The emission line intensity ratios at the nuclear region (Table 2)
clearly indicate that it is a LINER (Osterbrock
1989), which is compatible with the suggestion of
Shobbrock (1966) that this galaxy may be a Seyfert or
other emission type. Véron-Cetty & Véron (1986)
classified it as a Seyfert-like galaxy, a designation for either a Seyfert
or a LINER. However, from their measurements, they concluded that
NGC 2442 is a N galaxy, with H in absorption, because they
found that H
< 1.2
[N II]
6584 but no
other lines were present which would allow them to distinguish between a
Seyfert 2 and a LINER. Being effectively a LINER, the dominant excitation
mechanism should be photoionization by a continuous spectrum similar to
but weaker than those present in Seyfert 2 galaxies (Osterbrock & De Robertis
1985).
The electron temperature
and the density
at the
nuclear region were derived from the [N II] (
6548 +
6584)/
5755 and [S II] (
6717/
6731)
ratios, obtaining
14 000 K and
530
cm-3, respectively, which are normal values for this type of
objects. We might have here the situation mentioned by MB97
of seeing simultaneously the emission of a central point source (Seyfert)
and of a star forming ring. Since the ring, in the H
maps,
appears quite thick and the inclination is about 69
(as estimated by MB97), an appreciable amount of its emission should be
present in the nuclear spectrum. We shall come back later to this possibility
since we find also the central ring and we believe that our double peaked
spectra in the nuclear region are produced by the two H
sources
detected by H98 and by the central ring.
![]() |
Figure 3:
Spectrum of an emission region located on the major axis at
87![]() ![]() |
In Fig. 3 a spectrum
of the region at the NE, mentioned above, is shown. Its spectral
characteristics are typical of an H II region. The internal reddening
is much higher than in the nuclear region. For this region the N(O)/N(H)
and N(N)/N(H) abundance ratios were obtained. The N(O)/N(H)
abundance was derived from empirical calibrations from Edmunds & Pagel
(1984). Because the [O II] 3727 line lies
in a noisy zone of the spectrum, the [O II]
3727/H
ratio was calculated from the first predicting equation given by McCall
et al. (1985). Assuming N(O)/N(H) = (N(O+) +
N(O++))/N(H+) and N(N)/N(O) = N(N+)/N(O+), the
nitrogen abundance is N(N)/N(H) =
(N(O+) + N(O++))/N(O+)
(N(N+)/N(H+)).
The corresponding electron temperature was deduced from the equation for
N(O)/N(H) searching for the required value of
for the
previously derived N(O)/N(H) abundance. The resulting oxygen and
nitrogen abundances, N(O)/N(H) = 9.7 10-4 and N(N)/N(H) =
7.1 10-5, are 1.20 and 0.72 of the corresponding solar
abundances. The ratio N(N)/N(O) = 0.07 reflects a relative deficiency of
N with respect to O but it is very close to usual values in galactic emission
regions (Shaver et al. 1983). Its electron temperature
and density:
6500 K and
10 cm-3, are slightly low but within the range of normal
values.
Our results consist of spectra at scattered points over NGC 2442,
along the slit positions. They do not allow
us to build contour maps for the comparison with the results of observations
at other frequencies which did produce such maps. We can, however,
make such a comparison along those slit positions. In particular, we
were interested in the position at PA = 40 and through the nucleus,
which we assumed to coincide with the line of nodes, because there are several
important features along it. We shall start comparing the intensities.
Figure 4 shows the flux densities of H (our results),
in erg cm-2 s-1, and of the 843 MHz (from Plate 2 of Harnett
1984) and 1415 MHz continuum (from Fig. 5.4 of H98), in mJy
beam-1, and the areas of the 12CO(1-0) (from Fig. 6b of Bajaja
et al. 1995) and 21 cm H I (from Fig. 5.7a of H98)
velocity profiles, in K
, along the line at PA =
40
, as a function of the distance to the center. In spite of the
different angular resolutions, which are evident in the widths of the
central peaks, the correlation between the five components is clear.
Except for H I, which shows a rather flat curve in the central
region, all the curves show three peaks, one at the center, one at about
80
towards the NE and a third one, much less defined, at about
125
towards the SW. The differences between the distances to the
NE and SW peaks are another evidence for the asymmetry of the galaxy.
