A most unusual galaxy
MARLA GEHA DIDN’T WANT TO get her hopes up. She was poring over images of distant dwarf galaxies that surround hosts galaxies similar to our own Milky Way, when she realised that the dwarfs looked surprisingly different from the ones in our backyard. It was the first hint that the most-studied galaxy in the universe — our own — might not be as typical as previously thought.
“Whenever I see a result, I basically try and figure out what ways it could be wrong before getting excited,” says Geha (Yale). “I think I’ve been hurt too many times.”
But it didn’t take long before she realised that this was no mistake. The dwarf galaxies imaged as part of the Satellites Around Galactic Analogs (SAGA) Survey are studded with young, brilliant, blue stars. That’s a far cry from the old, red stars in the faint satellites circling our galaxy.
Apart from the Magellanic Clouds (which are large, recent arrivals), all 60-some known dwarfs around the Milky Way are ‘dead’ — and have been for at least 1 billion years. The same story is true around Andromeda, where only two of the galaxy’s 30-odd satellites are currently forming stars.
Such a stark contrast could be a problem. Much of our knowledge about galaxy formation and even cosmology is based on our understanding of the Milky Way’s satellites. Most of these small galaxies are also crowded with dark matter, making them useful laboratories for studying the mysterious substance. “There have been hundreds of papers written about the Milky Way satellites and using them to infer big things,” Geha says. But if the Milky Way’s satellites aren’t the norm, then many of those conclusions might need a rethink.
Enter SAGA, with the goal
