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 Mark290, which probes the high-velocity cloud (HVC) complex C, one of the largest HVCs. We find that this object has a metallicity of 0.094pm0.020(+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 D<30 kpc, putting the HVC in the Galactic Halo, as was the case for HVC complex~A (for which z=2.5--7 kpc). We find that, on this sightline, H+ represents 23pm10% of the hydrogen in the HVC. The total gaseous mass is 2x10^6 (D/5 kpc)^2 Msun and, depending on whether the space velocity is completely radial or vertical, the HVC represents a downward mass flux of ~0.036-0.083 (D/5 kpc) Msun/yr, or 2.9--6.7x10^{-3} (D/5 kpc)^{-1} Msun/yr/kpc^2. The low metallicity and large mass suggest that complex C is unlikely to be part of a Galactic Fountain, but rather represents accreting low-metallicity material. It may be a present-day analogue of the damped Ly-alpha absorbers seen in QSO spectra. Our abundance result provides the first direct observational evidence for the infall of low-metallicity gas on the Milky Way, required in models of galactic chemical evolution. It remains to be seen whether ultimately complex C is a remnant of the formation of the Milky Way (Oort 1970), a gas cloud orbiting the Galaxy (Kerr & Sullivan 1969), a Local Group object (Verschuur 1969, Blitz et al. 1996), the result of tidal interactions between the Magellanic System and the Galaxy >3 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). 
B.P. Wakker (1), J.C. Howk (1), B.D. Savage (1), S.L. Tufte (1) R.J.   Reynolds (1), H. van Woerden (2), U.J. Schwarz (2), R...