Hubble and the Giant Bubbles of Gas

An artist's concept of the double lobes of gas expanding from the center of the Milky Way Galaxy. NASA/STScI

It's an ancient galactic mystery with a modern explanation: two million years ago, something happened at the center of our Milky Way galaxy. Something energetic. Something that sent two huge bubbles of gas billowing out to space. Today, they stretch out across more than 30,000 light-years of space, extending above and below the plane of the Milky Way. No one was around to see it then -- at least no humans on Earth. Our earliest primate ancestors were just learning to walk upright, and astronomy was not likely on their list of activities.

So, this major explosion went unnoticed. Yet, it was a titanic event, driving gases and other material outward at two million miles per hour, didn't affect our plane then and it won't likely affect us in the future. However, it does show us what happens when a massive explosion occurs some 25,000 light-years away from our planet.

Hubble Sleuths the Cause of the Explosion

Astronomers used Hubble Space Telescope to look through one lobe of the bubbles toward a very distant quasar. That's a galaxy that is very bright in both visible and other wavelengths of light. The quasar's passed through the bubbles of gas, which allowed Hubble to peer inside the bubble to learn more about it— like looking at a distant light shining through a fog bank. 

The enormous structure illustrated in this image was discovered five years ago as a gamma-ray glow on the sky in the direction of the galactic center. The balloon-like features have since been observed in x-rays and radio waves. The Hubble Space Telescope presented a good way to measure the velocity and composition of the mystery lobes. With the data from HST, astronomers will work on calculating the mass of the material being blown out of our galaxy. That might also let them figure out just what happened to send all this gas billowing out of the galaxy in the first place.

What Caused this Massive Galactic Explosion?

The two most likely scenarios that explain these bipolar lobes are 1) a firestorm of star birth at the Milky Way's center or 2) the eruption of its supermassive black hole

This isn't the first time that gaseous winds and streams of material have been seen coming from the centers of galaxies, but it's the first time astronomers have detected evidence for them in our own galaxy. 

The giant lobes are called Fermi Bubbles. They were initially spotted using NASA's Fermi Gamma-ray Space Telescope to track gamma-rays. These emissions are a powerful clue that a violent event in the galaxy's core aggressively launched energized gas into space. To provide more information about the outflows, Hubble's Cosmic Origins Spectrograph (COS) studied the ultraviolet light from a distant quasar that lies beyond 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 COS data show that the gas is rushing from the galactic center at roughly 3 million kilometers an hour (2 million miles an hour). of the gas at approximately 17,500 degrees Fahrenheit, which is much cooler than most of the 18-million-degree gas in the outflow. This cooler gas means that some interstellar gas could be getting caught up in the outflow. 

COS observations also reveal that the clouds of gas contain the elements silicon, carbon, and aluminum. These are produced inside stars. 

Does this mean that star formation or star death is involved in the original event that formed the bubbles? Astronomers think that one possible cause for the outflows is a star-making frenzy near the galactic center. Eventually, those hot, young massive stars die in supernova explosions, which blow out gas. If a lot of them exploded at once, it might spur the formation of a huge gas bubble. 

Another scenario has a star or a group of stars falling onto the Milky Way's supermassive black hole. When that happens, gas superheated by the black hole blasts deep into space and that could be what filled out the bubbles. 

Those bubbles are short-lived compared to the age of our galaxy (which is more than 10 billion years old). It's possible that these aren't the first bubbles to billow out from the core. It could have happened before. 

Astronomers will continue to look at these bubbles using distant quasars as "illuminators", so it might not be too long before we hear just what it was that caused a huge commotion at the heart of the Milky Way Galaxy. They may well also be interested in studying smaller such bubbles that form as a result of supernova explosions and the actions of hot young stars. Such bubbles actually work to protect systems encased within. One example is the Local Interstellar Cloud, which encases the solar system today. In a few tens of thousands of years, the Sun and planets will move outside of it, exposing our system to radiation levels it hasn't experienced for a long time.