Over the past two years I’ve been working on a ginormous project archiving imaging data of nearby galaxies using the Hubble Space Telescope. We had taken a bunch of wide-field multi-color panoramas of some lovely nearby galaxies before ACS (the spiffy optical imaging camera) failed, and while looking at one of the outer regions of NGC 253, my collaborator Ben Williams noticed this peculiar pair of galaxies lurking in the background. On an image from the ground, the galaxies looked like:
But in the HST image, now released by the Hubble Heritage team, the galaxies looked like:
What’s striking about this pair of galaxies is that one of them is partially occluding the other. Galaxies contain a lot of dust which can block light, particularly at optical and bluer wavelengths. Because the background galaxy is relatively smooth, you get a wonderful view of the location of dust in the foreground galaxy. It’s clear from the image that the dust is found waaaaaay out from the center of the galaxy, in spiral arms that go well past one would call the edge of the galaxy (based on where you see light on the un-occulting side of the foreground galaxy, to the right on the image above). It’s rare that you ever find two galaxies of comparable size occulting one another, and rarer still that the alignment is so perfect for tracing out the dust to such large radii. Previous ISO studies showed that there was likely to be dust at large radii, but its signature could only be detected by stacking images of many different galaxies to build up sufficient signal-to-noise. Here, you actually get to see the presence of dust directly!
Now, you may not be wired to care about dust. But you should. The key to thinking about dust in this context is that dust extinction is an excellent tracer of the presence of molecular gas, as you can see in this beautiful animation from the millimeter wave group at the CfA, which blinks between an optical image of the Milky Way, and a map of the Milky Way’s molecular gas in the same region:
Given the correlation between dust extinction and molecular gas, the HST image shows that molecular gas is probably present at very large radii, and that it’s still in spiral arms, just as you’d expect. Since molecular gas is the immediate precursor to star formation, this potentially helps to understand where the isolated spots of star formation seen in outer disks are coming from.
In addition, collaborators Benne Holwerda (who’s lead author) and Bill Keel have developed all sorts of clever tricks for measuring the reddening law for this dust, which can potentially give constraints on what the dust is made of, and how it might be affecting other Big Important Measurements like high-z supernovae.
All in all, a pretty cool image, with lots of nifty science behind it.
September 17th, 2008 at 3:12 am
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.
September 17th, 2008 at 3:56 am
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September 17th, 2008 at 6:06 am
Read this post this morning, then by complete coincidence your collaborator Holwerda gave a lunch talk here on the galaxy pair. Very random!
September 17th, 2008 at 8:57 am
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.
September 17th, 2008 at 9:20 am
Such a coincidence! I am just coming back from a lunch talk in which Benne was talking about his work!
September 17th, 2008 at 1:01 pm
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…
September 17th, 2008 at 1:33 pm
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!
September 17th, 2008 at 2:07 pm
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?
September 17th, 2008 at 2:38 pm
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.
September 18th, 2008 at 3:08 am
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.
September 18th, 2008 at 11:31 pm
Whats the approximate distance between those two objects?
September 19th, 2008 at 12:21 am
Haelfix — We have no idea. I suspect they’re at comparable distances (i.e. a loose binary pair)
September 19th, 2008 at 9:08 am
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.
September 25th, 2008 at 5:10 pm
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?
September 25th, 2008 at 5:32 pm
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.
September 25th, 2008 at 7:36 pm
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)
September 25th, 2008 at 10:50 pm
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.
September 25th, 2008 at 11:32 pm
What’s a reddening law?
September 27th, 2008 at 4:27 pm
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.