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The ELG candidates were selected from the digitized objective prism
plates of the Hamburg Quasar Survey. The original plates were obtained
with the 80 cm Schmidt telescope at Calar Alto, Spain. The telescope
is equipped with an 1.7
objective prism allowing to obtain
spectra with a dispersion of
Å/mm at H
.
The size
of plates is 24
24 cm covering a field of
on the sky. The Kodak IIIa-J emulsion is used,
giving a spectral range from 3400 to 5400 Å. The plates were
digitized with a PDS 1010G microdensitometer in a low-resolution mode
using 100
20
m slit and 10
m step size and the extracted
spectra are stored on optical disks. Once spectra of
candidates are taken from this database, high-resolution PDS scans
(with a slit size of 20
20
m) are
made of them for further refinement of the classification. Additional
information about the HQS can be obtained from Hagen et al. ([1995]).
The low-resolution spectra, which have 10-20 spectral points per
object, already allow the detection of strong emission lines, in
particular for ELGs with prominent
[O III]
4959,5007 Å and H
lines.
The faintest ELG candidates have B magnitudes up to 20m.
In general their redshifts are limited to z
0.075 because the
[O III]
4959,5007 Å lines are shifted for more
distant galaxies outside the spectral range of the photoplates. Sometimes
prominent H
may allow detection up to z
0.1, and in
rare cases galaxies may be selected due to their strong
[O II]
3727 Å emission line up to a redshift of
z = 0.43.
The selection is biased for the [O III] lines
compared to the [O II] line because of linear dispersion, favoring the
detection of narrow lines near the red end of the objective prism spectra.
Only in bright galaxies (B < 17.5 the [O II] line was well detected
(for example, HS 0132+0610 and HS 0935+4726, see Vogel et al.
[1993]).
Quasars with strong emission lines near
5000 Å can also
be detected, in particular, if Ly
or Mg II
2798 Å enter to this wavelength region (corresponding to redshifts of
3.0 and 0.9, respectively).
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The adopted search method consists of various selection techniques.
It is applied first to the low-resolution and subsequently to the
high-resolution density spectra.
The low-resolution density spectra are characterized by several
parameters, and we chose after some experiments the integral density and
the slope at 4000 Å as the main selection parameters. The integral
density is the sum of the densities of all pixels contributing to a spectrum
and the slope is determined by a 2nd order polynomial fitting of
the spectrum.
The length of the selected spectra was limited to 5-20 pixels to avoid
spectra dominated by noise or affected by overlaps. To determine the range
of slopes shown by ELGs for a given brightness (represented by the integral
density), we used a training sample of
50 blue compact galaxies
with
known spectral properties taken from the SBS (Izotov et al. [1993a],
[1994]), which were separated on HQS plates in the zone of the
SBS.
This sample contains galaxies with a wide range of sizes, luminosities and
[O IIi]
5007 Å emission line equivalent widths between
10 and more than 1000 Å.
Figure 1 shows the distribution of the training sample in a photographic
magnitude - equivalent width plane. For magnitudes fainter than about 18.0,
our training sample is running out of objects with moderate or weak [OIII]
line strength. This yielded some uncertainties at the fainter end for our
first definition of the selection rules (see below).
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Figure 2 shows the location of the SBS BCGs in an integral
density - slope diagram (in the relative units of the digitisation of
the HQS plates). According to the distribution of BCGs in this diagram we
chose limits
for the slope and
for
the densities.
The slope range is rather wide due to both the intrinsic properties
of the galaxies and variations of the plates quality.
Nearly 30 % of the typically
30000-50000 spectra per plate pass this
filter. This number of spectra is still too large to proceed with
high-resolution scans. Therefore, all preselected low-resolution spectra
were visually classified on a vector graphics screen and candidates with
density peaks close to the green head of the spectrum were selected as
possible [O III]
5007 Å hits.
This procedure is quite efficient to keep good ELG candidates and to reduce
the total number of selected candidates to
.
Tests with several
plates of the same field have shown that 5 to 15 % of good candidates are
lost, if the selection was not made on the best plate.
All selected low-resolution spectra were then rescanned with high resolution
(PDS scanning time
3-4 hours/plate) in order to select visually
first and second priority candidates according to the following criteria:
a) Objects showing a clear density peak near 5000 Å and blue continuum in the high resolution spectra are adopted as first priority candidates.
b) Some bright (B
)
BCGs with weaker emission lines
(EW([O III]
5007 Å)
50 Å) in the training sample
have density spectra with blue continuum but no clearly detectable emission
peak. Therefore, candidates with a blue continuum but without prominent
emission features or candidates with indications of emission peaks but with
an unusual continuum shape are kept as second priority candidates in
the attempt to avoid losses of true ELGs.
Examples of the high resolution spectra of the first and the second priority candidates are shown in Fig. 3 (small boxes include the low resolution spectra). As a result we have got up to 30 first priority candidates per plate and a similar number of second priority objects for follow-up spectroscopy. The resulting surface density of emission-line candidates for each priority varies from 0.3 to 1 candidate per square degree.
It is reasonable to outline here the ELG types which we wish to discriminate
with the selection procedure used.
First, due to the adopted slope limits of the spectra in the parameter
space, we miss ELGs with redder continuum distribution. This is usually
the case for galaxies dominated by the emission of an old stellar population.
Such objects are found efficiently using wide-field objective prism surveys
sensitive to the H
spectral range (Zamorano et al. [1994],
[1996]) or by surveys based on narrow-band H
imaging
(Boroson et al. [1993]).
Second, due to the combined effect of the upper limit on the redshift of
[O III]
4959,5007 Å and the lower limit on
the object's apparent magnitude (corresponding to the upper limit in integral
density), we miss all objects with absolute magnitudes brighter than
MB
-21. Therefore, many Seyfert galaxies are not selected into
our candidate lists.
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We apply the above selection procedure to the first set of 38 fields with
boundaries
and
,
comprising an area of about 900 square degrees. This survey strip partly
overlaps with the fields of Popescu et al ([1996],
[1997]). The resulting list consists of about 1000 first priority
candidates, and a similar number of second priority ones. To check
the contents of these categories we observed both first and second priority
candidates in two pilot follow-up spectroscopy runs. The candidate lists
contain also many ELGs with known redshifts but with somewhat uncertain
classification. Due to the lack of time we could observe only 11 of them.
After the analysis of the results of these observations (see Section 5), we improved the efficiency of the overall selection procedure by applying additional criteria, which resulted in significant increase of ELG detection efficiency during follow-up spectroscopy.