Galaxy evolution, cosmology and dark energy

The most striking feature of the distribution of galaxies that surrounds us is “clustering”. Galaxies are not sprinkled uniformly through the Universe, but are concentrated into a complex network of dense knots, walls and filaments. This pattern, called large-scale structure, is an important source of data for cosmologists. It encodes a wealth of valuable information about the underlying contents of the Universe, and about the processes by which galaxies formed and grew under gravity.

The local galaxy distribution has been mapped in exquisite detail over the last few years by projects such as the HI Parkes All Sky Survey (HIPASS), Arecibo Legacy Fast ALFA (ALFALFA) survey, the 2dF Galaxy Redshift Survey and the Sloan Digital Sky Survey (SDSS). Our challenge now is to provide equally good measurements in the distant Universe. Owing to the time taken for light from distant galaxies to reach us, such observations see the Universe at an earlier epoch. Comparing the properties of galaxies then and now enables us to test models which describe the formation of structure in the Universe, and to link different classes of galaxy in an evolutionary sequence.

The SKA can be used to observe radio waves originating from atomic processes in neutral hydrogen atoms (21 cm emission). Hydrogen is the most common atomic element in the Universe, and a critical component in the formation of galaxies. However, observations of 21 cm emission from distant galaxies have hitherto been almost impossible owing to the lack of sensitivity of current instruments. The vast collecting area of the SKA will make such observations possible for the first time.

Simulation of bayonic oscillations in the power spectrum of the clustering of HI emission galaxies as a function of redshift. Credit: Chris Blake

The results will be astounding: during its operation, the SKA will be able to identify 21 cm emission from a billion galaxies over the whole sky. Radio waves from the most distant of these objects will have taken 9.5 billion years to reach us, two-thirds of the age of the Universe (which is 13.7 billion years). Furthermore, mapping the 21 cm emission line also determines an accurate distance to each galaxy: as these radio waves travel through the cosmos to our telescope, the overall expansion of the Universe stretches their wavelength. The more distant the galaxy, the greater the stretching factor, and the higher the observed wavelength. Hence as it collects data, the SKA forms a three-dimensional picture of the cosmic web of galaxies.

This unprecedented map will enable a host of interesting measurements. In particular, we will be able to probe the mysterious “dark energy” which is believed to constitute the majority of today’s Universe. In recent years, astronomers have made the astonishing discovery that the overall expansion of the Universe is not slowing down (due to the pull of gravity) but is actually accelerating! This remarkable fact was deduced from observations of distant supernovae, which appear significantly dimmer than one would expect in a decelerating Universe. This accelerating expansion must be driven by a new component of the Universe which is gravitationally repulsive, known as “dark energy”. Using the exquisite galaxy clustering maps produced by the SKA, we will be able to investigate the nature and effects of dark energy.

The SKA will enable revolutionary progress in the fields of galaxy evolution and large-scale structure.  The cosmic history of neutral hydrogen is a critical ingredient of galaxy evolution, but it has been charted only in the very local Universe. An HI emission-line survey is able to map out galaxies independently of dust extinction, with one additional advantage: once the galaxy has been located on the sky, the observed wavelength of the emission line automatically provides an accurate redshift, locating the object’s position in the three-dimensional cosmic web. The SKA will hence become the premier machine for delineating the large-scale structure of the Universe. With its exquisite sensitivity, in very deep observations over small areas of the sky, the SKA will be able to image typical galaxies at redshifts z ~ 3 via 21cm emission, when the Universe was only 2 billion years old. Moreover, with a wide enough field-of-view (> 10 sq deg), the SKA will conduct a somewhat shallower survey, to redshifts z ~ 1.5, over the entire sky. Such a survey encompasses an immense volume of the Universe, and is likely to contain one billion H I emission galaxies, and will be a powerful tool for cosmology.

The result would be the most accurate measurement of the clustering pattern of galaxies ever achieved, testing theoretical models for the growth of structure in the Universe and pinpointing the cosmological parameters (in conjunction with Cosmic Microwave Background data from the Planck satellite). For example, this survey would permit an accurate quantification of the properties of the mysterious “dark energy”, which is believed to compose 70 per cent of the current energy density of the Universe, and which is driving the cosmic expansion to accelerate (as evidenced by observations of distant supernovae). One of the cleanest methods of measuring dark energy in the Universe is by accurately delineating the small-amplitude “acoustic oscillations” in the clustering power spectrum. This baryonic signature has an identical physical origin to the acoustic peaks already identified in the CMB, which act as an accurate standard ruler for the experiment. Their recovery in the SKA HI survey, as a function of redshift, permits an extremely accurate determination of the rate of evolution of the equation of state of dark energy with cosmic time, discriminating between theoretical dark energy models. Thus an SKA cosmic structure survey can potentially disprove Einstein’s cosmological constant.

In addition to detecting vast numbers of HI emission line galaxies, the SKA can also perform the deepest-ever radio continuum survey, probing the star formation history of the Universe as a function of redshift in a manner independent of the dust extinction that confuses experiments in optical wavebands. It will be of great interest to link the star formation properties of galaxies to their HI contents, as a function of redshift and environment. Furthermore, a high-resolution radio continuum survey over wide areas allows a precise measurement of the coherent shape distortions of distant galaxies imparted by the foreground cosmic web. This “weak gravitational lensing” encodes a vast body of cosmological information, and its exploitation will become one of our key cosmological probes within the next decade. Radio wavebands are particularly advantageous for this experiment because the point-spread function is well-determined and stable (being simply the interferometer baseline distribution), solving the principle systematic difficulty inherent in the method.