Star formation drives huge bubbles in the Milky way
Bubbles' electrons move at 1,000km a second, cover 1/4 of Milky Way's diameter.
by Matthew Francis
Fermi gamma ray image of the huge bubbles originated at the center of the Milky Way. Follow-up radio and microwave observations revealed magnetic fields and ridgelike structures in these bubbles.
NASA/DOE/Fermi LAT/D. Finkbeiner et al.
In 2010, data from the Fermi Gamma-Ray Space Telescope revealed two huge gamma-ray-emitting bubbles extending 25,000 light-years in each direction from the Milky Way's center. Since this region of the galaxy is home both to a supermassive black hole and star formation activity, it was uncertain which of them produced the structures. A new analysis of radio and microwave observations has confirmed these bubbles exist - but found additional features suggestive of star formation, rather than black hole activity.
The new results looked at the polarization of the radio emission coming from the lobes. As reported by Ettore Carretti and colleagues, the polarized light revealed relatively strong magnetic fields. These are of a kind associated with rapid star formation, which is known to occur in the region 700 light-years around the galactic center. Additionally, the bubbles contain spiraling ridges that indicate fluctuations in the history of the star formation, which may be used to study the evolution of the Milky Way's nucleus over the last 10 million years.
Gamma rays and radio waves are on the opposite ends of the energy spectrum, but they are often produced by the same physical phenomena. However, they convey different information about their source. In particular, when electrons are accelerated by magnetic fields, they emit synchrotron radiation, radio light with a distinctive polarization signature - information not carried by gamma radiation.
The authors of the present study used data from the Parkes Radio Telescope in Australia and from the orbiting Wilkinson Microwave Anisotropy Probe (WMAP). All of these revealed lobes at the same location and of the same size as the Fermi gamma ray observations. Gamma ray, microwave, and radio emission from the same structure reveals the presence of a gas of electrons moving very rapidly, accelerated by magnetic fields. In this case, the data are consistent with electrons moving about 1000 km/s - fast enough to escape from the galaxy's gravitational pull. When background light collided with the energetic electrons, it was boosted to higher energies, explaining the gamma ray emission.
All three sets of data confirm that the bubbles are huge - each is about 1/4 of the diameter of the Milky Way - yet they originate in a tiny region at the center of the galaxy. In the absence of the radio and microwave data, astronomers and writers (including myself!) had speculated the lobes could be quasar jets from the Milky Way's central black hole. However, the distinctive magnetic field characteristics and ridge structures in the bubbles point to star formation as being the real culprit.
Intense star formation pumps a lot of matter into its surroundings, mostly in the form of electrons. It also results in strong magnetic fields, which carry energy outward. In the case of the Milky Way bubbles, the estimated magnetic fields radiate about 1032 Watts - a phenomenal amount of power by any standard, but consistent with the observed star formation rate within a 330 light-year radius of the galactic center.
The authors suggested the ridge features in the bubbles are the immediate output from the star formation. As these regions orbited the galactic center, their output spiraled, much like a lawn sprinkler. Over time, the entire bubble expanded, leaving the features seen by the Fermi observatory.
The researchers also proposed that the inflow of gas necessary to feed star formation may also be why the Milky Way's black hole is relatively quiet. Instead of landing on the black hole and producing jets, the gas is eaten up by new stars or blow out of the galaxy into the huge bubbles. The overall picture they present is consistent with the wealth of data from across the spectrum, and promises to clear up a number of questions.
Reader comments 9
zato_ichiWise, Aged Ars Veteran
I thought that Quasar jets only came from young black holes.
DermoplasmArs Scholae Palatinae
God is blowing Milk(y Way) Bubbles
TuishimiArs Centurion
There's a song in this, somewhere.
StuckInaCubeArs Scholae Palatinae
If that isn't fascinating, I don't know what is.
Great, you have me thinking of an enormous amnesiac fish swimming in circles through the galaxy and occasionally singing "Bubbles" (between the "swimming swimming swimming").
JarronArs Centurion
zato_ichi wrote:I thought that Quasar jets only came from young black holes.Yes, for the most part. I think he simply means a quasar-like jet (ie, it would be silly to suggest that the Milky Way is a quasar). The emission mechanism is the same, though: accretion of material onto the black hole. Even though the Milky Way's central supermassive black hole (SMBH) is considered to be "quiescent," there may still be some amount of matter being accreted onto the SMBH (likely supplied by the stellar winds from the massive stars orbiting the black hole). As this material is accreted, one might expect a polar jet composed of high-energy electrons.
Thanks a bunch (of bubbles)! I would never have thought star formation was the cause.But it clears up a lot for me personally. Electron flows that balances the solar wind proton flows; vigorous star formation in the pauses of the SMBH.So the spiral pattern would presumably be an imprint of the SMBH at work, and our "core engine" has some kind of cycle. Going further out on that precarious and untested limb, it could be a feedback oscillation.
AkaweSmack-Fu Master, in training
There's always some form of a jet as long as there is some matter being consume by a blackhole or any other star type. It differs on the amount, and hence, the type of jet phenomenon. It's been long known that star formation create a galactic shockwave. We are still save for some time but it's coming.
Torbjörn Larsson, OM wrote:So the spiral pattern would presumably be an imprint of the SMBH at work, and our "core engine" has some kind of cycle. Going further out on that precarious and untested limb, it could be a feedback oscillation.Um the article seems to say the spiral is simply a consequence of the motion these star-forming regions which are presumably not completely homogeneous. So you have various spot-sources of electrons moving in a circle, which creates a spiral in the resulting cloud. Makes sense to me.