Astron. Astrophys. 363, 917-925 (2000)

9. The L/M ratio as parameter to measure the normalized energy content

In order to compare knots in M 31 and dust clouds in the Milky Way, a suitable parameterisation is required. We use the ratio as parameter. The advantage over using the temperature alone lies in the fact that accounts simultaneously for an increase of L due to the warm dust and an increase of M due to the cold dust. Note that the 175 µm data now allow for a reasonable estimate of the cold dust component, which was not possible from the 60 and 100 µm data alone. In the case of one single modified Planckian, is equivalent to using the temperature (see Eq. 1). So far the ratio provides a measure for the normalized energy content of the dust. Power sources are the interstellar radiation field (ISRF) and star formation (SF) in the knots. Note that our ratio considers only the dust, thus it differs from previous concepts of / derived for galaxies, compact H II regions, and various molecular clouds and star forming regions (Chini et al. 1986, 1987; Wall et al., 1996; Boulanger et al. 1998). We will first consider only SF.

In Fig. 6 the FIR luminosity of each knot is plotted against its dust mass. The knots fill a continuous range in this diagram, but the three SED types are well separated. A least square fit

yields = 1.03 0.10 but a different value C for each SED-type. With being one, C is equal to the average -ratio, listed in Table 2. Thus within each SED type the ratio seems to be independent of the absolute values of L or M, which allows for a direct comparison of large and small cloud complexes via .

Fig. 6. FIR luminosity against dust mass. The symbols refer to the three knot types and to comparison regions as indicated. For each group, the linear fit is shown by the lines, with the offsets illustrating the different values. For the Chameleon clouds and the whole galaxies the data values lie outside the box, thus the positions of their average lines are indicated by arrows. The values and references are given in Table 2.

Table 2. The FIR luminosity to dust mass ratio in solar units averaged for the knots and comparison objects. The data for Orion have been recalculated with our model parameters.

In order to disentangle the power contribution of the ISRF and SF in the knots, we try to find some reasonable values for the ISRF. One can safely assume that the regions outside the rings are mainly heated by the ISRF. This is equivalent to the assumption that in M 31 the SF is confined to the three ring-like structures at radii of 5, 10 and 14 kpc which is consistent with observations (Devereux et al. 1994). As the ISRF variates along the galaxy, i.e. it is higher in the bulge region than in the outer parts (Pagani et al. 1999), we have determined the -ratios for several fields in the following regions:

1. in the central bulge region (0.8-1.3 kpc)

2. in the disk between 3 and 6 kpc, excluding the 5 kpc ring

3. in the disk between 8 and 12 kpc, excluding the 10 kpc ring

4. along the 5 kpc ring but avoiding the knots.

5. along the 10 kpc ring but avoiding the knots.

The resulting average values for these regions are given in Table 2. Except for the central part, the -ratio of the inter-ring regions are extremely low. In the rings, instead, the average value outside the knots is quite high. This might be explained by an increase of the ISRF in the ring or more likely by the assumption that the ring contains more unresolved dust clouds and/or star forming region.

Inside the knots the ISRF cannot be stronger, rather, the knots might be shielded against radiation from outside. Hence, for dust heated only by the ISRF, values between 20 and 50 are expected for the ratio. The power of the warm and probably also the medium knots (having ratios of more than 600) is therefore certainly dominated by SF. The cold knots have an average ratio of 345, which is still too high for a heating of the ISRF only, but the difference to the surrounding inner-ring regions is small. This can be interepreted in the way that in these sources also the ISRF plays a major role, which must then be higher in the rings than outside. However, it does not account for the whole power. Hence, even the cold knots require additional heating, which could come from a high number of low mass stars or from young massive stars. Their UV radiation, however, has to be absorbed effectively within the clouds, since signatures of a strong UV field are not observed (Cesarsky et al. 1998and Pagani et al. 1999).

© European Southern Observatory (ESO) 2000

Online publication: December 5, 2000
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