3.1. Overall distribution of the dust emission
Fig. 2 shows the CO-corrected continuum map of M 82 at 1.2 mm, superimposed onto an optical image. At 1.2 mm M 82 has a more or less oval shape (ellipticity e 0.6), with the major axis parallel to the galactic plane (molecular gas). This corresponds to a position angle of approximately 70o. The maximum of the continuum emission is located at , o54´55". Within the errors this position is identical to the peaks of the distributions at 800 µm and 1.1 mm (Hughes et al. 1990). Besides this regular shape there are some spur-like features extending above and below the galactic plane, which might be associated with the outflow seen at optical and IR wavelengths.
3.2. Major- and minor-axis profiles
The contours in Fig. 2 indicate some extended emission also along the minor axis of the galaxy, but at first glance the extent in this direction is not as large as suggested by the maps of Kuno & Matsuo (1997) and Alton et al. (1999). We have therefore computed the intensity as a function of distance from the galactic center along the minor (major) axis, averaging over some 30" in the major-axis (minor-axis) direction. These profiles are displayed in Fig. 3. For comparison the 12CO(2-1) line intensity, the 850 µm (Alton et al. 1999) and 20 cm continuum profiles are shown, too.
The 1.2 mm emission shows the most concentrated distribution, significantly smaller than that at 850 µm. Especially the scale height along the minor axis is very small with respect to the other wavelengths and the molecular emission.
Contrary to the expectation the molecular emission is the most extended component. Possible explanations for this phenomenon are discussed in Sect. 4.2.
3.3. Integrated flux density
Integrating over the continuum map we obtain a total flux density of Jy. This is lower than the value measured by Krügel et al. (1990a), but their flux density was much more uncertain, owing to the lower sensitivity of their 1-channel bolometer.
Fig. 4 shows a radio-to-IR spectrum of M 82 with the individual values listed in Table 1. Our new 240 GHz measurement is completely consistent with the slope of the spectrum at this frequency (indicated by the dotted line), whereas the value derived by Kuno & Matsuo (1997) at 157 GHz lies much too high with respect to all adjacent data points.
Table 1. Flux densities of M 82 from radio through IR wavelengths.
The CO-corrected flux density is significantly lower than the flux density in our original map. This demonstrates that a line correction for continuum measurements in the relevant bands is indispensible.
The total flux can be turned into a gas mass via
With a distance of D = 3.25 Mpc, and using the same parameters as Krügel et al. (1990a), i.e. a dust temperature of = 30 K (Chini et al. 1989) and a dust absorption coefficient (Krügel et al. 1990b), we derive a gas mass of . If we adopted the Draine & Lee value (Draine & Lee 1984) the resulting gas mass would be somewhat lower ().
In this calculation we assumed a gas-to-dust ratio of approximately /. The corresponding dust mass is M.
© European Southern Observatory (ESO) 2000
Online publication: June 26, 2000