A HIGH-METALLICITY, HIGH-VELOCITY CLOUD ALONG THE Mrk 421 SIGHT LINE: A TRACER OF COMPLEX M?

We present a new measurement, 0.85-3.5Z(circle dot), 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(-1) is not visible in 21 cm emission (logN(HI) < 18.38; 3 sigma), it is detected in ultraviolet absorption lines of CII, NI, OI, OVI, SiII, SiIII, SiIV, FeII, and HI. By referencing velocities to the intermediate-velocity cloud at -60 km s(-1) and jointly analyzing Hi absorption from high-order HI Lyman lines, we measure logN(HI) = 16.84(-0.13)(+0.34) (1 sigma) in the HVC. Comparing HI and OI, we find an HVC metallicity [O/H] = 0.32(-0.39)(+0.22). Because the sight line passes similar to 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 suggests that Complex M is condensed returning gas from a Galactic fountain.