Suchergebnisse

[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. ; R.S. acknowledges support by grants AYA2009-13036, AYA2010-17631 and AYA2012-38491-CO2-02, cofunded with FEDER funds, by the Ramón y Cajal programme of the Spanish Ministry of Economy and Competitiveness, and by the ESO Scientific Visitor Programme. The research leading to these results has received funding from the European Research Council under the European Union's Seventh Framework Programme (FP/20072013)/ERC Grant Agreement No. [614922]. We thank Rick Arendt for his work on the Spitzer/IRAC maps. D.K. acknowledges support from the Czech Science Foundation – Deutsche Forschungsgemeinschaft collaboration project 13-00070J. ; Peer reviewed

Using the Very Large Array (VLA), we recently detected a large number of protoplanetary disc (proplyd) candidates lying within a couple of light years of the massive black hole Sgr A*. The bow-shock appearance of proplyd candidates points towards the young massive stars located near Sgr A*. Similar to Orion proplyds, the strong UV radiation from the cluster of massive stars at the Galactic centre is expected to photoevaporate and photoionize the circumstellar discs around young, low-mass stars, thus allowing detection of the ionized outflows from the photoionized layer surrounding cool and dense gaseous discs. To confirm this picture, ALMA observations detect millimeter emission at 226 GHz from five proplyd candidates that had been detected at 44 and 34 GHz with the VLA. We present the derived disc masses for four sources as a function of the assumed dust temperature. The mass of protoplanetary discs from cool dust emission ranges between 0.03 and 0.05 M-circle dot. These estimates are consistent with the disc masses found in star-forming sites in the Galaxy. These measurements show the presence of ongoing star formation with the implication that gas clouds can survive near Sgr A* and the relative importance of high-versus low-mass star formation in the strong tidal and radiation fields of the Galactic centre. ; This work is partially supported by the grant AST-1517246 from the NSF. RS acknowledges funding from the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement [614922]. ; Peer reviewed

5 pags., 7 figs. -- The Universe under the Microscope – Astrophysics at High Angular Resolution, 21–25 April 2008, Bad Honnef, Germany ; We report on recent near-infrared (NIR) and X-ray observations of Sagittarius A* (Sgr A*), the electromagnetic manifestation of the ∼4x106M⊙ super-massive black hole (SMBH) at the Galactic Center. The goal of these coordinated multi-wavelength observations is to investigate the variable emission from Sgr A* in order to obtain a better understanding of the underlying physical processes in the accretion flow/outflow. The observations have been carried out using the NACO adaptive optics (AO) instrument at the European Southern Observatory's Very Large Telescope (July 2005, May 2007) and the ACIS-I instrument aboard the Chandra X-ray Observatory (July 2005). We report on a polarized NIR flare synchronous to a 8x1033 erg/s X-ray flare in July 2005, and a further flare in May 2007 that shows the highest sub-flare to flare contrast observed until now. The observations can be interpreted in the framework of a model involving a temporary disk with a short jet. In the disk component flux density variations can be explained due to hot spots on relativistic orbits around the central SMBH. The variations of the sub-structures of the May 2007 flare are interpreted as a variation of the hot spot structure due to differential rotation within the disk. © 2008 IOP Publishing Ltd. ; Part of this work was supported by the German Deutsche Forschungsgemeinschaft, DFG via grant SFB 494. L. Meyer, K. Muzic, M. Zamaninasab, D. Kunneriath, and R.-S. Lu, are members of the International Max Planck Research School (IMPRS) for Astronomy and Astrophysics at the MPIfR and the Universities of Bonn and Cologne. RS acknowledges support by the Ramon y Cajal programme by the Ministerio de Ciencia y Innovación of the government of Spain.

7 pags., 3 figs. -- The Universe under the Microscope – Astrophysics at High Angular Resolution, 21–25 April 2008, Bad Honnef, Germany ; At the center of the Milky Way, with a distance of 8 kpc, the compact source Sagittarius A* (SgrA*) can be associated with a super massive black hole of 4x 106M⊙. SgrA* shows strong variability from the radio to the X-ray wavelength domains. Here we report on simultaneous NIR/sub-millimeter/X-ray observations from May 2007 that involved the NACO adaptive optics (AO) instrument at the European Southern Observatory's Very Large Telescope, the Australian Telescope Compact Array (ATCA), the US mm-array CARMA, the IRAM 30m mm-telescope, and other telescopes. We concentrate on the time series of mm/sub-mm data from CARMA, ATCA, and the MAMBO bolometer at the IRAM 30m telescope. © 2008 IOP Publishing Ltd. ; Part of this work was supported by the German Deutsche Forschungsgemeinschaft, DFG via grant SFB 494. L. Meyer, K. Muzic, M. Zamaninasab, D. Kunneriath, and R.-S. Lu, are members of the International Max Planck Research School (IMPRS) for Astronomy and Astrophysics at the MPIfR and the Universities of Bonn and Cologne. RS acknowledges support by the Ramon y Cajal programme by the Ministerio de Ciencia y Innovacion of the government of Spain.