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Astron. Astrophys. 321, 363-373 (1997)

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Nebular gas abundances and mixing processes in the ringed galaxy NGC 4736 *

P. Martin 1 and J. Belley 2

1 European Southern Observatory, Casilla 19001, Santiago 19, Chile (pmartin@eso.org)
2 CUSLM, Université de Moncton, 165 Blvd. Hébert, Edmundston, N.-B., E3V 2S8, Canada (jbelley@novl1.ci.cuslm.ca)

Received 19 June 1996 / Accepted 20 August 1996


Results of imaging spectrophotometry in the nebular lines H [FORMULA], H [FORMULA], [O iii] [FORMULA] 5007 and [N ii] [FORMULA] 6584 of 65 H ii regions in the early-type ringed galaxy NGC 4736 are presented. The oxygen abundance is derived using the line ratios [O iii]/H [FORMULA] and [N ii] /[O iii] as calibrated by Edmunds & Pagel (1984). Analysis of the O/H distribution in the bright resonance ring of star formation in NGC 4736 reveals that the O/H scatter is similar to that of the ISM of the disc of gas-rich galaxies, despite the very high star formation rate presently observed in the ring. The magnitude of azimuthal mixing induced by supernovae explosions and galaxy differential rotation is estimated in the ring; it is found that both can explain the small O/H dispersion observed in this structure.

The radial distribution of O/H in the disc of NGC 4736 is also studied. Although the number of H ii regions located outside the ring, and for which a reliable O/H value was obtained is limited, a slope for the global gradient is derived and found to be moderate ([FORMULA] -0.035 [FORMULA] 0.020 dex/kpc). This gradient is shallower than gradients observed in normal galaxies, and is comparable to the slopes found for galaxies with bars of medium strength. Two evolutionnary scenarios are proposed to explain this O/H distribution: i) Oval distortions, like bars, induce large-scale gas flows throughout the disc of NGC 4736, resulting in radial mixing of the chemical composition on a relatively short time scale ([FORMULA] 1 Gyr); ii) A strong bar was present in the recent past and has flattened the abundance gradient.

Key words: galaxies: abundances – galaxies: evolution – galaxies: individual (NGC 4736) – galaxies: ISM – galaxies: kinematics and dynamics – galaxies: spiral

* Tables 3 and 4 are only available in electronic form at CDS via anonymous ftp or via http://cdsweb.u-strasbg.fr/Abstract.html

Send offprint requests to: P. Martin

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998