Final Report Summary - CODE (Constraining Dark Energy: an observational study of the properties of dark energy and dark matter)
The discovery that the expansion of the Universe is accelarating due to an unknown "dark energy" and that most of the matter is invisible, highlights our lack of understanding of the major constituents of the Universe. The importance of this discovery was recognised by the 2011 Nobel Prise in physics. These surprising findings set the stage for research in cosmology at the start of the 21st century. The objective of the research supported by this grant is to advance observational constraints by improving techniques that probe the growth of large-scale structure in the Universe.
The main data set for CoDE is the Canada-France-Hawaii-Telescope Legacy Survey (CFHTLS), in which Dr. Hoekstra is a lead researcher. The main aim of this project is to study the matter distribution in the universe and constrain cosmological parameters. Although the next generation of surveys has now started, the recently completed analysis of the CFHTLS data represent the state-of-the-art. The lesson learned will help improve the results of new surveys. The first papers describing in detail the extensive testing that has been carried out have been published. Furthermore several papers with science results have been submitted. In addition, the resulting catalog will be made public to the wider community in November 2012. Key science results include a significant improvement in the constraints on the dark energy equation of state from lensing measurements and a unique test of alternative theories of gravity. The lensing group in Leiden has led to work on the study of dark matter halos around galaxies, which has led to improved scaling relations for blue and red galaxies.
The research supported by CoDE makes use of two other surveys that rely on CFHT data: the second generation Red-sequence Cluster Survey (RCS2) is a large survey for clusters of galaxies and the Canadian Cluster Comparison Project (CCCP; PI Hoekstra) is a detailed multi-wavelength study of massive clusters. The former data set formed the basis for the PhD thesis of Edo van Uitert (in Leiden), whereas the latter data are part of the thesis of Chris Bildfell (in Victoria). The RCS2 data have been used to study the properties of dark matter halos around galaxies, both their mass and shapes. The work on the shapes of dark matter halos is the most comprehensive to date, even though definitive results require an even larger data set. The study of the mass-richness relation of galaxy clusters and its evolution is nearly completed. This work has resulted in a nice thesis and has proven to be important preparatory work for the next generation of wide-field imaging surveys, most notably the recently started KiloDegree Survey (KiDS).
The results of the weak lensing analysis of the CCCP were published and a paper describing the X-ray analysis has been submitted. An important conclusion of this work is that the gas mass is the most robust indicator of cluster mass. In fact for low entropy systems we have accounted for all sources of scatter. However, the X-ray pseudo-pressure, i. e. the product of gas mass and temperature, may be more useful for cosmological studies because the intrinsic scatter appears to be independent of dynamical state. Finally we confirmed our earlier finding that hydrostatic masses are biased low. This is very important for cosmological cluster studies based on X-ray observations. Our results have convinced the wider community of the importance of a weak lensing mass calibration. This has already led to a collaboration with the South Pole Telescope team to calibrate their cluster masses using weak lensing. The CCCP data have also been used to study the properties of the brightest cluster galaxies as well as the evolution in the ratio of the number of faint and bright galaxies in clusters.
The reintegration has provided Dr. Hoekstra with the means to interact in person with researchers involved in CCCP and CFHTLS. The grant was also used to partially fund a PhD student to work with him in Leiden on the weak lensing analysis of the RCS2 data and to interact with a PhD student in Victoria. Dr. Hoekstra was granted tenure in 2010. Furthermore, in early 2011 Dr. Hoekstra was selected as one of two weak lensing cosmology science coordinators for the Euclid mission thanks to his experience in the field of weak gravitational lensing. The work carried out by Dr. Hoekstra, made possible in part by CoDE, has helped in the selection of this exciting project. As a result, Euclid is now the next cosmology selected by ESA for launch in 2019. This is a large European projecting involving nearly 1000 scientists from 13 (and increasing) European countries, and it is therefore safe to say the Dr. Hoekstra has been fully integrated in European astronomy.
The main data set for CoDE is the Canada-France-Hawaii-Telescope Legacy Survey (CFHTLS), in which Dr. Hoekstra is a lead researcher. The main aim of this project is to study the matter distribution in the universe and constrain cosmological parameters. Although the next generation of surveys has now started, the recently completed analysis of the CFHTLS data represent the state-of-the-art. The lesson learned will help improve the results of new surveys. The first papers describing in detail the extensive testing that has been carried out have been published. Furthermore several papers with science results have been submitted. In addition, the resulting catalog will be made public to the wider community in November 2012. Key science results include a significant improvement in the constraints on the dark energy equation of state from lensing measurements and a unique test of alternative theories of gravity. The lensing group in Leiden has led to work on the study of dark matter halos around galaxies, which has led to improved scaling relations for blue and red galaxies.
The research supported by CoDE makes use of two other surveys that rely on CFHT data: the second generation Red-sequence Cluster Survey (RCS2) is a large survey for clusters of galaxies and the Canadian Cluster Comparison Project (CCCP; PI Hoekstra) is a detailed multi-wavelength study of massive clusters. The former data set formed the basis for the PhD thesis of Edo van Uitert (in Leiden), whereas the latter data are part of the thesis of Chris Bildfell (in Victoria). The RCS2 data have been used to study the properties of dark matter halos around galaxies, both their mass and shapes. The work on the shapes of dark matter halos is the most comprehensive to date, even though definitive results require an even larger data set. The study of the mass-richness relation of galaxy clusters and its evolution is nearly completed. This work has resulted in a nice thesis and has proven to be important preparatory work for the next generation of wide-field imaging surveys, most notably the recently started KiloDegree Survey (KiDS).
The results of the weak lensing analysis of the CCCP were published and a paper describing the X-ray analysis has been submitted. An important conclusion of this work is that the gas mass is the most robust indicator of cluster mass. In fact for low entropy systems we have accounted for all sources of scatter. However, the X-ray pseudo-pressure, i. e. the product of gas mass and temperature, may be more useful for cosmological studies because the intrinsic scatter appears to be independent of dynamical state. Finally we confirmed our earlier finding that hydrostatic masses are biased low. This is very important for cosmological cluster studies based on X-ray observations. Our results have convinced the wider community of the importance of a weak lensing mass calibration. This has already led to a collaboration with the South Pole Telescope team to calibrate their cluster masses using weak lensing. The CCCP data have also been used to study the properties of the brightest cluster galaxies as well as the evolution in the ratio of the number of faint and bright galaxies in clusters.
The reintegration has provided Dr. Hoekstra with the means to interact in person with researchers involved in CCCP and CFHTLS. The grant was also used to partially fund a PhD student to work with him in Leiden on the weak lensing analysis of the RCS2 data and to interact with a PhD student in Victoria. Dr. Hoekstra was granted tenure in 2010. Furthermore, in early 2011 Dr. Hoekstra was selected as one of two weak lensing cosmology science coordinators for the Euclid mission thanks to his experience in the field of weak gravitational lensing. The work carried out by Dr. Hoekstra, made possible in part by CoDE, has helped in the selection of this exciting project. As a result, Euclid is now the next cosmology selected by ESA for launch in 2019. This is a large European projecting involving nearly 1000 scientists from 13 (and increasing) European countries, and it is therefore safe to say the Dr. Hoekstra has been fully integrated in European astronomy.