The distance to Complex M and the Intermediate Velocity Arch

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Abstract
The Intermediate Velocity Arch is an association of HI clouds lying within the broad limits l≈110°–210°b≈50°–80°. We use high-resolution optical, and single-dish 21-cm, observations of 10 early-type halo star sightlines in this region to establish distance limits (0.4 ≤z≤3.5 kpc) for seven IVCs that are part of the Arch. We re-examine a previous determination of upper and lower distance limits on the IVC Complex M using the closely aligned halo stars BD +38 2182 and HD 93521. Our Hi spectra show no evidence of emission from Complex M, leading us to conclude that, while an upper distance Hmit of z≤ 3.5 kpc may be established, no useful lower distance determination can be made. Our new H i observations allow new limits to be established on the metallicity of Complex M, and show C, O and Si variations with respect to solar abundances in the range ±0.3 dex.

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