A&A 390, 439-448 (2002)
DOI: 10.1051/0004-6361:20020797
Thüringer Landessternwarte Tautenburg, 07778 Tautenburg,
Germany
Received 26 November 2001 / Accepted 13 May 2002
Abstract
We study a sample of 23 narrow-emission line galaxies (NELGs)
which were selected by their strong variability
as QSO candidates in the framework of a
variability-and-proper motion QSO survey on digitised Schmidt plates.
In previous work, we have shown that variability is an
efficient method to find AGNs. The variability properties of
the NELGs are however significantly different from those
of the QSOs. The main aim of this paper is to clarify
the nature of this variability and to estimate the fraction of
AGN-dominated NELGs in this sample.
New photometric and spectroscopic observations
are presented, along with revised data from the
photographic photometry. The originally measured high variability
indices could not be confirmed. The diagnostic line-ratios of
the NELG spectra are consistent with H II region-like spectra.
No AGN could be proved, yet we cannot rule out the existence of
faint low-luminosity AGNs masked by H II regions from
intense star formation.
Key words: galaxies: active - galaxies: starburst - quasars: emission lines
The variability of flux densities is a common property of high-luminosity AGNs. We have performed a QSO search based on variability and proper motion (VPM survey) measured on a large number of digitised Schmidt plates in two fields (Meusinger et al. 2002). The work in the M 92 field is the subject of the present series of papers. In the first paper (Brunzendorf & Meusinger 2001; hereafter Paper 1), we discussed the motivation, the observational data, the data reduction procedure, and the selection of QSO candidates. The results from the follow-up spectroscopy and the properties of the resulting QSO sample were the subject of Paper 2 (Meusinger & Brunzendorf 2001). The primary goal of the present study is to improve the understanding of the selection effects of this survey.
An object is considered a VPM-QSO candidate if it appears star-like,
has no significant proper motion, and shows significant overall variability
and long-term variability. The variability is
expressed by the indices
(overall variability) and
(long-term variability). For instance, an object with
has
a probability of
to be variable.
It is well known that high-luminosity AGNs vary on long timescales
(years and longer). High priority QSO candidates have therefore to meet
both
and
.
On the other hand, we found several QSOs with strong overall variability
but without significant long-term variability (Paper 2). The long-term
variability constraint may introduce a bias in the VPM QSO search, and
it is therefore important to study also the subsample of
variable, star-like objects with zero proper motion showing
no significant long-term variability. In particular, we found
27 narrow emission line galaxies (NELGs) with redshifts
in this subsample. Most of these galaxies show
strong emission lines. NELGs may be dominated by
narrow-emission line AGNs (Seyfert 2, narrow-emission line Seyfert 1,
LINERs), intense starbursts, or a mixture of both. For example,
Ho et al. (1997) found that about half of
the NELGs from their magnitude-limited sample show some form of AGN or
composite spectra. In Paper 2, we have speculated that at least some
of the VPM NELGs are dominated by AGNs, although the available data did
not allow a clear-cut conclusion.
The present paper is concerned with the sample of the NELGs from the VPM survey. The main question is whether the measured strong overall variability as well as the strong emission lines are related to AGNs or not. It is not our intention to provide a large and well-defined sample of NELGs useful for further detailed studies. Much larger samples (e.g., Terlevich et al. 1991; Ho et al. 1997; Popescu & Hopp 2000) are available and are better suited to the investigation of the overall NELG population. In Sect. 2, we present new spectroscopic and photometric observations. Section 3 is concerned with the variability properties of the NELGs. The spectroscopic properties are discussed in Sect. 4, and Sect. 5 reviews further properties of the galaxy sample. Sect. 6 concludes. As in the previous papers of this series, we adopt H0 = 50 km s-1 Mpc-1 and q0=0.
![]() |
Figure 1:
Sample averaged structure function SF as a function of the time-lag ![]() |
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Low resolution spectra of VPM QSO candidates were described in
Paper 2. Unfortunately, the spectra for the NELGs did not
allow a clear-cut separation between the principal ionisation sources
(AGNs versus massive stars). New
observations were performed with CAFOS at the 2.2 m telescope
on Calar Alto, Spain, during six nights in July 2000.
