Accretion of low-metallicity gas by the Milky Way

Models of the chemical evolution of the Milky Way suggest that: the observe d abundances of elements heavier than helium ('metals') require a continuou s infall of gas with metallicity (metal abundance) about 0.1 times the sola r value. An infall rate integrated over the entire disk of the Milky Way of similar to 1 solar mass per year can solve the 'G-dwarf problem'-the obser vational fact that the metallicities of most long-lived stars near the Sun lie in a relatively narrow range(1-3). This infall dilutes the enrichment a rising from the production of heavy elements in stars, and thereby prevents the metallicity of the interstellar medium from increasing steadily with t ime. However, in other spiral galaxies, the low-metallicity gas needed to p rovide this infall has been observed only in associated dwarf galaxies(4) a nd in the extreme outer disk of the Milky Way(5,6). In the distant Universe , low-metallicity hydrogen clouds (known as 'damped Ly alpha absorbers') ar e sometimes seen near galaxies(7,8). Here we report a metallicity of 0.09 t imes solar for a massive cloud that is falling into the disk of the Milky W ay. The mass now associated with this cloud represents an infall per unit a rea of about the theoretically expected rate, and similar to 0.1-0.2 times the amount required for the whole Galaxy.