Alright, startswithabang-ers, Ben, my most avid commenter, saw me online while I was eating breakfast this morning, and pointed me to this new press release. Now, before you get started clicking on everything, the guy who the release is about is Brian Gaensler, who’s a really nice guy, lives in Australia, whom I met at the AAS (American Astronomical Society) meeting in Austin, TX last month. Bryan’s also brilliant.

Basically, what he did was he said, “well, we know what the rough density of hot gas in our galaxy is, and we can measure the timing of these pulsars to extraordinary accuracy.” And that’s true. What he then did was he realized that light of different wavelength gets affected differently by the gas. By realizing just how much, and calculating the arrival time difference between different wavelengths, he figured out how much gas these pulses traveled through, and therefore how far away they are. Now, here’s where it gets interesting (from the press release):

Astrophysicist Professor Bryan Gaensler led a team that has found that our galaxy – a flattened spiral about 100,000 light years across – is 12,000 light years thick, not the 6,000 light years that had been previously thought.

What does this mean? It means our galaxy is twice as thick as we thought! “Our galaxy” means the part of it where gas and stars are, or the size of the galactic disk. Learning new stuff every day.

Now, Ben remembered that I happen to be the world expert (somehow) on using pulsar timing to search for dark matter. And so he asked me this:

I’m guessing since missing galactic mass is one of the motivations for dark matter that this might revise our thinking on that somewhat, too? Maybe help figure out where and what it is?

Does this help me? Unfortunately, no. The pulsars that he used to do this are the ones found in globular clusters.

While there are plenty of them, globular clusters are too dense for me to use them to search for dark matter; very dense environments mess up precision timing, since the gravitational signature of the nearby stars would swamp any gravitational signal from dark matter. How dense are globular clusters? If I drew a sphere 4 light-years in radius, centered on our Sun, I would come almost to the next closest star. But if I did this to a star in a globular cluster, I would find that there are up to 1,000,000 other stars in my sphere! That, my friend, is dense!


  1. #1 benhead
    February 20, 2008

    I was actually wondering about its implications more generally than for your pulsar-timing method for searching. If the galaxy is bigger than previously believed, is it also more massive? And if it’s more massive, does that change the “missing mass” calculations that motivate the dark matter search in the first place? Does it change calculations about where dark matter is, how much there is, etc, and if so, does it change them in a way that might help us to pin down just WHAT it is?

  2. #2 ethan
    February 20, 2008

    Nope. It just means that the galactic disk is thicker, in terms of how much gas, stars, and dust is in it, than we thought. It doesn’t change the “missing mass” problem any more than it would change Earth’s orbit if we doubled its mass but kept its velocity the same. In other words, we still need all the same dark matter junk to make the disk rotate as it does.

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