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.
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 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
and an
inclination of 69
. 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
emission regions, at both sides of the
nucleus, separated by less than 5
.
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 and
30
, 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
, 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
. 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
which indicates the pre-eminence of the synchrotron emission over the thermal
radiation.
Besides the H, 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
16
and
a synthesised beam of 24
6
26
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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