Free Astronomy Magazine March-April 2015

SPACE CHRONICLES from a distant quasar that lies be- hind the base of the northern bub- ble. Imprinted on that light as it travels through the lobe is informa- tion about the velocity, composi- tion, and temperature of the ex- panding gas inside the bubble, which only COS can provide. Fox's team was 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 travelling away. COS spectra show that the gas is rushing from the galactic cen- ter at roughly 2 million miles an hour (3 million kilometers an hour). "This is exactly the signature we knew we would get if this was a bi- polar outflow," explained Rong- mon Bordoloi of the Space Tele- scope Science Institute, a co-author on the science paper. "This is the closest sightline we have to the gal- axy's center where we can see the bubble being blown outward and energized." The COS observations also measure, for the first time, the composition of the material being swept up in the gaseous cloud. COS detected silicon, carbon, and alumi- num, indicating that the gas is en- riched in the heavy elements pro- duced inside stars and represents the fossil remnants of star forma- tion. COS measured the tempera- ture 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 de- grees Fahrenheit. "We are seeing cooler gas, perhaps interstellar gas in our galaxy's disk, being swept up into that hot outflow," Fox explained. This is the first result in a survey of 20 faraway quasars whose light passes through gas in- side or just outside the Fermi Bub- bles — like a needle piercing a balloon. An analysis of the full sam- ple will yield the amount of mass being ejected. The astronomers can then compare the outflow mass with the velocities at various loca- tions in the bubbles to determine the amount of energy needed to drive the outburst and possibly the origin of the explosive event. One possible cause for the outflows is a star-making frenzy near the ga- lactic center that produces superno- vas, which blow out gas. Another scenario is a star or a group of stars falling onto the Milky Way's super- massive black hole. When that hap- pens, gas superheated by the black hole blasts deep into space. Because the bubbles are short-lived compared to the age of our galaxy, it suggests this may be a repeating phenomenon in the Milky Way's history. Whatever the trigger is, it likely occurs episodically, perhaps only when the black hole gobbles up a concentration of material. "It looks like the outflows are a hic- cup," Fox said. "There may have been repeated ejections of material that have blown up, and we're catching the latest one. By study- ing the light from the other quasars in our program, we may be able to detect the fossils of previous out- flows." Galactic winds are common in star- forming galaxies, such as M82, which is furiously making stars in its core. "It looks like there's a link be- tween the amount of star forma- tion and whether or not these outflows happen," Fox said. "Al- though the Milky Way overall cur- rently produces a moderate one to two stars a year, there is a high con- centration of star formation close to the core of the galaxy." T his graphic shows how NASA's Hubble Space Telescope probed the light from a distant quasar to analyze the so-called Fermi Bubbles, two lobes of material being blown out of the core of our Milky Way galaxy. The quasar's light passed through one of the bubbles. Imprinted on that light is information about the outflow's speed, composition, and even- tually mass. The out- flow was produced by a violent event that hap- pened about 2 million years ago in our gal- axy's core. [NASA, ESA, and A. Feild (STScI)] n