The stellar-free emission component in galactic nuclei : at low-levels, evidence for shock ionization

Abstract

We have isolated the emission-line component in a sample of 92 galaxies using stellar absorption templates built from emission-free star cluster spectra. Ali sources of reddening extrinsic to the line emitting regions have been properly taken into account. A statistical analysis of the emission line proper­ ties has been carried out. We conclude, in particular, that the [N II] λλ 6548, 84 lines are slightly more sensitive to metallicity effects than the [S II] λλ 6717, 31 lines. However, both [N II] and [S II] lines are as well dependent on the excitation mechanism. The intrinsic reddening inferred from the Balmer emission line ratios is rather homogeneous as the Hα/Hβ line intensity ratio is peaked around the value 4.2. This indicates a remarkably uniform dust content in those regions building up the emission spectrum for a wide variety of objects: LINERS, nuclear H II regions, normal H II regions, Seyfert 2 nuclei and the narrow line region in Seyfert 1 nuclei. From a subsample of 76 galaxies we have built three charac­ teristic average spectra corresponding to (i) nuclear HII regions, (ii) objects identified as LINERS and (iii) extreme low-level emission galaxies with W em (Hα) ≤ 2 Å. For the latter class of objects we have been able to push down line measurements to an unprecedent levei. We conclude from the [S II] λλ 6717, 6731/ [SIII] λλ 9069, 9532 line ratio that shock ionization is the mechanism at work in extreme low-level emission galaxies.