Up: Spectroscopic observations of southern galaxies
We made long slit spectroscopic observations of the galaxy NGC 2442
positioning the 3
3
348
slit
on six different positions (Fig. 1), five at PA = 40
and
one at PA = 79
. One of the five parallel positions was centered on
the nucleus of the galaxy and the one at PA = 79
, was positioned
along the external northern arm. From the analysis of the reduced spectra,
we arrived to the following conclusions:
- 1.
- The emission line intensity ratios at the nuclear region indicate that
it is a LINER. The electron temperature
and the density
in this region are
14 000 K and
530 cm-3, respectively, which are normal
values for this type of objects.
- 2.
- The spectral characteristics of a region 87'' to the NE are
typical of an H II region. N(O)/N(H) = 9.7 10-4
and N(N)/N(H) = 7.1 10-5. The ratio N(N)/N(O) = 0.07
is very close to usual values in galactic emission regions.
6500 K and
10 cm-3 are slightly low but within the range of normal values.
- 3.
- The intensities of H
, 12CO(1-0), 21 cm H I, and
the continuum at 843 and 1420 MHz, along a line at PA = 40
and
through the nucleus (Fig. 4), show all, except one, three peaks:
one at the center, another one at about 80
NE and the third one at
about 125
SW. H I does
not show the peak at the center. The sources at the positions of the peaks
are not resolved. The intensities of the peaks and their ratios (Tables
3 and 4) show that the spectral indices for the two
continuum frequencies and for the three peaks are large, indicating
pre-eminence of non-thermal radiation. The H2/H I column density
ratio at the center is higher than at the other two places while the ratios
H2/H
and H I/H
are
much lower at the center, indicating different conditions for the conversion
of atomic to molecular Hydrogen and molecular gas to stars.
- 4.
- There is a good correlation between the mean velocities of H
and CO (Fig. 5). The velocity curve along the line at
PA = 40
(assumed
to be the line of nodes) shows a linear and steep velocity gradient within
12
5 from the center, with terminal velocities of 1262 and 1694
(Fig. 6). This type of velocity
curve may be caused by a fast rotating
ring, in this case with a projected velocity of 216
.
We adopt 1478
for the systemic heliocentric
velocity and, using H0 = 75
Mpc-1, a
distance of 16.1 Mpc. The central ring would have then a radius of
1 kpc. All these values are similar to those found by MB97.
The optical velocity curve extends up to 137
(10.7 kpc
at 16.1 Mpc) where the measured velocity is 275
with respect to the center.
- 5.
- Using an inclination angle of 69
(MB97), the rotation
velocity of the central ring, at a radius of 12
5, would be
231
and the mass within its radius
1.2 1010
. The total mass within a
radius of 137
, assuming an inclination of 24
, would be
11 1011
.Therefore, the mass within the central ring would be about 1% of the
estimated total mass, i.e. proportional to the areas.
- 6.
- The three optical spectra in the nuclear region, at 3
to the
NE and 5
1 to the SW, show double velocity components separated by
145
4
. These components and their relative values
might be explained by the presence of two H
small regions (H98)
separated 4
5, symmetrically positioned at both sides of the nucleus
and along a line at PA
97
, and of the central ring. The
velocities given by H98
for the central features do not agree neither with the velocities derived
from our spectra nor with the systemic velocities found by us and by MB97.
An explanation for these velocity differences should be found
in the procedures for deriving them.
- 7.
- The LINER nature of the nuclear region could be the result of the
integration of the emissions from a central point Seyfert source and
from star formation regions.
- 8.
- The velocities along the external northern arm are incompatible
with a disc in normal galactic rotation with the line of nodes at a
PA = 40
. Different parameters are required which must be
obtained by modelling. In this modelling it is also necessary to include the
effects of the probable interaction with some of the galaxies seen in the
neighbourhood of the galaxy.
- 9.
- The final conclusion is that NGC 2442 requires still much
work to be done to fully understand its morphology, dynamics and internal
processes.
Acknowledgements
We acknowledge the cooperation of the Director and of the technical staff
of the CASLEO observatory
which made possible these observations.
We also acknowledge the use of the CCD and data acquisition
system supported under U.S. National Science Foundation grant AST-90-15827 to
R.M. Rich. We are grateful to Sally Houghton for having access to her Thesis
before publication.
Up: Spectroscopic observations of southern galaxies
Copyright The European Southern Observatory (ESO)