CAFOS was equipped with a SITe1d CCD. The grism was chosen
dependent on the redshift: low-z NELGs were observed
with G-100 in order to achieve a good separation between
H
and [N II]
6583 Å. For the
higher-z NELGs, G-200 was used because of its higher
transmission at longer wavelengths. For some objects, spectra were taken
with both grisms. Total integration times between 30 and 60 min were
necessary to obtain spectra of reasonable signal-to-noise.
The weather conditions were mainly good.
The seeing was stable (
to
)
and the slit width was
kept constant, resulting in a linear resolution of 10 Å (G-200)
and 5 Å (G-100), respectively. The orientation of the slit was always
North-South. Wavelength calibration spectra were taken
by means of Hg-He-Ar calibration lamps.
We omitted two of the 27 NELGs that are located close to brighter galaxies since their measured variability is very likely not real. Further, the two NELGs of lowest priority could not be observed due to poor weather on the last night of the observing run. For the remaining 23 NELGs, spectra of good quality were obtained. In addition, six comparison galaxies with well-known spectroscopic data and spectral classification (see Table 3) were observed. All spectra were reduced on the basis of ESO-MIDAS routines, in particular the MIDAS package LONG. The resulting one-dimensional spectra are dominated by the emission from the central regions of the NELGs. The spectra were not flux-calibrated.
A subset of NELGs were photometrically monitored during the CAFOS campaign.
We selected the 10 galaxies with highest variability indices and small
deviations from a star-like image structure. On each of
the six nights, we took a 180 s direct image of a
field around each of the 10 galaxies through a
Johnson B filter. The CCD frames were reduced using MIDAS
standard routines.
![]() |
Figure 2: New and old variability indices for the NELGs for overall variability (top) and long-term variability (bottom). |
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The variability indices derived from the magnitude measurements in Paper 1 show two fundamental differences between NELGs and QSOs. First, there is no indication for significant long-term variability in the NELG sample, contrary to the QSOs. On the other hand, strong variability of the NELGs is indicated at short timescales of a few days or less. These differences are clearly illustrated by the comparison of the sample-averaged structure functions in Fig. 1 (for definitions and properties of the structure function see e.g. Simonetti et al. 1985). The absence of long-term variability is probably consistent with the presence of low-luminosity AGNs (LLAGNs) that are suspected to have shorter variability timescales than high luminosity AGNs (Filippenko 1992; Lira et al. 1999; Moran et al. 1999).
Strong variability on short timescales is provable
by means of CCD time-series observations with a baseline of a few days.
The results from our CCD-monitoring campaign do not indicate
significant variability at the 0.02 mag level.
We adopt the null-hypothesis
H0:
,
i. e. the photometric standard deviation
is the same
for the NELGs and the stars of comparable magnitude. The F test
shows that H0 should not be rejected on a significance level
for 9 of the 10 NELGs. For the remaining object,
the measured F is close to the critical value
.
Hence, there is no evidence of significant short-term variability
from the CCD time-series.
The photographic photometry described in Paper 1 was
based on a two-dimensional
Gaussian profile-fitting procedure. Deviations of the image profile from
the Gaussian leads to an increased measurement error and, combined with
variations of the observing conditions from plate to plate, to
artificial variability. This effect is clearly reflected by the high
variability indices measured for galaxies with extended images. However,
all but three NELGs appear star-like even on the deepest plates, and
none of the NELGs was classified as extended on the basis of the
index
.
In order to solve the discrepancy between the strong variability
from the Schmidt plate data and the results of the CCD time-series,
we have completely revised the reduction of all 162 B Schmidt
plates. The SExtractor package (Bertin & Arnouts 1996)
was used and magnitudes were measured by aperture photometry instead of
profile fitting. At the faint end, the photometric accuracy of the revised
data is improved by a factor of about two. An additional
improvement is achieved by averaging the measured magnitudes
of an object over adjacent epochs. This was done if
(a) the single-epoch data have too low a photometric accuracy, i.e.
or
,
and
(b) there are several plates available of close-by epochs.
Thus, we finally have 54 data points of different epochs spanning 33.2 years.
