Surface brightness profile of the Milky Way’s nuclear star cluster

Abstract

[Context] Although the Milky Way nuclear star cluster (MWNSC) was discovered more than four decades ago, several of its key properties have not been determined unambiguously up to now because of the strong and spatially highly variable interstellar extinction toward the Galactic centre.

[Aims] In this paper we aim at determining the shape, size, and luminosity/mass of the MWNSC.

[Methods] To investigate the properties of the MWNSC, we used Spitzer/IRAC images at 3.6 and 4.5 μm, where interstellar extinction is at a minimum but the overall emission is still dominated by stars. We corrected the 4.5 μm image for polycyclic aromatic hydrocarbon (PAH) emission with the help of the IRAC 8.0 μm map and for extinction with the help of a [3.6−4.5] colour map. Finally, we investigated the symmetry of the nuclear cluster and fit it with Sérsic, Moffat, and King models.

[Results] We present an extinction map for the central ~ 300 × 200 pc2 of the Milky Way, as well as a PAH-emission- and extinction-corrected image of the stellar emission, with a resolution of about 0.20 pc. We find that the MWNSC appears in projection to be intrinsically point-symmetric, that it is significantly flattened, with its major axis aligned along the Galactic plane, and that it is centred on the black hole, Sagittarius A*. Its density follows the well known approximate ρ ∝ r-2-law at distances of a few parsec from Sagittarius A*, but becomes as steep as ρ ∝ r-3 at projected radii around 5 pc. We derive a half light radius of 4.2 ± 0.4 pc, a total luminosity of LMWNSC,4.5 μm = 4.1 ± 0.4 × 107 L⊙, and a mass of MMWNSC = 2.5 ± 0.4 × 107 M⊙.

[Conclusions] The overall properties of the MWNSC agree well with the ones of its extragalactic counterparts, which underlines its role as a template for these objects. Its flattening agrees well with its previously established rotation parallel to Galactic rotation and suggests that it was formed by accretion of material that tended to fall in along the Galactic plane. Our findings support the in situ growth scenario for nuclear clusters and emphasise the need to increase the complexity of theoretical models for their formation and for the interaction between their stars and the central black hole in order to include rotation, axisymmetry, and growth in recurrent episodes.