Astronomers using NASA’s Hubble Space Telescope have measured for the first time the velocity and composition of giant, balloon-like features known as the Fermi Bubbles.
This graphic shows how Hubble probed the light from the quasar PDS 456 to analyze the Fermi Bubbles. The quasar’s light passed through one of the bubbles; imprinted on that light is information about the outflow’s speed, composition, and eventually mass. Image credit: NASA / ESA / A. Field, STScI.
Fermi Bubbles – clouds of gas towering about 30,000 light-years above and below the plane of our Milky Way Galaxy – were discovered in 2010 by NASA’s Fermi Gamma-ray Space Telescope.
The detection of high-energy gamma rays suggested that about 2 million years ago a massive eruption in the galactic core drove gases and other material outward at several million km per hour.
Scientists proposed two possible scenarios: a firestorm of star birth at the Galactic center or the eruption of the Galaxy’s supermassive black hole.
Although they have seen gaseous winds, composed of streams of charged particles, emanating from the cores of other galaxies, they are getting a unique, close-up view of our Galaxy’s own fireworks.
“When you look at the centers of other galaxies, the outflows appear much smaller because the galaxies are farther away,” said Dr Andrew Fox of the Space Telescope Science Institute in Baltimore, Maryland, who is the lead author of a paper accepted for publication in the Astrophysical Journal Letters (arXiv.org preprint).
“But the outflowing clouds we’re seeing are only 25,000 light-years away in our galaxy. 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.”
To provide more information about the Milky Way’s Fermi Bubbles, Dr Fox and his colleagues the United States, Italy and Australia used Hubble’s Cosmic Origins Spectrograph (COS) to probe the UV light from the distant quasar PDS 456 that lies behind the base of the northern bubble.
Imprinted on that light as it travels through the lobe is information about the velocity, composition, and temperature of the expanding gas inside the bubble, which only COS can provide.
The astronomers were able to measure that the gas on the near side of the bubble is moving toward Earth and the gas on the far side is traveling away.
COS spectra show that the gas is rushing from the Galactic center at roughly 3 million km per hour.
The observations also measure, for the first time, the composition of the material being swept up in the gaseous cloud.
Hubble’s spectrograph detected silicon, carbon, and aluminum, indicating that the gas is enriched in the heavy elements produced inside stars and represents the fossil remnants of star formation.
The instrument measured the temperature of the gas at approximately 17,500 degrees Fahrenheit, which is much cooler than most of the super-hot gas in the outflow, thought to be at about 18 million degrees Fahrenheit.
“We are seeing cooler gas, perhaps interstellar gas in our Galaxy’s disk, being swept up into that hot outflow,” Dr Fox said.
The team now seeks to calculate the mass of the material being blown out of our Galaxy, which could lead them to determine the outburst’s cause from two competing scenarios.
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Andrew J. Fox et al. 2015. Probing the Fermi Bubbles in Ultraviolet Absorption: A Spectroscopic Signature of the Milky Way’s Biconical Nuclear Outflow. ApJ Letters, accepted for publication; arXiv: 1412.1480
