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Astron. Astrophys. 347, 55-62 (1999) 5. Composite spectraAs it was found in the previous section, only 4 galaxies could be classified as LINERs and 1 as Seyfert 2 in at least one of the diagnostic-diagrams, but several galaxies are in the transition zone. These transition objects could have a composite spectrum due to the simultaneous presence on the slit of a Seyfert nucleus and an H II region (V97f). In order to test this hypothesis we have built composite spectra, combining emission-pure spectra of a typical Seyfert 1 (J 15.22), Seyfert 2 (NGC 1068) and LINER (AM 2054-433S) with the H II region spectra of solar abundance R1a, R5a and R10a, of the Gumm 38a complex. These galactic H II regions have been selected because they are as metallic as the circumnuclear H II regions of nearby galaxies (Storchi-Bergmann et al. 1996a), are purely photoionised by stars, and also show a wide range of excitation (G97). For J 15.22 and NGC 1068, suitable stellar population templates were also subtracted following the method described in Sect. 3. The composite spectra were built adding different fractions of the
H II region spectrum to that of the AGN . To do this, the
stellar-population subtracted AGN and H II region spectra have been
normalised at the peak of the
The resulting emission line ratios of the composite spectra are
plotted in the diagnostic-diagrams (long dashed lines) in Figs. 5
and 6, where the effect of the H II region contamination can be seen:
for the composite model shown in Fig. 7, the 70% H II region
contribution shifts the line ratios of the Seyfert 1 to the typical
H II region zone. In terms of In order to check if the selection of H II regions with different
properties would affect the results, we calculated using the CLOUDY
code (Ferland 1993), a series of H II region models photoionised by
black-body spectra with effective temperatures T=35 000K, 40 000K and
50 000K. Since we are dealing with circumnuclear H II regions, and the
low spectral resolution prevents a direct calculation of the electron
density ( From Figs. 5 and 6, all the points to the right of the
Gumm 38a H II regions (heavy-dashed line) can be reproduced by a
combination of an AGN spectrum with that of an H II region. In
this sense, only the points to the left of that line would be pure
H II regions. Notice that for our sample galaxies, the
Taking into account the position of the observed line ratios with respect to the Gumm 38a H II regions zone in the 3 diagnostic-diagrams (Figs. 5 and 6), we give in column 6 of Table 2 a revised classification for each galaxy. The objects having a composite spectrum (CS) are those with line ratios lying to the right of the H II region line in all 3 diagnostic-diagrams. Objects with a possible composite spectrum (CS?) are those with line ratios satisfying the above criterion in at least one diagnostic-diagram. From this approach, we conclude that 15 objects (28% of the sample) may be AGN contaminated by some fraction of H II region emission due to the simultaneous presence on the slit of a Seyfert nucleus and an H II region. Including the possible composite spectra (CS?) this number is increased to 20, which corresponds to 37% of the present galaxy sample, which is consistent with previous results (Ho, Filippenko & Sargent 1997; Liu & Kennicutt 1995). ![]() ![]() ![]() ![]() © European Southern Observatory (ESO) 1999 Online publication: June 18, 1999 ![]() |