Quote:
Originally Posted by sjastro
Opacity is caused by scattering of photons which is a function of photon wavelength and the particle size of the scattering medium. (......) NUV wavelengths and particle size has maximized the degree of opacity.
Long wavelength UV (NUV) is classified in the region of 300-400nm.
I'm not sure for the reasons behind the Galex definition but the NUV data in the Galex image isn't really NUV but more like MUV.
Steven
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(( The above post was transferred from the IIS imaging forum, as we were discussing the effects of the dust lane in Centaurus A, as seen at various different wavelengths ))
Hello Steven,
This is a fairly basic discussion, but I do try to bear in mind that our IIS readership is composed of people with various amounts of astronomical knowledge.
The terminology regarding the dimming and reddening (due to the wavelength-dependent amount of extinction) of electromagnetic radiation as it passes through the interstellar medium is not always accurately or consistently used in the astronomical literature. In general, there is direct
absorption of light by dust grains, as well as
scattering of the light by the dust grains.
A word in common use for the total effect of absorption
and scattering, which expresses the fact that the interstellar dust
dims the light of an object, is
extinction.
[[[
But Alessandro Boselli, in his recent (2012) textbook "A Panchromatic View of Galaxies", uses the
non-standard word
attenuation instead of the word
extinction. This is a very accurate descriptor, though it will be a hard one to use for "vocabulary-poor people who can't spell"(an ever-increasing fraction of the population!)
]]]
Bruce Draine, in his book I mention below, presents some detailed physics (several chapters!) for the interstellar dust grains; there is nothing simple about trying to model the nature (size ; shape ; composition ; evolution) of the grains themselves, though their effects on electromagnetic radiation are well known.
Here are three quotations (edited for clarity) about interstellar dust, taken from some of my favourite reference handbooks about galaxies and astrophysics:
(1) "Normal spiral galaxies have dust masses in the order of 1 million to 100 million solar masses, with ~3 percent in PAHs, this fraction being slightly smaller in low-metallicity galaxies. The dust mass represents ~0.5 percent of the gaseous phase. Normal galaxies have a dust temperature of ~15-25 Kelvin in the diffuse Interstellar Medium.
(this quotation is from "A Panchromatic view of Galaxies"(2012) by Alessandro Boselli)
(the dust is warmer near to HII regions)
(Other authors give 1 percent of the ISM mass of a non-dwarf galaxy as being in the form of dust. But small galaxies without enough gravity to hold on to the products of mass ejection from stars can contain very little dust.)
(cold as it is, the interstellar dust emits very brightly at Far-infrared to sub-millimetre wavelengths)
(2) Interstellar grain material appears to be quite varied. Large organic molecules, and graphite and silicate grains are prominent constituents. The graphite grains appear to originate in the dense atmospheres of carbon stars. Water and other volatiles freeze out on these grains in the cold interior of dense clouds, where the grains are shielded from heating by starlight.
[ this quotation is from "Astrophysical Concepts (4th edition)"(2006) by Martin Harwit ]
(3) Based on observations of ultraviolet extinction, scattering of visible light, and polarization of starlight, it is clear that the interstellar grain population must have a broad size distribution, extending from radii as small as about 0.01 micrometers (or less) to radii of about 0.2 micrometers.
(this quotation is from "Physics of the Interstellar and Intergalactic Medium"(2011) by Bruce T. Draine)
[[ extinction of
visible light is due to dust grains which are about 0.1 micron in radius ]]
Characteristically opinionated commentary.......
Comment No. 1
Draine says that some of the dust grains might be as small as 50 atoms! Maybe Fred Hoyle wasn't as 'cracked' as other astronomers thought he was when he posited the existence of interstellar viruses?
Reading Draine's book (he is a well-known modeller of the dust) gives me the impression that there is no 'typical' or 'characteristic' size for the interstellar dust particles, but much of the total mass of the dust in a galaxy will be in the larger grains (0.1 microns in diameter, and greater).
According to one popular model, the grains become progressively more numerous with progressively decreasing grain radius.
Comment no. 2:
Figuring out the nature of the interstellar dust grains is very much a 'black box' problem;
the 'box' of matter we are trying to look into is at a
vast distance (x light years) from the observer and contains
tiny particles no bigger than 1/3000 millimeter. So astronomers will never actually
see these interstellar particles in a microscope or telescope or any other type of scope! (unless we can build a telescope that is much bigger than the solar system.....)
Therefore, humans can only observe the
effects of this dust on the light that reaches it or the light that originates from it. This implies a significant probability that current ideas about the nature of the interstellar dust could be substantially wrong! So astronomers who talk about the composition and size-distribution of the Interstellar Dust very prudently talk about
models rather than actual confirmed facts.
Comment No. 3 :
My "lady friend" (call her Virginia, for argument's sake) was very dismissive and sarcastic when I just told her I was writing about dust in outer space......
"Studying dust in outer space? Why not study the dust in the carpet?"
- Virginia's response on hearing that I am studying
the dust between the stars