Possible perp found in mystery of Milky Way’s missing galaxy pals

SAN DIEGO — The long-standing mystery of the Milky Way’s missing satellite galaxies has a credible culprit, new research suggests. Supernovas, the vigorous explosions of massive stars, might have shoved much of the matter surrounding our galaxy deep into space, preventing a horde of tiny companion galaxies from forming in the first place.

Millions of teeny galaxies should be buzzing around the Milky Way, according to theories about how galaxies evolve, but observations have turned up only a few dozen (SN: 9/19/15, p. 6). And the brightest of those that have been found are lightweights compared with what theorists expect to find. But new computer simulations designed to track the growth of galaxies down to the level of individual stars reveal the critical role that supernovas might play in resolving these conundrums.

Philip Hopkins, an astrophysicist at Caltech, presented the results June 13 during a news briefing at a meeting of the American Astronomical Society.

“Galaxies don’t just form stars and sit there,” Hopkins said. “If you [add] up all the energy that supernovae emitted during a galaxy’s lifetime, it’s greater than the gravitational energy holding the galaxy together. You cannot ignore it.”

Simulations are typically limited by computing power, and efforts to simulate galaxy evolution have to brush over some details. For instance, rather than capture everything that’s going on in a galaxy, simulations slap on the additive effects of supernovas in an ad hoc fashion. These limitations don’t fully capture all the physics of stellar winds and supernova shocks that ripple through a galaxy.

Hopkins’ simulations grow a galaxy organically within a computer, tracing the evolution of a system such as the Milky Way over 13 billion years. Within a massive virtual blob of dark matter — the elusive substance thought to bind galaxies together — gas collects and fragments into stellar nurseries. Stars are born and die in this digital universe. A volley of life-ending explosions from the most massive of these stars lead to a turbulent galactic history, Hopkins finds.

“As these stars form rapidly in the early universe, they also live briefly and explode and die violently, ejecting material far from the galaxy,” he said. “They’re not just getting rid of gas.” They’re stirring up the dark matter as well, preventing a multitude of satellite galaxies from forming, and whittling away at those few that survive. “It’s not until quite late times … that [the galaxy] settles down and forms what we would call a recognizable galaxy today,” Hopkins said.

The idea that stellar tantrums could chip away at the gas and dark matter around a galaxy is not new, says Janice Lee, an astronomer at the Space Telescope Science Institute in Baltimore. But Hopkins’ simulations bring a lot more detail to that story and show that it’s a plausible reason for our galaxy’s satellite shortfall.

Before declaring that the mystery of the missing satellite galaxies is solved, however, astronomers need to run a few more checks against reality, says Lee. There are still assumptions in the calculations about how energy from dying stars interacts with interstellar gas, for example. The precise details of that interaction can affect how many stellar runts versus behemoths form in star clusters.

NASA’s James Webb Space Telescope, scheduled to launch in 2018, could probe star clusters in several relatively nearby galaxies, she says. Those observations could be compared with virtual clusters that appear in the simulations to see how close they match the real universe.