Astron. Astrophys. 325, 124-134 (1997)

1. Introduction

The evolution of spiral galaxies and their properties are determined by many processes. One of the most important points is the occurrence and time-scale of star formation, which depends mainly on the amount, composition, and distribution of the available raw material. This raw material consists of neutral gas in either atomic or molecular form.

The amount of atomic hydrogen can easily be derived from measurements of the 21 cm line of HI. The mass of molecular hydrogen, on the other hand, is usually determined indirectly by observing the lowest rotational transition of the CO molecule. The measured line intensities are converted into column densities using a conversion factor . The estimated molecular mass, however, is quite uncertain because X is still a matter of debate (e.g. Maloney & Black 1988; Combes 1991; Arimoto et al. 1996).

Observations of thermal dust emission at mm-wavelengths provide an alternative way to estimate the total mass of the interstellar matter of a spiral galaxy. Although this emission is difficult to observe due to its weakness, it has several advantages. Firstly, at millimeter wavelengths the dust is optically thin. Secondly, the emission scales roughly with the first power of the temperature. Thirdly, at this wavelength the dust absorption cross sections do not depend very much on physical dust properties like grain size, shape, composition, and surface properties (Hildebrand 1983; Draine & Lee 1984). And, finally, interstellar dust is about 2 orders of magnitude more abundant than the most abundant CO molecule in normal spiral galaxies.

In the last few years, the advent of multi-channel bolometer arrays has made it possible to obtain high quality maps (i.e. high angular resolution and high sensitivity) of the cold dust emission of nearby spiral galaxies. In order to study the relation of the dust to the atomic and molecular hydrogen in a normal late-type spiral, we decided to observe the non-interacting edge-on galaxy NGC 5907.

This galaxy shows a relatively low level of star formation from the weak infrared emission measured by IRAS (Young et al. 1989). Therefore it is - together with NGC 4565 observed by Neininger et al. (1996) - some kind of counterpart to the more active galaxies already observed in the radio continuum at mm-wavelengths: NGC 891 (Guélin et al. 1993), NGC 3079 (Braine et al. 1997), NGC 3627 (Sievers et al. 1994), NGC 4631 (Braine et al. 1995), and M 51 (Guélin et al. 1995).

NGC 5907 was observed in the HI-line twenty years ago and was the first non-interacting galaxy where a galactic warp could be detected (Guélin et al. 1974; Sancisi 1976). Former CO measurements (Sofue 1994; García-Burillo & Guélin 1995) have shown that this galaxy is relatively weak in CO. Although its nearly edge-on orientation of does not allow us to observe the emission of individual star forming regions or spiral arms in the disk, this inclination increases the column density along the line of sight to a significant and easier measurable value. Therefore NGC 5907 is a good candidate to study the radial distribution of the dust and its correlation with the molecular and atomic hydrogen distributions. This will be done especially in comparison with the other non-interacting edge-on galaxies mentioned above, namely NGC 891 and NGC 4565. The latter one shows an even lower star formation activity than NGC 5907 and is also CO-weak, whereas NGC 891 contains a considerable amount of molecular gas and shows a relatively high level of star formation. These two galaxies are classified as Sb and Sbc, respectively, in contrast to the Sc-galaxy NGC 5907.

Furthermore edge-on galaxies are suitable to investigate the thickness of the gas and dust layer. Since NGC 5907 is lacking strong star formation, one does not expect to detect a thick disk or halo in this galaxy because star formation and the existence and structure of a gaseous halo seem to be directly connected (e.g. Dahlem et al. 1995).

In this paper we present our molecular line and bolometer observations of NGC 5907. The following section describes the observations and the data reduction. Sect. 3 presents the results we obtained from the CO observations. In Sect. 4 we discuss the thermal dust emission. We estimate dust temperatures and absorption cross sections, and compare the distribution of the cold dust with that of the atomic and molecular gas. From this we get some hints on how the CO-H₂ -conversion factor may vary in galactic disks. In Sect. 5 the results for NGC 5907 are compared with those obtained for other edge-on galaxies, and the final section gives a summary of our results and some concluding remarks.

Table 1 lists some basic parameters of NGC 5907 which will be used throughout this paper.

Table 1. Some basic parameters of NGC 5907

© European Southern Observatory (ESO) 1997

Online publication: May 5, 1998

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