The radial gradient of heavy-element abundances in disk galaxies

Various origins for the formation of the heavy-element abundance gradients observed in nearly all disk galaxies are analyzed in the framework of evolu tionary models. In an isolated galaxy, there is a radial gradient of the ab undance of heavy elements only early in its evolution (the first several bi llion years), which Subsequently practically disappears. The gradients of c hemical compositions Of Young Objects and the interstellar gas require that typical disk galaxies be open systems (i.e., that they eject some heavy el ements into circumgalactic space and/or accrete intergalactic gas) and that the rates of both processes be dependent on galactocentric distance. (C) 2 001 MAIK "Nauka/Interperiodica".