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 800 MHz due to thermal absorption (Israel et al., 1992). A nonthermal (synchrotron) spectral index of nt (nt -1.0 for M31 and nt -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 nt is constant across the galaxy, the nonthermal and thermal components were separated (Berkhuijsen et al., in prep). The average thermal fractions at 6 cm are 35% and 30% in M31 and M33, respectively. The large value in M31 is surprising in view of its relatively low star-formation rate.
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 |
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).