For each individual epoch, the photometric accuracy is better than
0.1 mag at B=19 and better than 0.2 mag at B=20. This is a significant
improvement compared to the original data (cf. Fig. 5 in Paper 1).
![]() |
Figure 3:
The new object classification parameter class for all objects
(dots) in the magnitude range of the NELGs.
The white curve represents the running median,
the various symbols designate visually identified galaxies (bullets),
Seyfert 1 galaxies with ![]() |
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no. | NELG |
![]() |
MB |
![]() |
![]() |
![]() |
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![]() |
1 |
J171122.0+440721 | 19.4 | -20.5 | 5.53 | 0.33 | 1.44 | 0.69 | 3.7 | -0.45 | 0.80 |
2 | J171124.1+433117 | 19.7 | -21.0 | 5.19 | -0.57 | 2.87 | -1.38 | 5.1 | -1.38 | 0.29 |
3 | J171241.1+430512 | 19.4 | -21.0 | 13.66 | -2.00 | 3.11 | 0.11 | 6.7 | -0.17 | 1.10 |
4 | J171319.5+435216 | 19.1 | -20.6 | 11.68 | 1.54 | 4.20 | -0.14 | 10.8 | -0.90 | 0.38 |
5 | J171323.0+431230 | 18.7 | -20.6 | 9.19 | 1.75 | -0.72 | -0.29 | 8.7 | -0.33 | 0.65 |
6 | J171326.8+440117 | 19.1 | -17.1 | 6.16 | 0.91 | -0.03 | -0.22 | 4.7 | -0.52 | 0.76 |
7 | J171448.3+434455 | 18.8 | -20.7 | 8.96 | 0.37 | 0.69 | 1.66 | 7.4 | -0.24 | 0.43 |
8 | J171459.0+434327 | 19.4 | -18.4 | 7.57 | 1.38 | 0.07 | 1.21 | 5.6 | -0.07 | 0.85 |
9 | J171510.7+430506 | 19.4 | -20.1 | 6.41 | 1.30 | 0.74 | 0.17 | 8.9 | -0.63 | 1.15 |
10 | J171520.1+433427 | 18.7 | -19.5 | 10.61 | -0.22 | 3.32 | -1.75 | 11.3 | -0.25 | 0.66 |
11 | J171610.9+422333 | 18.8 | -21.6 | 32.47 | 0.45 | 5.70 | -2.05 | 12.0 | -0.50 | 1.06 |
12 | J171652.4+433528 | 19.7 | -18.1 | 6.26 | 2.04 | 2.11 | -0.21 | 7.2 | -0.12 | 0.51 |
13 | J171734.2+432824 | 19.9 | -18.8 | 7.92 | 1.31 | 0.87 | -0.04 | 2.9 | -0.79 | 0.67 |
14 | J171734.9+425643 | 19.8 | -20.3 | 6.78 | -0.18 | -1.00 | 0.26 | 2.0 | -0.62 | 0.73 |
15 | J171747.3+432550 | 19.2 | -18.8 | 10.82 | 0.44 | 2.85 | -0.86 | 8.2 | -0.52 | 1.05 |
16 | J171828.1+442727 | 19.4 | -18.8 | 5.82 | -1.49 | 1.89 | 1.71 | 4.2 | -0.44 | 0.65 |
17 | J171908.9+423111 | 19.3 | -21.0 | 10.37 | 1.39 | 2.97 | 0.77 | 8.8 | -0.76 | 1.06 |
18 | J171955.6+442244 | 18.9 | -20.4 | 11.85 | -1.25 | 4.23 | 0.41 | 9.7 | 0.34 | 0.62 |
19 | J172156.0+441912 | 19.3 | -17.0 | 6.74 | -1.79 | -2.02 | -0.08 | 4.8 | -0.71 | 0.53 |
20 | J172256.1+425447 | 19.4 | -21.2 | 9.76 | 0.51 | 2.28 | 2.57 | 6.0 | -0.64 | 1.17 |
21 | J172340.6+434102 | 19.6 | -19.4 | 14.51 | -2.41 | 2.66 | -0.20 | 5.8 | -0.19 | 0.56 |
22 | J172348.1+432907 | 19.0 | -21.4 | 9.54 | -0.40 | 2.86 | -1.21 | 5.5 | -1.01 | 0.93 |
23 | J172407.6+424037 | 19.2 | -21.3 | 3.65 | 0.55 | 1.46 | 0.65 | 5.0 | -0.75 | 0.69 |
The variability indices
and
were computed in
exactly the same way as in Paper 1. In Fig. 2,
we compare the new with the old variability indices of the NELGs.
