KiDS Scientific goals
The central science case for KiDS and its near-infrared sibling VIKING is mapping the matter distribution in the universe. This will be done through weak gravitational lensing and photometric redshift measurements. However, the enormous data set that KiDS will produce, can also be used for many different applications.
Here follow brief descriptions of the main scientific topics that the KiDS team will focus on: dark energy, structure of galaxy halos, evolution of galaxies and clusters, and stellar streams and the Galactic halo.
We live in an expanding universe, but the exact nature of this expansion and how it evolves through time is not yet understood. Dark energy manifests itself in the expansion history of the universe, as a repulsive term that appears to accelerate the expansion. Understanding its properties is one of the central quests of present-day cosmology. With KiDS we intend to push this question as far as possible, while at the same time the serving as a learning curve for future (space-based?) experiments.
The main techniques that will be used to map the matter distribution in the universe are weak gravitational lensing and photometric redshift measurements. The power of weak lensing as a cosmological probe relies on two facts: gravitational lensing is a very geometric phenomenon, and it is sensitive to mass inhomogeneities along the line of sight. This makes it a good probe of the growth of structure with time (redshift), as well as being a purely geometric distance measure. As it happens, the distance-redshift relation and the speed with which overdensities grow with cosmic time are the two most fundamental measures of the energy content of the universe: both depend directly on the rate with which the universe expands.
Making this measurement is therefore of great interest. Although weak lensing is in principle an excellent method for this, the challenges are great. Lensing measurements need to be free of systematics to better than 1% accuracy, and photometric redshifts unbiased to a similar level. This is not easy to achieve, but with KiDS we are in the optimal position to attempt this, by ensuring the best image quality in our instrument, by choosing a survey depth and area appropriately, and having a wide wavelength coverage that will make the photometric redshifts as free of errors as is possible with wide-band photometry. The strength of KiDS in terms of the influence of systematics is illustrated in Figure 1.
Comparison of the formal statistical power and sensitivity to
systematic errors in the photometric redshifts for the KiDS/VIKING and
DES/VHS surveys in cosmological parameter estimation (here the matter
density parameter Ωm and the dark energy equation of state parameter
w0), based on a tomographic analysis of simulated, realistic
photometry in each of the surveys (Szomoru, Hildebrandt and Hoekstra,
private comm., from Szomoru's MSc thesis.) The '+' represents the
input truth. The coloured contours assume perfect redshift
information, and the dashed contours show the effect of redshift
errors. Flat geometry was assumed here but otherwise no external
information (such as CMB analysis) were included. Once external
information is folded in constraints tighten and systematic effects
become even more significant.
Structure of galaxy halos
On large scales the structure and shape of galaxy halos is well constrained by simulations of structure formation. However, the inner regions of galaxy halos are not realistically represented by these simulations due to the influence of complex physics such as star formation, cooling, and feedback. The relation between light (baryons) and mass (dark matter) is crucial for our understanding of the influence of the dark matter on galaxy formation and evolution, and vice-versa.
Galaxy-galaxy lensing, the gravitational lensing effect of foreground galaxies on the images of background galaxies, provides a unique way to study this relation between galaxies and their halos. Since the effect is very weak, it can only be measured statistically, namely by stacking large numbers of foreground galaxies and measuring the net image distortion of background galaxies. The potential rewards make the effort worth-while, because by measuring this distortion at different scales, galaxy-galaxy lensing can probe halos over a large range of scales: from the inner regions of galaxy halos to group halos.
Also for galaxy-galaxy lensing the strength of KiDS is two-fold. The vast size and thus enormous numbers of available galaxies allow the foreground galaxies to be split in bins of different galaxy types. Because of the accurate photometric redshifts they can also be split into different redshift bins, thus allowing the redshift dependence to be studied. Compared to earlier studies, the image quality and sensitivity delivered by KiDS will provide more foreground-background pairs, more accurate shape measurements, and the ability to probe the galaxy population up to higher redshifts.
Evolution of galaxies and clusters
Within the current cosmological paradigm of Cold Dark Matter, structures form hierarchically and dark matter halos are the same at all scales. Some of the ramifications of this picture have alluded rigorous observational testing. For example, the influence of mergers on the evolution of the galaxy population at redshifts higher than ~0.5 is poorly constrained, with various observational constraints differing up to an order of magnitude. Also, galaxy clusters probe the highest mass peaks in the universe, but at redshifts higher than 1 the number of known galaxy clusters is still too small to constrain cosmological models.
Owing to its photometric sensitivity, KiDS can play a major role in this field. The estimated 100 million galaxies KiDS will detect will have a median redshift of z=0.8, with approximately 20% having redshifts between 1 and 1.5. Such a sample will enable the evolution of the galaxy luminosity function, the build-up of stellar mass, and the assembly of early-type galaxies to be traced back to unprecedented look-back times. Cluster finding will also be directly possible from the KiDS catalogs, with an estimated yield of 10 to 20 thousand clusters. With the red sequence detectable out to a redshif of z~1.4, approximately 5% of these clusters will be located beyond a redshift of 1, providing an important sample for constraining cosmological parameters.
Another perspective of galaxy evolution and galaxy populations in clusters will be provided by KiDS by virtue of the fact that the KiDS-S field overlaps with two nearby superclusters: Pisces-Cetus and Fornax-Eridanus. This allows detailed studies of galaxy properties as function of environment, all the way from cluster cores to the infall regions, and the filaments that connect clusters in the cosmic web.
Stellar streams and the Galactic halo
Studies of the stellar halo of the Milky Way require photometry of faint stars over large areas of sky. The Sloan Digital Sky Survey, although aimed primarily at cosmology and high-redshift science, has proved to be a milestone in Milky Way science as well. Its sensitive photometry over a quarter of the sky has unveiled many stellar streams and previously unknown faint dwarf Spheroidal galaxy satellites of the Milky Way. While KiDS will image a smaller area, it is significantly deeper and therefore will provide a more distant view into the Galactic halo. Perhaps more importantly, while Sloan covered the Northern Sky, KiDS will target the Southern hemisphere which until now is still uncharted territory. Particularly in the KiDS-S field, new discoveries may lie in waiting in the direct vicinity of our own Galaxy.