A High-Metallicity High Velocity Cloud along the MRK 421 Sight Line: A Tracer of Complex M?

TL;DR

This study measures the metallicity of a high velocity cloud near Mrk 421, revealing it has high, possibly super-solar metallicity, indicating it may be gas returning to the Galaxy from a fountain process.

Contribution

First measurement of the metallicity of HVC Complex M along the Mrk 421 sight line using ultraviolet spectroscopy, suggesting its origin as Galactic fountain returning gas.

Findings

HVC detected in ultraviolet absorption lines but not in 21 cm emission.

Measured metallicity of the HVC is approximately solar or higher.

HVC is likely part of a Galactic fountain process.

Abstract

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.