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Astrophysics

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  • Introduction
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Introduction

The KIAS Astrophysics Group is interested in many aspects of astrophysics, particularly in extragalactic astronomy and theoretical and observational cosmology.

Cosmology is the study of the origin and evolution of the Universe. Together with a tremendous growth in observational data and computational technology, cosmology has been growing rapidly during the past 30 years. Since the 1980's cosmology has changed radically and became a truly quantitative science requiring precision measurements and theoretical predictions. A goal of the modern cosmology is to construct a physical universe model that is best consistent with astronomical observations. There have been continuous efforts for this aim by observing the properties and distribution of galaxies, large-scale structures, and cosmic microwave background radiation to constrain cosmological parameters within a few percent uncertainties through joint analyses of various observational data. The KIAS astrophysics group has been contributing to this great job.

Topics of our current interest include cosmic microwave background, cosmological parameter estimation from large-scale structures, and galaxy formation and evolution. Activities in each research area are summarized below.

Cosmic microwave background

 

Figure 1: The CMB temperature fluctuations from the WMAP 7-year data release. The average temperature is 2.725 K. Colors represent the tiny temperature fluctuations: red regions are warmer, and blue regions are colder by about 0.0002 degrees.

The answers to many of the fundamental questions in cosmology can be obtained from the fluctuations in the cosmic microwave background radiation (CMBR). This radiation is a snapshot of the universe when it was only 380,000 years. The small temperature fluctuations of the CMBR across the sky tell us about the seeds of the cosmological structures we see today. By measuring these minute fluctuations, we can estimate many major cosmological parameters with unprecedented precisions. Recently, the Wilkinson Microwave Anisotropy Probe (WMAP) satellite mission opened a new door to this precision cosmology. The WMAP has measured temperature anisotropy and polarization in CMBR with high resolution and sensitivity. The WMAP data implies that the observed CMB fluctuations are consistent with predictions of the concordance Lambda-dominated CDM model with scale-invariant and adiabatic fluctuations which have been generated during the inflationary epoch.

The KIAS Astrophysics Group investigates the statistical properties of the CMB anisotropy with novel techniques, such as the genus or the excursion set andpixel clustering correlations. The group has a particular interest in non-Gaussian signatures, which are investigated by constructing detailed simulations with primordial Gaussianity or non-Gaussianity. The simulations implement all the observational artifacts (i.e. noise, foreground contamination from synchrotron, dust, and free-free emission, and from experimental uncertainty etc.), and reflect the characteristics of the various spacecraft instruments such as WMAP or Planck. A new direction of study of the group is the polarization of the CMB, and the detection of gravitational waves.

Large-Scale Structure

Figure 2. A wedge of the redshift-space distribution of the SDSS MAIN galaxies.

The study of the large-scale structure of the universe is also undergoing a tremendous growth. In the early 1980's there were less than 5,000 galaxy redshifts known. However, several new redshift surveys such as 2-Degree Field (2dF) Survey and Sloan Digital Sky Survey (SDSS) have recently been completed, and we now have more than 1,000,000 redshifts mapping the 3-dimensional distribution of galaxies in our local universe. Furthermore, those structures can be matched to the early time snapshot of density perturbation (temperature fluctuations in CMBR). Since any model of structure formation must explain both the temperature fluctuations in CMBR and the large-scale structures we observe today in the universe, the combination of these two probes is especially powerful.

The standard cosmological model assumes two unknown components, dark matter and dark energy, that collectively account for 95% of the Universe's total energy budget, and yet whose nature is entirely unknown. Dark matter and dark energy cannot be explained by modern physics, the illumination of the nature of these fundamental constituents of our Universe will mark a revolution in physics impacting particle physics and cosmology and will require new physics beyond the standard model of particle physics, general relativity or both.

The information of underlying science about the universe is given by looking at structure formation on large scales. While current observations revealed the breakdown of our knowledge of physical science on cosmological scales, the future observation of large scale structure formation will provide us with a clue as to which part of our physical science should be modified - matter or gravity.

The next generation of cosmological observations will be launched to scan inhomogeneity on large scales, such as BOSS, DES, EUCLID and LSST. These experiments be optimally designed to answer the question whether our understanding of nature on the earth is universal or not. We plan to work on how to read signatures of our fundamental knowledge of nature imprinted on large scale structures, how to falsify different approaches - dark materials or violation of fundamental laws - and how to develop computational and observational techniques to transform raw data sets into understandable physical knowledge. We are at a critical moment of understanding of Nature, and we need inter-communicating leadership between theoretical knowledge and observational techniques. We have developed research to meet such a need.

