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Galaxies In Multiple Wavelengths

We customarily classify galaxies and attempt to describe their morphological characteristics primarily on the basis of visual light images. This is in part a historical accident in that we first examined galaxies (spiral nebulae or elliptical nebulae) 200 years ago using visual inspection of telescopic images. Beginning 100 years ago photographic emulsions began to be used to record astronomical images. While the spectral sensitivity of photographic emulsions is slightly different from the spectral sensitivity of the eye, the two are close enough so that they are both commonly referred to as "visual light" images. Of course, it turns out that the visual-light region is the most advantageous one for studying galaxies in general since this is the region where stars (and hence galaxies) usually emit the most radiation.


However, as technological advances have made it possible to image astronomical sources outside the visible band, it has become apparent that some morphological structures are more pronounced in the IR, radio or even x-ray than they are in the visible. Indeed, some relatively featureless galaxies have revealed vast amounts of structure in non-visible bands. For example, in spiral galaxies the dust scatters and obscures visible light while allowing IR, radio and x-ray photons to pass through relatively unimpeded. Furthermore, the dust becomes a stronger emitter of IR photons at the temperatures of the interstellar medium. In addition, neutral atomic hydrogen can be observed in the radio (1.414 MHz/21 cm) and yields views of galaxy interactions that are not at all obvious in other bands. As this technology has improved, we have come to recognize that a far clearer understanding of galaxies is possible when we observe both visible and non-visible photons.

Optical Image of M81

Optical Image of M81. Image Credit: NASA/ESA Hubble Space Telescope.

UV Image of

GALEX UV Image of M81. Image Credit: NASA, JPL-Caltech, GALEX Team, J. Huchra et al. (Harvard-Smithsonian Center for Astrophysics)

IR Image of M81 IR Image of M81
IR Image of M81 IR Image of M81 IR Image of M81

Spitzer IR Data: NASA/JPL-Caltech/S. Willner (Harvard-Smithsonian Center for Astrophysics)

X-Ray/UV Image of M81

XMM-Newton X-ray/UV Image of M81 Credit: A. Breeveld, M.S.S.L. and RGS Consortium and ESA

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This page was last modified on Thursday 01st October 2009 @ 10:36am

Science Mission Directorate Universe Division

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