TGQR 2010Q4 Report.pdf - Teragridforum.org

2.1.18
2.1.19
Physics: The Geophysical High Order Suite for Turbulence (GHOST) (PI: Annick
Pouquet, NCAR)
Student Training: Use of TeraGrid Systems in the Classroom and Research Lab (PI:
Andrew Ruether, Swarthmore College)
2.2 Science and Engineering Highlights
2.2.1
Astronomical Sciences: Detecting HI In Emission At z = 1: The Effect of Substructure
(PI: Tiziana Di Matteo, Carnegie Mellon)
Observations show that the cosmic star formation rate has declined by more than one order of
magnitude since redshift z = 1 (about 7.7 billion years ago). However a combined census of the
cold gas (the fuel for star formation), and stellar components is largely missing in observations.
This cold gas fraction is a crucial ingredient in models of galaxy formation and will suggest how
galaxies obtain gas and subsequently convert it to stars. A census of neutral hydrogen (HI) will
put tight constraints on different models of galaxy formation.
Based on current observations, Di Matteo’s group modeled the distribution of HI using dark
matter simulations with 14.6 billion particles at z = 1, using the hybrid MPI/Pthreads and
massively parallel code GADGET on ~22k cores of TeraGrid’s Kraken system at NICS. This
simulation is around 1.5 times larger than the Millennium Simulation (Springel et al. 2005) with 3
times finer mass resolution. This was made possible by their NSF PetaApps project with PSC and
the University of Washington, which has scaled GADGET to ~100k cores on Kraken.
A slice from Di
Matteo’s group’s
simulation
illustrating the
rich network of
nodes, filaments
and voids in the
distribution of
dark matter at z
= 1 is shown in
the left side of
the figure. The
high resolution
and large
volume allow
the group to
resolve the
smallest
subhalos that host HI, residing in the largest dark matter halo. The group makes predictions for
existing surveys and radio interferometers to determine the HI mass function at this redshift. They
predict that small galaxies (undetected in an optical survey) embedded in their parent halo can be
detected through their contribution to the total HI signal when seen in a radio telescope. This is
shown for one of the many large halos in their simulation in the right side of the figure.
Figure 2.1. (Left) A slice from the dark matter simulation at z = 1 enclosing the largest halo. (Right)
Contribution of subhalos to the HI signal. The measured signal (red line) of a single large halo and the
modeled signal (black line). Data points are the contribution of subhalos within pixel, the large halo
giving rise to a discrepancy between the theoretical and the observed signal.
Even though individual halos cannot be detected in HI at z = 1, the group can detect the effect of
substructures by stacking thousands of halos whose redshifts have been determined in an optical
survey. They find that the contribution of substructure can be seen in the cumulative HI signal
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