The smallest width for the central source is
shown by H
with a FHMW of
16
, which indicates that
it is not resolved by the other observations.
Differences in the positions of the peak centers at the NE, and
probably at the SW, may be due
to the fact that the data were taken from the figures and these may have
untraceable scale errors, but the intensities are supposed to be correct.
![]() |
Figure 4:
Intensities as a function of the distance to the center,
along a line at PA = 40![]() ![]() |
In Table 3 are listed the peak values for the three regions
(NE, Center and SW) and for the five types of emission. In order to be
able to compare the emissions in the CO and H I lines, the profile
areas were converted to column densities using for the conversion factors
the values 3 1020 and 1.823 1018
(cm2 K )-1,
respectively. For CO the conversion factor is the same as the one used
by Bajaja et al. (1995) although there is not a general
agreement about the value that should be used. The continuum emissions are
given in mJy beam-1 so, for the
comparison of these intensities, observing extended sources, the values
should be corrected for the beams. In our case, however, we have seen that
at both frequencies, 843 MHz and 1415 MHz, the NE and central sources are not
resolved. As for the southern part, the peaks in the
curves are badly defined (values enclosed in brackets in Table 3)
except for the H I whose peak has a FHMW similar to the one on the NE source.
Some of the ratios between the emission intensities are listed in
Table 4. We used the flux densities quoted in Table 3,
for the continuum emissions at 843 and 1415 MHz, to estimate the spectral
indices which are also quoted in Table 4.
The ratio is larger at the center than at the NE and SW but all the
spectral indices are indicating steep gradients in the flux densities
for the three regions between both frequencies. As mentioned in Sect. 1,
Harnett (1984) derived for the
whole galaxy a spectral index value of 0.92 0.08. All these values
indicate a large pre-eminence of non-thermal radiation in the galaxy.
The column density ratios for the lines also differ greatly in the three
regions. The H2/H I column density ratio is higher
at the center, as in the case of the continuum, but the ratios
H2/H
and H I/H
are much lower
at the center. Evidently, the conditions for the conversion of atomic to
molecular Hydrogen and from this to stars at the center are quite different
from the conditions at the other regions.
The velocities derived from our observations are indicated in Fig. 5 superposed on the CO velocity field (Fig. 6c in Bajaja et al. 1995). There is, in general, a good agreement between both if we take into account the errors. There are, however, differences which are due to the much better angular resolution of the optical observations and to the different types of emission sources. This is particularly evident at the three regions mentioned in the previous section.
![]() |
Figure 5:
Optical heliocentric radial velocities (small numbers at the
positions indicated with dots) in ![]() ![]() |
In Fig. 6 the optical velocities along the PA = 40 are
shown superposed on the position-velocity diagram obtained with the CO
spectra (Fig. 6d in Bajaja et al. 1995).
The optical and the radio rotation curves, as derived from this figure, show
similar velocity gradients in the central part in spite
of the very different angular and velocity resolutions. The optical gradient,
within 12
5 from
the center, is 17
per arcsec and it is approximately
constant along its length. The velocities at the ends of this part of the
velocity curve are 1262 and 1694
and, from its
symmetry, we adopt for the systemic
velocity the value of 1478
(the errors of these
velocities are
4
). This feature
may be associated either to a disk in solid rotation or to a
ring rotating at 216/sin(i)
. Our spectra,
along a line, do not allow us to have a two-dimensional picture of the object.
MB97 derived similar parameters for a ring for which they obtained
a radius of 8
and a rotation velocity of 220/sin(i),
and for the systemic velocity a value of 1475
. All
these values are practically the same as ours since the differences are within
the errors. H98, however, derived for the systemic velocity, from the
H I velocity field, the value of 1431
which is much lower than the velocities mentioned above.
![]() |
Figure 6:
Optical heliocentric radial velocities (dots) and CO
velocity position diagram (from Bajaja et al. 1995)
along the PA = 40![]() |
Our velocity for the galaxy, with respect to the centroid of
the Local Group, is 1208 so, using a Hubble
constant H0 = 75
Mpc-1, the
distance to NGC 2442 would be 16.1 Mpc and
the radius of the central disk or ring would be
1 kpc.
