THE INFLUENCE OF BARS ON THE CHEMICAL-COMPOSITION OF SPIRAL GALAXIES

Imaging spectrophotometry in the nebular lines Halpha, Hbeta, [O III] lambda5007 and [N II] lambda6584, using narrow-band interference filte rs, has been performed of 82, 55, and 79 H II regions in the barred sp iral galaxies NGC 925, NGC 1073, and NGC 4303, respectively. The O/H a bundance gradients were derived from the abundance indicators [O III]/ Hbeta and [N II]/[O III], calibrated by Edmunds & Pagel (1984). The gl obal O/H gradients in NGC 925 [DELTA log (O/H)/DELTAR = -0.033 dex kpc -1] and NGC 1073 [DELTA log (O/H)/DELTAR = -0.048 dex kpc-1] are flatt er than the gradients observed in normal galaxies of the same morpholo gical type. The abundance gradient in NGC 4303 is identical to that fo und in normal spiral galaxies. A comparison is made between the O/H gr adients of normal galaxies and of a sample of galaxies showing a barre d morphology. It is concluded that the global abundance gradients of s piral galaxies with a barred structure are in general shallower than g radients of normal galaxies. The slopes of O/H gradients are analyzed as a function of two properties of bars: the relative length of the ba r with respect to the size of.the disk, and the bar ellipticity, defin ed by the axis ratio of the bar. It is found that gradients are flatte r when the length or the ellipticity of the bar increases. This result is consistent with recent models of radial flows. These observations indicate that large-scale mixing of the interstellar gas occurs across the disks of barred spiral galaxies and affects the radial distributi on of elements.