A High-metallicity, High-velocity Cloud Along the Mrk 421 Sight Line: A Tracer of Complex M?

Abstract

We present a new measurement, 0.85-3.5 Z _sun, 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 (log N _{H I} < 18.38; 3σ), 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 H I. By referencing velocities to the intermediate-velocity cloud at -60 km s^-1 and jointly analyzing H I absorption from high-order H I Lyman lines, we measure log N _{H I} = 16.84^+0.34 _-0.13 (1σ) in the HVC. Comparing H I and O I, we find an HVC metallicity [O/H] =0.32^+0.22 _-0.39. Because the sight line passes ~4° 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.