Inconsistencies in supercomputer simulations to be compared in the AGORA
project are clearly evident in this test galaxy produced by each of
nine different versions of participating codes using the same
astrophysics and starting with the same initial conditions. [Credit:
Simulations performed by Samuel Leitner (ART-II), Ji-hoon Kim (ENZO),
Oliver Hahn (GADGET-2- CFS), Keita Todoroki (GADGET-3), Alexander Hobbs
(GADGET-3-CFS and GADGET-3-AFS), Sijing Shen (GASOLINE), Michael Kuhlen
(PKDGRAV-2), and Romain Teyssier (RAMSES)]. Large image
AGORA, an international collaboration led by UC Santa Cruz, will perform
systematic comparisons of high-resolution computer simulations of
galaxy formation and evolution
One of the most powerful tools for understanding the formation and
evolution of galaxies has been the use of computer
simulations--numerical models of astrophysical processes run on
supercomputers and compared with astronomical observations. Getting
computer simulations to produce realistic-looking galaxies has been a
challenge, however, and different codes (simulation programs) produce
inconsistent results.
Now, an international collaboration led by astrophysicists at the
University of California, Santa Cruz, aims to resolve these issues
through an ambitious multi-year project named AGORA (Assembling Galaxies
of Resolved Anatomy). AGORA will run direct comparisons of different
codes using a common set of initial conditions and astrophysical
assumptions. Each code treats some aspects of the physics differently,
especially the way that energy from stars and supernovas is fed back
into the simulated galaxies. The simulations are being run at the best
resolutions currently possible, and they are using the same input
physics as much as possible. The simulation results will be
systematically compared with each other and against a variety of
observations using a common analysis and visualization tool.
These comparisons will help researchers determine which of their
simulation results are due to a particular code platform and which are
due to the underlying theoretical assumptions common to all of the
simulations.
"The physics of galaxy formation is extremely complicated, and the
range of lengths, masses, and timescales that need to be simulated is
immense," explained Piero Madau, professor of astronomy and astrophysics
at UCSC and co-chair of the AGORA steering committee. "You incorporate
gravity, solve the equations of hydrodynamics, and include prescriptions
for gas cooling, star formation, and energy injection from supernovae
into the code. After months of number crunching on a powerful
supercomputer, you look at the results and wonder if that is what nature
is really doing or if some of the outcomes are actually artifacts of
the particular numerical implementation you used."
Dark matter
The AGORA project will explore the fundamental astrophysics of galaxy
formation in the cosmological context of a "cold dark matter" universe.
Although the nature of dark matter remains a mystery, it accounts for
about 84 percent of the matter in the universe. As a result, the
evolution of structure in the universe has been driven by the
gravitational interactions of dark matter ("dark" because it can't be
seen, and "cold" because the particles are moving slowly). The ordinary
matter that forms stars and planets has fallen into the "gravitational
wells" created by clumps of dark matter, giving rise to galaxies in the
centers of dark matter halos.
The project's first major task will be to model a realistic isolated
disk galaxy using various codes and their feedback recipes, varying both
the feedback parameters and the resolution. The second task will be to
compare the codes in cosmological simulations. Specifically, all the
participating codes will model the evolution of eight individual
galaxies from the big bang to the present, resulting in final masses
representing a range of galaxy sizes, from that of a dwarf galaxy to one
more massive than the Milky Way. For each mass, one set of simulations
would model a galaxy having a quiescent merger history (having few
mergers with another galaxy its own size) and another would model a
galaxy having a violent merger history with many major mergers. The
final task will be to compare the results, including such observable
characteristics as the shape, internal structure and velocities, and
spectral energy distribution (distribution of light at different
wavelengths) between simulations and with observations of real galaxies.
The AGORA project will take advantage of new infrastructure for
computational astrophysics at UC Santa Cruz, including the "Hyades"
supercomputer and a high-capacity data storage system. "Our ability to
store and analyze the data here, and make the output of the simulations
available to the community at large, has made it possible for us to set
up such a large project," Madau said.
The project was initiated in a workshop at UCSC in August 2012, under
the sponsorship of the University of California High-Performance
AstroComputing Center (UC-HiPACC). A second workshop was held at UCSC a
year later. The project now involves more than 90 astrophysicists from
45 institutions in eight nations.
A paper describing the project in detail has been accepted for publication in the Astrophysical Journal Supplement.
The first author is Ji-hoon Kim, formerly a postdoctoral researcher at
UCSC and now at CalTech, who is coordinating the project along with the
steering committee led by Madau and Joel Primack, a professor of physics
at UCSC and director of the UC-HiPACC. Other members of the AGORA
steering committee are Tom Abel (Stanford), Nick Gnedin (Fermilab and
University of Chicago), Lucio Mayer and Romain Teyssier (University of
Zurich), and James Wadsley (McMaster University, Canada).
AGORA is not the first such comparison of supercomputer simulations
of galaxy evolution, but it is the most comprehensive and the
highest-resolution (finest detail). Previous astronomical simulation
comparison studies were the Santa Barbara Cluster comparison project
(1999) and the Aquila comparison project (2012). The AGORA project is an
open collaboration and welcomes new participants. AGORA is making all
of its initial conditions and common assumptions public, both to make it
easy for astrophysicists to join the collaboration and also to raise
the level of galaxy simulations worldwide.
"This project will tell us what are the key ingredients that produce
realistic galaxies regardless of the numerical codes. It will also
challenge the community to put more effort in cross-checking their
results against others'," Kim said.
More information about AGORA is available in the paper, "The AGORA
High-Resolution Galaxy Simulations Comparison Project," preprint
available at arxiv.org/abs/1308.2669. The official AGORA website is at www.agorasimulations.org.
By Tim Stephens
