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Formation of Structure in the Universe

The Universe consists mostly of Dark Matter and Dark Energy but the structures which we can see are of course formed from the baryonic component. The formation, structure, evolution and distribution of galaxies provide essential data on the nature of Dark Matter and Dark Energy. The leading theme in the OKC research are the earliest stages of galaxy formation some 13 billion years ago, the frontier of modern cosmological research. Starburst

We are participating in the LOFAR Epoch of Reionization project which tries to detect the neutral hydrogen from this period through its redshifted 21cm line. This detection will open a whole new research field in cosmology allowing us to study the matter distribution in the Universe directly and is eagerly awaited by the international community. At the OKC we have developed analysis methods for the interpretation of this complex signal and studied the impact of bright quasars on the neutral hydrogen.

Observations of the earliest generations of galaxies is one of the key science drivers for the next generation of ground-based (E-ELT) and space-based (JWST) telescopes. OKC research is predicting what these telescopes will be able to deliver using so-called population synthesis models, including exotic objects such as Population III stars and Dark Stars (stars producing energy through dark matter annihilation).

The galaxies from this period are often characterized by their prominent Lyman-alpha emission line. However, the interpretation of this line is complicated due to radiative transfer effects. To guide the interpretation of the data from the earliest galaxies, an international survey of nearby galaxies was initiated at the OKC (Lyman-Alpha Reference Survey, LARS) collecting data from a wide range of wavelengths.

Members of this working group are also attempting to constrain the properties of dark matter on subgalactic scales, by studying gravitationally lensed quasars and galaxies with high-resolution radio interferometers like the European VLBI network and the soon-to-be completed Atacama Large Millimeter/sub-millimeter Array (ALMA).

 

Epoch of reionization. The first galaxies formed some 13 billion years ago from the mixture of normal and dark matter contained in our Universe. The ionizing radiation from these galaxies caused the last global phase transition of the Universe - from a cold neutral gas to a hot ionized plasma. Investigation of this poorly understood epoch of reionization (EoR) is at the frontier of modern cosmological research. OKC members are part of the LOFAR EoR experiment which aims to detect the redshifted 21cm radio emission produced by neutral hydrogen during this period. Observations are ongoing and the first statistical detection is expected for the end of 2014. Contributions from OKC involve simulating the signal in order to provide a framework for interpretation of the observational results. The detectability of the effects from a supermassive black hole has been explored and the importance of redshift space distortions revealed. OKC members will contribute with advanced signal simulations for the interpretation of LOFAR detections. OKC members are also involved in the design of the next generation Square Kilometer Array (SKA) telescope. The baseline design for the low frequency part of SKA is mostly based on recommendations defined at an OKC-supported workshop held in January 2012. OKC research allows predictions of what the next generation of telescopes, both ground-based (European Extremely Large Telescope) and space-based (James Webb Space Telescope) will be able to discover, including exotic objects such as Population III stars, Dark Stars (stars producing energy through dark matter annihilation) and Lyman-a emitting galaxies. (Lyman-a, or simply Ly-α, is the transition from the first excited state of hydrogen to the ground state.)

 

Lyman-a detection at the Hubble Space Telescope (HST). Two thirds of ionizing photons are reprocessed by Ly-α which is the intrinsically strongest emission line in astrophysical nebulae. With a rest wavelength in the far ultraviolet (UV) at 122 nm it is furthermore observable from the ground for very distant galaxies (characterized by a high redshift, z > 2). Ly-α is a resonant line and becomes optically thick for moderate neutral hydrogen densities, meaning that the produced Ly-α radiation scatters before escaping a galaxy. The Lyman Alpha Reference Sample (LARS) aims at quantitatively and qualitatively improving our understanding of how Ly-α emission forms and is transported out of galaxies. It uses the UV capability of the Hubble Space Telescope (HST) to obtain high resolution Ly-α images and spectra of 14 nearby galaxies (with resolution 2 orders of magnitude higher than possible for distant galaxies). LARS is the largest Swedish-led project on HST (with a leader from OKC), and data collection was recently completed. The first results (which have featured prominently, e.g. on the HST web pages) show that Ly-α emission is considerably more extended than the continuum or other recombination lines, due to scattering. In June 2013, a proposed extension of LARS (eLARS) was awarded substantial time on HST which will increase the total number of galaxies by a factor of three. The LARS team developed the methods for Lyα imaging with HST and is responsible for all such images available in the literature. These results are important for interpreting observations of distant Ly-α emitting galaxies, which themselves are crucial probes for the evolution of structure in the universe.