What's the farthest thing you can see with the naked eye? For most of us that would be the Andromeda Galaxy, located 2.5 million light years away. But a new study shows that it may be a whole lot closer, maybe even close enough to rub shoulders with our own galaxy, the Milky Way.
Scientists using the Hubble Space Telescope have mapped an immense envelope of gas, called a halo, surrounding the Andromeda galaxy, our nearest large galactic neighbor. To their surprise they discovered that a tenuous cloud of ionized gas or plasma extends 1.3 million light years from the galaxy — about halfway to our own Milky Way — and as far as 2 million light-years in some directions. Because the Milky Way is a big spiral galaxy like Andromeda it likely to possess its own heavenly halo that extends a similar distance, implying the two galaxies may be literally touching at this very moment.
The Hubble data also revealed that Andromeda's halo has a layered structure with two distinct, nested shells. The inner shell extends about 500,000 light years beyond the main galaxy and has a complex, dynamic structure probably due exploding supernovae. The outer shell is smoother and hotter.
A supernova occurs when a massive star in the bright disk of the galaxy runs out of fuel at the end of its life. With no "fire" in its belly to beat back gravity's inexorable pull, the star implodes and then rebounds in a titanic explosion that rips it apart. The blast ejects star stuff into space at extremely high speeds and sends shock waves reverberating across the inner halo.
A key clue that supernovae are responsible for the inner halo's dynamic structure is the discovery of a large amount of heavy elements there. Stars fuse light elements such as hydrogen and helium to generate the energy to shine. When a star "cooks" lighter elements the ash left behind is made of heavier stuff like carbon and oxygen. Massive stars like Betelgeuse create even heavier materials all the way up to iron. During its violent death, these elements — and others created during the explosion itself such as uranium and gold — are blasted into space and "contaminate" the inner halo.
The Andromeda galaxy is a beautiful spiral galaxy like the Milky Way but nearly twice as large and possessing far more stars, up to 1 trillion! You can see the galaxy without optical aid on a dark, moonless night. It looks like a little, oval-shaped smudge of light not far from the familiar W of Cassiopeia. But if its gaseous halo could be viewed with the naked eye, it would be about three times the width of the Big Dipper and easily the biggest thing in the nighttime sky.
Although the illustration makes the halo look substantial it's made of extremely rarified gas that emits very little light. To map the halo's extent and structure astronomers used the light of 43 quasars — very distant, brilliant cores of active galaxies powered by black holes — located far beyond Andromeda. Looking through the halo at the quasars' light, the team observed how the light is absorbed by the gas and how the amount of absorption changes from place to place. Imagine someone swinging a flashlight beam in your direction through a cloud of smoke rising from a fire. As you followed the light with your eye you'd be able to make out the texture and thickness of the smoke.
Using quasars as flashlights astronomers detected the signature of ionized gas from carbon, silicon and oxygen in the ultraviolet (UV) light emitted by the quasars. An atom becomes ionized when radiation strips one or more electrons from it. Because UV is absorbed by the atmosphere, Hubble was uniquely suited to make this discovery because it orbits above it.
Because we live inside the Milky Way, scientists can't easily tease out the signature of our own galaxy's halo, but they believe the it must be similar to Andromeda's. While the two may be touching in the gentlest way imaginable now, they're on a collision course and ultimately will merge to form a giant elliptical galaxy beginning about 4 billion years from now.
Andromeda is unique because it's close by. It's much more difficult to probe the halos of more distant galaxies because they appear so much smaller in the sky — there aren't enough quasars in these tiny regions to map them.
"Understanding the huge halos of gas surrounding galaxies is immensely important," explained co-investigator Samantha Berek (Yale University). "This reservoir of gas contains fuel for future star formation within the galaxy. It's full of clues regarding the past and future evolution of the galaxy."You'll find the full (and free) report in the Aug. 27 Astrophysical Journal.
I like to think of Andromeda's touch as a prelude to the time when the two galaxies will be inseparable — a marriage that gravity will guarantee lasts a lifetime.