The values are listed in Table 1,
along with the other photometric data.
As for the original data, no significant long-term variability
is found from the revised data. The overall variability indices
from the revised photometry are considerably reduced: only about
50% of the NELGs have
.
An outstandingly high
is found
for an NELG that is projected onto an extended foreground galaxy.
These facts illustrate that the way of measuring magnitudes
is of major importance for the assessment of variability.
In Paper 1, image profile indices were derived from the
radius-magnitude relation. The SExtractor package allows a more
sophisticated morphological classification based on a trained neural
network. According to the classification parameter class derived by
SExtractor, NELGs are clearly separated from star-like objects
(Fig. 3). On the other hand, the classification parameters of some
of the Seyfert galaxies from the VPM survey are similar to NELGs.
Analogously with the nonstellar index from Paper 1,
we define a new nonstellar index
![]() |
Figure 4:
Overall variability index
![]() ![]() ![]() |
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The spectra cover the wavelength interval between about 4800 Å
and 8200 Å in the observer frame. Prominent lines are H,
[O III]
5007,
[O I]
6300,
H
,
[N II]
6583,
and the [S II]
6717, 6731 blend.
The accuracy of the emission line equivalent widths (EWs) is
determined primarily by the uncertainty in the level of the continuum.
The problem of a subjective bias was partly overcome by a
non-interactive measurement procedure. Briefly,
the level of the local continuum is estimated by means of median
and low-pass filters. Then, the EW is estimated at the position calculated
from the previously measured redshift. The redshifts and the
resulting EWs are listed in Table 2.
The errors given there are the random
errors derived by the measuring procedure.
In general, the Balmer lines will be blended.
We follow the approach by Popescu & Hopp (2000)
to correct for the absorption by the underlying older stellar population:
for all galaxies with strong continuum emission (
Å)
the measured
is increased by an assumed constant
absorption EW. A constant value of
Å
was found by McCall et al. (1985) and is in good agreement with
the data given by Ho et al. (1997). H
absorption was corrected
for in the same way with
Å
derived from the Ho et al. sample.
![]() |
Figure 5: Diagnostic line-ratio diagrams for the NELGs (symbols as in Fig. 3) and the comparison galaxies (galaxies classified in the literature as Seyferts or LINERs are marked as asterisks, transition types as open boxes). |
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no. | grism | z | H![]() |
[O III] | [O I] | H![]() |
[N II] | [S II] | spectral |
4861 Å | 5007 Å | 6300 Å | 6565 Å | 6584 Å | 6716/31 Å | class | |||
1 | G100 | 0.1457 | 14.2![]() |
6.7![]() |
1.5![]() |
75.4![]() |
24.7![]() |
27.6![]() |
H II |
2 | G200 | 0.2087 | 30.7![]() |
77.0![]() |
0.1![]() |
171.5![]() |
27.8![]() |
45.5![]() |
H II |
3 | G200 | 0.1867 | 2.1![]() |
0.5![]() |
2.3![]() |
33.6![]() |
9.9![]() |
10.9![]() |
H II |
4 | G200 | 0.1362 | 22.3![]() |
74.5![]() |
6.1![]() |
129.1![]() |
14.9![]() |
10.0![]() |
H II |
5 | G100 | 0.1150 | 4.1![]() |
11.7![]() |
1.9![]() |
35.7![]() |
10.8![]() |
10.9![]() |
H II |
6 | G100 | 0.0270 | 5.5![]() |
9.0![]() |
0.2![]() |
36.3![]() |
7.6![]() |
11.1![]() |
