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 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.

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

	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

	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).