Galactic winds push researchers to probe galaxies at unprecedented scale

Rather than getting pushed, the simulation shows the cold
material instead becomes gradually heated until it is fully
incorporated into the hot wind. Credit: Credit: Evan Schneider,
Princeton University

When astronomers peer into the universe, what they see often
exceeds the limits of human understanding. Such is the case
with low-mass galaxies—galaxies a fraction of the size of our
own Milky Way.

These small, faint systems made up of millions or billions of
stars, dust, and gas constitute the most common type of galaxy
observed in the universe. But according to astrophysicists’
most advanced models, low-mass galaxies should contain many
more stars than they appear to contain.

A leading theory for this discrepancy hinges on the
fountain-like outflows of gas observed exiting some galaxies.
These outflows are driven by the life and death of stars,
specifically stellar winds and supernova explosions, which
collectively give rise to a phenomenon known as “.” As star activity expels gas into
intergalactic space, galaxies lose precious raw material to
make new stars. The physics and forces at play during this
process, however, remain something of a mystery.

To better understand how galactic wind affects star formation
in galaxies, a two-person team led by the University of
California, Santa Cruz, turned to high-performance computing at
the Oak Ridge Leadership Computing Facility (OLCF), a US
Department of Energy (DOE) Office of Science User Facility
located at DOE’s Oak Ridge National Laboratory (ORNL).
Specifically, UC Santa Cruz astrophysicist Brant Robertson and
University of Arizona graduate student Evan Schneider (now a
Hubble Fellow at Princeton University), scaled up their Cholla
hydrodynamics code on the OLCF’s Cray XK7 Titan supercomputer
to create highly detailed simulations of galactic wind.

“The process of generating galactic winds is something that
requires exquisite resolution over a large volume to
understand—much better resolution than other cosmological
simulations that model populations of galaxies,” Robertson
said. “This is something you really need a machine like Titan
to do.”

After earning an allocation on Titan through DOE’s INCITE
program, Robertson and Schneider started small, simulating a
hot, supernova-driven wind colliding with a cool cloud of gas
across 300 light years of space. (A light year equals the
distance light travels in 1 year.) The results allowed the team
to rule out a potential mechanism for galactic wind.

Now the team is setting its sights higher, aiming to generate
nearly a trillion-cell of an entire galaxy, which would be
the largest simulation of a galaxy ever. Beyond breaking
records, Robertson and Schneider are striving to uncover new
details about galactic wind and the forces that regulate
galaxies, insights that could improve our understanding of
low-mass galaxies, dark matter, and the evolution of the
universe.

Simulating cold clouds

About 12 million light years from Earth resides one of the
Milky Way’s closest neighbors, a disk galaxy called Messier 82
(M82). Smaller than the Milky Way, M82’s cigar shape
underscores a volatile personality. The galaxy produces new
stars about five times faster than our own galaxy’s rate of
star production. This star-making frenzy gives rise to galactic
wind that pushes out more gas than the system keeps in, leading
astronomers to estimate that M82 will run out of fuel in just 8
million years.

Analyzing images from NASA’s Hubble Space Telescope, scientists
can observe this slow-developing exodus of gas and dust. Data
gathered from such observations can help Robertson and
Schneider gauge if they are on the right track when simulating
galactic wind.

“With galaxies like M82, you see a lot of cold material at
large radius that’s flowing out very fast. We wanted to see, if
you took a realistic cloud of cold gas and hit it with a hot,
fast-flowing, supernova-driven outflow, if you could accelerate
that cold material to velocities like what are observed,”
Robertson said.

Answering this question in high resolution required an
efficient code that could solve the problem based on well-known
physics, such as the motion of liquids. Robertson and Schneider
developed Cholla to carry out hydrodynamics calculations
entirely on GPUs, highly parallelized accelerators that excel
at simple number crunching, thus achieving high-resolution
results.

In Titan, a 27-petaflop system containing more than 18,000
GPUs, Cholla found its match. After testing the code on a GPU
cluster at the University of Arizona, Robertson and Schneider
benchmarked Cholla under two small OLCF Director’s
Discretionary awards before letting the code loose under
INCITE. In test runs, the code has maintained scaling across
more than 16,000 GPUs.

“We can use all of Titan,” Robertson said, “which is kind of
amazing because the vast majority of the power of that system
is in GPUs.”

The pairing of code and computer gave Robertson and Schneider
the tools needed to produce high-fidelity simulations of gas
clouds measuring more than 15 light years in diameter.
Furthermore, the team can zoom in on parts of the simulation to
study phases and properties of galactic wind in isolation. This
capability helped the team to rule out a theory that posited
close to the galaxy’s center could be
pushed out by fast-moving, hot wind from supernovas.

“The answer is it isn’t possible,” Robertson said. “The hot
wind actually shreds the clouds and the clouds become sheared
and very narrow. They’re like little ribbons that are very
difficult to push on.”

Galactic goals

Having proven Cholla’s computing chops, Robertson and Schneider
are now planning a full-galaxy simulation about 10 to 20 times
larger than their previous effort. Expanding the size of the
simulation will allow the team to test an alternate theory for
the emergence of galactic in disk like M82. The theory suggests that
clouds of cold gas condense out of the hot outflow as they
expand and cool.

“That’s something that’s been posited in analytical models but
not tested in simulation,” Robertson said. “You have to model
the whole galaxy to capture this process because the dynamics
of the outflows are such that you need a global simulation of
the disk.”

The full-galaxy simulation will likely be composed of hundreds
of billions of cells representing more than 30,000 light years
of space. To cover this expanse, the team must sacrifice
resolution. It can rely on its detailed gas cloud simulations,
however, to bridge scales and inform unresolved physics within
the larger simulation.

“That’s what’s interesting about doing these simulations at
widely different scales,” Robertson said. “We can calibrate
after the fact to inform ourselves in how we might be getting
the story wrong with the coarser, larger simulation.”

Explore further:
Milky Way’s origins
are not what they seem

More information: Evan E. Schneider et al,
HYDRODYNAMICAL COUPLING OF MASS AND MOMENTUM IN MULTIPHASE
GALACTIC WINDS, The Astrophysical Journal (2017).
DOI: 10.3847/1538-4357/834/2/144

Journal reference: Astrophysical
Journal

Provided by: Oak
Ridge National Laboratory