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B.P. Wakker, J.C. Howk, B.D. Savage, S.L. Tufte, R.J. Reynolds (University of Wisconsin-Madison), H. van Woerden, U.J. Schwarz (Rijks Universiteit Groningen), R.F. Peletier (University of Durham), P.M.W. Kalberla (Universit\"at Bonn)
We find evidence for a hitherto unknown component of the Milky Way: infalling gas with low heavy element abundances. Infall of metal-poor matter has been the preferred solution to the long-standing ``G-dwarf problem'', the fact that the abundance distribution of unevolved stars is narrower than expected in a simple model (e.g.\ Pagel 1997). \par Our conclusion is based on the metallicity we measure for the high-velocity cloud complex~C of 0.070±0.020 times solar, using the Seyfert galaxy Mark~290 as a background probe. We also determine a lower limit of 5\,kpc to its distance and argue that it is likely that D<25\,kpc. These results were obtained by combining S\,II absorption-line data taken with the Hubble Space Telescope (HST), H\,I data taken with the Effelsberg and Westerbork (WSRT) radio telescopes, H\alpha and S\,II emission-line data taken with the Wisconsin H\alpha Mapper (WHAM) and Ca\,II absorption-line data taken with the William Herschel Telescope (WHT) at La Palma. \par We derive a total gaseous mass for complex~C of 2.0x106 (D/5kpc)2 M\odot and find that the HVC represents a downward mass flux of ~3--7x10-3 (D/5kpc)-1 M\odot yr-1 kpc-2. This is close to the rate suggested theoretically to solve the G-dwarf problem (Giovagnoli & Tosi 1995). \par The HVC may be a present-day analogue of the damped Ly\alpha absorbers seen in QSO spectra. It remains to be seen whether ultimately it is a remnant of the formation of the Milky Way (Oort 1970), a gas cloud orbiting the Galaxy (Kerr & Sullivan 1969), a remnant of the formation of the Local Group (Verschuur 1969, Blitz et al.\ 1999), or the result of tidal interactions between the Magellanic System and the Galaxy >3 Gyr ago (Toomre, in Kerr & Sullivan 1969). \par This work was supported by NASA under grant #GO6590.01-95A
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