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The advent of modern astronomy about 20 years ago has seen a huge shift from data-starving science to data-flooding science. Galaxy surveys have become a key tool in this transition.
Spectroscopic surveys provide precise information about the radial modes of galaxies, which are important for calibrating photometric redshift uncertainty. They also provide information about galaxy shapes and surface brightnesses which can be exploited for sample selection.
In the coming decade, sky surveys will image and spectroscopically probe the vast expanse of the extragalactic sky to unprecedented depths and resolution. Such surveys will be able to transform the understanding of cosmology and galaxy evolution, in particular the epochs when the Universe began to form and evolve, and when the universe began to become dominated by dark matter.
Among the science drivers for these surveys are the search for high redshift quasars, galaxy clusters, and stellar masses. They will also be a key source of data for studies of the dark energy equation of state by examining the 'baryon wiggles' in the matter power spectrum, and by observing the 'weak lensing' effect on galaxy luminosity.
The GOGREEN survey builds on the strengths of multi-object spectroscopy to study the rich early environments of the Universe, when galaxies were only a third of their present age. The GOGREEN sample includes spectroscopically confirmed members that span a wide range of mass, representing the building blocks from which today's clusters were formed.
Photometry is the study of the propagation of light through space or material. It is a subset of radiometry, and the radiation studied is confined to the visible part of the spectrum (VIS).
In photometric measurements, four physical quantities are used: luminous flux, illuminance, luminance and intensity. These are adapted or specialized to represent the response of the human eye to different wavelengths of light.
Typically, modern galaxy surveys rely on the blending of source populations to reduce the survey area needed for galaxy number density estimates and therefore shape or photometry measurements. For example, in the Legacy HST Archival proposal a fraction of galaxies are surveyed using images from different ground- and space-based surveys.
Spectroscopy is one of the most important techniques used in galaxy surveys to map the redshifts of nearby galaxies. These data can be used to study large-scale structure and the properties of the galaxy population in the local universe.
However, the spectroscopic survey time required to obtain a high-precision redshift of each galaxy is typically very expensive. This is because the slitless (grism) spectroscopy used in these surveys relies on a very large number of exposures, each of which takes time to integrate.
In order to minimize the number of passbands required for a high-resolution spectroscopic survey, it is often necessary to slice the galaxy distribution into concentric redshift bins. This is referred to as a thick redshift binning, but in practice this approach can lead to loss of information due to the correlations between adjacent z-bins that are not taken into account.
Galaxy surveys have provided a wealth of data on galaxy clustering and dark matter distribution in the Universe. Modern surveys have increased the number of galaxies covered by tenfold, allowing high-order correlation functions to be measured for the first time on scales that can constrain the cosmological models of matter formation.
These measurements can have significant implications for our understanding of the Universe and its physics. They can be used to test modified theories of gravity by measuring the growth rate of structure, the CDM model by measuring the halo mass function and galaxy formation models by measuring the star formation efficiency in groups.
To exploit this data for cosmological parameter estimation, the survey design should be optimised in such a way as to maximise its information content on the relevant parameters of a given cosmological model. Here, we develop new analytical expressions for the cosmic variance of key cosmological parameters constrained from the galaxy power spectrum of any survey, and we present a technique for survey optimisation.
