Anisotropic high-excitation emission and chemical abundances in the Seyfert 2 galaxy NGC 5643

Abstract

We present narrow-band images in the lines [O III] λ5007 and Hα(+ [N II] λλ6548, 6584 and long-slit spec­ troscopy along the bar of the Seyfert 2 galaxy NGC 5643. The continuum subtracted [O III] and Hα( images show that the emission of the high excitation gas is elongated along the bar direction and extends to ≈1 kpc to each side of the nucleus. Farther out, there is low-excitation emission due to H II regions along the bar. Our data confirm previous suggestions of a biconical morphology for the high-excitation gas and indicate that the cone axis is slightly tilted relative to the bar. We study the reddening in the central regions and find that the most obscured region is located at ≈3" W from the peak of the continuum, which is also the region with the highest gas density. We speculate that the active nucleus is hidden and located there. The available data is consistent with the nuclear obscuration being due to the presence of a dusty torus, which would also be responsible for collimating the ionizing radiation and producing the biconical morphology. The stellar population derived from the spectra is moderately old in the central region and young in the H II regions along the bar. Within 10" of the nucleus we find that the absorption lines are diluted by a blue continuum. We discuss the origin of this continuum. The fluxes of the emission lines are obtained and plots showing the distribution of the line fluxes and ratios along the bar are constructed. The ionic and total abundances of the H II regions are calculated. The oxygen and nitrogen abundances increase toward the center reaching, respectively, solar and 2 times solar values. Photoionization model calculations for the high-excitation extended narrow-line region gas confirm these values and indicate that the sulphur has the same abundance relative to solar as nitrogen.