Observational Evidence for the Infall of Low-Metallicity Gas onto the Milky Way

Details Observational Evidence for the Infall of Low-Metallicity Gas onto the Milky Way Observational Evidence for the Infall of Low-Metallicity Gas onto the Milky Way

May 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999aas...194.4604w&link_type=abstract

American Astronomical Society, 194th AAS Meeting, #46.04; Bulletin of the American Astronomical Society, Vol. 31, p.887

Astronomy and Astrophysics

Astronomy

2

Scientific paper

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). 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, Hα and S II emission-line data taken with the Wisconsin Hα Mapper (WHAM) and Ca II absorption-line data taken with the William Herschel Telescope (WHT) at La Palma. We derive a total gaseous mass for complex C of 2.0x10(6) (D/5kpc)(2) M_sun and find that the HVC represents a downward mass flux of ~ 3--7x10(-3) (D/5kpc)(-1) M_sun yr(-1) kpc(-2) . This is close to the rate suggested theoretically to solve the G-dwarf problem (Giovagnoli & Tosi 1995). The HVC may be a present-day analogue of the damped Lyalpha 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). This work was supported by NASA under grant #GO6590.01-95A

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