The Stellar Cusp around the Supermassive Black Hole in the Galactic Center

Abstract

We analyze deep near-IR adaptive optics imaging (taken with NAOS/CONICA on the VLT) 1 as well as new proper motion data of the nuclear star cluster of the Milky Way. The surface density distribution of faint (H≤ 20, K s ≤ 19) stars peaks within 0.2 ′′ of the black hole candidate SgrA ⋆ . The radial density distribution of this stellar 'cusp' follows a power law of exponent α ∼ 1.3 − 1.4. The K-band luminosity function of the overall nuclear stellar cluster (within 9 ′′ of SgrA ⋆ ) resembles that of the large scale, Galactic bulge, but shows an excess of stars at K s ≤ 14. It fits population synthesis models of an old, metal rich stellar population with a contribution from young, early and late-type stars at the bright end. In contrast, the cusp within ≤ 1.5 ′′ of SgrA ⋆ appears to have a featureless luminosity function, suggesting that old, low mass horizontal branch/red clump stars are lacking. Likewise there appear to be fewer late type giants. The innermost cusp also contains a group of moderately bright, early type stars that are tightly bound to the black hole. We interpret these results as evidence that the stellar properties change significantly from the outer cluster (≥ a few arcsecs) to the dense innermost region around the black hole.We find that most of the massive early type stars at distances 1-10" from SgrA ⋆ are located in two rotating and geometrically thin disks. These disks are inclined at large angles and counter-rotate with respect to each other. Their stellar content is essentially the same, indicating that they formed at the same time. We conclude that of the possible formation scenarios for these massive stars the most probable one is that 5-8 million years ago two clouds fell into the center, collided, were shock compressed and then formed two rotating (accretion) disks orbiting the central black hole. For the OB-stars in the central arcsecond, on the other hand, a stellar merger model is the most appealing explanation. These stars may thus be 'super-blue-stragglers', formed and 'rejuvenated' through mergers of lower mass stars in the very dense (≥ 10 8 M ⊙ pc −3 ) environment of the cusp. The 'collider model' also accounts for the lack of giants within the central few arcseconds.The star closest to SgrA ⋆ in 2002, S2, exhibits a 3.8 µm excess. We propose that the mid-IR emission either comes from the accretion flow around the black hole itself, or from dust in the accretion flow that is heated by the ultra-violet emission of S2.1 Based on observations obtained at the European Southern Observatory, Chile

Orbits and origins of the young stars in the central parsec of the galaxy

“…Our first attempt at determining individual orbits for young stars that reside outside the central arcsecond shows definitive evidence for the clockwise-rotating disk that was suggested by [6] and was subsequently refined by [9] and [3]. Our results do not show a statistically significant second disk.…”

“…5 and 14 (0.02-0.6 pc) projected distance from Sgr A* exhibit coherent rotation [8]. Analyses of the statistical properties of the three-dimensional velocity vectors for these stars suggest that they lie in a clockwise-rotating disk [6] and perhaps also a counter-clockwise-rotating disk [9] that is nearly perpendicular to the first, with the disks extending from ∼0. 8 to at least 7 [3].…”

“…The distribution of young stars resulting from our study supports the hypothesis of a single, clockwise disk plus a more isotropically distributed population. Over-plotted in black are the candidate orbital planes as proposed by [6] and [9] with updated values from [3] for the candidate plane normal vector and uncertainties (solid black) and the disk thickness (dashed black) shown as solid angles of 0.05 sr and 0.09 sr for the clockwise and counter-clockwise disks respectively. Figure 5.…”

15 years of high precision astrometry in the Galactic Center

“…The density profile of the central star cusp as obtained from number counts (Genzel et al 2003, Schödel et al 2007) predicts that in the central 100 mas there should reside a few (∼5) stars with K-band magnitudes from…”

The AGN Jet Model of the Fermi Bubbles

“…Black hole accretion events 2 Fulai Guo often release winds and jets, as observed in quasars and radio galaxies (Fabian 2012). Observational evidence for recent AGN activity at the GC includes (1) there appear to be two young stellar disks within 0.5 pc of Sgr A * with typical stellar ages of 6 ± 2 Myr (Genzel et al 2003, Paumard et al 2006, which may be remnants of recent accretion flows onto Sgr A * ; (2) the Hα emission of the Magellanic Stream peaks toward the south Galactic pole, which may be energized by ionizing photons from Sgr A * about 1 − 3 Myrs ago (Bland-Hawthorn et al 2013). …”