Numerical simulations of structure formation


Figure 3. A 64 Mpc/h thick slice through the Horizon Run simulation showing the matter density field in the past light cone as a function of look-back time all the way to the horizon. The thickness of the wedge is constant and the opening angle is 45◦.. The Earth is at the vertex and the upper boundary is the big bang surface at a look-back time of 13.7 billion years.

In order to study how the cosmological structures evolve from those small fluctuations at early times into the large-scale structures, numerical simulations are required because gravity acts non-linearly and analytic calculation becomes intractable beyond a certain stage. Gravitational collapse takes the initial pattern of small density fluctuations in the early universe and transforms them into regions that are either more dense or more underdense, with overdense regions eventually forming stars, galaxies, galaxy clusters, etc. Due to advances in computational power, numerical simulations can now reliably capture many properties of this evolution, providing many mock universes whose statistical properties can be directly compared to the observational data. Numerical simulation can also be applied to the studies of weak gravitational lensing, Lyman alpha forest, and Sunayev-Zel'dovich effect.

We are simulating the formation of individual dark halos and of large-scale structures of the universe using the GOTPM-II code. To identify dark matter halos from the distribution of simulation particles, we developed a new halo finding method, the Physically Self-Bound (PSB) halo finding algorithm adopting the total energy and tidal boundary criteria which delineates halo/subhalo regions even in crowded environment. In 2008 we made a large cosmological N-body simulation, The Horizon-Run, using 4120^3 particles. We also made many 2048^3 particle simulations with various simulation box sizes ranging from the local universe scale to the super-Hubble scale. These are extensively used to understand the internal and collective properties of galaxies and to calibrate the measurements of various cosmological parameters.

Galaxy Formation


Figure 4. A HST image of the Stephan's Quintet

At present we have a number of observations to establish the Lambda CDM model as the de facto standard model of cosmology. The fundamental cosmological parameters are now known with uncertainties of only a few per cent, removing a large part of parameter space in galaxy formation studies. While the basic theoretical paradigm appears to be well-established, our understanding of the physical processes that lead to the varieties of observed galaxy properties is still far from complete. For better understanding of galaxy formation and evolution, we are actively working on many important areas in this field.

Objects like galaxies form when gas condenses at the center of dark matter halos after radiative cooling of baryons. In semi analytic models of galaxy formation we model the evolution, of baryonic components of galaxies, using simple yet physically and observationally motivated prescription. Taking advantage of high resolution N-Body simulations, we specify the location and evolution of dark matter haloes, which are assumed to be the birth places of galaxies. Since pure N-Body simulations like the Horizon-Run can handle very large number of particles, this approach can access very large dynamic ranges in mass and spatial resolutions. In addition, the computational costs are limited so that the method allows a fast exploration of the parameter space and an efficient investigation of different specific physical assumptions.

In addition to this theoretical modeling of galaxy formation and evolution, we are also investigating galaxy evolution using observational data and through the comparison of them to theoretical models. These days, we are experiencing the boom of extra-galactic data -- both of photometric and spectroscopic -- of a wide redshift range from various galaxy surveys, including the Sloan Digital Sky Survey. Applying various analysis methods -- including Spectral Energy Distribution-fitting methods and comparison to semi-analytic models -- to these vast observational data, we can investigate various physical properties of galaxies and their evolution, as well as the environmental effects on these evolution.

We are also actively working on the effects of galaxy interactions on galaxy evolution. Galaxies interact with neighboring galaxies and matter, and are affected by the environment over the course of their lives. In these processes, a variety of phenomena can occur in the involving galaxies such as the morphological changes and enhanced star formation and nuclear activities. We study those effects of interactions on galaxy properties and overall galaxy evolution in the connection to the structure formation theories, by both analyzing observational data, specifically from the SDSS, and performing numerical hydrodynamical simulations.

Besides, we are also interested in other subjects that are directly related to cosmology and galaxy formation such as galaxy peculiar velocity field, gravitational lensing, Lyman alpha forest clouds, and Type Ia SN, and so on. Our interest also resides on astro-hardron physics and relativistic astrophysics, extending our research interest to the formation and evolution of blackholes and neutron stars, and supernova.