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 \sigma 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 21cm emission .
This high-velocity ridge has a similar morphology to , and is roughly centered on , Complex C proper .
The similarity of the absorption lines ratios in the HVR and Complex C suggest that these structures are intimately related .
In Complex C proper we find [ O/H ] = -0.76 ^ { +0.23 } _ { -0.21 } .
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 _ { \odot } 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 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 to 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 .