The Nuclear Star Cluster of the Milky Way

Abstract

Abstract. This contribution briefly summarizes the current state of our knowledge about the Milky Way's nuclear star cluster. The center of the Milky WayThe Galactic center (GC) is located at a mere 8 kpc from Earth, about a hundred times closer than the next comparable galaxy. It can thus provide us with unique data about the structure and evolution of a galactic stellar nucleus. Adaptive optics assisted observations with 8-10m-class telescopes in the near-infrared (NIR) allow us to resolve scales as small as 0.05", corresponding to about 8 mpc at the GC. On the downside, we must cope with the extreme interstellar extinction toward the GC that is caused by the line-of-sight through the Galactic Disk. At A V ≈ 30, observations in the visual regime are impossible. Fortunately, extinction decreases steeply throughout the NIR. With A K ≈ 2.7 toward the central parsec at λ = 2.2 μm (Schödel et al., 2010) we can study the distribution and kinematics of the stars in our Galaxy's nucleus. Figure 1 presents an overview of the GC. The emission is dominated by stars in the the nuclear bulge, a disk-like region that stands out from the kpc-scale Galactic Bulge through intense far-infrared emission, ongoing star formation, a strong ionizing radiation field, large amounts of clumpy molecular gas, and an extremely high density of stars The nuclear bulge (sometimes also called Nuclear Stellar Disk ) has a radius of ∼230 pc, a scale height of ∼45 pc and a total mass of ∼1.4 × 10 9 M (Launhardt et al., 2002). Properties of the nuclear star clusterThe nuclear star cluster (NSC) is a compact entity at the center of the nuclear bulge.

Loss-cone dynamics

“…The NSCs in these galaxies have masses below ∼10 7 M and half-light radii 10 pc. The NSC in the Milky Way has properties that are close to these limiting values [53], consistent with the fact that its relaxation time is ∼10 Gyr. Unfortunately no NSC beyond the Local Group is near enough that the presence of a Bahcall-Wolf cusp could be confirmed even if it were present; furthermore the typical NSC may not contain a massive BH, although some clearly do [54].…”

“…Figure 10 shows estimates of the capture rate in spherical and axisymmetric galaxies [60]. The left panel is based on solutions to the Fokker-Planck equations ( 13), (53), assuming two sets of galaxy models: having either steep (ρ ∝ r −3/2 ) central density cusps (M • 10 8 M ) or shallower (ρ ∝ r −1 ) 'cores' (M • 10 8 M ). The M • -σ relation:…”