Three million years ago: Three million km/h

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  • Published: Jan 8, 2015
  • Author: David Bradley
  • Channels: UV/Vis Spectroscopy
thumbnail image: Three million years ago: Three million km/h

An outburst of galactic proportions

The Hubble Space Telescope probed the light from a distant quasar to analyse our galaxy's Fermi Bubbles (Credit: NASA, ESA, and A. Feild (STScI))

The Hubble Space Telescope's Cosmic Origins Spectrograph (COS) has probed ultraviolet light from a distant quasar to investigate the "bubble" of cosmic debris formed by a 3 million kilometres per hour outburst from our galaxy, the Milky Way, that occurred between 2.5 and 4.0 million years ago.

Our Milky Way galaxy underwent a gigantic eruption, spewing gas and other materials outwards in two plumes at an estimated speed of more than 3 million kilometres per hour (2 million mph). The aftermath of this eruption exists today as two clouds of gas reaching out from the galactic plane some 30 000 light years above and below the spiral. The structure was discovered only five years ago evidenced by an afterglow of energetic gamma rays in the direction of the galactic centre and subsequently observed in the X-ray and radio regions of the electromagnetic spectrum. However, astronomers have only now obtained sufficient fine detail, thanks to NASA's Hubble Space Telescope, to give them a measure of the velocity and composition of the eruptive lobes. Moreover, they are currently calculating the mass of the material being blown out of the galaxy, in the hope of determining the cause of this galactic outburst and pinning the blame squarely on one of several competing explanations.

Bipolar eruption

There are two tenable explanations for the Milky Way's bipolar lobes. The first is that a firestorm of star birth at the galaxy's centre reached critical proportions. The second suggests that the some kind of disturbance or perturbation caused by the supermassive black hole at the centre of the galaxy is to blame, perhaps as stars plummet into the black hole. Astronomers have had a sniff of gaseous wind composed of streams of charged particles emanating from the core of other galaxies, but this is the first close-up view of our galaxy's own eruptive activity.

"The outflowing clouds we're seeing are only 25 000 light-years away in our galaxy," explains Andrew Fox of the Space Telescope Science Institute in Baltimore, Maryland. "We have a front-row seat. We can study the details of these structures. We can look at how big the bubbles are and can measure how much of the sky they are covering." Fox and colleagues presented their findings at the American Astronomical Society meeting in Seattle, Washington and will also publish details in a forthcoming issue of Astrophysical Journal Letters, currently available as an "arXiv" preprint.

The giant galactic lobes, are known as Fermi Bubbles, as they were first detected by NASA's Fermi Gamma-ray Space Telescope. The detection of high-energy gamma rays suggested that a violent event in the galaxy's core ejected the highly energized gas into space. Fox and colleagues have used Hubble's Cosmic Origins Spectrograph (COS) to probe the ultraviolet light from a distant quasar that lies behind the base of the northern bubble to get a clearer perspective on the chemical and physical nature of the Fermi Bubble - the velocity, composition and temperature.

Fermi question

The researchers have demonstrated that the gas on the near side of the bubble is moving towards the Earth and the gas on the far side is travelling away from us. "This is exactly the signature we knew we would get if this was a bipolar outflow," explains co-worker Rongmon Bordoloi of the Space Telescope Science Institute. "This is the closest sightline we have to the galaxy's centre where we can see the bubble being blown outward and energized."

The COS observations also reveal the presence of aluminium, carbon and silicon in the Fermi Bubble thus suggesting that the gas with such a heavy element enrichment is the fossil remains of star formation. The gas is at almost 10 000 Celsius, whereas the temperature of the super-hot gas of the outflow is at about 10 million Celsius suggesting that the cooler gas, is perhaps interstellar gas being sucked into the hot outflow.

"It looks like the outflows are a hiccough," Fox explains. "There may have been repeated ejections of material that have blown up, and we're catching the latest one. By studying the light from the other quasars in our program, we may be able to detect the fossils of previous outflows."

"The next step is for us to analyse over twenty quasar directions in our Hubble Space Telescope sample," Fox told spectroscopyNOW. "This first paper from our program focuses on the quasar direction closest to the Galactic centre, where the wind is launched. The other directions extend further up into the Fermi bubbles and outside them. We need to see how the wind properties change with position. Ultimately, we aim to determine the energetics of the galactic wind - how much energy is being carried away from the Galactic centre in the outflow?  This may help us to understand whether the outflow is driven by nuclear star formation or by the supermassive black hole."

Related Links

Astrophys J Lett, 2015, in press: "Probing the Fermi bubbles in ultraviolet absorption: a spectroscopic signature of the milky way’s biconical nuclear outflow"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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