Hydrogen gas moving at different velocities within a galaxy will be detected at slightly different frequencies becuase of the Doppler effect. Astronomers can infer how quickly a galaxy is rotating by measuring the range of frequencies over which a galaxy’s 21 cm radiation occurs.
Measurements such as these provided some of the first evidence in support of dark matter. That is, astronomers measured how fast various galaxies were rotating. From the rotation rate, one can deduce a 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 observe in the galaxies.
Often the hydrogen gas is rotating faster, sometimes much faster, than the amount of mass contributed by the stars would suggest. There has to be some other kind of matter within galaxies – dark matter – that produces no light, but produces gravitational attraction so that the galaxy does not fly apart.
As another example, 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. How did that gas get there?
One possibility is that the powerful winds from hot, young stars can blow some of a galaxy’s gas to large distances from the galaxy. Once sufficiently far away from the winds, the gas then falls back into the galaxy.
An alternate possibility is that this gas represents ‘pristine’ or ‘primordial’ material from the very early Universe. It is possible that not all of the hydrogen in the Universe has been 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. Observations of many more galaxies are required to distinguish between these possibilities.
A final example concerns ‘satellite galaxies’. Numerical simulations of how galaxies form suggest that a major galaxy like the Milky Way Galaxy should be surrounded by many smaller galaxies. However, various attempts to find such smaller satellite galaxies have been unable to find enough galaxies.
One possibility is that some of these satellite galaxies have not yet formed stars, but consist only of gas. Another possibility is that the numerical simulations are not fully describing all of the physics. Are there undiscovered star-less satellite galaxies lurking around major galaxies or are the numerical simulations incomplete? One way to resolve this question is to use the SKA to conduct surveys for clouds of gas and search for galaxies that have not yet formed stars.
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 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. 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.
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