@article{RichterWinkelWakkeretal.2018,
  author    = {Richter, Philipp and Winkel, Benjamin and Wakker, Bart P. and Pingel, N. M. and Fox, Andrew J. and Heald, G. and Walterbos, Rene A. M. and Fechner, C. and Ben Bekhti, N. and Gentile, G. and Zschaechner, Laura},
  title     = {Circumgalactic Gas at Its Extreme},
  series = {The astrophysical journal : an international review of spectroscopy and astronomical physics},
  volume    = {868},
  journal   = {The astrophysical journal : an international review of spectroscopy and astronomical physics},
  number    = {2},
  publisher = {IOP Publ. Ltd.},
  address   = {Bristol},
  issn      = {0004-637X},
  doi       = {10.3847/1538-4357/aae838},
  pages     = {12},
  year      = {2018},
  abstract  = {We present a detailed analysis of the absorption properties of one of the tidal gas streams around the "Whale" galaxy NGC 4631 in the direction of the quasar 2MASS J12421031+3214268. Our study is based on ultraviolet spectral data obtained with the Cosmic Origins Spectrograph (COS) on board the Hubble Space Telescope (HST) and 21cm-data from the HALOGAS project and the Green Bank Telescope (GBT). We detect strong H I Ly alpha absorption in the velocity range +550 to +800 km s(-1) related to gas from a NGC 4631 tidal stream known as Spur 2. We measure a column density of log (N(H I/cm(-2))) = 18.68 +/- 0.15, indicating that the quasar sightline traces the outer boundary of Spur 2 as seen in the 21 cm data. Metal absorption in Spur 2 is detected in the lines of O I, C II, Si II, and Si III in a complex absorption pattern that reflects the multiphase nature of the gas. We find that the average neutral gas fraction in Spur 2 toward 2MASS J12421031+3214268 is only 14\%. This implies that ionized gas dominates the total mass of Spur 2, which then may comprise more than 10(9)M(circle dot). No significant depletion of Si is observed, showing that Spur 2 does not contain significant amounts of dust. From the measured O I/H I column density ratio, we determine an alpha abundance in Spur 2 of 0.131(-0.05)(+0.07) solar ([alpha/H] = -0.90 +/- 0.16), which is substantially lower than what is observed in the NGC 4631 disk. The low metallicity and low dust content suggest that Spur 2 represents metal-deficient gas stripped off a gas-rich satellite galaxy during a recent encounter with NGC 4631.},
  language  = {en}
}