A High-Metallicity High Velocity Cloud along the MRK 421 Sight Line: A
  Tracer of Complex M?
  We present a new measurement, 0.85-3.5 Z_solar, of the metallicity of high
velocity cloud (HVC) Complex M by analyzing ultraviolet spectroscopic
observations of the blazar Mrk 421 taken with the Cosmic Origins Spectrograph
on the Hubble Space Telescope and the Far Ultraviolet Spectroscopic Explorer.
Although an HVC at V_LSR = -131 km/s is not visible in 21 cm emission
(logN_HI<18.38; 3sigma), it is detected in ultraviolet absorption lines of C
II, N I, O I, O VI, Si II, Si III, Si IV, Fe II, and HI. By referencing
velocities to the intermediate velocity cloud at -60 km/s and jointly analyzing
HI absorption from high-order HI Lyman lines, we measure
logN_HI=16.84(0.13,+0.34) (1sigma) in the HVC. Comparing HI, and O I, we find
an HVC metallicity [O/H]=0.32(-0.39, +0.22). Because the sight line passes 4
degrees from the HVCs in Complex M, the detected HVC may represent the highest
velocity component of the Complex, and our measurements provide a lower limit
to its metallicity. The high, possibly super-solar metallicity, together with
the low distance, z<3.5 kpc, above the Galactic plane suggest that Complex M is
condensed returning gas from a Galactic fountain.