MB97 estimated, from the geometry of the central ring, an inclination angle
of 69
. Using this value the rotation velocity would be
231
and the mass within the ring
1.2 1010
.
The optical rotation curve,
as determined from our observations, extends up to 137 (10.7 kpc
at 16.1 Mpc) where the observed velocity, with respect to the center, is
275
.From the geometry of the optical image of NGC 2442 we may not assume
a value as high as 69
for the inclination angle of the outer parts of
the galaxy. Although, most probably, it is not 24
(Bajaja &
Martin
1985; Baumgart & Peterson 1986) either,
in the absence of a better value we adopt this one for the mass estimation.
In consequence, the mass within 10.7 kpc would be about
11 1011
. From these values we may estimate then
that, roughly, the mass within the ring is about 1% of the total mass,
i.e. approximately proportional to the areas.
The use of very different values of i for difeerent distances from
the center is a consequence of the large distortions present in
NGC 2442.
Our optical lines in the nuclear region show, as mentioned in the
previous section, double velocity components. The spectra at 3 NE
and 5
1 SW also show two velocity components, with about the same
separation as in the
nucleus, but with some differences: a) the mean velocities of the spectra
agree with the central rotation curve;
b) the intensities of the velocity components at the NE are lower than at
the nucleus but higher than at the SW, and c)
while the intensities of both velocity components are approximately
the same at the nucleus, they are different in the spectra at both sides of
it and their intensity ratios are
inverse respect to each other, i.e. in the spectrum at the NE the component with
the shortest wavelength has the largest intensity and the opposite happens in
the SW spectrum.
The explanation for these features has been searched for in the objects
already mentioned: the two small H regions detected by H98
in the nuclear region and the central ring found by MB97
and by us. The H
regions are about 2
in
diameter and their peaks are separated by
about 4
5. The regions are, apparently, symmetrically positioned at
both sides of
the nucleus along a line at a PA of about 97
so the angle between
this line and our slit (at PA = 40
) is 57
. The extension of
the line seen by the 3
3 aperture is 4
so
a large part of the emission of the H
regions should have been
detected and be present in the spectra on the nuclear region. The
regions are displaced by
2
2 along the slit. Our H
spectrum at the nucleus of the
galaxy encompasses both
H
regions symmetrically positioned but the two spectra at each
side, to the NE and to the SW, cross them at different distances, i.e. with different weights.
About the central ring, the H maps of MB97 and of H98
show emission distributed over an oval region, centered in the nucleus and
with the major axis at a PA of about 40
. If it is a ring then it is
quite thick. The H
velocity field of H98 shows
clearly the rotation of this central feature and indicates that the PA of the
line of nodes is the same as the PA of the major axis of the oval region.
From the axis ratio of this region, MB97 derived an inclination
angle of 69
.The two H
regions and the points corresponding to our spectra in the
nuclear region are inside this central fast rotating thick ring so, besides
encompassing the two H
regions, our three spectra
should cover also the emission from the ring.
Therefore, if the intensities of both H regions are approximately the
same, we would expect the following effects on our H
spectra:
a) the two velocity components at the nuclear position should be similar
in intensity
and displaced in velocity by the difference in the projected radial
velocities of the two H
regions; b) since the spectrum at the NE
is closer to the two H
regions its integrated line intensity should be larger than in the spectrum
at the SW; c) in each spectrum, the velocity component corresponding to
the closest H
region, should be the strongest; d) the velocity difference for the two
components should be
approximately the same for the three spectra, and e) in each of our spectra,
the material of the ring which is closer to its position will be seen
with higher weight, pushing the mean velocity accordingly.
Qualitatively, the five effects are present
in our spectra. Even quantitatively we have been able to reproduce the peak
intensities on each of the three spectra assuming, for the sensitivity
along the slit, a gaussian with a standard deviation = 3
1
1
.We might say then that the interpretation made above could be acceptable.
There is a problem, however, with the central velocities given by H98
for the two H
regions: 1367 and 1458
,with an average of 1412.5
and a difference of
91
. We have, for
the velocity components in the spectrum at the nuclear region, 1373 and
1518
, with an average of
1445.5
and a difference of
145
. The separations between the component velocities
differ by 54
and the averages by
48
, differences which are much larger
than the errors in our velocities.
