Astronomy and Astrophysics – Astrophysics
Scientific paper
1998-11-20
Astronomy and Astrophysics
Astrophysics
28 pages (LaTeX 2e) with 8 tables and 6 figures (Figs. 1 and 2 were bitmapped; they are given with full resolution in the OHP-
Scientific paper
10.1051/aas:1999183
In a previous paper (Veron et al. 1997) we presented medium resolution (3.4 A FWHM) spectroscopic observations of 15 "transition objects", selected for having an ambiguous location in the Veilleux & Osterbrock (1987) diagnostic diagrams, and showed that most of them were in fact "composite", this being due to the simultaneous presence on the slit of both a Seyfert or Liner nucleus and a HII region. Here, we report new spectroscopic observations of 53 emission-line galaxies with a "transition" spectrum, bringing up to 61 the total number of observed objects in an unbiased sample of 88 "transition objects". Almost all of the observed galaxies have a "composite" nature, confirming the finding that true "transition" spectra may not exist at all. By eliminating "composite objects" from the diagnostic diagrams, a clear separation between the different classes of nuclear emission-line regions (Seyfert 2s, Liners and HII regions) becomes apparent; by restricting the volume occupied by the different line-emitting regions in the 3-dimensional diagnostic diagrams, we are also restricting the range of possible physical parameters in these regions. There seems to be no continuity between Seyfert 2s and Liners, the two classes occupying distinct volumes in the 3-dimensional space defined by [OIII]5007/Hbeta, [NII]6583/Halpha, and [OI]6300/Halpha.
Goncalves Alexandre C.
Véron Pascal
Veron-Cetty M.-P.
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