H II |
6 | G200 | 0.0275 | 7.9![]() |
10.9![]() |
0.2![]() |
39.5![]() |
4.6![]() |
3.5![]() |
H II |
7 | G100 | 0.1265 | 13.6![]() |
15.1![]() |
2.4![]() |
72.9![]() |
23.0![]() |
43.5![]() |
H II, (AGN) |
8 | G100 | 0.0576 | 3.8![]() |
9.3![]() |
1.6![]() |
21.3![]() |
1.6![]() |
5.3![]() |
H II, (T) ? |
9 | G100 | 0.1256 | 3.8![]() |
0.1![]() |
0.3![]() |
25.7![]() |
10.9![]() |
5.8![]() |
H II |
10 | G100 | 0.0690 | 2.4![]() |
0.0![]() |
2.3![]() |
29.6![]() |
12.9![]() |
2.1![]() |
H II |
10 | G200 | 0.0690 | 3.4![]() |
2.6![]() |
0.5![]() |
33.0![]() |
12.5![]() |
3.5![]() |
H II, (T) ? |
11 | G200 | 0.1824 | 0.8![]() |
0.4![]() |
0.6![]() |
33.1![]() |
12.8![]() |
8.2![]() |
H II |
12 | G200 | 0.0576 | 0.1![]() |
0.1![]() |
2.4![]() |
7.0![]() |
1.4![]() |
4.4![]() |
AGN, (H II) ? |
13 | G100 | 0.0892 | 0.8![]() |
12.5![]() |
0.0![]() |
2.1![]() |
0.6![]() |
1.8![]() |
H II, (AGN) ? |
14 | G100 | 0.1698 | 10.0![]() |
18.7![]() |
1.8![]() |
65.9![]() |
19.6![]() |
22.8![]() |
H II, (T) |
15 | G100 | 0.0655 | 10.2![]() |
28.2![]() |
1.0![]() |
44.6![]() |
3.9![]() |
9.3![]() |
H II |
15 | G200 | 0.0659 | 4.6![]() |
30.3![]() |
0.1![]() |
48.7![]() |
2.1![]() |
6.6![]() |
H II |
16 | G100 | 0.0680 | 3.3![]() |
16.0![]() |
1.3![]() |
15.4![]() |
0.0![]() |
0.6![]() |
H II, (AGN) ? |
17 | G200 | 0.1828 | 8.6![]() |
19.9![]() |
1.8![]() |
125.7![]() |
42.2![]() |
22.4![]() |
H II |
18 | G200 | 0.1147 | 2.4![]() |
4.4![]() |
1.5![]() |
48.9![]() |
14.8![]() |
21.2![]() |
H II, (AGN) |
19 | G200 | 0.0278 | 229.8![]() |
1813.8![]() |
0.7![]() |
1066.7![]() |
10.2![]() |
12.1![]() |
H II |
20 | G200 | 0.2051 | 5.3![]() |
4.4![]() |
0.1![]() |
52.3![]() |
18.9![]() |
25.3![]() |
H II, (AGN) |
21 | G100 | 0.1002 | 4.6![]() |
1.9![]() |
0.2![]() |
29.1![]() |
7.6![]() |
6.0![]() |
H II |
22 | G200 | 0.1871 | 8.4![]() |
13.6![]() |
1.6![]() |
78.3![]() |
20.8![]() |
27.2![]() |
H II |
23 | G200 | 0.2056 | 7.2![]() |
33.9![]() |
0.1![]() |
76.0![]() |
13.6![]() |
22.8![]() |
H II |
galaxy | grism | H![]() |
[O III] | [O I] | H![]() |
[N II] | [S II] | class | class. | class |
name | 4861 Å | 5007 Å | 6300 Å | 6565 Å | 6584 Å | 6716,6731 Å | (here) | type | (lit) | |
NGC3031 | G100 | 0.0![]() |
11.6![]() |
5.3![]() |
11.8![]() |
15.4![]() |
5.5![]() |
S | u | S1 |
NGC3031 | G200 | 0.1![]() |
8.9![]() |
5.4![]() |
19.8![]() |
20.3![]() |
5.1![]() |
S | u | S1 |
NGC5678 | G200 | 0.1![]() |
0.1![]() |
0.3![]() |
2.2![]() |
2.4![]() |
0.9![]() |
T | u | T1 |
NGC6323 | G100 | 0.0![]() |
41.5![]() |
1.1![]() |
14.0![]() |
19.8![]() |
7.0![]() |
S | u | S2 |
NGC6323 | G200 | 1.9![]() |
31.6![]() |
1.4![]() |
22.0![]() |
21.2![]() |
6.1![]() |
S | u | S2 |
NGC6500 | G200 | 2.7![]() |
14.0![]() |
10.5![]() |
45.8![]() |
29.9![]() |
36.4![]() |
L, (S) | a | L1 |
NGC7177 | G200 | 0.1![]() |
0.1![]() |
0.1![]() |
5.2![]() |
3.0![]() |
5.1![]() |
T? | a | T1 |
NGC7743 | G200 | 0.1![]() |
5.1![]() |
1.1![]() |
9.2![]() |
13.3![]() |
7.4![]() |
T, (S) | u | S1 |
