ABUNDANCES IN THE STARBURST GALAXY II ZW 40

Low-dispersion optical imaging spectroscopy has been performed of the starburst galaxy II Zw 40, employing the area spectroscopy system at t he Anglo-Australian Telescope. A procedure was applied to correct for the effects of atmospheric differential refraction arising from the la rge zenith distance of the observations. Maps of interstellar reddenin g, electron temperature and abundances of elements (O/H, N/H, He/H and Ne/H) were constructed. Comparisons are made with similar maps of the amorphous galaxy NGC 5253. II Zw 40 suffers from large intrinsic abso rption due to dust, with the greater values of A(V) being found in the northern half of the galaxy. Electron temperatures were measured at 2 4 points over the bright core of the galaxy from the [O III] 5007/4363 -angstrom line ratio. The mean value of T(e) derived from the integrat ed spectrum of the emission-line region is 12 000 K, and [O/H] = 8.25. Three points covering an area of approximately 160 x 100 pc2 have sig nificantly lower T(e), and abundances of O/H and N/H appear higher. It is shown that the original calibration proposed by Pagel, Edmunds & S mith for the line ratio ([O II] + [O III])/Hbeta to derive O/H agrees with our direct measurements of O/H for the metallicity range observed in II Zw 40 and NGC 5253. N/O is found to be weakly inversely correla ted with O/H in II Zw 40, as for NGC 5253. Various mechanisms have bee n explored in order to explain this anticorrelation. Radiative recombi nation in the O2+ region is not found to be a significant contributor to the [O II] surface brightness; thus it cannot account for the antic orrelation. Excitation or charge exchange reaction effects can also be excluded. The observed behaviour of N/O in II Zw 40 and NGC 5253 is p ossibly due to a mix of secondary nitrogen with primary nitrogen of un known origin.