Monthly Archives: September 2015

Dark Axion Stars

In a post from 4 years ago I discussed “Dark Matter Powered Stars”.

The context here was neutralino dark matter, which is a possible explanation for very massive stars in the early universe. The idea is that the very first stars could be thousands of solar masses, much greater than is possible with ordinary matter dominated stars. They would be powered by dark matter annihilation in their cores during the early part of their life. They would eventually collapse to black holes and could be candidates to seed supermassive black holes found at the center of many galaxies.


Hubble Space Telescope image of Sirius A and Sirius B (lower left) 

NASA, ESA, H. Bond (STScI), and M. Barstow (University of Leicester)

Another dark matter candidate apart from the neutralino is the axion. While the neutralino is expected to have masses in the several to tens of GeV (Giga-electron-Volts), the axion mass is a tiny fraction of an eV, at least a trillion times smaller than the expected neutralino mass. So there would be many more of them, of course, to explain the amount of dark matter we detect gravitationally.

Neither neutralinos nor axions have been discovered to date. The axion does not require supersymmetry beyond the Standard Model of particle physics, so in that sense it is a more conservative proposed candidate.

Currently we detect dark matter only through its gravitational effects – in galaxies, in clusters of galaxies, and at the very large scale by looking at thermal variations in the cosmic microwave background radiation.

In addition there are three main direct methods to try to ‘see’ these elusive particles. One is to directly detect dark matter (e.g. neutralinos) here on Earth when it collides with ordinary matter – or in the case of axions – generates photons in the presence of a magnetic field. Another is to attempt to create it at the Large Hadron Collider, and the third is to look in space for astrophysical signals resulting from dark matter. These could include gamma rays produced in the galactic center when dark matter mutually annihilates.

In a paper recently published in the journal Physical Review Letters and titled “Accretion of dark matter by stars”, Richard Brito, Vitor Cardoso and Hirotada Okawa discuss a different kind of dark star, one whose dark matter component is axions. The paper is available here.

There are two formation scenarios envisaged. The first is that dark matter (axion) stellar cores form and then these accrete additional dark matter and ordinary matter. In the second scenario, a star forms primarily from ordinary matter, but then accretes a significant amount of dark matter.

We are talking about dark matter fractions which may be say 5% or 20% of the total mass of the star.

The authors find that stable configurations seem to be possible and that the axion dark matter may lead to stellar oscillations in the microwave band. So looking for stellar oscillations in the Gigahertz range may be another astrophysical detection method for dark matter. They intend to explore the idea more deeply in future research.


TAIPAN: A Million Galaxy Survey

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.