Hello there, all you Friends of the Coalsack.
Here are three images of the Coalsack......these three images were made in three different wavelength regimes. The three images are approximately in registration, and the imaging data has been displayed with the "Aladin Lite" tool found on the internet (
http://aladin.u-strasbg.fr/AladinLite/ ).
The most evident difference between the three images is the tendency for the Coalsack dark nebula to become
ever more transparent to the background light coming from objects that are situated behind the dark nebula,
with progressively increased (longer) wavelength of observation. This is as one would expect for sub-micron sized dust particles which are
absorbing and
scattering the light of background objects.
[[ for instance, if you look at the Coalsack at 22 micrometers (displayed as red in the WISE image), the dark nebula seems to be "not there".......because the light from background objects passes through it so easily at this wavelength ]]
However,
the dust cloud itself is not emitting meaningful amounts of electromagnetic radiation in these three images;
the wavelength of the maximum
emission of light coming from the
cold interstellar dust particles at a temperature of 6 to 20 degrees Kelvin (that is, the peak of the Spectral Energy Distribution) is well longwards of 100 microns, so this thermal emission from the smoke-like dust particles that live in the cold "Molecular Hydrogen
plus admixed Dust" clouds like the Coalsack was detectable with the now-deceased Herschel Space Observatory at far-infrared wavelengths.
For instance, Pagani et al. (2004, A&A, 417, 605) showed that 200 micron emission traces the outer regions of a cold dust cloud very well , though imaging at this wavelength does not pick up the coldest dust in the centre of a dust cloud.
[[[ in the following paragraph, I restate some information from Pagani et al. (2015, A&A, 574, L5) ( http://www.aanda.org/articles/aa/pdf...aa25095-14.pdf
) ]]]
In principle, it is possible to use an infrared telescope to detect
all of the dust in the Coalsack and in other similar Cold & Dark & potentially Star Forming clouds ,
emitting electromagnetic radiation at far-infrared wavelengths, if we observe at wavelengths longwards of 200 microns. For instance, according to this Pagani et al. paper, interstellar dust at 10 degrees Kelvin is brightest at 300 micrometers, while the coldest dust near to the cloud centres is at a temperature of about 6 degrees Kelvin and emits Electromagnetic Radiation most brightly at a wavelength of 500 micrometers.
(( The dust in dark nebulae is COLD, some of it not that much above absolute zero, which is why it emits only in the thermal far-infrared ! For the physicists among you, I note that the infrared spectrum of the dust in dark clouds is close to a Black Body curve, though with some discrepancies from a standard Black Body. ))
The longest wavelength of observation of the Spitzer Telescope, 160 microns, should also pick up at least some of the dust emission from the Coalsack.....but , would you believe, it, the researchers chose (and this is all too usual!)
not to publish about far southern objects (like the Coalsack) in their first 6 papers (The Spitzer Survey of Interstellar Clouds in the Gould Belt). You would think that research astronomers actually have an aversion to the Coalsack.
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[[[ Dana, you presented some really good and useful maps of the space distribution of visual extinction in various dark nebulae. It would be a good idea to put a copy of them in the observational astronomy forum. (
as a lot of people avoid the science forum..... as if it were the plague). Also, an explanation of how to use them would be helpful, for the benefit of the "lesser brethren". ]]]
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Anyhow, here are the three images........
Image 1: Visible light (Digitized Sky Survey 2, Red image)
Image 2 : Near infrared (image at 1 to 2.4 micrometers , from the 2MASS sky survey)
Image 3 : Longer wavelength Infrared image, from the WISE satellite:
- Blue displays emission near 3.4 micrometers
- Green displays emission near 4.6 micrometers
- Red displays 22 micrometer emission
Note that extinction from dust in the Coalsack is so low at 22 microns that some of the red objects seen within the sky outline of this dark nebula are thousands of light years
behind the Coalsack!
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