hide
Free keywords:
Astrophysics, Galaxy Astrophysics, astro-ph.GA
Abstract:
In this paper we aim at determining the shape, size, and
luminosity/mass of the Milky Way Nuclear Star Cluster (MWNSC). We use
Spitzer/IRAC images at 3.6 and 4.5 micrometer, where interstellar extinction is
at a minimum but the overall emission is still dominated by stars. We correct
the 4.5 micrometer image for PAH emission with the help of the IRAC 8.0
micrometer map and for extinction with the help of a [3.6-4.5] colour map. We
present an extinction map for the central ~300x200 pc^2 of the Milky Way, as
well as a PAH-emission and extinction corrected image of the stellar emission,
with a resolution of about 0.2 pc. We find that the MWNSC appears in projection
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
rho~r^{-2}-law at distances of a few parsecs from Sagittarius A*, but may
become as steep as rho~r^{-3} at projected radii around 5 pc. We derive a half
light radius of 4.2+-0.4 pc, a total luminosity of L_MWNSC=(4.1+-0.4)x10^{7}
L_Sun, and a mass of M_{MWNSC}=(2.5+-0.4)x10^{7} M_Sun. 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 has formed by accretion of material that fell in
preferentially along the Galactic Plane. Our findings support the in situ
growth scenario for nuclear clusters and emphasize 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.
