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2. GLOBAL PROPERTIES OF M31 AND M33

2.1. Radio continuum spectra

Tables 1 and 2 give the radio continuum surveys of M31 and M33 with sufficiently small telescope beams to resolve the spiral arms. The flux densities (integrated to 16 kpc and 12 kpc radius, respectively) give an average spectral index in M31 and M33 of 0.76 ± 0.04 and 0.75 ± 0.10, respectively. The spectrum of M33 flattens below appeq 800 MHz due to thermal absorption (Israel et al., 1992). A nonthermal (synchrotron) spectral index of alphant (alphant appeq -1.0 for M31 and alphant appeq -0.9 for M33) was derived from spectral index maps in regions with low star-forming activity where the thermal contribution is negligible. Assuming that alphant is constant across the galaxy, the nonthermal and thermal components were separated (Berkhuijsen et al., in prep). The average thermal fractions at lambda6 cm are appeq 35% and appeq 30% in M31 and M33, respectively. The large value in M31 is surprising in view of its relatively low star-formation rate.

Table 1. Radio continuum surveys of M31 with resolutions leq 5'

lambda Telescope Resolution rms noise Reference
(cm)     (mJy/beam)  

91.6 Westerbork 53" × 81" 1 Golla (1989)
73.5 Cambridge + Effelsberg 4' / 5' 10 Pooley (1969), Beck & Gräve (1982)
49.1 Westerbork 54" × 82" 0.7 Bystedt et al. (1984)
21.2 Westerbork 23" × 35" 0.2 Walterbos et al. (1985)
20.5 VLA (N only) 5" 0.03 Braun (1990)
20.5* VLA+Effelsberg 45" 0.05 Beck et al. (1998)
11.1 Effelsberg 4'.8 2 Berkhuijsen & Wielebinski (1974)
11.1* Effelsberg 4'.4 1.5 Beck et al. (1980)
6.3 Effelsberg 2'.6 1.4 Berkhuijsen et al. (1983)
6.2* Effelsberg 2'.4 0.6 Berkhuijsen et al. (in prep.)

*including polarization

Table 2. Radio continuum surveys of M33 beyond 800 MHz

lambda Telescope Resolution rms noise Reference
(cm)     (mJy/beam)  

35.6 Effelsberg 15' ~ 50 Beck (1979)
21.1* Effelsberg 9'.2 17 Buczilowski & Beck (1987)
21.1 VLA + Westerbork 7" 0.05 Duric et al. (1993)
17.4* Effelsberg 7'.7 9 Buczilowski & Beck (1987)
11.1* Effelsberg 4'.4 3 Buczilowski & Beck (1987)
11.1* Effelsberg 4'.4 0.6 Beck (unpubl.)
6.3* Effelsberg 2'.4 2 Buczilowski & Beck (1987)
6.2* Effelsberg 2'.4 0.6 Niklas & Beck (unpubl.)
6.2 VLA + Westerbork 7" 0.05 Duric et al. (1993)
2.8* Effelsberg 1'.2 1.8 Buczilowski & Beck (1987)

*including polarization

The radial distributions of the total radio continuum emission from the diffuse disk of M33 (Fig. 6) and beyond the "ring" of M31 (Fig. 1) can be fitted well by exponentials (Table 3).

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