Annu. Rev. Astron. Astrophys. 1992. 30: 575-611
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2. THE POPULATION OF NORMAL GALAXIES

Radio sources exist in nearly all normal spiral and dwarf irregular galaxies, in many peculiar or interacting systems, and even in a small number of E/SO galaxies with ongoing star formation (Dressel 1988; Wrobel & Heeschen 1988, 1991). Unbiased samples of these sources reveal the types of sources to be found; the range of luminosities, sizes, brightnesses, morphologies, spectra, etc. that they span; and their similarities and differences. Normal galaxies range in power from L 10 18 h -2 W Hz -1 to L ~ 10 23 h -2 W Hz -1 at = 1.49 GHz,where h H0 / (100 km s -1 Mpc -1). The total brightness of the fainter extended sources in normal galaxies is not much larger than the rms brightness fluctuation produced by discrete background sources, so confusion is a serious observational problem. All spiral and irregular galaxies brighter than the BT = 12 limit for 100% completeness of the Revised Shapley-Ames Catalog (Sandage & Tammann 1981) and north of = -45° have been mapped at 1.49 GHz by the VLA with ~ 0´.9 FWHM resolution; most were detected (Condon 1987). Since the FIR and radio luminosities of normal galaxies are tightly correlated, flux-limited samples of normal galaxies selected at FIR and radio wavelengths are nearly identical. The largest and most complete is the IRAS Revised Bright Galaxy Sample (Soifer et al. 1989) of 313 extragalactic sources stronger than S = 5.24 Jy at = 60 µm. Multiconfiguration VLA maps at 1.49 GHz (Condon et al. 1990) detected all but one, and 8.44 GHz maps with 0".25 FWHM resolution (Condon et al. 199lc) resolved most of the sources not already resolved at 1.49 GHz. Large samples of radio sources in normal galaxies have also been generated by position-coincidence identifications of sources found in general radio surveys with optically-selected UGC (Nilson 1973) galaxies (Condon & Broderick 1988, Condon et al. 1991b). Radio sources powered by monsters in active galactic nuclei (AGNs) can be excluded on the basis of radio morphology FIR spectral index (25 µm, 60 µm) < 1.5 (de Grijp et al. 1985, 1987) and FIR / radio flux density ratio (Condon & Broderick 1988). Direct radio identifications of faint FIR-selected galaxies (Condon & Broderick 1986, 1991) also yield complete, but small, samples of radio sources in normal galaxies. The FIR- and radio-selected samples of ``normal'' galaxies favor luminous starbursts and contain a large proportion of disturbed, peculiar and interacting systems.

Figure 2 illustrates the range of radio sizes and morphologies seen in normal galaxies. The faintest dwarf irregulars have radio luminosities comparable with the Galactic SNR Cassiopeia A and little or no detectable emission extending beyond known H II regions and SNRs. Their radio morphologies are lumpy and irregular (see IC 10 in Figure 2), and their radio spectra are often relatively flat (Klein & Gräve 1986). The thick radio disk / halo of the edge-on galaxy NGC 891 and the fairly smooth radio disk with bright spiral arms of the face-on galaxy NGC 6946 are typical of the larger spiral galaxies. The central radio sources in normal galaxies like NGC 6946 are brighter but usually much less luminous than the disk sources and it is no longer thought that they are significant sources of disk cosmic rays. Luminous radio sources with complex morphologies may be found in colliding galaxies (e.g. NGC 1144). There is a tendency for fairly compact (diameter D 1 kpc) central starbursts to dominate at higher radio luminosities, as in M82. The most luminous radio sources in normal galaxies are frequently quite compact, D ~ 200 pc (Condon et al. 1991c), and confined to the nuclei of strongly interacting (Condon et al. 1982, Hummel et al. 1990) systems (e.g. IC 694+NGC 3690). Hybrid Nbody / gasdynamics simulations show that a prograde collision involving a disk galaxy can drive about half of the disk gas ( 10 10 M ) within 200 pc of the nucleus (Hernquist 1989, Barnes & Hernquist 1991), where the gas density becomes quite high before a powerful starburst is triggered (Kennicutt 1989). The resulting massive stars and their SNRs then produce intense radio emission.

Figure 2 Figure 2. Contour maps (Condon 1987, Condon et al. 1990) illustrating the range of source morphologies, sizes, and luminosities found in normal galaxies. The bars are 2h-1 kpc long. The logarithmic contours are separated by 21/2 in brightness, and the 1.49 GHz brightness temperatures Tb of the lowest contours are 0.25 K (IC 10, NGC 891, NGC 6946), 0.5 K (M82), 8 K (NGC 1144), and 128 K (IC 694+NGC 3690).

The 1.49 GHz local luminosity function of normal galaxies (Condon 1989) specifies the differential number of normal galaxies per Mpc 3 per magnitude (factor of 10 0.4) of 1.49 GHz luminosity (Figure 3). The FIR / radio correlation (see Figure 8 later) ensures that the FIR and radio luminosity functions have the same form. The dashed line in Figure 3 is the = 60 µm luminosity function (Saunders et al. 1990) directly transformed to 1.49 GHz assuming only <(log(S60 µm / S1.49 GHz)> = 2.24 from Figure 8. If the radio sources in normal galaxies evolve on cosmological time scales (implying higher star-formation rates in the past), they may account for the inflection in the normalized radio source counts below S ~ 1 mJy at 1.49 GHz (Condon 1989) and a significant fraction of the discrete source background (Condon 1989 ). The = 60 µm faint source counts are consistent with the radio counts and evolution of normal galaxies (Lonsdale et al. 1990).

Figure 3 Figure 3. The 1.49 Ghz local luminosity function of normal spiral and irregular galaxies (data points and solid line) is indistinguishable from their lambda = 60 µm luminosity function scaled to 1.49 GHz if < log(S60 µm / S1.49 GHz) > = 2.24 (dashed line). Abscissa: log luminosity h2L (W Hz-1). Ordinate: low density h-3 rho (mag-1 Mpc-3).

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