Dust within the nuclear star cluster in the Milky Way

Abstract

The mean absolute extinction towards the central parsec of the Milky Way is AK ≃ 3 mag, including both foreground and Galactic Centre dust. Here we present a measurement of dust extinction within the Galactic old nuclear star cluster (NSC), based on combining differential extinctions of NSC stars with their υl proper motions along Galactic longitude. Extinction within the NSC preferentially affects stars at its far side, and because the NSC rotates, this causes higher extinctions for NSC stars with negative υl, as well as an asymmetry in the υl-histograms. We model these effects using an axisymmetric dynamical model of the NSC in combination with simple models for the dust distribution. Comparing the predicted asymmetry to data for ~7100 stars in several NSC fields, we find that dust associated with the Galactic Centre mini-spiral with extinction AK ≃ 0.15-0.8 mag explains most of the data. The largest extinction AK ≃0.8mag is found in the region of theWestern arm of themini-spiral. Comparing with total AK determined from stellar colours, we determine the extinction in front of the NSC. Finally, we estimate that for a typical extinction of AK ≃ 0.4 the statistical parallax of the NSC changes by ~0.4 per cent.