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Paper: Observational Evidence for the Accretion of Low-metallicity Gas onto the Milky Way: Metallicity, Physical Conditions and Distance Limit for HVC Complex C
Volume: 166, Stromlo Workshop on High-Velocity Clouds
Page: 26
Authors: Wakker, Bart P.; Howk, J. Chris; Savage, Bart D.; Tufte, Steve L.; Reynolds, Ron J.; van Woerden, Hugo; Schwarz, Ulrich J.; Peletier, Reynier F.
Abstract: 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 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; van Woerden et al. 1998). We find that, on this sightline, H^+ represents 23+/-10% of the hydrogen in the HVC. The total gaseous mass is 2.0tdex6\ (D/5 kpc)^2 M_odot 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) M_odot yr^{-1}, or 2.9-6.7tdex{-3} (D/5 kpc)^{-1} M_odot yr^{-1} 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).
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