Taipan is an ambitious survey planned for southern hemisphere galaxies, with the goal of mapping and measuring as many as one million galaxies in our Milky Way’s neighborhood. This will provide a deeper understanding of cosmology and galaxy evolution in the relatively nearby region of our universe.
There are more than a hundred billion galaxies in our visible universe. In order to refine our understanding of galaxies, their distribution and evolution, and of the overall cosmological properties of the universe, we want to sample a very large number of galaxies.
It is naturally easier to detect galaxies that are relatively nearby, and those that are more luminous.
Since the universe is expanding in an isotropic and homogeneous manner, galaxies are in general receding away from one another – in accordance with the Hubble relation below. The Taipan survey will explore our local neighborhood, with redshifts up to about 0.3.
For nearby galaxies,
V = cz = H*d
where V is the recession velocity, c is the speed of light, z is the redshift, H is the Hubble constant, and d is the galaxy’s distance. If we evaluate for z = 0.3 and the best estimate of the Hubble constant of 68 kilometers/second/Megaparsec, this implies a survey depth of 1300 Megaparsecs, or over 4 billion light-years.
The Taipan galaxy survey will begin next year and run for four years, using the UK Schmidt telescope, which is actually in Australia at the Siding Springs Observatory. Up to 150 galaxies in the field of view will be observed simultaneously with a fibre optic array. Of course the positions of galaxies is different in each field to be observed, so the fibers are robotically placed in the the proper positions. Many thousands of galaxies can thus be observed each night.
Short video of a Starbug fiber robot
One expected result will be refinement of the value of the Hubble constant, now uncertain to a few percent, reducing its uncertainty to only 1%.
The Taipan galaxy survey will also provide a better constraint on the growth rate of structure in the universe, decreasing the uncertainty down to about 5% for the low-redshift data points. This is a factor of 3 improvement and will provide a stricter test on general relativity.
The Taipan survey will also look at galaxies’ peculiar velocities, which are the deviations away from the general Hubble flow described in the equation above. These peculiar velocities reflect the details of the gravitational field – that is dominated by the distribution of dark matter primarily, and ordinary matter secondarily. On average galaxies are moving according to the Hubble equation, but in regions where the density of matter (dark and ordinary both) is higher than average they are pulled away from the Hubble flow toward any concentrations of matter. Bound galaxy groups and clusters form in such regions.
The mapping of peculiar velocities and the details of local variations in the gravitation field will enable fundamental tests of gravity on large scales.
Another of the important areas that Taipan will explore is how galaxies evolve from young active star-forming blue galaxies to older reddish, less active galaxies. Ordinary matter cycles through stars and the interstellar medium of a given galaxy. As stars die they shed matter which ends up in molecular clouds that are the sites of new star formation. Taipan will help to increase our understanding of this cycle, and of galaxy aging in general. Star formation slows down as more and more gas is tied up in lower mass, longer-lived stars, and the recycling rate drops. It also can be quenched by active galactic nuclei events (AGN are powered by supermassive black holes found at galactic centers).
Taipan will be the definitive survey of galaxies in the southern hemisphere, and is expected to significantly add to our understanding of galaxy evolution and cosmology. We look forward to their early results beginning in 2016.