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.

By: Julianne

Julianne — Wed, 17 Sep 2008 19:38:07 +0000

Region 1 is much further out in the galaxy, so there are simply fewer stars out there. There’s probably dust too, but I think lack of stars is the biggest reason it’s darker. In Region 2, there are two things going on, but I’m not sure which dominates. The first is spiral arms, which tends to cause stars to pile up into dense arms, making things dimmer between the arms. The other effect is dust, which also tends to go along with spiral arms (since the gas gets piled up as well). Some combination of those are causing the structure you see in the background galaxy.

By: Moh

Moh — Wed, 17 Sep 2008 19:07:18 +0000

Julianne, please click on the link on my name! There are two regions, 1 and 2. It seems that the density of stars is not much different (maybe I am wrong), however, it’s dark between the stars in region 1 and glowing in region 2. Based on what you’ve said, that’s not dust effect. so what is it?

Or, could it be that the star density is really different in 1 and 2, so that we do not get to have a good look at the region between stars in region 2 because of resolution restrictions?

By: Julianne

Julianne — Wed, 17 Sep 2008 18:33:47 +0000

Moh — The background galaxy also has dust within it, which can indeed produce grey features even without the foreground galaxy. The optical emission (i.e. anything that isn’t dark in the image) comes almost entirely from starlight. Dust does emit light too, but only at longer infrared wavelengths — so it glows, but not at wavelengths you can see here.

Not sure I actually answered your question though!

By: Moh

Moh — Wed, 17 Sep 2008 18:01:09 +0000

Julianne, a silly question! Isn’t the gray cloudy thingy existing allover the center of the galaxy also dust? Are there two kinds of dust one glowing and one isn’t? maybe I miss something…

By: Ali

Ali — Wed, 17 Sep 2008 14:20:43 +0000

Such a coincidence! I am just coming back from a lunch talk in which Benne was talking about his work!

By: Julianne

Julianne — Wed, 17 Sep 2008 13:57:50 +0000

Hi Eric — The dust is only a tiny fraction of a galaxy’s mass, so it wouldn’t influence a lensing map. For galaxies like this, dark matter dominates pretty much everywhere outside the center of the galaxy.

By: Sarah

Sarah — Wed, 17 Sep 2008 11:06:10 +0000

Read this post this morning, then by complete coincidence your collaborator Holwerda gave a lunch talk here on the galaxy pair. Very random!

By: Populär Astronomi - » Galaxskugga

Populär Astronomi - » Galaxskugga — Wed, 17 Sep 2008 08:56:44 +0000

[...] mer på engelska i pressmeddelandet eller på Cosmic Variance, en förträfflig [...]

By: eric gisse

eric gisse — Wed, 17 Sep 2008 08:12:41 +0000

I wonder what a weak lensing profile of that little system would look like.

Since dust is appearing to be slightly important, this could be a good way to not only _see_ the dust in a direct fashion but use lensing and other measurements to obtain the mass of the overall galaxy and get a good guesstimate on what the dust weighs. Or so goes the thinking.