Visible: DSS and Visible: AAO
Both of the visible-light images (above) reveal the spectacular pinwheel structure of M100, with the prototypical spiral arms and dust lanes that characterize this type of spiral galaxy. The spiral arms contain luminous knots of light, known as supergiant HII regions, where active star formation is occurring. These grand spirals often contain hundreds of millions of stars.
Near-Infrared: 2MASS, Mid-Infrared: IRAS and Far-Infrared: IRAS
The near-infrared image (above, left) displays a bright central nucleus and the grand spiral structure of the arms. There are two aspects of this photo that deserve mention. First, the dust lanes that were so obvious in the visible pictures are not evident at near-infrared wavelengths. This is because near-IR light can peer through all but the densest of dust clouds. Second, the image itself is very grainy. This is partly a result of the relatively short exposure of the near-IR photo. It is also a product of OH airglow, an upper atmospheric phenomena associated with the hydroxyl (OH) molecule. This airglow primarily affects near-infrared data taken in the 1.65 micron bandpass, one of the components of the three-color composite 2MASS image.
The mid- and far-infrared images were taken with the space-borne IRAS satellite in 1983. In each case, red denotes the brightest emission, and blue corresponds to the weakest. The most notable feature of these photos is that the relatively poor spatial resolution of the IRAS detectors conspires to smear the wonderful features seen in the visible-light photos above into a singular blob of IR light. Future IR telescopes, such as the Spitzer Space Telescope, will have far better resolution and will be capable of producing amazing images of spiral galaxies like Messier 100. Both of the IRAS images reveal a bright central source of infrared emission, stretched into an ellipse by the peculiar shape of the IRAS detectors.
The radio image (above) displays the bright central nucleus (in red). The pixelation of the radio image immediately suggests that the image is of low re-radiating at lower energies (longer wavelengths) in the infrared. Second, the spatial resolution. By counting the equivalent of 24 pixels across the image, one concludes that the effective angular resolution (seeing) is no better than (6 arcmin/24) = (360 arcsec/12) = 15 arcseconds. Ground-based visible-light telescopes often achieve 1 arcsec seeing, limited primarily by fluctuations within the atmosphere. In truth, the VLA array of telescopes telescope used to make the radio image is capable of far better resolution. However, the NVSS survey was intended to cover the entire sky visible from New Mexico. Astronomers often degrade the resolution of their telescope intentionally (that is, zoom out in a photographic sense) in order to cover wide fields of view and hasten large-area surveys of the sky.
Ultraviolet: ASTRO-1 UIT
The fascinating element of the ultraviolet image is that the central nucleus is almost invisible, whereas the brightest UV emission is found in the spiral arms. The UV light marks the position of massive young stars, which typically form as the result of density shocks within a spiral galaxy's arms. The photo suggests that more star formation is taking place on the western (right) side of the galaxy than on the east.