-resolution near-infrared (NIR) imaging observations of the stellar cluster within 20" (about 0.75 pc) of Sgr A*, the massive black hole at the centre of the Milky Way to derive the structure of the nuclear star cluster via stellar number counts. Our findings are: (a) We demonstrate that an analysis of the stellar surface brightness is not the adequate tool for determiningmthe density of the cluster because the NIR flux is dominated by amminority of very bright stars and does not reflect the structure ofmthe underlying cluster. Therefore stellar number counts must be used. (b) Several significant density clumps aremdetected, at least one of them is probably a co-moving dense cluster, similar to the well-known IRS˜13E complex. (c) A detailed map of the variation of interstellar extinction in the central ˜0.5 pc of the Milky Way is presented. The extinction map in combination with possible shock structures in an L'-band map provides support for the assumption of an outflow from the central arc seconds. (d) The overall structure of the GC nuclear cluster can be described well by a broken power-law. The break radius is located between 8"-12" (0.30-0.44 pc). (e) In agreement with previous observations, a density excess, the 'cusp', around Sgr A* is detected with a high significance. Such a structure is the expected signature that a massive black hole imprints on a surrounding dense stellar cluster. (f) Effects of mass segregation can be seen in the surface density of the horizontal branch/red clump stars. Their distribution shows a significantly flatter power-law index than that of the other, heavier stars.

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