Astron. Astrophys. 358, 65-71 (2000)

3. Results

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 70^o. 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.

Fig. 2. CO-corrected map of M 82 at 1.2 mm overlayed on a R-band image made with the 1.2-m telescope on Calar Alto. The contours start at the 2 (10 mJy/beam) level. The beam size of the IRAM 30-m telescope is given in the lower left corner.

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.

Fig. 3. Profiles along the minor and major axis of M 82. For each direction the 1.2 mm continuum emission (this paper), the integrated CO(2-1) line intensity (this paper), the 850 µm continuum emission (Alton et al. 1999) and the radio continuum emission at 20 cm (Reuter et al. 1992) are shown. All data are smoothed to a beam of 15" HPBW.

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.

Fig. 4. Radio-to-IR spectrum of M 82. Our new value is indicated by the filled circle, the CO-corrected value by the open circle, and the 115 GHz point of Kuno & Matsuo 1997 by the filled square. The slope of the spectrum in the submm wavelength range is indicated by the dotted line. The error bars of our data points are smaller than the used symbols.

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
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