Complex C: A Low-Metallicity, High-Velocity Cloud Plunging into the Milky Way

We present evidence that high-velocity cloud (HVC) complex C is a low-metallicity gas cloud that is plunging toward the disk and beginning to interact with the ambient gas that surrounds the Milky Way. This evidence begins with a new high-resolution (7 km s-1 FWHM) echelle spectrum of 3C 351 obtained with the Space Telescope Imaging Spectrograph (STIS). 3C 351 lies behind the low-latitude edge of complex C, and the new spectrum provides accurate measurements of O I, Si II, Al II, Fe II, and Si III absorption lines at the velocity of complex C; N I, S II, Si IV, and C IV are not detected at 3 σ significance in complex C proper. However, Si IV and C IV as well as O I, Al II, Si II and Si III absorption lines are clearly present at somewhat higher velocities associated with a "high-velocity ridge" (HVR) of 21 cm emission. This high-velocity ridge has a similar morphology to and is roughly centered on complex C proper. The similarities of the absorption-line ratios in the HVR and complex C suggest that these structures are intimately related. In complex C proper we find [O/H] = -0.76. For other species the measured column densities indicate that ionization corrections are important. We use collisional and photoionization models to derive ionization corrections; in both models we find that the overall metallicity Z = 0.1–0.3 Z⊙ in complex C proper, but nitrogen must be underabundant. The iron abundance indicates that the complex C contains very little dust. The size and density implied by the ionization models indicate that the absorbing gas is not gravitationally confined. The gas could be pressure confined by an external medium, but alternatively we may be viewing the leading edge of the HVC, which is ablating and dissipating as it plunges into the Milky Way. O VI column densities observed with the Far Ultraviolet Spectroscopic Explorer (FUSE) toward nine QSOs/AGNs behind complex C support this conclusion: N(O VI) is highest near 3C 351, and the O VI/H I ratio increases substantially with decreasing latitude, suggesting that the lower latitude portion of the cloud is interacting more vigorously with the Galaxy. The other sight lines through complex C show some dispersion in metallicity, but, with the current uncertainties, the measurements are consistent with a constant metallicity throughout the HVC. However, all of the complex C sight lines require significant nitrogen underabundances. Finally, we compare the 3C 351 data with high-resolution STIS observations of the nearby QSO H1821+643 to search for evidence of outflowing Galactic fountain gas that could be mixing with complex C. We find that the intermediate-velocity gas detected toward 3C 351 and H1821+643 has a higher metallicity and may well be a fountain/chimney outflow from the Perseus spiral arm. However, the results for the higher velocity gas are inconclusive: the HVC detected toward H1821+643 near the velocity of complex C could have a similar metallicity to the 3C 351 gas or it could have a significantly higher Z, depending on the poorly constrained ionization correction.