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We’re Not Alone–but the Universe May Be Less Crowded than We Think

Study Suggests there are Fewer Faint Galaxies than Expected

Published July 1, 2015

Matter overdensity (top row) and ionized fraction (bottom row) for the three regions simulated in the Renaissance Simulations. The red triangles represent the locations of galaxies that are detectable with the Hubble Space Telescope. Its successor, the James Webb Space Telescope, will detect many more distant galaxies, shown by the blue squares and green circles. These first galaxies reionized the universe only one billion years after the Big Bang, shown in the image with ionized (blue) bubbles around the galaxies. Image: Brian W. O’Shea (Michigan State University), John H. Wise (Georgia Tech); Michael Norman and Hao Xu (UC San Diego)

There may be far fewer galaxies further out in the Universe than might be expected, suggests a new study based on simulations conducted on the Blue Waters supercomputer at the National Center for Supercomputing Applications, with resulting data transferred to SDSC Cloud at the San Diego Supercomputer Center at the University of California, San Diego for future analysis.

The study, published this week in the Astrophysical Journal Letters, shows the first results from the Renaissance Simulations, a suite of extremely high-resolution adaptive mesh refinement (AMR) calculations of high redshift galaxy formation.

Moreover, these simulations show hundreds of well-resolved galaxies, allowing researchers to make several novel and verifiable predictions ahead of the October 2018 launch of the James Webb Space Telescope (JWST), a new space observatory that succeeds the Hubble Space Telescope.

“Most critically, we show that the ultraviolet luminosity function of our simulated galaxies is consistent with observations of redshift galaxy populations at the bright end of the luminosity function, but at lower luminosities is essentially flat rather than rising steeply,” wrote researchers in their paper, called ‘Probing the Ultraviolet Luminosity Function of the Earliest Galaxies with the Renaissance Simulations.’

“Our work suggests that there are far fewer faint galaxies than one could previously infer,” said principal investigator and lead author Brian W. O’Shea, an associate professor at Michigan State University, with a joint appointment in the Department of Computational Mathematics, Science and Engineering, the Department of Physics and Astronomy, and the National Superconducting Cyclotron Laboratory. “Observations of high redshift galaxies provide poor constraints on the low-luminosity end of the galaxy luminosity function, and thus make it challenging to accurately account for the full budget of ionizing photons during that epoch.”

“The Hubble Space Telescope can only see the what we might call the tip of the iceberg when it comes to taking inventory of the most distant galaxies,” said SDSC Director Michael Norman, who was part of the research team for this study. “A key question is how many galaxies are too faint to see. By analyzing these new, ultra-detailed simulations, we find that there are 10 to 100 times fewer galaxies than a simple extrapolation would predict.”

Because these simulations are so costly to generate, the team moved the entire output of the Renaissance Simulations to SDSC Cloud – some 100 terabytes of data, or the equivalent of about 150,000 audio compact discs. “A data access portal is being set up so that others can investigate their properties in more detail,” added Norman, also a distinguished professor of physics at UC San Diego and a faculty member with the Center for Astrophysics & Space Sciences at the university.

“The flattening at lower luminosities is a key finding in the study and significant to researchers’ understanding of the reionization of the universe, when the gas in the universe changed from being mostly neutral to mostly ionized,” said John H. Wise, Dunn Family Assistant Professor with the School of Physics at the Georgia Institute of Technology.

The term ‘reionized’ is used because the universe was ionized immediately after the fiery Big Bang. During that time, ordinary matter consisted mostly of hydrogen atoms with positively charged protons stripped of their negatively charged electrons. Eventually, the universe cooled enough for electrons and protons to combine and form neutral hydrogen. They didn’t give off any optical or UV light and without that light, astrophysicists aren’t able to see traces of how the cosmos evolved during these Dark Ages using conventional telescopes. The light returned when reionization began.

In an earlier paper, simulations conducted by two researchers who were part of this new study concluded that about 300 million years after the ‘Big Bang’, the universe was 20 percent ionized, 50 percent ionized at 550 million years, and fully ionized at 860 million years after its creation. 

While the James Webb Space Telescope will give cosmic researchers the ability to view and record substantial numbers of galaxies, the telescope has a relatively small field of view, according to the researchers. As a result, interpretation of any JWST survey must by necessity take into account cosmic variance – the statistical variation in the number of galaxies from place to place. A deeper understanding based on theory may be necessary to correctly interpret high redshift survey results.”

These simulations were done on the National Science Foundation-funded Blue Waters supercomputer, which is one of the largest academic supercomputers in the world. “These simulations are physically complex and very large – we simulate thousands of galaxies at a time, including their interactions through gravity and radiation, and that poses a tremendous computational challenge,” said O’Shea. “Without the large memory, fast inter-node network, and powerful file system of Blue Waters, this calculation would not have been possible.”

In addition to O’Shea, Wise, and Norman, the research team also included Dr. Hao Xu, a postdoctoral research associate with the Center for Astrophysics & Space Sciences, at the University of California, San Diego. The research was funded by the National Science Foundation and NASA.

About Blue Waters

Blue Waters is one of the most powerful supercomputers in the world, used to tackle a wide range of challenging problems, from predicting the behavior of complex biological systems to simulating the evolution of the cosmos. Using the latest technologies from Cray, Inc., Blue Waters has more than 1.5 petabytes of memory, enough to store 300 million images from one’s digital camera, and can achieve a peak performance level of more than 13 quadrillion calculations per second. Blue Waters, based at the National Center for Supercomputing Applications, is supported by the National Science Foundation and the University of Illinois.

About SDSC Cloud

Launched in 2011, SDSC Cloud is one of the first ever large-scale academic deployment of cloud storage in the world, based at the San Diego Supercomputer Center at UC San Diego.  SDSC Cloud’s high-speed connectivity, coupled with its cloud architecture, provides a flexible resource for hosting large-scale archives such as the Renaissance Simulations.