Comments on: A Galaxy Silhouette

By: mollishka

mollishka — Sat, 27 Sep 2008 21:27:27 +0000

Lab Lemming: Dust tends to “redden” light; longer-wavelength (i.e., redder) light will pass through dust more easily than bluer light; so if you shine “white” light through a bunch of dust, it will appear red. A reddening law is just the relationship between the wavelength of light and the typical change in magnitude of the light passing through the galaxy. Like Julianne said, this relationship varies depending on what kind of dust is doing the absorbing/scattering of the light, so we can use it to learn about the properties of the dust itself.

By: Lab Lemming

Lab Lemming — Fri, 26 Sep 2008 04:32:09 +0000

What’s a reddening law?

By: Julianne

Julianne — Fri, 26 Sep 2008 03:50:19 +0000

We’ve measured the average reddening law for the dust (which turns out to be not quite the same as the Milky Way), which can give you some constraints on composition — PAH’s have different scattering/absorption behaviors than larger silicate grains.

By: Lab Lemming

Lab Lemming — Fri, 26 Sep 2008 00:36:34 +0000

Can you get compositional information on the dust? Perhaps from a closer galaxy than this one… (I should confess that the only thing I know about dust comes from presolar grains in meteorites- which are all refactory due to preservation selection bias)

By: Julianne

Julianne — Thu, 25 Sep 2008 22:32:09 +0000

LL — Where that dust came from is an extremely interesting question. We think there is a fair bit of processing of gas through “galactic fountains”, where gas is blown out of the galaxy and then falls back down onto it. Since the dust is probably made primarily in stellar winds, and then is coupled to the gas, it can form in the inner regions, and then circulate out to large radii when dragged along by the “fountain”. The radius where it peters out might give you a limit on how far out the fountain throws material. Interesting thing to think about. There are also stars at large radii, though not so many — I’m not sure if there are enough to produce the needed dust.

By: Lab Lemming

Lab Lemming — Thu, 25 Sep 2008 22:10:55 +0000

So is the dust made in the middle and pushed out, or was it made by stars that have since died, or are the stars out there simply too dim to see?

By: TomC

TomC — Fri, 19 Sep 2008 14:08:31 +0000

That is a truly fantastic image — and comparing it with the ground-based data just makes it even more impressive. COBE and WMAP may have contributed more to fundamental physics, but HST has to be the finest astronomical instrument we’ve ever had.

By: Julianne

Julianne — Fri, 19 Sep 2008 05:21:07 +0000

Haelfix — We have no idea. I suspect they’re at comparable distances (i.e. a loose binary pair)

By: Haelfix

Haelfix — Fri, 19 Sep 2008 04:31:31 +0000

Whats the approximate distance between those two objects?

By: swety

swety — Thu, 18 Sep 2008 08:08:11 +0000

The article is so informative. In future I want to study on ASTRO PHYSICS. So this type of article can help me to grow up my knowledge. The picture is superb. Thank you julianne.