Galactic cirrus clouds
ometimes, the largest mysteries are solved by understanding the smallest details.
For example, when you go to a dark place and gaze at the sky above, you will often see our home galaxy, the Milky Way, arching overhead from horizon to horizon. Against this background brimming with billions of far away stars, dark areas may be noticed - places that seem to have no stars. Actually, the stars are there, but their light is blocked by gigantic clouds of infinitesimally small interstellar dust particles
- NGC 891 is an iconic example of a galaxy seen edge on from our planet's perspective. The spiral galaxy spans about 100 thousand light-years and is approximately 30 million light-years distant in the constellation Andromeda.
- Image credit: R. Jay GaBany
The dark spaces between the stars is not a perfect vacuum; it's filled with molecules of gas and dust. The gas is mostly hydrogen and helium- the most abundant elements in the Universe. In addition, there are tiny dust grains released by mighty suns that ended their lives in titanic explosions, known as super nova, long ago in the distant past. The dust absorbs (blocks) and scatters (reflects) optical light. It also emits FIR (Far Infra-Red) radiation at a wavelength between 100-250 microns. Because of this, interstellar dust is often considered a nuisance by astronomers.
For example, when light passes through interstellar dust, some is reflected and some is absorbed. This dims the light by the time it reaches Earth. Because interstellar dust reflects blue light better than red, the color of light from distant stars also appears reddened. This effect is similar to the scattering of light in our atmosphere that causes blue skies and crimson sunsets. So, when astronomers measure the distance to stars and galaxies, they must adjust the color and amount of light their instruments capture for the amount of dust through which it has traveled.
at the top of this page was produced from data collected by two space telescopes launched in the nineteen eighties: the Infrared Astronomical Satellite (IRAS) and the Cosmic Background Explorer (COBE). The two datasets have been merged and individual point sources, from stars and distant galaxies, were removed to isolate the vast structures formed by interstellar dust.
Because our planet is located within the middle of the Milky Way's flattened disk
, our view of the galaxy is always from
Therefore, the disk of our galaxy is always seen edge-on, as a broad band, similar to looking at the thin edge of CD or DVD
. In this picture, the bright band running through the middle is an image of the dust within our galaxy's disk and it contains most of the Milky Way's dust particles. The fainter filaments above and below this band represent only the very closest dust clouds as seen from our vantage point. Because of their thin, wispy nature, similar to high altitude fair weather clouds here on Earth, they are collectively known as galactic cirrus.
Galactic cirrus clouds are veils that surround our galaxy, partially block and confuse our view into the great beyond.
The presence of galactic cirrus was first noticed on optical glass plates recorded at Palomar Observatory and subsequently cataloged by B. T. Lynds, in 1965. However, their infrared properties were not discovered until the early 1970's and studied in detail following the launch of the IRAS mission in 1983.
Earlier this decade, astronomer Steve Mandel
unexpectedly noticed faint filaments of cirrus on deep, wide field optical images he exposed near the northern galactic pole. His picture led to further research that uncovered two photographically significant new properties of interstellar dust:
it could be seen in optical light by its reflection of visible and ultraviolet light released from the combined stars in our galaxy and
it was also capable of producing a faint ruddy emission.
Thus, because interstellar dust within the cirrus reflects blue light but also glows red, it often displays a mauve hue in deep optical images. Mandel labeled these ghostly optical filaments Integrated Flux Nebula
Galactic cirrus has intruded on images produced by other astronomers, too. For example, over the years, astrophotographers have noticed faint patterns in the darkest portions of their images that appeared to be random noise. Concluding this to be the case and following the common wisdom espoused by others, these signals were subsequently removed during image processing. But, its now understood, their instruments were probably recording faint light signatures from galactic cirrus clouds.
Until very recently, galactic cirrus was also misidentified as evidence of galactic interaction between M 81
and M 82
. Seen as dark vertical lines to the right of the M 81's central region, these curious linear features remained a mystery until they were recognized to be a portion of galactic cirrus back lit by the iconic northern galaxy. Recent research
revealed they are part of
, a cirrus structure that appears next to M 81 but actually located close to the Milky Way and along our line of sight to the more distant galaxy by chance alignment.
Although I cannot prove it beyond a shadow of a doubt, I believe a similar situation exists with the galaxy
. A familiar subject for both visual observers and astrophotographers, NGC 891 can be glimpsed through small telescopes prior to mid-night by looking toward the constellation of Andromeda during the northern late fall and winter seasons. The galaxy appears edge-on from our vantage point and is located about 30 million light years from Earth.
Color pictures of NGC 891 typically show a reddened hue that, many have suspected, is due to intervening dust. But no images, until now, have conclusively revealed its presence. Many telescopic photographs have also revealed an extensive set of unusual dark, thin vertical, horizontal and arch shaped
structures extending above and below the entire length of this distant star system- far beyond the range of features expected for a galaxy with this orientation and proximity to Earth. These structures have been described as evidence of furious star formation and other explanations have also been postulated throughout published literature.
But, this very deep
reveals the galaxy is situated behind broad, faint filaments of galactic cirrus. As a result, as in the case of M 81, I strongly suspect portions of cirrus are back illuminated by the glow from the much more distant star system and appear to be associated with it, when in fact, they most likely are not. Instead, line of sight chance alignment has created the illusion of association. Of course, a separate investigation would need to be mounted that measures local cirrus properties in this sky area, its color characteristics and the effect it imposes on our view of the galaxy to prove this assertion.
None the less, understanding the true nature of galactic cirrus requires an appreciation for the smallest building blocks in creation- dust particles. Armed with this knowledge, science now has a map to adjust their measurements of distant phenomena, stars and galaxies thus paving the way for our ever increasing realization about the Universe in which we exist.