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Up: Spectroscopic observations of southern galaxies


1 Introduction

NGC 2442 is a southern SAB(s)bc pec (de Vaucouleurs et al. 1991) galaxy belonging to a small group in Volans. A visual inspection of an optical image of the galaxy (Fig. 1) permits to appreciate that it is a very asymmetrical object, mainly due to the differences in the lengths and shapes of the northern and southern spiral arms. The main body consists of a central region from which two arms originate at each side of it and run along rather straight lines in the NE-SW direction (inner parts) till they change the direction, rather abruptly, starting the external parts of the arms. The inner parts are asymmetrical in the sense that the southern part is better defined and longer than the northern part. In the external parts the asymmetry appears in the opposite order: the northern arm is narrow, well defined and very extended towards the west while the southern arm is much shorter, broader and messy. Dust is prominent and can be seen projected everywhere against the luminous parts of the galaxy and sharing the characteristics described above for the optical appearance. In particular, the external part of the narrow northern arm is split into two parts along most of its extension by a narrower dust lane seen projected against it.

  
\begin{figure}
\resizebox {8.8cm}{!}{\includegraphics{eb8111f1.eps}}
\\ end{figure} Figure 1: Optical image of NGC 2442 from a SERC IIIa J plate obtained from the Digital Sky Survey (DSS). North is at the top and East to the left. The lines represent the positions of the slit: 1 to 5 at PA = 40$\hbox{$^\circ$}$, 6 at PA = 79$\hbox{$^\circ$}$
There have been several photometric observations of this galaxy in the past (Wegner 1979; Griersmith 1980; Peterson 1982; Baumgart & Peterson 1986). More recently Ryder & Dopita (1993) and Sersic & Donzelli (1993) made more detailed photometric studies. Ryder & Dopita (1993) made CCD images through narrow band H$\alpha$ filters and a red continuum (6676 Å). Their H$\alpha$ map displays very clearly the large number of H II regions and their nearly continuous distribution along the arms. Sersic & Donzelli (1993) used photographic plates and a narrow H$\alpha$ filter. They concluded that NGC 2442 is a rather normal barred spiral galaxy which should be classified as SAB(rs)bc. They derived a distance modulus of 30.8 mag and a total blue magnitude for the galaxy of $B_{\mathrm{T}} = -20.5$ mag. The nucleus, according to Sersic & Donzelli (1993), is elongated with a position angle (PA) which varies from 40$\hbox{$^\circ$}$ at 5$\hbox{$^{\prime\prime}$}$ to 60$\hbox{$^\circ$}$ at 20$\hbox{$^{\prime\prime}$}$ and the bar lies at a PA of 97$\hbox{$^\circ$}$.

From spectroscopic observations, Shobbrock (1966) suggested that NGC 2442 might be a Seyfert or other emission type nucleus and Véron-Cetty & Véron (1986) classified it as Seyfert-like galaxy. Recently, Mihos & Bothun (1997) (MB97) and Houghton (1998) (H98) observed NGC 2442 in H$\alpha$ with Fabry Perot instruments on the CTIO 1.5 m telescope and on the 3.9 m Anglo-Australian Telescope, repectively. MB97 found evidence in the nuclear region of emission from a central source and from a ring with a radius of 8$\hbox{$^{\prime\prime}$}$ and an inclination of 69$\hbox{$^\circ$}$. They suggest that a combination of Seyfert (central source) and starburst activity (ring) might be the cause for the confusion about the galaxy being a Syfert or a Seyfert-like. H98 detected less regions than MB97 (because of shorter integration times) but she was able to detect two small H$\alpha$ emission regions, at both sides of the nucleus, separated by less than 5$\hbox{$^{\prime\prime}$}$.

NGC 2442 has been observed in the H I 21 cm line, with single dishes, by Reif et al. (1982) and by Bajaja & Martin (1985). The low angular resolutions (13$\hbox{$^\prime$}$ and 30$\hbox{$^\prime$}$, respectively) permitted to obtain only global velocity profiles which, nevertheless, provided information about the systemic velocities and about the H I and indicative masses. The observation of the 12CO(1-0) line (Bajaja et al. 1995) with the SEST telescope, with an angular resolution of 43'' and a velocity resolution of 7.2 $\rm \, km\, s^{-1}$, provided a more detailed picture of the distribution and velocity field of the molecular gas. In particular, the rotation curve showed a very steep velocity gradient at the center implying a fast rotation feature present there which, due to the angular resolution of the SEST, could not be properly resolved. The general distribution of the molecular gas over the main body of the galaxy is well correlated with the continuum emission at 843 MHz mapped by Harnett (1984), with the Molonglo Observatory Synthesis Telescope (MOST), also with an angular resolution of 43$\hbox{$^{\prime\prime}$}$. In both maps there is a N-S asymmetry, as seen in the optical image, and there are three main concentrations of emission, one at the center and one at each end of the main body, at the NE and the SW on the major axis. The spectral index derived by Harnett for this galaxy, on the basis of four frequencies between 408 and 5000 MHz, is $\alpha = -0.92 \pm 0.08$which indicates the pre-eminence of the synchrotron emission over the thermal radiation.

Besides the H$\alpha$, H98 observed also the H I 21 cm line on NGC 2442 using the Australian Telescope Compact Array (ATCA). The channel maps were made with a velocity resolution of $\sim$16 $\rm \, km\, s^{-1}$ and a synthesised beam of 24$\hbox{$.\!\!^{\prime\prime}$}$6 $\times$ 26$\hbox{$.\!\!^{\prime\prime}$}$2. A 20 cm (1415 MHz) continuum map was also produced. The H I distribution and the continuum maps are similar except for the central region where the continuum emission is strong and the H I emission is weak. Also in this case both maps reproduce the asymmetries noticed at other wavelengths.

In the next two Sects. we describe our optical observations and the results and in the last Sect. we discuss these results and their comparison with the results of observations at other frequencies.


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