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5 Conclusions

We made long slit spectroscopic observations of the galaxy NGC 2442 positioning the 3$\hbox{$.\!\!^{\prime\prime}$}$3 $\times$ 348$\hbox{$^{\prime\prime}$}$ slit on six different positions (Fig. 1), five at PA = 40$\hbox{$^\circ$}$ and one at PA = 79$\hbox{$^\circ$}$. One of the five parallel positions was centered on the nucleus of the galaxy and the one at PA = 79$\hbox{$^\circ$}$, 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 $T_{\mathrm{e}}$ and the density $N_{\mathrm{e}}$ in this region are $T_{\mathrm{e}} \sim$ 14 000 K and $N_{\mathrm{e}} \sim$ 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. $T_{\mathrm{e}}$ $\approx$ 6500 K and $N_{\mathrm{e}}$ $\approx$10 cm-3 are slightly low but within the range of normal values.

3.
The intensities of H$\alpha$, 12CO(1-0), 21 cm H I, and the continuum at 843 and 1420 MHz, along a line at PA = 40$\hbox{$^\circ$}$ and through the nucleus (Fig. 4), show all, except one, three peaks: one at the center, another one at about 80$\hbox{$^{\prime\prime}$}$ NE and the third one at about 125$\hbox{$^{\prime\prime}$}$ 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$\alpha$ and H I/H$\alpha$ 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$\alpha$ and CO (Fig. 5). The velocity curve along the line at PA = 40$\hbox{$^\circ$}$ (assumed to be the line of nodes) shows a linear and steep velocity gradient within 12$\hbox{$.\!\!^{\prime\prime}$}$5 from the center, with terminal velocities of 1262 and 1694 $\rm \, km\, s^{-1}$ (Fig. 6). This type of velocity curve may be caused by a fast rotating ring, in this case with a projected velocity of 216 $\rm \, km\, s^{-1}$. We adopt 1478 $\rm \, km\, s^{-1}$ for the systemic heliocentric velocity and, using H0 = 75 $\rm \, km\, s^{-1}$ Mpc-1, a distance of 16.1 Mpc. The central ring would have then a radius of $\sim$ 1 kpc. All these values are similar to those found by MB97. The optical velocity curve extends up to 137$\hbox{$^{\prime\prime}$}$ (10.7 kpc at 16.1 Mpc) where the measured velocity is 275 $\rm \, km\, s^{-1}$ with respect to the center.

5.
Using an inclination angle of 69$\hbox{$^\circ$}$ (MB97), the rotation velocity of the central ring, at a radius of 12$\hbox{$.\!\!^{\prime\prime}$}$5, would be 231 $\rm \, km\, s^{-1}$ and the mass within its radius $\sim$ 1.2 1010 $M_{\hbox{$\odot$}}$. The total mass within a radius of 137$\hbox{$^{\prime\prime}$}$, assuming an inclination of 24$\hbox{$^\circ$}$, would be $\sim$ 11 1011 $M_{\hbox{$\odot$}}$.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$\hbox{$^{\prime\prime}$}$ to the NE and 5$\hbox{$.\!\!^{\prime\prime}$}$1 to the SW, show double velocity components separated by 145 $\pm$ 4 $\rm \, km\, s^{-1}$. These components and their relative values might be explained by the presence of two H$\alpha$ small regions (H98) separated 4$\hbox{$.\!\!^{\prime\prime}$}$5, symmetrically positioned at both sides of the nucleus and along a line at PA $\approx$ 97$\hbox{$^\circ$}$, 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$\hbox{$^\circ$}$. 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.


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