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The emission-line galaxies are classified using the line ratio diagnostic diagrams proposed by Baldwin et al. ([1981], hereafter BPT81) and Veilleux & Osterbrock ([1987], hereafter VO87). These diagrams allow to distinguish between narrow-line AGN ionization by a non-thermal power law continuum and HII region-like galaxies with ionization of gas by a large number of hot OB-stars.
According to VO87 a confident separation of the two ionization mechanisms
and a classification of the galaxy can be obtained from its location on
the [O III]
Å/H
versus
[N II]
Å/H
diagram.
This diagram has the advantage of being reddening-insensitive and
is used to classify the ELGs from the Calar Alto observations which cover
a wide spectral region from 3700 Å to 8100 Å.
The observations of the 12 ELGs with the 6m telescope do not cover
the red region. For these objects we use the
[O III]
Å/H
versus
[O II]
Å/[O III]
Å diagram, which is also a good constraint on the ionization mechanisms,
but it is reddening-sensitive, so that extinction corrections would be
desirable. However this correction is unreliable for the 6m telescope
observations due to poor S/N ratios near the H
emission line.
Nevertheless for high-excitation BCGs the average reddening is not large
(Izotov et al. [1993a], [1994], [1997b]),
and therefore, even without extinction correction they occupy nearly the
correct positions in this diagnostic diagram. For lower-excitation
galaxies, however, the reddening is getting larger in average, and thus
the possible uncertainty of their classification can increase, especially if
an object falls into the transition area.
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In Fig. 4 emission-line galaxies from the HSS sample are plotted in comparison with some BCGs from the SBS (Izotov et al. [1994], [1997b]; Thuan et al. [1995]) and ELGs from the Case survey (Weistrop & Downes [1988], [1991]; Augarde et al. [1987]; Salzer et al. [1995]; Ugryumov et al. [1998]). The branch of HII-galaxies is separated from AGN by a short-dashed line running in both figures from the left-top corners populated by objects showing strong emission lines to the right-bottom corners populated by objects with a low level of gas ionization.
It is clear from the data in Fig. 4 that the major part of the observed HSS emission-line galaxies belongs to HII-galaxies and only a few objects can be classified as probable AGN.
>From the two objects in the AGN region of Fig. 4a only one -- HS 1609+4902 -- can be definitely classified in both diagrams as a Sy2 galaxy. The second probable AGN-type object is HS 1304+4710. In Fig. 4a it is located not too far from the border line separating HII-type objects from AGNs. We tentatively classify it as a possible LINER (low-ionization nuclear emission-line region galaxies; Heckman [1980]). In Fig. 4b this object falls however within the HII-type region. A possible reason for this is the reddenning due to the dust extinction.
Among the second priority candidates we find a number of
low-excitation massive galaxies with nondetected (in low S/N spectra)
H
and [OIII]
Å emission lines. They are
tentatively classified as possible LINERs (marked as LINER? in Table 1)
according to the flux ratios of [NII]
Å and H
(Heckman [1980]).
Although these galaxies are not shown in Fig. 4, they, likely,
populate the extreme right-bottom corner in Fig. 4a.
We use some additional criteria for the classification of ELGs in the cases where the S/N ratio is insufficient to use the diagnostic diagrams. As it was emphasized by Salzer ([1989a], hereafter S89a) there is a tight correlation between the ELG type and their mean global parameters such as luminosity and size. In particular, the ELGs with the highest excitation are low-luminosity, compact objects, while AGN-type objects are almost always high-luminosity, large galaxies.
Two more objects in Fig. 4b, HS 1038+4616 and HS 1443+5018,
fall into AGN region. We suspect that it is due to poor S/N ratio for
H.
Therefore, their large [O III]
Å/H
ratios are unreliable, and, probably these galaxies are low excitation BCGs.
Indeed, their low luminosities (
and
,
respectively)
are more compatible with a HII-type nature.
HII-galaxies can be splitted further into several classes depending on
physical parameters such as sizes, absolute magnitudes, colors, metal
abundance, equivalent widths (EW) of the [O III]5007 Å line and morphology. S89a have proposed the following sequence
of HII-galaxy types (starting with the least luminous and most compact
objects and ending with the most luminous, quite large spirals):
``Sargent-Searle (SS) objects'' (or ``Blue Compact Dwarfs (BCD)''),
``dwarf HII hotspot (DHIIH) galaxies'', ``HII hotspot (HIIH) galaxies'',
``Dwarf Amorphous Nucleus Starburst (DANS) galaxies'' and
``Starburst Nucleus (SBN) galaxies''. SBN-galaxies have seemingly also
some input from shock excitation.
There is also a significant correlation of the luminosity with other
parameters, such as metallicity,
EW of the [O III]
5007 Å line and color.
The insufficient quality of our spectroscopy does not allow to perform such detailed classification. Therefore, in our case SS, DHIIH and HIIH galaxies are taken together as one class of blue compact/ HII galaxies (BCGs), which are objects with strong and moderate emission lines, caused by the large number of young OB-stars in current burst of star formation.
Following S89a, we separate the BCGs and low-excitation ELGs regions
in the diagnostic diagrams with dotted lines (Fig. 4).
This border is derived from the analysis and generalization of good-quality
ELG observational data from the UM survey. To classify objects near the border we use additional
parameters, such as EW([O III]
5007 Å) and MB,
according to their characteristic values for corresponding ELGs from S89a.