Andromeda is the closest large galaxy to the Milky Way. And it is getting closer, as the two will merge in several billion years. It has been an object of study for centuries, but we have never seen it like this. The Hubble Space Telescope has just finished a campaign of observations that lasted for more than 10 years, creating the deepest observations of this galaxy ever.
The galaxy is so large that 600 separate fields of view had to be assembled in an extraordinary mosaic with 2.5 billion pixels. Hubble was able to resolve about 200 million stars, all hotter than our Sun. A large number, but still a tiny fraction of the galaxy’s total stellar population. Andromeda is estimated to have 1 trillion stars. Still, this drop in the ocean tells us a lot about the galaxy.
“With Hubble we can get into enormous detail about what’s happening on a holistic scale across the entire disk of the galaxy. You can’t do that with any other large galaxy,” principal investigator Ben Williams of the University of Washington said in a statement.
The work has revealed that Andromeda is a lot more chaotic than expected. Similar to what recently retired Gaia did for the Milky Way, Hubble spotted the hallmarks of a past collision: the presence of a large stellar population, coherent streams of stars, etc.
“Andromeda’s a train wreck. It looks like it has been through some kind of event that caused it to form a lot of stars and then just shut down,” added Daniel Weisz at the University of California, Berkeley. “This was probably due to a collision with another galaxy in the neighborhood.”
It is possible that the compact satellite of Andromeda known as Messier 32 was the culprit, and that Andromeda stole its little neighbor’s gas, using it all up to form a lot of new stars.
“Andromeda looks like a transitional type of galaxy that’s between a star-forming spiral and a sort of elliptical galaxy dominated by aging red stars,” said Weisz. “We can tell it’s got this big central bulge of older stars and a star-forming disk that’s not as active as you might expect given the galaxy’s mass.”
“This detailed look at the resolved stars will help us to piece together the galaxy’s past merger and interaction history,” added Williams.
You can see the full picture here, and it is worth having it on the biggest screen you can find to truly appreciate the level of detail. We have placed a high resolution version (but not the highest) vertically for ease of scrolling, partly inspired by the infamous Tumblr post “Do you love the color of the sky?” We promise you, the scrolling will be worth it.
Andromeda like we have never seen it before.
Image credit: NASA, ESA, Benjamin F. Williams (UWashington), Zhuo Chen (UWashington), L. Clifton Johnson (Northwestern); image processing: Joseph DePasquale (STScI)
A paper describing part of the observations, led by Zhuo Chen, is published in The Astrophysical Journal.