Observational evidence for the accretion of low-metallicity gas onto the Milky Way: metallicity, physical conditions and distance limit for HVC complex C

We present observations of the (field of the) Seyfert galaxy Mark 290, which probes the high-velocity cloud (HVC) complex C, one of the largest HVCs (Wakker & van Woerden 1991). We find that this object has a metallicity of 0.094+/-0.020^{+0.022}_{-0.019} times solar. We determine a lower limit to its distance of 5 kpc (z>3.5 kpc). If the gas is in thermal equilibrium with a hot halo, then it is also likely that D3 Gyr ago (an "Old Magellanic Stream"; Toomre, quoted in Kerr & Sullivan 1969), or was formed when hot, ionized intergalactic gas was compressed by the motion of the Milky Way (Silk et al. 1987)