Does Galactic extinction affect our observing

[Weltevreden SA]

The star charts we rely on such as Uranometria, Aladin, and SkySafari are very good, but they don’t show extinction maps of our Galaxy’s midplane and centre. Extinction is caused mainly by dust in the Galactic disc, and maps of it would be a nightmare on top of all the details already on the printed pages or your iWhatever. Extinction is a concern if you’re after objects within 4° of the Galactic midplane. While E. E. Barnard's 'dark nebulae' studies are fascinating and detailed, they are not really full-length maps in the way we think of them. There are three places to download useful extinction maps:

Schlegel et al, 2008 (http://arxiv.org/abs/astro-ph/9809230) is still considered something of a Gold Standard. It was compiled from far IR data, but are nonetheless useful, notably in the bulge region.

Nidever et al, 2012 (http://arxiv.org/abs/1206.5799) has useful colour maps you can use to determine how reddened your area of interest is. Though compiled using near and mid IR data, the maps are reliable in the visual spectrum as well.

Chen et al (http://arxiv.org/abs/1211.3092), 2012 leads you into a very large, detailed database that is probably of more use to astrophotographers than visual fans. If you are interested in yet another set of detailed info about the myriad complexities of our Galaxy, the internal (some might say infernal) links in this paper will keep you on your toes for weeks.

These maps mainly apply to <4° above and below the Galactic disc. If you after an object higher or lower, skip this thread and have a good time.

If you delve into the professional papers, extinction is generally referred to using the terms E(B - V) or E(V - I). This means—roughly—blue band magnitude minus visual band magnitude, and visual minus near infrared. The 'E' means 'extinction' to distinguish extinction values from other (B - V) or (V - I) values are used for other purposes.

It is hard to visualize what exactly this means. To convert these into visual band extinctions by magnitude (i.e., how much fainter the object appears than it would be if well away from the dusty disc), use these conversions:

Multiply E(B - V) times 3.1 to get visual extinction. Example, an object of E(B - V) 1.23 will appear 3.87 magnitudes fainter than it really is.

Multiply E(V - I) times 2.3 to get visual extinction. An object of E(V - I) 1.23 will appear 2.83 magnitudes fainter.

I hope this is a good start to the thread. =Dana in S Africa

[madbadgalaxyman]

[note : the first sentence was not clear, before my edit ]

Schlegel et al. extinction values work very well , when we compare them to the
' baseline truth' which is the numerical value of extinction that is derived individually for each individual field;
it is a long time since I looked into exactly how this is done, but you derive the reddening value & the extinction value for a specific field of interest by comparing the 'intrinsic'(real)(unreddened) B-V or U-B colour of a star of a specific spectral type (which is derived from measuring the colors of several stars of the same type which are not very reddened and extincted) with the actual observed B-V or U-B colour of a star in the desired field of the same spectral type.

While extinction is not much of a concern for visual observers , except near to the 'great circle' of the Milky Way in the sky, the numerical value of the foreground extinction in front of a galaxy is certainly something you always need to plug into the magnitude equation, so as to figure out more accurate distances and luminosities for external galaxies.

From practical experience, I do find that the observed surface brightnesses of many individual galaxies are noticeably reduced by extinction, even at 10 degrees from the Galactic Plane; a good example is NGC 4945.

Fortunately for us, the Sun is not situated in a very dusty part of the Milky Way Galaxy (just by chance, the local interstellar medium is of low density), so there is very little extinction at and near the Galactic Poles (+/- 90 degrees from the observed Milky Way). There are dusty locations near to the plane of most non-dwarf Spiral Galaxies where there is 1-4 magnitudes of extinction even at right angles to the plane of a galaxy!

Robert

I shall shortly post on what observational data can be used to model the interstellar dust; I have a draft document, but need to check that it makes sense.

[Merlin66]

Dana,
Interstellar reddening is a issue for spectroscopy.
The Miles data base has both non-reddened and reddened info.

[madbadgalaxyman]

Yes, it's a "power law" particle size-distribution in that model.
[[
number of particles of a particular Grain Radius
is proportional to
the reciprocal of (the radius to the power of 3.5 )
]]
(within the applicable range of Grain Radii)

As the source of this result, Bruce Draine cites: Mathis et al, 1977, ApJ, 217, 425

There are a large number of observational constraints available for models of the interstellar dust grains.

Traditionally, the most important (and practically useful) observable is the observed 'extinction law' which is the graph of how the extinction/dimming (e.g measured in magnitudes) varies with the wavelength (or frequency) of observation. This was first observed by R.J. Trumpler in 1930, who has the immortal honour of proving that a general diffuse & dusty medium pervades our own Galaxy. (E.E. Barnard's results in the first years of the 20th C. were strongly suggestive, but did not actually prove the existence of a dimming agent in the interstellar medium.)

Bruce T. Draine, who is a good explainer (though he has a habit of explaining things at a level suitable for a really good physics graduate), gives a list of observations of the interaction of the interstellar dust with electromagnetic radiation. This is observational evidence that provides constraints on any model of the interstellar dust:
[ a very-heavy book, but a comprehensive reference on the ISM] [ the following is a direct quotation from Draine's book, slightly edited for clarity]

"


Wavelength-dependent attenuation ("extinction") of starlight by absorption and scattering, now observable at wavelengths as long as 20 microns and as short as 0.1 microns. The extinction includes a number of spectral features that provide clues to grain composition.
Polarization-dependent attenuation of starlight, resulting in wavelength-dependent polarization of light reaching us from reddened stars.
Scattered light in reflection nebulae.
Thermal emission from dust, at wavelengths ranging from 2 microns to sub-millimeter.
Small-angle scattering of X-rays, resulting in "scattered halos" around x-ray point sources.
Microwave emission from dust, probably from rapidly spinning ultrasmall grains.
Luminescence when dust is illuminated by starlight; the so-called extended red emission.


In addition to these electromagnetic studies, our knowledge of interstellar dust is also informed by other, less direct, evidence:



Presolar grains preserved in meteorites - a selective but not well-understood sampling of the interstellar grains that were present in the solar nebula, 4.5 billion years ago.



"Depletion" of certain elements from the interstellar gas, with the missing atoms presumed to be contained in the dust grains.
The observed abundance of molecular (diatomic) hydrogen gas in the Interstellar Medium, which can only be understood if catalysis on dust grains is the dominant formation avenue.
The temperature of interstellar HI (neutral atomic hydrogen) and interstellar Molecular Hydrogen gas, in part a result of heating by photoelectrons emitted from interstellar grains.

" (end of quotation)


There are very many Remarkable ideas hidden in the above list,
e.g. that molecular hydrogen, which is the basic source material for the formation of new stars, forms on interstellar dust grains. Indeed, the interstellar gas can only get cold enough to contract into new stars when it is shielded from starlight by the interstellar dust in dense and dusty regions of galaxies.

Cheers,
Robert