How did our galaxy form? The short answer is “galaxy collisions”. That keeps astronomers busy as they develop new tools to “look back” at the birth of our own galaxy. We know that the infant Milky Way began taking shape relatively early in the history of the universe. It probably existed for quite some time as a tiny dwarf galaxy. Then, it began to merge with others to “plump up” and grow.

Today, the Milky Way continues to dance the galaxy formation tango with other dwarf galaxies, such as the Sagittarius Dwarf. And, we know that it will eventually do an interactive dance with the Andromeda Galaxy. That series of galaxy collisions and interactions will happen several billion years from now. For now, it seems, that our galaxy is still forming through mergers. And, astronomers have some interesting tools to probe the details of the stars that got brought into the Milky Way.

The First Big Galaxy Collision

A couple of weeks ago, Ohio State University researchers released new information about the earliest stages of the Milky Way’s birth and growth. It seems the young Milky Way experienced a major merger with an orbiting satellite galaxy called Gaia-Enceladus. That happened about 10 billion years ago and influenced the distribution of stars in the galaxy.

Fiorenzo Vincenzo, who works at OSU’s Center for Cosmology and Astroparticle Physics, and a team of astronomers, wanted to know exactly what happened as a result of that merger. Specifically, what can the stars of our galaxy tell us about that ancient event?

They used several techniques to answer that, including asterochronology, asteroseismology, and a spectroscopic survey called APOGEE. Asterochronology studies the ages of stars to trace the basic “mergers and acquisitions” that helped form the Milky Way. Asteroseismology looks inside stars by studying their oscillations to get precise ages. The spectroscopic studies reveal the chemical compositions of stars. The combination of all three approaches gives amazingly good estimates for the ages and origins of stars in our galaxy.

“Our evidence suggests that when the merger occurred, the Milky Way had already formed a large population of its own stars,” said Vincenzo.

By calculating the ages of stars throughout our galaxy, the researchers determined something interesting. The stars captured from Gaia-Enceladus have similar or slightly younger ages than the majority of stars that already existed there.

During the merger, many of the infant Milky Way’s batch of “homemade” stars ended up in the “thick disk” that occupies the middle of the galaxy. The stars that were captured from Gaia-Enceladus during the merger exist mostly in the outer halo of our galaxy.

What Galaxy Collisions Change

Today, we know that most galaxies grow through mergers and acquisitions. So, it seems like a fairly normal occurrence in the universe. The merger of the baby Milky Way with Gaia-Enceladus was one of the most important in the Milky Way’s history. It shaped the galaxy we see today. It “shook things up”, by changing the orbits of stars already in the Milky Way.

Many stars were pushed into eccentric orbits. The stars from the long-disappeared Gaia-Enceladus, move in different ways than the “native” stars. Upon closer examination, stars that came from Gaia-Enceladus have different chemical compositions from those born inside the Milky Way.

Next Steps

Now that researchers peeked at the past collision and figured out what happened to the stars, what next? Their next step is to take the same approach and apply it to larger groups of Milky Way stars. The evolution of the Milky Way is, after all, an ongoing process. The end goal is to get a much clearer idea of that evolution and the changes mergers make to its stellar populations.

As a science writer, and during my tenure as a cruise lecturer (pre-pandemic), I often get questions that are difficult to answer. I once got into a discussion with a doctor on board a ship who asked about sound in space. He had retired from a career in ear, nose, and throat and asked about what we would “hear” on another planet. From there, we talked about Jupiter’s sounds and the noise of a star exploding.

Detecting Sounds on Mars

Well, it turns out, if he’s still around, there are some good examples of sounds in space sent back from spacecraft recently. The first one comes from the Ingenuity helicopter on Mars. Its flights are wildly popular with viewers here on Earth. Each one takes the rover further and higher and they all provide new challenges for the tiny craft. During the April 30th flight, the microphone on the Perseverance rover picked up the actual sounds of Ingenuity’s rotors flipping around at about 2400 rpm (rotations per minute).

Ingenuity’s flight noises became the first recorded ones on a planet where only the sound of the wind existed heard for billions of years. (Yes, of course, each spacecraft landing made a sound, but there were no ears, or microphones, to pick up their noise.)

Now, I can imagine someone asking if it would sound like this to someone standing outside on the surface of the planet. Probably not. Anybody standing outside on Mars will have to wear a spacesuit (and make their own oxygen so they can safely walk around) and will listen to radio communications. That’s going to change what they hear and nobody will ever get to walk around on Mars without protection.

Sounds from Deep Space

Mars isn’t the only place where scientists record “sound” vibrations. And, that’s what sound is: a disturbance, a vibration through a medium. On Mars, the vibrations from the helicopter move through the thin Martian air. But, “sounds” exist in interstellar space (where there is no air, as such). And, they can be detected.

As it turns out, the venerable Voyager 1 (V1) spacecraft does a little sonic exploration of its own, as it passes through interstellar space. It uses its Plasma Wave System to detect a narrow band of vibrations in the plasma environment. V1 has been in interstellar space since 2012 and completed its planetary studies long before that. Studies of the ISM, as it’s called, comprise an important part of its long-term science mission.

Sounds of Silence? Not!

I once had an astronomy professor describe the environment of deep space as “the sounds of silence.” Today, we know that deep space is not empty and we know it’s not “silent.”

Interstellar space contains atoms and molecules of gases and other compounds, and those can move, vibrate, and spin. Those actions send out vibrations that can be detected by sensitive instruments onboard spacecraft passing through the region.

Sampling Sounds in Space

All the spacecraft headed out to interstellar space can be used to sample the density of this “stuff”. So, Voyager 2 can do this, as can New Horizons. In the case of V1, the Plasma Wave System detects a faint “hum” from the low-level actions of interstellar gas. Think of it like the sound made by the constant, gentle patter of rain on a roof. However, instead of water falling in a storm, low-level activity by interstellar gas molecules sends out a monotonic plasma-wave signal. That “hum” is what gets picked up by V1’s instrument.

From time to time, the Sun interrupts this hum with outbursts of its own. Those disturbances travel out to space and Voyager detects them. However, solar events only disturb the interstellar medium for a short time. The rest of the time, the faint hum of gases is all that Voyager can detect” in the plasma wave frequency range.

Now, this isn’t something that human ears are going to hear, even if we could figure out a way to exist in interstellar space without spacesuits and helmet radios. This signal would need to be boosted and processed quite a bit for human ears to hear it. That’s not the point of the study, however. Instead, what it really represents is our exploration of interstellar space in a very direct and evocative way.

(Want to read more about the V1 finding? Check out the research paper where the team involved in monitoring V1’s signals reports their work.)