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arXiv · 1810.07700

Circumgalactic gas at its extreme: tidal gas streams around the Whale galaxy NGC 4631 explored with HST/COS

P. Richter·B. Winkel·B. P. Wakker·N. M. Pingel·A. J. Fox·G. Heald·R. A. M. Walterbos·C. Fechner·N. Ben Bekhti·G. Gentile·L. Zschaechner
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Abstract

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.

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P. Richter, B. Winkel, B. P. Wakker, N. M. Pingel, A. J. Fox, G. Heald, R. A. M. Walterbos, C. Fechner, N. Ben Bekhti, G. Gentile, L. Zschaechner. 2018-10-17. Circumgalactic gas at its extreme: tidal gas streams around the Whale galaxy NGC 4631 explored with HST/COS. https://doi.org/10.3847/1538-4357/aae838

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