'Super' Dwarf Galaxies Protected by Dark Matter Force Fields

The scoop: Dwarf galaxies 250 million light-years away are surviving the gravitational torture of larger galaxies, which are themselves being ripped to shreds.

What's protecting these tiny, ancient clouds of stars? We chat with University of Nottingham astrophysicist Chris Conselice to find out.

Dave on Earth (11:09 AM): Hello Dr. Conselice -- or do you prefer Chris?

DwarfGalaxyGuy (11:10 AM): Chris is fine.

Dave on Earth (11:11 AM): Excellent.
Before we dive into things such as dark matter, mysterious galaxies, etc., I wanted to ask you what the Canary Islands are like.
You said in your e-mail that you're out there doing some astronomy?

DwarfGalaxyGuy (11:12 AM): The Canary Islands are pretty nice, both as a place to visit and to do astronomy.
And yes, I'm at the William Herschel telescope examining the star formation rates of distant galaxies.

Dave on Earth (11:14 AM): Speaking of distant galaxies, you and some of your astronomer friends just published a paper...
You found something really unusual going on with old dwarf galaxies? Or should I say something not going on?

DwarfGalaxyGuy (11:16 AM): Correct. Using the Hubble Space Telescope, we examined dwarf galaxies in the nearby Perseus cluster.
We found that these galaxies were ALL smooth and symmetric without any signs of tidal disturbances -- yet they live in one of the highest density regions a galaxy has been found within.
The only conclusion that's possible to reach: these galaxies are embedded in large dark matter halos, which allow them to survive.

Dave on Earth (11:17 AM): Ok, lots to talk about there!

DwarfGalaxyGuy (11:17 AM): Yes, sorry!

Dave on Earth (11:17 AM): No worries -- when I get excited about stuff, my mouth/fingers also go a million miles an hour.
Dwarf galaxies: What makes one?
And you mention "smooth" -- do you mean the stars are all pretty uniform?

DwarfGalaxyGuy (11:21 AM): Dwarf galaxies are defined in a number of ways, but they're all low-mass galaxies, and usually they contain a low surface density of stars.
In other words, they're "fluffy."
We don't really know what makes them like that -- but they are likely amongst the oldest galaxies in the universe.
Oh, and by smooth I mean these galaxies have no sub-structure. They're symmetrical blobs. Like all galaxies they are brighter at their centers, but they are uniform in brightness at a given radius.

Dave on Earth (11:22 AM): So big, uniform fuzz balls of stars, then.

DwarfGalaxyGuy (11:22 AM): Exactly.

Dave on Earth (11:23 AM): Next order of business: Dark matter haloes.
So dark matter hangs out near the edges of galaxies?

DwarfGalaxyGuy (11:25 AM): That's right -- although there is dark matter in the inner parts as well.
You just see the effects more in the outer parts because there are few to no stars out there, and there's a larger volume -- dark matter is thought to be relatively uniform, so more volume gives you more dark matter.
The behavior dark matter originally was "invented" for was to explain why galaxy motions didn't slow when there are fewer stars.

Dave on Earth (11:26 AM): I see -- there has got to be more mass there to account for how galaxies behave, hence dark matter.

DwarfGalaxyGuy (11:27 AM): Right. It took a long time for the idea to be generally accepted, but that was the idea from the start.

Dave on Earth (11:27 AM): So you found some old dwarf galaxies out there among some heavyweight galaxies, but somehow they're pretty unaffected
Almost like they have a "shield" of dark matter protecting them from larger, hungrier galaxies?

DwarfGalaxyGuy (11:28 AM): That's right -- it's almost as if they don't notice!

Dave on Earth (11:28 AM): Were you surprised when you figured this out?

DwarfGalaxyGuy (11:29 AM): You bet. The beauty of this is that it is independent of galaxy kinematics, which some believe doesn't give correct measurements of galaxy mass.
I was surprised to find that they were SO smooth, but we had some ideas from previous data that these guys had to have a high ratio of dark matter.

Dave on Earth (11:30 AM): Hmm. So there are there other ideas out there to explain what you saw?
Or is dark matter pretty much the guy sticking out of the line-up here?