Quote:
Originally Posted by Weltevreden SA
Another question, Robert: Have there been secular extinction studies of the Magellanics? All the extinctions I came across referred to Galactic figures and were in the 0.35 to 0.45 range. Does anything like the Schlegel 1998 map set exist for the LMC-SMC? I wonder if we really know what we're seeing once outside the bar region. =Dana
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You are lucky I am on holiday in the last two days, so I have time to try to answer this question!
Because of the lower mass of "sub-giant" galaxies like M33 and the LMC and NGC 300 and NGC 7793 and NGC 1313, compared to the likes of luminous spiral galaxies like MW and M31 and M51 and M83, the significantly smaller and less luminous and less massive galaxies
invariably have a much
lower content of heavy elements in their interstellar medium than the bigger galaxies;
the enriched "heavy element" products of gas expulsion by supernovae and OB stars and Planetary Nebulae and Red Giant stars are presumably
expelled from small galaxies, as the energies in their ISM are large enough to presumably expel a lot of gas from a low mass galaxy (perhaps permanent expulsion)
So galaxies like the LMC and M33 and NGC 300 tend to
look nearly dust free..... because they
do in fact have little dust in their interstellar medium.
The effects of internal extinction from the host galaxy on the objects found within small galaxies are not entirely negligable, but they tend to be eclipsed by other larger sources of error in our measurements.
However, dust extinction (in this case I refer to the extinction due to the LMC itself, rather than the extinction due to the dust screen within our own Galaxy's disk component) across the face of the LMC is much better studied than in just about any other galaxy, because many of the basic "standard candles" (Cepheids, absolute magn. of the tip of the red giant branch, planetary nebulae luminosity function) for the extragalactic distance ladder are
calibrated using these objects in the LMC.
I wouldn't be too obsessed about extinction in the outermost parts of galaxies, if I were you;
it gets markedly less with progressively increasing galactocentric radius in a galaxy;
indeed, the outer disks of
even very massive and metal-rich spirals can be virtually transparent to light in the visual regime (but this is not the case for the inner disks of these galaxies)
[[ I have
massive numbers of papers on extinction within galaxies at home on my computer, but I am away from base at present; it is something I have often studied, in the practical sense of calculating luminosities and distances of extragalactic objects]]
When I googled on the search terms "extinction + LMC", I came up with plenty of papers and even calculators to estimate extinction. However, I do believe that a lot of the papers give the
total extinction (that from our Galaxy
plus that from the LMC.)
This is a recent study of extinction over the face of the LMC:
http://iopscience.iop.org/0004-637X/...1004.text.html
The mean
internal extinction for the LMC in the V-band was found to be only 0.3 to 0.4 magnitudes, and if LMC is like any other galaxy, then it will be less at large radii.
All of which is to say that the appearance of the LMC at visual wavelengths is not substantially falsified by the effects of dust within it.