NASA/IPAC EXTRAGALACTIC DATABASE
Date and Time of the Query: 2019-08-18 T02:18:00 PDT
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For refcode 1989A&A...226...23B:
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Copyright by European Southern Observatory (ESO). Reproduced by permission
1989A&A...226...23B The stellar-free emission component in galactic nuclei: at low-levels, evidence for shock ionization C. Bonatto, E. Bica, and D. Alloin Instituto de Fisica, UFRGS, C.P. 15051 Av. Bento Goncalves 9500, Porto Alegre, RS, 91500, Brazil Observatoire de Paris, Departement d'Astrophysique Extragalactique et de Cosmologie, F-92195 Meudon Cedex, France Received August 9, 1988; accepted March 20, 1989 Summary. We have isolated the emission-line component in a sample of 92 galaxies using stellar absorption templates built from emission-free star cluster spectra. All sources of reddening extrinsic to the line emitting regions have been properly taken into account. A statistical analysis of the emission line properties has been carried out. We conclude, in particular, that the [N II] {lambda}{lambda}6548, 84 lines are slightly more sensitive to metallicity effects than the [S II] {lambda}{lambda}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 H{alpha}/H{beta} 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 characteristic average spectra corresponding to (i) nuclear H II regions, (ii) objects identified as LINERS and (iii) extreme low-level emission galaxies with W_em_(H{alpha}} <= 2 A. For the latter class of objects we have been able to push down line measurements to an unprecedent level. We conclude from the [S II] {lambda}{lambda}6717, 6731/[S III] {lambda}{lambda}9069, 9532 line ratio that shock ionization is the mechanism at work in extreme low-level emission galaxies. Key words: galaxies: active - nuclei - stellar content - interstellar medium: abundances - lines: formation
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