It is a common practice to discriminate between AGNs and massive stars by means of line-ratio diagrams (Baldwin et al. 1981; Vielleux & Osterbrock 1987; Ho et al. 1997). We use the diagnostic line-ratios recommended by Vielleux & Osterbrock that are based on the measurements of the lines mentioned above. Since the wavelength separation of the considered lines are small, these line-ratios are relatively insensitive to reddening (cf. Dessauges-Zavadsky et al. 2000). Line-ratios are expressed by the ratios of the corresponding EWs. Figure 5 shows these diagnostic diagrams for all galaxies. Each object was individually classified on each of the three diagrams, and then the consistence of the three classifications was evaluated. We consider three spectral classes: H II galaxies to the left of the demarcation curves, AGNs to the right, and transition objects near the boundary (i.e., the error bars cross the demarcation), In addition, we define two classification types: unambiguous (u), i. e. all relevant data are consistent with one spectral class, or ambiguous (a), i.e. the object is classified both as AGN and H II galaxy on different diagrams. The results are given in Table 2. For ambiguous classifications the two most likely classes are given. There is good agreement between the spectral classes derived for the six comparison galaxies and the classification from the literature (Table 3).
The results can be summarised as follows:
for most of the NELGs the line-ratios correspond to H II region
spectra. None of the NELGs is unambiguously classified as an AGN.
Only for one object is more than one diagram compatible with
an AGN spectrum, but the signal-to-noise is relatively low in
this case. A substantial fraction of the sample (40%) has an
ambiguous classification, and a similar fraction is
located near the H II-AGN border.
It must be emphasised that the line-ratios are derived from
integrated spectra. At a redshift of z=0.1, a slit width of 1''covers about 2 kpc. In the direction of the long-slit,
the spectrum integrates over the whole galaxy.
For many of the NELGs rotation curves are seen in Hindicating the presence of extended emission regions like
circum-nuclear rings or spiral arms.
If an LLAGN is present, it can thus be masked by the emission
from H II regions connected to intense
star formation (Storchi-Bergmann et al. 1996;
Pastoriza et al. 1999).
A trend of decreasing line ratios with increasing effective aperture,
and therewith with increasing z, is expected if AGNs
substantially contribute to the spectra (Storchi-Bergmann 1991).
Such a trend is not indicated in our data.
We cannot exclude that there are LLAGNs in at least some
of the NELGs. However, the integrated spectra are not
dominated by AGNs.
![]() |
Figure 6:
Correlations with
![]() |
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Some trends with
are shown in
Fig. 6 (where NELG 19 with its exceptionally
high EWs was excluded for the sake of clearness). There is
a strong correlation between
)
and
).
The slope is similar to that of nearby galaxies and implies a
mean extinction
mag (cf. Kennicutt 1992).
Further, there are loose correlations with
)
for the
EWs of the lines
[O III]
5007,
[N II]
6583,
and [S II]
6717, 6731.
Such trends are known from samples of nearby galaxies
(e.g., Tresse et al. 1999;
Sodré & Stasinska 1999).
The large dispersions are likely due to
the variation in the mean nebular excitation in the
galaxies (Kennicutt 1992).
Our sample is magnitude-limited and the absolute magnitudes are
therefore strongly correlated with redshifts. Further,
due to the constraint of star-like images, the
characteristic scale-length of the dominant component
is of the order of a few arcseconds or less.
The sample thus comprises roughly two classes of objects:
dwarfs and sub-
galaxies at z<0.1 and
galaxies at
.
