4.4. A Mid-Infrared Look Within Galaxies

ISO-CAM CVF studies between 5 and 17 µm are turning out to be powerful diagnostics of the radiation field within the disks of nearby galaxies, allowing us to disentangle the variations in heating intensity and hardness of interstellar radiation. The approach is to relate the intensity to the shape of the continuum, and the hardness to the ratios of ionic fine-structure lines (Tran 1998; Contursi 1998). See also the overview on ISOCAM studies of nearby galaxies by Vigroux et al. 1999, and the studies of NGC 891 by Le Coupanec et al. (1999) and by Mattila et al. (1999). Such studies are valuable in establishing the local relation between mid-infrared emission and the star formation intensity, thereby guiding the interpretation of the global fluxes.

The ISOCAM images of galaxies show dust emission in nuclear regions, in the inner barred disk, outlining the spiral arms, and tracing the disk out to the Holmberg radius and beyond (Malhotra et al. 1996, Sauvage et al. 1996, Vigroux 1997, Smith 1998, Roussel et al. 1999, Dale et al. 2000b). There are clear color variations within spiral galaxies, some of which have not yet found satisfactory explanations (Helou et al. 1996; Tran 1998; Vigroux et al. 1999). Dale et al. (1999) describe behavior similar to the ISO-IRAS color diagram within the disks of three star forming galaxies, IC 10, NGC 1313, and NGC 6946, where the 6.75-to-15 µm color drops precipitously as the surface brightness exceeds a certain threshold. The point of inflexion in the color curve occurs at a surface brightness which is a function of the dust column density, whereas the shape of the curve seems invariant, and may result from a rise in both the hardness and intensity of the heating radiation (Figure 8). Dale et al. discuss these findings in the context of a two-component model for the interstellar medium, suggesting that star formation intensity largely determines the mid-infrared surface brightness and colors within normal galaxy disks, whereas differences in dust column density are the primary drivers of mid-infrared surface brightness variations among galaxy disks.

Figure 8. The mid-infrared color as a function of surface brightness for three disk galaxies well resolved by ISOCAM, and smoothed so the resolution corresponds to ~ 200 pc in each case (Dale et al. 1999). All three data sets show roughly the same behavior, indicative primarily of how color and surface brightness evolve as heating intensity increases. The data are consistent with the expectation that a change in total ISM dust column density will shift the curves along the surface brightness axis: NGC 6946 does indeed have an order of magnitude greater column density than the other two galaxies in the product of HI + (2/3)H2 and metallicity.

Rouan et al. (1996), Block et al. (1997) and Smith (1998) have combined ISOCAM and Br images with other broad-band and line images to estimate star formation rates, ISM parameters, obscuration and dust properties. These studies again point to AFE carriers as a ubiquitous component of interstellar dust, to the likely destruction of these carriers by ionizing UV, and to dust heating in non-starburst disk galaxies being derived from both old stars and OB stars.

In M31, Pagani et al. (2000) demonstrate a very close correlation between mid-infrared emission at both 6.75 and 15 µm and the distribution of neutral gas as traced by HI and CO maps. The correlation is poorer with ionized gas as traced by H, and poorest with UV emission, a result which they attribute to extinction. They conclude that AFE can be excited by visible and near-IR photons, the dominant dust heating vectors in this particular case, and therefore by older disk and bulge stars. They also find evidence that in this environment the AFE carriers are amorphous carbonaceous particles formed in the envelopes of carbon stars, and have not yet been graphitized by ultraviolet radiation.