No abstract

The Square Kilometre Array (SKA) will conduct the biggest spectroscopic galaxy survey ever, by detecting the 21cm emission line of neutral hydrogen (HI) from around a billion galaxies over 3 /4 of the sky, out to a redshift of z ∼ 2. This will allow the redshift-space matter power spectrum, and corresponding dark energy observables, to be measured with unprecedented precision. In this paper, we present an improved model of the HI galaxy number counts and bias from semi-analytic simulations, and use it to calculate the expected yield of HI galaxies from surveys with a variety of Phase 1 and 2 SKA configurations. We illustrate the relative performance of the different surveys by forecasting errors on the radial and transverse scales of the baryon acoustic oscillation (BAO) feature, finding that the full "billion galaxy survey" with SKA2 will deliver the largest dark energy figure of merit of any current or future large-scale structure survey.

The standard concordance model of the Universe is based on the cosmological constant as the driver of accelerating expansion. This concordance model is being subjected to a growing range of inter-locking observations. In addition to using generic observational tests, one can also design tests that target the specific properties of the cosmological constant. These null tests do not rely on parametrizations of observables, but focus on quantities that are constant only if dark energy is a cosmological constant. We use supernova data in null tests that are based on the luminosity distance. In order to extract derivatives of the distance in a model-independent way, we use Gaussian Processes. We find that the concordance model is compatible with the Union 2.1 data, but the error bars are fairly large. Simulated datasets are generated for the DES supernova survey and we show that this survey will allow for a sharper null test of the cosmological constant if we assume the Universe is flat. Allowing for spatial curvature degrades the power of the null test.

This chapter describes the assumed specifications and sensitivities for HI galaxy surveys with SKA1 and SKA2. It addresses the expected galaxy number densities based on available simulations as well as the clustering bias over the underlying dark matter. It is shown that a SKA1 HI galaxy survey should be able to find around 5 × 10 6 galaxies over 5,000 deg 2 (up to z ∼ 0.8), while SKA2 should find ∼ 10 9 galaxies over 30,000 deg 2 (up to z ∼ 2.5). The numbers presented here have been used throughout the cosmology chapters for forecasting.Advancing Astrophysics with the Square Kilometre Array

Extending deep observations of the neutral atomic hydrogen (H i) to the environment around galaxy groups can reveal a complex history of group interactions which is invisible to studies that focus on the stellar component. Hickson Compact Group 44 (HCG 44) is a nearby example and we have combined H i data from the Karoo Array Telescope, Westerbork Synthesis Radio Telescope, and Arecibo Legacy Fast ALFA survey, in order to achieve high column density sensitivity (N HI < 2 × 10 18 cm −2 ) to the neutral gas over a large field-of-view beyond the compact group itself. We find the giant H i tail north of HCG 44 contains 1.1 × 10 9 M ⊙ of gas and extends 450 kpc from the compact group: twice as much mass and 33% further than previously detected. However, the additional gas is still unable to account for the known H i deficiency of HCG 44. The tail likely formed through a strong tidal interaction and H i clouds in the tail have survived for 1 Gyr or more after being stripped. This has important implications for understanding the survival of neutral clouds in the intragroup and circumgroup medium, and we discuss their survival in the context of simulations of cold gas in hot halos. HCG 44 is one of a growing number of galaxy groups found to have more extended H i in the intragroup and circumgroup medium than previously measured. Our results provide constraints for simulations on the properties of galaxy group halos, and reveal a glimpse of what will be seen by future powerful H i telescopes and surveys.