Our Galaxy is Fatter Than Previously Thought

"We were tossing around ideas about the size of the Galaxy, and thought we had better check the standard numbers that everyone uses. It took us just a few hours to calculate this for ourselves. We thought we had to be wrong, so we checked and rechecked and couldn't find any mistakes."

A team of Australian astrophysicists pulled some readily available data off the internet, ran it through a spreadsheet model, and re-estimated the thickness of the 100,000 light year wide Milky Way Galaxy. The previously widely used estimate was that the Galaxy was 6,000 light years thick, but this new estimate suggests that it is twice that thickness.

The University of Sydney team's analysis differs from previous calculations because they were more discerning with their data selection. "We used data from pulsars: stars that flash with a regular pulse," Professor Gaensler explains. "As light from these pulsars travels to us, it interacts with electrons scattered between the stars (the Warm Ionised Medium, or WIM), which slows the light down.

"In particular, the longer (redder) wavelengths of the pulse slow down more than the shorter (bluer) wavelengths, so by seeing how far the red lags behind the blue we can calculate how much WIM the pulse has travelled through.

"If you know the distance to the pulsar accurately, then you can work out how dense the WIM is and where it stops - in other words where the Galaxy's edge is.

"Of the thousands of pulsars known in and around our Galaxy, only about 60 have really well known distances. But to measure the thickness of the Milky Way we need to focus only on those that are sitting above or below the main part of the Galaxy; it turns out that pulsars embedded in the main disk of the Milky Way don't give us useful information."

Choosing only the pulsars well above or below us cuts the number of measurements by a factor of three, but it is precisely this rejection of data points that makes The University of Sydney's analysis different from previous work.

My favorite part of this report is...

"Some colleagues have come up to me and have said 'That wrecks everything!'" says Professor Gaensler. "And others have said 'Ah! Now everything fits together!'"

And thus, science marches on.

The press report is here. The original results were presented last month at the Austin meeting of the American Astronomical Society.