From Dwarf Planets to Hobbit Galaxies

dwarf galaxyI learned from an article in New Scientist by David Shiga that there have been recently found four more small satellite galaxies of our Milky Way galaxy.

The satellites are dwarf galaxies a few hundred to a few thousand light years across. The tiny galaxies are thought to be the building blocks of large galaxies, such as our own Milky Way – which is about 100,000 light years wide.

As you may know, we’ve known for some time that there have been such satellites (the number knwon has gone from 10 to 20 in the last two years, and some models expect as many as 50), but the small ones are very hard to detect. How do you distinguish them from other stars in the way? As part of the Sloan Digital Sky Survey (SDSS), the researchers have been looking for….

the particular types of stars expected to lie in dwarf galaxies, then detect the dwarfs as slight “overdensities” in these types of stars – patches of the sky where there are more of the stars than in surrounding areas.

The Cambridge team named them after the constellations in which they were found:

[...] Coma Berenices, Canes Venatici II, Hercules, and Leo IV, all of them lie between roughly 100,000 and 500,000 light years from Earth.

[...]The largest and smallest are Hercules and Coma Berenices, which are about 1000 and 200 light years across, respectively. Like most of the other dwarfs discovered by SDSS, the new finds are much smaller and fainter than the 10 dwarfs that were known previously, [Vasily] Belokurov [the team leader] says. “They should not really be called dwarfs – they are more like hobbits,” he told New Scientist.

Hobbits. Right. Does that make the Milky Way a Cave Troll? Or maybe a Numenorian? (Picture above is one of them. This one’s Farmer Maggot, I think. They’re not as obvious as that… the foreground stars have been removed.)

Sure this stuff is fun, but what’s the larger point, you might ask? Well, just as the discussion about Pluto’s status as a planet (see also here) touches on hugely important issues such as formation and evolutions of planetary systems, so the satellite galaxy issue touches on similar questions for galaxies (although it I were them I’d not take too seriously being called galaxies, in case the name is snatched away from them one day… poor things.).

Once you get to those concerns, you quickly realise that this is probably more indirect information about the nature of Dark Matter, whose properties -given how much of it there is in galaxies- is intimately involved in the details of formation and evolution of galaxies. Recall my post on computer simulations of structure formation from last year? See also Risa Weschsler’s home page for a lot of information and some lectures on that sort of approach.

This sort of observational information about the details of how the structure is distributed, coming from the SDSS is crucial for pinning down some of the properties of Dark Matter, by allowing the outcomes of computer simulations (where you input the Dark Matter properties at the start and then let the universer evolve) to be checked against what it’s really like out there from real observations of the sky (see whether your input properties were reasonable or not).


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