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George
2014-Feb-25, 10:38 PM
I am curious what is the favored current dust percentage for clouds and galaxies in the ISM (interstellar medium).

Gerrit Verschuur's book, Interstellar Matters, is a bit dated but it was helpful and enjoyable (even the title is a pun). He gave a 1% figure for the amount of dust in galaxies. The dust being found on the edge of spiral arms.

The next book, by coincidence, is a newer book (2008), Shrouds in the Night, by Block & Freeman, who were part of the team that revealed that the galactic dust is far more ubiquitous with 9 or 10 times as much matter in galaxies than had been seen before (thanks to IR studies, of course). I think this was in 1994.

They used a 5% figure for the amount of dust, though this was stated for GMCs (Giant Molecular Clouds).

This seems higher than I have seen before or expected, so am I missing something? Do GMCs have this much dust and is this true for all the ISM. [Just seems a lot more than medium, pun, yes.]

ngc3314
2014-Feb-25, 11:03 PM
A gas/dust ratio near 160 by mass still seems to apply to environments like ours (outer disks of luminous spirals, based as well on absorption studies which do not have the temperature favoritism that's an issue in the far-IR. This could be determined for, among others, the foreground spiral in a silhouetted galaxy system whose ID is on the tip of my tongue. The potentially missing piece would be dust clumps which are both very cold and so small that they leave essentially no absorption signature, but we would recognize these in the Milky Way where there is nearly continuous background illumination (as in large nebulae). The dust fraction does seem to vary with galaxy luminosity, which makes sense since the heavy-element abundance changes and that must be reflected in the gain population.

George
2014-Feb-26, 03:46 AM
A gas/dust ratio near 160 by mass still seems to apply to environments like ours (outer disks of luminous spirals, based as well on absorption studies which do not have the temperature favoritism that's an issue in the far-IR. Ok, and it feels right given stellar compositions for Pop I, too.

I see my mistake -- one too often when shooting from the hip (memory) during a work break. The 5% was not a dust amount at all (ug!) but the percent of the total luminosity that comes from the young Population I stars, which masks the spiral arms, blinding us optically to the other 95%. [Looks like big NGC309 was their breakthrough case in a composite.]


This could be determined for, among others, the foreground spiral in a silhouetted galaxy system whose ID is on the tip of my tongue. It is unwise to lick the screen, though it keeps getting more scientifically appetizing, admittedly.


The dust fraction does seem to vary with galaxy luminosity, which makes sense since the heavy-element abundance changes and that must be reflected in the gain population. That's logical given the need for carbons and silicate needed to give stuff a place to make dusty. :) I would guess large spirals would favor a higher dust to gas ratio assuming many firey visitors that often never leave.

Thanks

Van Rijn
2014-Feb-26, 09:35 AM
This reminded me of a somewhat related question I'm asking over here:

http://cosmoquest.org/forum/showthread.php?149545-What-percentage-of-ISM-metallicity-is-NOT-due-to-supernovae&p=2196729#post2196729

Along with the general question about metallicity, I'm wondering how much of that dust isn't due to supernovae.