The Galactic Feast
Mary E. Putman, University of Michigan
Somewhere on a distant frontier of astronomy, someone discovered that galaxies
have to eat too. The big ones are always munching on the little ones.
In the Milky Way, the banquet table is the galaxy’s gaseous halo clouds.
But over the last billion or so years, exactly how has this played out? What
is on the menu? That’s where Mary E. Putman comes in.
From her lab in Ann Arbor, she talks of our galaxy as “us.” “We
know we need the fuel,” she said. “We don’t know where it comes
from. Does it come from all these smaller galaxies, being gobbled up? Or recycled
material from our own galaxy?”
The Magellanic System, HI column density map
It’s been her quest ever since her love of travel and astronomy took
her to Australia and Mount Stromlo, the noted observatory complex west of
Canberra. She became hooked on the halo clouds that surround our giant disk
of a galaxy.
When she returned to the United States and eventually a position at Michigan,
she was among the top experts on how galaxies evolve, consuming their smaller
neighbors. She has produced striking photographs documenting their evolution
in hydrogen gas that only hint at the persistent questions about the galactic
cannibalism. “We want to look at how they gobble up their fuel,” she
said. “The way they do it is still unclear.”
She calls what’s going on “hierarchical merging.” Larger structures
with stronger gravity assimilate what they need to grow, and the process continues.
What makes the inquiry more intriguing is the fact that most of the universe
is dark energy, or for galaxies, dark matter, dark in the sense of unknown and
invisible. “What we see is only 4 percent of what’s out there,
the matter,” Putman said.
To explain what’s happening as the small structures get consumed would
have an immense impact on what students learn in general astronomy.
“If we can explain that gobbling, we will know how galaxy formation is
going on today. We know there is fuel coming in to galaxies. We are trying to
understand continuous star formation. What’s needed is a new way of
understanding where that fuel comes from.”
Is the medium something left over from galaxy formation? Or something our
galaxy blew out into the medium? To learn more, she’ll employ images from the
NSF’s 305-meter wide Arecibo radio dish, the world's largest and most sensitive
radio telescope, in Puerto Rico. It played a sinister part in the James Bond
film “GoldenEye” and played its scientific self in “Contact,” the
movie based on the Carl Sagan novel.
Her description of the halo has its own intrigue. A picture emerges of clouds,
appearing like raindrops or tadpoles, falling through some kind of mysterious
layer. In standard pictures of formation, the hot, diffuse gas is elusive. “It’s
not easy to probe its properties. The clouds come in as fuel, then pass through
a larger medium that we don’t know much about,” she said.
To try to pinpoint the halo’s medium, her team, working with collaborators
at the University of California Berkeley and Wisconsin, will carry out simulations,
then compare the findings to their actual observations.
And along the way, they may extend our understanding of where our galaxy came from, and the 125 billion others as well.
Nov. 1, 2007
