Saturday, 16 February 2013

The Milky Way is on a diet

Watching the Russian meteor explosion sent a shiver down my spine. When I was young, I avidly devoured a magazine called The Unexplained, about the paranormal and mysterious events (although, as a good friend of mine said, the only unexplained thing is why I paid for the rag) and so was familiar with the Tunguska Event in 1908. The rock that exploded in 1908 was roughly 10x larger than yesterday's explosion, but seeing the effect of the shock-wave gives us a feeling of what Tunguska must have been like!

I wrote a little about a result by my student, Prajwal, on measuring the mass of the Milky Way, but we just wrote up a short media story on it. I've popped the entire story below. We're not expecting the story to be travel to far and wide, but it has appeared on the roof of the world, at Nepal News.

The Milky Way is on a diet

A team of University of Sydney astronomers, led by international PhD student, Prajwal Kafle, and his collaborators, Joss Bland-Hawthorn, Geraint Lewis and Sanjib Sharma have shown that the Milky Way is a lot slimmer than we previously thought.

Like all galaxies, the mass of The Milky Way Galaxy is dominated by an immense "dark halo" that can only be studied through detecting its gravitational pull, and our location, deep in the disk of the Milky Way, makes detecting this large scale pull very difficult.
Artists impression of the Milky Way Galaxy. The blue halo of material surrounding the galaxy indicates the expected distribution of the mysterious dark matter.(Credit: ESO/Calcada)
Artists impression of the Milky Way Galaxy. The blue halo of material surrounding the galaxy indicates the expected distribution of the mysterious dark matter.(Credit: ESO/Calcada)
The dark halo is comprised of dark matter, the dominant form of matter in the Universe, and in the early 1970s, Australian researcher, Professor Ken Freeman of the Mount Stromlo Observatory realised it actually dominates motion of individual stars, including our Sun. This work was recently rewarded with Professor Freeman receiving the Prime Minister's Prize for Science.
"Living inside the Milky Way provides some unique opportunities to estimate how much dark matter there is in the dark halo of a typical large spiral", said Professor Freeman.
The amount of dark matter that surrounds our Galaxy and how is it distributed became the target of Kafle's research.
"We still struggle to answer these questions," said Kafle
He called on a technique derived by British astronomer, James Jeans, in 1915, to attempt to weigh the amount of dark matter. While the approach is mathematically robust, it requires the detection of very distant stars in the halo.
"There were so many assumptions going into those calculations, I would take almost all the previous estimates that use this technique with a pinch of salt," said Kafle.
"I turned to a massive survey of the sky, the Sloan Digital Sky Survey, delivering an unprecedented view of our Milky Way. This magnificent survey means we can do away with many of the previous assumptions," said Kafle.
Professor Freeman adds "Kafle's new analysis uses robust techniques, and the sample of tracer stars is much larger than anyone has used before. His new estimate is a big step forward from previous work."
To make the calculation more accurate, Kafle used the speeds of the stars, as measured by the Doppler effect, to see how fast they are moving in the halo. With this, the Jeans's technique revealed that the mass of dark matter within 0.8 million light years radius of our Galaxy, is almost half the amount reported previously, weighing 1.2 trillion times that of the mass of the Sun.
While this represents one of the most accurate measurement, Kafle admits that "The uncertainties in the quoted number is still around 30%. But this is an indication of how well we understand the amount of mass in the Milky Way".
"This is challenging work", said Kafle's supervisor, Professor Geraint Lewis, "but it is great to uncover how much dark matter is there in the Milky Way one of the great astronomical mysteries."
Now, Kafle is looking forward to settle the controversy about the rotating stellar components in the Halo, where it appears that there are two populations of stars moving in opposite directions and its direct implication on the formation history of our Galaxy.
After 3 years of devoted work far far away from his home and family in Nepal, Kafle said, "It was worth traveling all the way from the roof of the world, Nepal, to the country with beaches, BBQs and snags. With icons of the Dark Force, Professor Freeman and Nobel Laureate Professor Brian Schmidt around, I am more motivated as I push our understanding one step further."
Kafle's candidature is supported by the University of Sydney International Scholarship.
Read the paper in The Astrophysical Journal at: http://iopscience.iop.org/0004-637X/761/2/98/ orastro-ph at: http://arxiv.org/abs/1210.7527

Report by Tom Gordon

2 comments:

  1. "The mass of dark matter within 0.8 million light years radius of our Galaxy, is almost half the amount reported previously, weighing 1.2 trillion times that of the mass of the Sun."

    I thought the amount of dark matter in the halo was previously established by the visible effect it had on the orbital velocities of stars at various radii from the nucleus. If the mass is halved, what effect does this new finding have on the cause of galactic rotation?

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    1. Excellent question. Roughly speaking, the Sun feels the gravitational pull to the mass *within* its orbit, and so its orbital speed depends on the mass within ~8.5 kiloparsecs from the centre of the galaxies. However, the dark matter halo, the dominant mass of the Milky Way, may extend out to 150-250 kiloparsecs, and even though the density may be rapidly falling off, there is a lot of mass out there. So, to get the *total* mass of the Milky Way, we need things *out there*, such as the stars we looked at.

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