From the sun to the Galactic Center
The centers of galaxies are their own ultimate gravitational sinks. \nMassive black holes and star clusters as well as gas are especially likely to fall into the centers of galaxies by \ndynamical friction or dissipation. Many galactic centers harbor supermassive black holes (SMBH)\nand dense nuclear (star) clusters which possibly arrived there by these processes.\nNuclear clusters can be formed in situ from gas, or from smaller star clusters which fall to the center. \nSince the Milky Way harbors both an SMBH and a nuclear cluster, both can be studied best in the Galactic Center (GC), \nwhich is the closest galactic nucleus to us. \nIn Chapter 1, I introduce the different components of the Milky Way, and put these into the context of the GC.\nI then give an overview of relevant properties (e.g. star content and distribution) of the GC.\nAfterwards, I report the results of four different studies about the GC. \n\nIn Chapter 2, I analyze the limitations of astrometry, one of the most useful methods for the study of the GC. Thanks to the \nhigh density of stars and its relatively small distance from us it is possible to measure the motions of thousands of stars in the GC with images,\nseparated by few years only. I find two main limitations to this method: (1) for bright stars the not perfectly correctable distortion of the \ncamera limits the accuracy, and (2)\n for the majority of the fainter stars, the main limitation is crowding from the other stars in the GC. The position uncertainty of faint stars is mainly \ncaused by the seeing halos of bright stars. In the very center faint unresolvable stars are also important for the position uncertainty.\n\n\nIn Chapter 3, I evaluate the evidence for an intermediate mass black hole in the small candidate cluster IRS13E within the GC.\nIntermediate mass black holes (IMBHs) have a mass between the two types of confirmed black hole: the stellar remnants and the \nsupermassive black holes in the centers of galaxies.\nOne possibility for their formation is the collision of stars in a dense young star cluster. Such a cluster \ncould sink to the GC by dynamical friction. There it would consist of few bright stars like IRS13E. \nFirstly, I analyze the SEDs of the objects in IRS13E. The SEDs of most objects can be explained by pure dust emission. \nThus, most objects in IRS13E are pure dust clumps and only three young stars.\nThis reduces the significance of the 'cluster' IRS13E compared to the stellar background. \nSecondly, I obtain acceleration limits for these three stars. The non-detection of accelerations makes an IMBH an\nunlikely scenario in IRS13E. However, since its three stars\nform a comoving association, which is unlikely to form by chance, the nature of IRS13E is not yet settled.\n\n\nIn the third study (Chapter 4) I measure and analyze the extinction curve toward the GC. The extinction is a contaminant for GC \nobservations and therefore it is necessary to know the extinction toward the GC to determine the luminosity properties of its stars. \nI obtain the extinction curve by measuring \nthe flux of the HII region in the GC in several infrared HII lines and in the unextincted radio continuum.\nI compare these ratios with the ratios expected from recombination\nphysics and obtain extinctions at 22 different lines between 1 and 19 micron. For the K-band I derive A_Ks=2.62+/-0.11. \nThe extinction curve follows a power law with a steep slope of -2.11+/-0.06 shortward of 2.8 micron. At longer wavelengths the extinction is grayer and \nthere are absorption features from ices. The extinction curve is a tool to constrain the properties of cosmic dust between the sun \nand the GC. The extinction curve cannot be explained by dust grains consisting of carbonaceous and silicate grains only. In addition\ncomposite particles, which also contain ices are necessary to fit the extinction curve.\n\nIn the final part of this thesis (Chapter 5) I look at the properties of most of the stars in the GC.\nThese are the old stars that form the nuclear cluster of the Milky Way. I obtain the mass \ndistribution and the light distribution of these stars.\nI find that the flattening of the stellar distribution increases outside 70''. This indicates that inside a nearly spherical nuclear cluster \ndominates and that the surrounding light belongs mostly to the nuclear disk. I dissect the light in two components and obtain \n for the nuclear cluster L_Ks=2.7*10^7 L_sun. I obtain proper motions for more than 10000 stars and\nradial velocities for more than 2400 stars. \nUsing Jeans modeling I combine velocities and the radial profile to obtain within 100'' (4 pc) a mass of \n 6.02*10^6 M_sun and a total nuclear cluster mass of 12.88*10^6 M_sun. \nThe Jeans modeling and various other evidence weakly favor a core in the extended mass compared to a cusp.\nThe old star light shows a similar core. The mass to light ratio of the old stars of the nuclear cluster is\nconsistent with the usual initial mass function in the Galaxy. This suggests that most stars in GC formed in the\n usual way, in a mode different from the origin of the youngest stars there.