An Accurate Distance to High‐Velocity Cloud Complex C

Abstract

We report an accurate distance of d=10+/-2.5 kpc to the high-velocity cloud Complex C. Using high signal-to-noise ratio Keck HIRES spectra of two horizontal-branch stars, we have detected Ca II K absorption lines from the cloud. Significant nondetections toward a further three stars yield robust lower distance limits. The resulting H I mass of Complex C is MHI=4.9+2.9-2.2×106 Msolar a total mass of Mtot=8.2+4.6-2.6×106 Msolar is implied, after corrections for helium and ionization. At 10 kpc, Complex C has physical dimensions 3×15 kpc, and if it is as thick as it is wide, then the average density is log ~=-2.5. We estimate the contribution of Complex C to the mass influx may be as high as ~0.14 Msolar yr-1.