The Andromeda Galaxy Is Not Nearly As Big As We Thought

The Andromeda Galaxy Is Not Nearly As Big As We Thought

The closest galaxy to our own is the majestic Andromeda galaxy, a collection of a trillion stars located a “mere” two million light years away. New research suggests that, contrary to previous estimates, this galaxy isn’t much bigger than the Milky Way, and is practically our twin. This means our galaxy won’t be completely devoured when the two galaxies collide in five billion years.

This is what the Andromeda Galaxy would look like in the night sky if it were bright enough to be seen with the naked eye. (Photo credit: Tom Buckley-Houston & Stephen Rahn.)

There are billions of galaxies in the observable universe, yet only a few are close enough to be studied in any kind of detail — Andromeda one of these. In addition to its close proximity, Andromeda is a spiral just like the Milky Way, so it can teach us a lot about our own galaxy.

As new research published yesterday in Monthly Notices of the Royal Astronomical Society now shows, Andromeda is even more like the Milky Way than we realised. It’s roughly the same size as the Milky Way, and not two to three times larger as conventionally assumed.

A portion of Andromeda. (Credits: NASA, ESA, J. Dalcanton, B.F. Williams, and L.C. Johnson (University of Washington), the PHAT team, and R. Gendler)

A portion of Andromeda. (Credits: NASA, ESA, J. Dalcanton, B.F. Williams, and L.C. Johnson (University of Washington), the PHAT team, and R. Gendler)

This carries serious implications for our very distant future. In about five billion years, the Milky Way and Andromeda are scheduled to meet in a collision of cosmological proportions. The revised estimate of Andromeda’s weight now means that models of the merger will likewise have to be revised. Current simulations show the Milky Way getting engulfed by the “larger” Andromeda galaxy, but if these new calculations are correct, it will be more like the merger of equals.

The end-result of the collision will likely yield a giant, elliptical galaxy.

During the collision, many stars will be thrown out into interstellar space, and the two galaxies’ central supermassive black holes will be unable to resist each other’s allure, producing strong gravitational waves as they get closer together, and eventually merging into one. Our Sun will still be around during the merger, but it’s now unclear what’ll happen to our solar system. Worst case scenario is that we’re dragged into the tumultuous galactic centre towards the merging black holes, which, if we were actually around to witness it, would be bad.

Another important implication of the new study is that it could improve our understanding of how the Andromeda galaxy formed, how it’s evolving, and how its role in shaping the so-called Local Group of galaxies (a conglomeration of galaxies in relatively close proximity that includes the Milky Way).

Measuring the size of a distant galaxy is obviously not a simple process. For one, we’re buried deep inside the Milky Way, making observations of objects outside of our galaxy difficult but not impossible. And because we don’t have a celestial scale at our disposal, astronomers have to rely on mathematical and observational techniques to create size estimates. Over the years, methods used to measure Andromeda’s size include the rotation curve method (measuring the mass of stars in relation to the galactic centre), measuring velocity dispersion (tracking the speed of stars within the galaxy), and other techniques.

To date, these techniques have yielded estimates of Andromeda’s size that vary wildly, with some estimates suggesting it’s smaller than the Milky Way, and others suggesting it’s as much as two to three times larger.

Troubled by the lack of consensus, University of Western Australia astronomer Prajwal Kafle decided to use a technique that was employed to revise the size estimate of the Milky Way in 2014. The technique is based on observations of fast-moving stars within the Andromeda galaxy and estimates of the speed required for those stars to escape the galaxy’s gravitational pull. In other words, he figured out Andromeda’s escape velocity, which in turn yielded a new estimate of the galaxy’s mass.

Escape velocity is the speed required for an object to escape the gravitational bounds of another object. For example, a rocket needs to travel at 11 kilometers per second (11km/s) to escape Earth’s gravity. In the new study, Kafle calculated the entire weight of Andromeda as being 800 billion times heavier than the Sun — a size figure that’s a close match to the Milky Way.

“Our home galaxy, the Milky Way, is over a trillion times heavier than our tiny planet Earth so to escape its gravitational pull we have to launch with a speed of 550 km/s [341 miles/second],” said Kafle in a statement.

The paper claims a virial mass of Andromeda at 8 x 1011, which is at the very low end of the range predicted by previous estimates, and a virial radius at 782,775 light years across (by comparison, the Milky Way features a virial mass of ~4.8 x 1011 solar masses and virial radius of ~652,313 light years). By virial, the astronomers are referring to the range of a galaxy’s dark matter halo, which extends well beyond the edge of the visible galaxy, and dominates its total mass.

Part of the problem with the previous estimates is that astronomers had overestimated Andromeda’s dark matter — a mysterious mass that can interact with gravity but not with the other forces of nature, such as electromagnetism. “By examining the orbits of high-speed stars, we discovered that this galaxy has far less dark matter than previously thought, and only a third of that uncovered in previous observations,” he said.

Astronomer Heidi Newberg of the Rensselaer Polytechnic Institute, who wasn’t involved with the new study, told Gizmodo that the method used by Kafle “seems to be at least as good as any other method for determining the mass of a galaxy … They claim to have smaller errors that pin it down to a smaller total mass.”

Newberg says it’s important to know the mass of galaxies to better than a factor a three [the margin of error], in part because that changes the amount of dark matter required to explain the mass. “Dark matter is expected to dominate the mass of galaxies (and the Universe), but has not yet been successfully observed,” she said.

The finding also changes what we know about the Local Group of galaxies, which contains more than 54 galaxies, the bulk of them dwarf galaxies. Instead of Andromeda being the biggest, we now know — assuming the new calculations are correct — that the Local Group consists of two giants, the other now being the Milky Way.

[Monthly Notices of the Royal Astronomical Society]