Galaxy Evolution, Cosmology and Dark Energy
Our Universe is expanding, and we have recently discovered that this expansion is accelerating. Scientists are still trying to understand why this is happening, with theories such as a force known as “dark energy” being put forward. The detection of this acceleration won the Nobel Prize for physics in 2011.
This mysterious acceleration, however, is still one of the most fundamental mysteries of modern science and also one of the key questions the SKA will try to address. One of the main science goals of the SKA is to investigate why this acceleration is taking place, by looking at the distribution of the most basic element, Hydrogen, throughout the cosmos. The SKA’s unrivalled sensitivity will be able to track young, newly forming galaxies at the edge of the known Universe, and by mapping the distribution of Hydrogen, help us unravel this key mystery, and understand more how the earliest galaxies evolved.
Hydrogen is the most abundant element in the Universe and is the raw material from which stars form. Over 70% of our Sun is Hydrogen, and as a radio source, the Sun is one of the brightest objects not only in the visible sky but also in the radio.
Hydrogen atoms produce radio emission at a wavelength of 21cm or a frequency of 1420 MHz. This emission was first discovered from the hydrogen gas clouds within our Milky Way Galaxy in the 1930s by Karl Jansky, an engineer at Bell Laboratories.
Since then, hydrogen gas has been found in tens of thousands of galaxies, most of which are relatively near to the Milky Way. Generally, astronomers find that spiral galaxies, like our Milky Way Galaxy, and irregular galaxies, like the Magellanic Clouds, often contain large amounts of hydrogen gas. These galaxies also form stars and astronomers believe that hydrogen gas provides the raw fuel for star formation.
Aside from dark energy, this mysterious force thought to be causing the acceleration of the expansion in the Universe, there is also Dark Matter, which astronomers believe makes up a large fraction of all matter in the Universe. The study of this came about after anomalies were found in the rotation rates and masses of galaxies, based on their visible characteristics and the values derived from observation.
The SKA will revolutionise our study of how galaxies form and transform their gas into stars by detecting hydrogen gas in surveys which encompass as many as a billion galaxies, at distances much greater than is possible to detect today.
Cosmology is the study of the origin and fate of the Universe. Prior to the discovery that the expansion of the Universe is actually accelerating, astronomers had thought that the expansion should be slowing under the mutual gravitational attraction of galaxies. Instead, it seems that the Universe contains some additional component, which astronomers have termed “dark energy”
This mysterious force appears to counteract and even surpass the mutual gravitational attraction causing acceleration in the expansion. It could however indicate that our understanding of gravity, as described by Einstein’s General Theory of Relativity, is incomplete.
There are two broad approaches that the SKA will pursue in studying cosmology:
- The first approach involves a large survey of galaxies, searching for their (redshifted) 21 cm hydrogen emission. An extremely large survey of galaxies is required in order to sample a large enough of volume in the Universe that one can detect relatively subtle effects.
- Another approach by which the SKA can study cosmology and dark energy is to observe the gravitational effects of galaxies and clusters of galaxies on the path of radio waves through the Universe.
Astronomers typically measure distances to very distant galaxies using the redshift effect. Consider observing the hydrogen gas in a distant galaxy. To an astronomer in that galaxy, the hydrogen gas emits radio emission at its characteristic 21cm wavelength.
However, because the galaxy is distant, it takes some time for the radio emission to travel to astronomers here on Earth. During that time, the Universe expands, which has the effect of increasing the wavelength at which we detect the radio emission from the hydrogen gas. The difference between the observed wavelength and expected wavelength is then a measure of how distant the galaxy is as well as being a measure of how much time has elapsed since the light was emitted.
For instance, with a large sample of galaxies, one can track how galaxies form into clusters. How quickly the clusters form is partly a balancing act between gravity, which causes galaxies to fall together into a cluster, and dark energy, which acts to separate the galaxies. How big, and how rapidly, clusters form is a measure of the strength of dark energy, which can help astronomers understand better what it is.
Einstein’s Theory of Relativity relates mass to energy and shows that both mass and energy contribute to gravitation. Concentrations of mass have the effect of disturbing the path that radio waves take in their path to the Earth. In effect, concentrations of mass, such as galaxies and clusters of galaxies, can act as giant lenses in space.
Thus, if there is a galaxy behind another galaxy, or behind a cluster of galaxies, the shape of the background galaxy will appear distorted because the path(s) that the radio waves have taken has been distorted by the foreground galaxy or cluster.
By measuring the amount of distortion of background galaxies, astronomers can infer how much mass (both regular matter and dark matter) is between the background galaxies and us and a measure of how this mass is distributed. In turn, how the mass is distributed can be affected by the properties of dark energy, and aspects of cosmology. Thus, measurements of the shapes of large numbers of galaxies can be used to constrain models of the cosmology of the Universe.