H98 also obtained, for the systemic velocity of the galaxy,
1431
which is 47
and
44
lower than the values derived
by us and by MB97, respectively. We do not know the reasons for these
differences. We are inclined to think that our values are correct but
the errors involved are much too high and strongly dependent on the procedure
used for deriving the velocities.
There are further questions related to the two H regions detected
by H98. Being
inside the central ring, it is natural to assume that they participate
of its rotation and that the velocity difference is due to the difference
in the projected rotational velocities. The question is then: why both
regions are along a line with a PA of 97
and not at the edges of the
ring, along the line of nodes? Furthermore, a difference of
145
and a separation of 4
5 on a plane
inclined 69
, would imply that the H
regions are rotating
with a velocity of about 345
.This velocity is 50% larger than the velocity of the ring.
With a velocity separation of 91
(as given by H98)
that velocity
would become 216
but the distance to the center
would be then twice the value measured in the H
map of H98.
The possibility that the two H
regions are on the edges of
a ring on a different plane, with the line of nodes at 97
and with an
unknown inclination angle, is not supported by the emission
distribution in the H
maps. There might be still another possibility:
two H
regions with expanding movements. It
should be recalled that Sersic & Donzelli (1993)
specified for the bar also a PA of 97
. Might be there a connection?
The questions are open.
We have to consider how the results described above could affect the previous
conclusion, derived from the intensity ratios at the nuclear region, that
it is a LINER. As was said then, this would imply that the dominant excitation
mechanism is photoionization by a power-law continuum similar to but weaker
than in Seyfert 2 galaxies. However, from the fact that our
nuclear region spectrum sees the H regions, and that H
is also
detected in absorption, it would be possible that this spectrum is the result
of the integration of the emissions from a central point Seyfert source and
from the star formation regions. This combination, which weakens the
effect of a pure Seyfert source, would give as a result the characteristics
of a LINER.
The CO Velocity-Position diagram in Fig. 6 shows a tendency
for double
velocity components at both ends of the line at PA = 40. The optical
velocities along this line, in those regions, seem to follow the CO
components with the highest absolute velocities and the lowest intensities.
It is not clear what this double velocity would mean. A warp could easily
produce this kind of effect along the minor axis but it would have to be very
strong to be visible along the major axis.
More data are necessary for modelling it.
Figure 5 shows the velocities obtained for ten points along
the external northern arm and it may be found that
the velocities, at the far end, are between 1360 and
1370 . Since the
errors are of the order of 3 to 10
, the conclusion is
that these velocities are not compatible with a normally rotating inclined
disc, with a PA for the line of nodes of 40
and a systemic velocity of
1478
, because the points are then close to the minor
axis. There might be several possible explanations for the abnormal
velocities:
a) a smaller PA; b) a velocity component normal to the
galactic plane (warp?), c) a radial component along the plane
(expansion?), etc, some of which might be ocurrying simultaneously.
This is another evidence for the distortion of the galaxy. Almost certainly
there are no single values for the PA of the line of nodes and for
the inclination that may be applied to the whole galaxy.
Interaction with other galaxies might be playing a fundamental role in
determining the matter distribution and the velocities on this galaxy. There
are in fact several galaxies around that might be interacting with
NGC 2442,
but none of them is so close and with such a clear evidence of interaction
as to be defined as the candidate. The consequence is that
different authors suggest different galaxies: NGC 2434
(Elmegreen et al. 1992), AM 0738-692 (MB97).
H98 has considered several of the galaxies in
the neighbourhood of NGC 2442 without pointing to any one in
particular as the most probable candidate.
The number of free parameters involved in the modelling of this galaxy
requires many more data than
those available for this work. The model, however, is largely necessary
before any discussion about the dynamics of this galaxy may be attempted.
MB97 considered the encounter with a nearby galaxy
modelling the evolution of the stellar and gaseous components. H98
used two programs within the AIPS package, GAL and ROCUR, for deriving
some basic parameters and then to make a three dimensional model of the
galaxy using a constant inclination of 42 and fitting the PA.
Both models are important but still partial contributions to the
knowledge of NGC 2442.
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