Astronomers have discovered a dwarf galaxy orbiting our nearest major galaxy, the Andromeda Galaxy, and the discovery could change what we know about how galaxies develop and evolve over time.
The newfound dwarf galaxy orbiting the Andromeda Galaxy is called Andromeda XXXV and is about 3 million lightyears away from Earth.
It rests on the outskirts of the Andromeda Galaxy, our galactic neighbour, and is the smallest and dimmest satellite galaxy of Andromeda ever seen.
Satellite galaxies?
Yes, just as planets have natural satellites – also known as 'moons' – major galaxies have smaller satellite galaxies orbiting them.
Satellite galaxies are smaller than their larger host galaxies, and quite distinct from them, but are caught in the larger galaxy's gravitational pull.
Even our own Milky Way has satellite galaxies like the Large and Small Magellanic Clouds, which can be seen from the Southern Hemisphere as smudges in the sky.
In fact, the Milky Way has dozens of known satellite galaxies, also known as 'companion galaxies', and so discovering more about other satellites can reveal the secrets of our own Galaxy and the wider Universe.
Why Andromeda XXXV is important
Scientists at the University of Michigan made the discovery, led by Marcos Arias, lead author of the accompanying study published in Astrophysical Journal Letters.
"There are so many things that we still need to learn — even about what's near to us — in terms of galaxy formation, evolution and structure before we can reverse engineer the history of the Universe and understand how we came to be where we are today," he says.
Arias worked on the study while still an undergraduate student in the university's Department of Astronomy.
"These are fully functional galaxies, but they're about a millionth of the size of the Milky Way," says the study's senior author Eric Bell, U-M professor and associate chair of astronomy.
"It's like having a perfectly functional human being that's the size of a grain of rice."
Understandably, satellite galaxies are tricky to spot, because they're so much dimmer and smaller than their massive galactic hosts.
But astronomers are making great strides in discovering and studying them due to an increase in the technological observing prowess available.
Astronomers can now even see faint satellite galaxies around major galaxies further away than the Andromeda Galaxy.
Prior to this, the Milky Way's satellites were the only satellite galaxies available for study.
As you might have guessed by its name, Andromeda XXXV is not the first satellite discovered around the Andromeda Galaxy, but this is one is notable by how small and faint it is.
"This is, in part, why Marcos's discovery is so important. This type of galaxy was only discoverable around one system, the Milky Way, in the past," Bell says.
"Now we're able to look at one around Andromeda and it's the first time we've done that outside our system."
Discovering Andromeda XXXV
To discover Andromeda XXXV, the team looked at data to try and find signs of satellite galaxies around the Andromeda Galaxy.
Once they found a few promising candidates, they then used the Hubble Space Telescope for a closer look.
Hubble revealed that Andromeda XXXV was a satellite galaxy, and that it's small enough to challenge existing ideas of how galaxies evolve and form stars
"It was really surprising," Bell says. "It's the faintest thing you find around, so it's just kind of a neat system. But it's also unexpected in a lot of different ways."
"Most of the Milky Way satellites have very ancient star populations. They stopped forming stars about 10 billion years ago," Arias says.
"What we're seeing is that similar satellites in Andromeda can form stars up to a few billion years ago: around 6 billion years."
Murder mystery
Why do satellite galaxies stop forming stars?
Do they simply run out of the gas and dust necessary for stars to form, or do the larger host galaxies steal those star-forming ingredients from them?
Astronomers say the Milky Way's satellites seem to have simply run out of fuel, but that the Andromeda Galaxy satellites' fuel seems to have been sapped by their hosts
"It's a little dark, but it's either did they fall or did they get pushed? These galaxies appear to have been pushed," Bell says.
"With that, we've learned something qualitatively new about galaxy formation from them."
Standing the heat
One key question surrounding satellite galaxies is how they survived the extreme temperatures of the early Universe, without losing the fuel necessary for star formation within them.
It's thought that very small galaxies – those with less mass than about 100,000 times the mass of our Sun – simply wouldn't survive.
"We thought they were basically all going to be fried because the entire Universe turned into a vat of boiling oil," Bell says.
But Andromeda XXXV is evidence that these smaller satellite galaxies can survive.
"We thought that it would completely lose its gas, but apparently that doesn't happen, because this thing is about 20,000 solar masses and yet it was forming stars just fine for a few extra billion years."
"I don't have an answer," Bell says. "It is also still true that the Universe did heat up, we're just learning the consequences are more complicated than we thought."
"It's the Universe," says Arias. "There will always be something new to discover."
The study on Andromeda XXXV also included researchers from the University of Chicago, Utah Valley University, the Vatican Observatory, the University of La Serena in Chile, the University of Alabama, Montana State University and the Leibniz Institute for Astrophysics Potsdam in Germany.
Read the full paper at iopscience.iop.org/article/10.3847/2041-8213/adb433