How do galaxies evolve?
Radio telescopes have played a pivotal role in the understanding of galactic evolution. Their ability to “see” regions beyond the optical view of a galaxy have brought significant insights into how galaxies form and develop. However, despite this progress, it is still a mystery as to how the early galaxies, in the millions of years following the Big Bang, began to evolve: where did they get their material? What drives their rotation? What has shaped them? The SKA’s unrivalled sensitivity and resolution will be able to track young, newly forming galaxies at cosmological distances, and, through mapping the distribution of Hydrogen, help us unravel these key mysteries.
Rotation & dark matter
Hydrogen gas moving at different velocities within a galaxy will be detected at slightly different frequencies because of the Doppler effect. Astronomers can infer how quickly a galaxy is rotating by measuring the range of frequencies over which a galaxy’s 21cm radiation occurs. From these measurements, it is possible to deduce the total mass of the galaxy, as it must have enough mass to ensure that it remains whole (and essentially does not fly apart!). This amount of mass can be compared with that estimated from the stars and gas that we can visibly observe in the galaxies.
Often the hydrogen gas is rotating faster, sometimes much faster, than the amount of mass contributed by the stars and gas would suggest, implying there has to be some other kind of matter within galaxies – so-called dark matter – that produces no light, but produces gravitational attraction such that the galaxy does not fly apart.
Origin of gas & satellite galaxies
Observations of the hydrogen gas in spiral galaxies, like our Milky Way Galaxy, are revealing small clouds of hydrogen at large distances from the galaxy, but how did that gas get there? There are at least three possibilities:
- The gas has been blown out of the galaxy by powerful winds from hot, young stars, and once sufficiently far away from the influence of the stars, has started to fall back onto the galaxy.
- The gas represents ‘pristine’ or ‘primordial’ material from the very early Universe. It is possible that not all of the hydrogen in the Universe was captured within galaxies. Some of it is likely to still be in the space between the galaxies. Over time, that gas may slowly fall into galaxies, probably in the form of small clouds.
- The gas represents starless satellite galaxies. Numerical simulations of how galaxies form suggest that a major galaxy like the Milky Way should be surrounded by many smaller galaxies. However, various attempts to find such a quantity of smaller satellite galaxies have been unsuccessful. It is possible that some of these satellite galaxies have not yet formed stars, but consist only of gas (or the numerical simulations are not fully describing all of the physics!).
Observations of many more galaxies are required to distinguish between the possibilities, and the SKA will conduct surveys for clouds of gas and search for undiscovered star-less satellite galaxies lurking around major galaxies.
Both the star light and the hydrogen gas in galaxies in the relatively local Universe have been mapped in exquisite detail over the last few years by projects which are providing key scientific input into the SKA 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 with the SKA is to provide equally good measurements in the distant Universe. Such measurements will enable astronomers to track how galaxies acquired the hydrogen gas, from which stars could form, as well as track the various processes by which galaxies might gain or even lose gas.
How does the increased sensitivity and resolution of the SKA play a role in this work?
Sensitivity is a measure of the minimum signal that a telescope can distinguish above the random background noise. The more sensitive a telescope, the more light it can gather from faint and distant objects.
The SKA’s sensitivity stems from the huge number of radio receivers at low, mid and high frequencies, which will combine in each frequency range from the locations in Africa and Australia to form a collecting area equivalent to a single radio telescope 1km wide.
Resolution is a measure of the minimum size that a telescope can distinguish, effectively going from a blurry image to discerning the detail. The large distances between receivers of the SKA will provide the ability to distinguish the details. The combined factors of sensitivity and resolution will dwarf all existing telescopes currently in operation, and give the SKA an unparalleled view of the early formation of our Universe.
- A small portion of the hiss you get when you detune an old style analogue TV was due to radio waves from the Big Bang itself, that is the birth of the Universe.
- Data from space telescopes looking at the Cosmic Microwave Background sky has shown the Universe is approximately 13.8 billion years old.
- The SKA will enable scientists to study this galactic formation at distances much greater than is possible to detect today.
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.
In addition to detecting hydrogen emission in galaxies, the SKA will 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 (dust hiding objects behind it, which affects optical telescopes).
With radio surveys, the dust which blocks a large proportion of the visible light is penetrated, revealing the complex structures and individual stars which lie within.
It will be of great interest to scientists who will be able to link the star formation properties of galaxies to their hydrogen 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, distortions similar to that created by a lens being placed in front of an object being seen in visible light, but using the force of gravity instead of a lens.
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.