The wavelength region of interest for studying important physical processes tends to fall outside the domain accessible with ground-based telescopes. This applies to both the mid-IR and the space-UV. Past generations of space instruments could not afford the resolution required by the relevant astrophysical spatial scales. The IR in particular suffered from this shortcoming, as the IRAS mission had very limited spatial resolution, and fully panchromatic SED studies were restricted to the spatially integrated light.
With ISO and Spitzer, we have entered a new era. Panchromatic imagery of M81 over six decades in wavelength space is shown in Fig. 14 (). The proximity of M81 has made it a favorite target for many investigations of galaxy properties from the X-ray to radio. Many well-resolved images have been taken in the past, with the notable exception of the IR. Previously, well-resolved far-IR images of galaxies have only been possible for Local Group galaxies such as the Magellanic Clouds. With the successful launch of Spitzer, it is now possible to map many large galaxies in the far-IR with good spatial resolution, good sensitivity, and in a reasonable amount of time. M81 is one of the key galaxies in the Spitzer Infrared Nearby Galaxies Survey (). Two of the questions that can be probed with these new observations are the variation of star-formation indicators and the IR-radio correlation across the disk of M81. MIPS, Spitzer's mid-IR imager, permits comparisons with UV and H images to probe the behaviors of the IR, H, and UV star-formation indicators across M81. Such comparisons have been made for global galaxy fluxes (), but rarely has it been possible to resolve all three indicators in a single galaxy. By studying their resolved behavior, we will greatly improve the accuracy of these star formation indicators for both resolved and global galaxy measurements.
Figure 14. Panchromatic view of M81 using Spitzer, UIT, and ground-based images at wavelengths of 1500 Å, 2500 Å, H, R, 24 µm, 70 µm, 160 µm, and 20 cm. The field of view of the images is 15" × 23" ().