These two subclasses are
marked by different symbols in all relevant figures.
The most significant difference between the sub-
and the
galaxies are the mean EWs
(Fig. 6, bottom):
NELGs with fainter absolute magnitudes show a clear tendency to
have smaller
). This trend is opposite to
what is found in a representative sample of galaxies in the
local universe (Tresse et al. 1999).
If the trend in Fig. 6 is real, it is probably
related to the selection effects of the VPM search.
However, a plausible explanation for this trend
has not yet been found.
Most of the NELGs are blue (Table 1). Using the
K corrections for Sbc galaxies (Coleman et al. 1980),
the corrected sample-averaged colour indices are
and
.
(The U-B colour is probably slightly underestimated due to a bias
towards brighter magnitudes at the faint end of the Umeasurements.) The blue colours are likely a
selection effect: the object selection is based on the
morphological classification done on the deepest red plate and on the
variability measured on B plates (Paper 1). Galaxies that are
compact on the red plate but more extended, and therewith brighter
on the blue plates, have therefore a good
chance to be selected as (nearly) star-like and variable objects.
The MB distribution of the NELGs is similar to that
of galaxies selected for their compact nuclei (Sarajedini
et al. 1999).
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Figure 7:
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Open with DEXTER |
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Figure 8:
Logarithmic gray scale presentation of 10 NELGs (in the middle
of each panel) in the B band after Lucy-Richardson deconvolution.
The image size is 1 ![]() |
Open with DEXTER |
The
vs. B-V diagram for the NELGs of the
present study (Fig. 7) resembles the one for the
starburst galaxies discussed by Moy et al. (2001).
According to these authors, the most attractive explanation
for this diagram is provided by galaxy models with continuous star
formation + starbursts where the ages and metallicities
of the underlying population are similar to those in normal
spirals.
Direct 100 s exposures were obtained in the V band for all 23 NELGs. There is little information about morphological details for most of the galaxies. Deeper images are available for the 10 NELGs from the photometric series where six 180 s B band exposures for each object were co-added. Since we are interested in the extended, non-star-like structure components, the images were PSF-deconvolved applying the Lucy-Richardson algorithm. The resulting images (Fig. 8) show that all NELGs have dominant, more or less compact components. In addition, a more extended fainter component is indicated in many cases. Structure details like spiral arms, bars, or rings cannot be recognised. Some NELGs (4, 11, 19) clearly show asymmetric light distributions, perhaps indicating morphological distortions. Many of the NELGs have several close-by neighbour galaxies (in projection). For instance, NELG 11 seems to be a highly disturbed galaxy in the centre of a group, and the dwarf NELG 13 seems to have a light-bridge toward a nearly edge-on giant spiral at a projected distance of less than 20 kpc. A further interesting object is NELG 19 which is probably a cometary blue compact dwarf galaxy with EWs similar to Mkn 71 (for the latter, see Kennicutt 1992; Noeske et al. 2000).
The VPM NELGs have properties characteristic of starbursts and are thus
expected to be strong emitters in the far infrared (FIR).
There are no entries in the NED1
at the positions of the NELGs, and there are no
IRAS counterparts. The latter is simply explained by the IRAS
detection limits: galaxies with
must have
to be found in the IRAS catalogues. Such a strong
FIR excess is characteristic of only a few ultra-luminous IR
galaxies (Sanders & Mirabel 1996) which have
much stronger internal extinction in the optical than is
indicated for the NELGs (Fig. 6).
We have studied the sample of NELGs from the VPM survey in the M 92 field. These objects have been selected as QSO candidates because of their high variability indices (Paper 1) and have been classified later as NELGs (Paper 2). However, it was not clear from the previous data to what extent the variability and the spectral properties of the NELGs are related to AGNs. In the present paper, we re-investigated the variability and analysed the emission line-ratios, as well as the photometric and morphological properties of the NELGs. The main conclusions are the following.
Acknowledgements
This research is based on observations made with the 2.2 m telescope of the German-Spanish Astronomical Centre, Calar Alto, Spain. We acknowledge financial support from the Deutsche Forschungsgemeinschaft under grants Me1350/8 and /10. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.