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 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 Lyα 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 10⁹ M _⊙. 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 α abundance in Spur 2 of {0.13}_-0.05^+0.07 solar ([α/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.