Circumgalactic gas at its extreme: tidal gas streams around the Whale
  galaxy NGC 4631 explored with HST/COS
  We present a detailed analysis of the absorption properties of one of the
tidal gas streams around the Whale galaxy NGC4631 in the direction of the
quasar 2MASSJ12421031+3214268. Our study is based on ultraviolet spectral data
obtained with the Cosmic Origins Spectrograph (COS) onboard the Hubble Space
Telescope (HST) and 21cm-data from the HALOGAS project and the Green Bank
Telescope (GBT). We detect strong HI Ly alpha absorption in the velocity range
+550 to +800 km s^-1 related to gas from a NGC4631 tidal stream known as Spur
2. We measure a column density of log N(HI)=18.68pm0.15, indicating that the
quasar sightline traces the outer boundary of Spur 2 as seen in the 21cm data.
Metal absorption in Spur 2 is detected in the lines of OI, CII, SiII, and SiIII
in a complex absorption pattern that reflects the multi-phase nature of the
gas. We find that the average neutral gas fraction in Spur 2 towards
2MASSJ12421031+3214268 is only 14 percent. This implies that ionized gas
dominates the total mass of Spur 2, which then may comprise more than 10^9
M_sun. No significant depletion of Si is observed, showing that Spur 2 does not
contain significant amounts of dust. From the measured OI/HI column-density
ratio we determine an alpha abundance in Spur 2 of 0.13pm0.07 solar
([alpha/H]=-0.90pm 0.16), which is substantially lower than what is observed in
the NGC4631 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 NGC4631.
