Clues on the obscured active nucleus of NGC 1365

Abstract

We have analyzed optical spectra (ESO-CASPEC) from the composite starburst-Seyfert galaxy NGC 1365 taken on the nucleus and the following positions (relative to nucleus): 2 arcsec N, 2 arcsec W; 2 arcsec S; 4 arcsec S; 5 arcsec S; 10 arcsec S and 20 arcsec W. A nuclear broad-line component indicative of the AGN source is confirmed in Hβ and Hα . Narrow-line widths vary between 150 and 200 km s(-1) . Extranuclear line ratios in the observed regions are mostly consistent with the lines being formed in HII regions. One of the exceptions is a rise of [OIII]lambda5007 /Hβ from 0.5 to 5 within 5 arcsec (from west to east) across the nucleus suggesting the transition from gas ionized by stars to gas ionized by the active nucleus. Faint emission-line gas observed 20 arcsec W of the nucleus (called region II) shows line ratios lying in the AGN part of diagnostic diagrams. While at first glance shocks by bar streaming motions appear to be a suggestive explanation for these line ratios we stress that there is no positive evidence for the velocities > 300 km s(-1) required for this mechanism. Another, presently more likely, explanation is that region II belongs to the far cone of the bipolar nuclear outflow which can be glimpsed through the dusty disk. Photoionization of a single-density cloud system by a diluted AGN continuum reproduces the measured line ratios, but leads via the ionization parameter to an intrinsic Hα luminosity of the obscured AGN of ~ 10(42) erg s(-1) of which less than ~ 4% are observed in the central few arcseconds. Dust obscuration could explain Hα but its concomitant gas column cannot account for the lack of Seyfert-1 typical hard X-rays from the nucleus. Based on observations collected at the European Southern Observatory, La Silla, Chile