@article{WinkelBenBekhtiDarmstaedteretal.2011,
  author    = {Winkel, B. and Ben Bekhti, Nadya and Darmstaedter, V. and Floeer, L. and Kerp, J. and Richter, Philipp},
  title     = {The high-velocity cloud complex Galactic center negative as seen by EBHIS and GASS I. Cloud catalog and global properties},
  series = {Astronomy and astrophysics : an international weekly journal},
  volume    = {533},
  journal   = {Astronomy and astrophysics : an international weekly journal},
  number    = {18},
  publisher = {EDP Sciences},
  address   = {Les Ulis},
  issn      = {0004-6361},
  doi       = {10.1051/0004-6361/201117357},
  pages     = {13},
  year      = {2011},
  abstract  = {Using Milky Way data of the new Effelsberg-Bonn HI Survey (EBHIS) and the Galactic All-Sky Survey (GASS), we present a revised picture of the high-velocity cloud (HVC) complex Galactic center negative (GCN). Owing to the higher angular resolution of these surveys compared to previous studies (e.g., the Leiden Dwingeloo Survey), we resolve complex GCN into lots of individual tiny clumps, that mostly have relatively broad line widths of more than 15 km s(-1). We do not detect a diffuse extended counterpart, which is unusual for an HVC complex. In total 243 clumps were identified and parameterized which allows us to statistically analyze the data. Cold-line components (i.e.,Delta upsilon(fwhm) < 7.5 km s(-1)) are found in about 5\% only of the identified cloudlets. Our analysis reveals that complex GCN is likely built up of several subpopulations that do not share a common origin. Furthermore, complex GCN might be a prime example for warm-gas accretion onto the Milky Way, where neutral HI clouds are not stable against interaction with the Milky Way gas halo and become ionized prior to accretion.},
  language  = {en}
}
@article{BenBekhtiWinkelRichteretal.2012,
  author    = {Ben Bekhti, Nadya and Winkel, B. and Richter, P. and Kerp, J. and Klein, U. and Murphy, M. T.},
  title     = {An absorption-selected survey of neutral gas in the Milky Way halo New results based on a large sample of Ca II, Na I, and H I spectra towards QSOs},
  series = {Astronomy and astrophysics : an international weekly journal},
  volume    = {542},
  journal   = {Astronomy and astrophysics : an international weekly journal},
  number    = {2},
  publisher = {EDP Sciences},
  address   = {Les Ulis},
  issn      = {0004-6361},
  doi       = {10.1051/0004-6361/201118673},
  pages     = {17},
  year      = {2012},
  abstract  = {Aims. We aim at analysing systematically the distribution and physical properties of neutral and mildly ionised gas in the Milky Way halo, based on a large absorption-selected data set. Methods. Multi-wavelength studies were performed combining optical absorption line data of Ca II and Na I with follow-up H I 21-cm emission line observations along 408 sight lines towards low-and high-redshift QSOs. We made use of archival optical spectra obtained with UVES/VLT. H I data were extracted from the Effelsberg-Bonn H I survey and the Galactic All-Sky survey. For selected sight lines we obtained deeper follow-up observations using the Effelsberg 100-m telescope. Results. Ca II (Na I) halo absorbers at intermediate and high radial velocities are present in 40-55\% (20-35\%) of the sightlines, depending on the column density threshold chosen. Many halo absorbers show multi-component absorption lines, indicating the presence of sub-structure. In 65\% of the cases, absorption is associated with H I 21-cm emission. The Ca II (Na I) column density distribution function follows a power-law with a slope of beta approximate to -2.2 (-1.4). Conclusions. Our absorption-selected survey confirms our previous results that the Milky Way halo is filled with a large number of neutral gas structures whose high column density tail represents the population of common H I high-and intermediate-velocity clouds seen in 21-cm observations. We find that Na I/Ca II column density ratios in the halo absorbers are typically smaller than those in the Milky Way disc, in the gas in the Magellanic Clouds, and in damped Lyman a systems. The small ratios (prominent in particular in high-velocity components) indicate a lower level of Ca depletion onto dust grains in Milky Way halo absorbers compared to gas in discs and inner regions of galaxies.},
  language  = {en}
}