5. Comparison with other edge-on galaxies
In this section the results we obtained from the present study of NGC 5907 are compared with the Milky Way and two other edge-on galaxies already observed at and in the CO emission, namely NGC 891 and NGC 4565. The reason why we restrict ourselves here to these few galaxies is that they have not too different properties (i.e. Hubble type, star formation, interaction state) and the analyses are based on comparable data sets. The data that correspond to this section are collected in Table 2. NGC 891, NGC 4565, and the Milky Way have already been compared in a similar way by Neininger et al. (1996).
Table 2. Dust and gas parameters of several edge-on galaxies and the Milky Way in comparison
5.1. Molecular gas distribution
One similarity between the galaxies is that the molecular gas distributions (as traced by the CO intensity) all show a central peak and a ring with a radius of kpc. But the ratio in the major axis intensity distribution differs from galaxy to galaxy. The Sb galaxy NGC 4565 has a relatively strong ring compared to the central peak, in the Sbc galaxy NGC 891 it is less pronounced, and in the Sc galaxy NGC 5907 it is just marginal detectable. Whether this variation is due to the different Hubble types or occurs just by chance, is not known yet. Since galaxies of later Hubble types in general tend to have flatter rotation curves in their inner disks, there may be dynamical reasons for a less pronounced ring near the turnover radius.
A second ring-like feature in the CO distributions, which may trace a spiral arm structure, is only detected in NGC 4565 (at ) and NGC 5907 (at ).
Compared to NGC 891 and NGC 4565, the kinematics of the central regions point to NGC 5907 being somehow intermediate between these two. NGC 4565 just shows a marginal indication for a central bar (Neininger et al. 1996), whereas in NGC 891 molecular gas was detected at forbidden velocities and the kinematics in the central region could be successfully modelled by orbits driven by a bar potential (García-Burillo & Guélin 1995). NGC 5907 seems also to exhibit a central bar, but much less pronounced than NGC 891 (García-Burillo & Guélin 1995; García-Burillo et al. 1997).
5.3. Dust properties
Compared to other galaxies, the continuum emission at 1.2 mm of NGC 5907 is surprisingly strong. Because of the similar distances of the galaxies to be considered here, it is sufficient to compare the measured intensities. In Fig. 4 (NGC 5907) they exceed values of 24 mJy per beam (or 35 mJy per beam in a smoothed map), whereas in NGC 4565 the map maxima reach just values of about 22 mJy per beam (see Fig. 3 of Neininger et al. 1996). Although they did not give a value for the total flux density of NGC 4565, this seems to be much lower than in NGC 5907, too. Guélin et al. (1993) determined a value of 850 mJy for the non-interacting Sbc-galaxy NGC 891. This points also to an unexpectedly high value for NGC 5907 since the infrared emission of NGC 891 is about 3 times higher (Young et al. 1989, Table 2).
This strong thermal dust emission may be partially due to a nuclear point source which shows some characteristics of a weak starburst (Skrutskie et al. 1985). Since star formation and the subsequent supernova events always lead to enhanced turbulence in the ISM, this may be the reason for stronger beam depolarization and the non-detection - in contrast to NGC 891 and NGC 4565 - of ordered magnetic fields at (with a resolution of HPBW) by Dumke et al. (1995).
© European Southern Observatory (ESO) 1997
Online publication: May 5, 1998