Stellar Activity in the Seyfert Nucleus of NGC 1808

Summary

Abstract

Although claimed to possess a Seyfert nucleus, NGC 1808 reveals radio properties, line widths and ratios that are consistent with a few, albeit powerful, SNRs. There is as yet no compeling evidence for Seyfert activity, and this galaxy should be reclassified as a starburst galaxy (it shows many similarities to NGC 253 and M82).

Introduction

NGC 1808 is a ‘hotspot’ or Sersic–Pastoriza galaxy at a distance of 16.4 Mpc (1″ = 80 pc). Although this galaxy shows optical hotspots, these hotspots have largely disappeared at near–infrared wavelengths. It is a highly inclined and dusty Sbc spiral, with evidence for a burst of star formation in the circumnuclear region about 5 × 10⁷ yrs ago. Two pieces of evidence suggested that NGC 1808 habours a Seyfert nucleus:

• High resolution spectra revealed broad lines (Veron–Cetty & Veron 1985).

• The 6cm radio luminosity of the compact nucleus is 500 times that of the most luminous Galactic supernova remnant (SNR), suggesting a non–stellar nucleus (Saikia et al. 1990).

Line Ratios

We have obtained a red spectrum of NGC 1808 (see Forbes, Boisson & Ward 1992 for details) containing the [SIII] lines at 9069 and 9532Å. Fig. 1 shows a diagnostic diagram based on the sulphur line ratios. We also show the mixing curve between the location of HII regions and Galactic SNRs. This line ratio diagram (and those for [OI]6300Å, [NII]6583Å, [SII]6717+6731Å) are consistent with a high abundance HII region and a ∼ 20% contribution from SNRs, i.e. no Seyfert nucleus is required to explain the line ratios.