Every astronomy student learns that in galaxies stars form from huge gas clouds. However, the details of the accretion and distribution of gas in galaxies is still unclear. Therefore, an international group of scientists at MPA and ASTRON in the Netherlands joined forces and carried out the Bluedisk project to map neutral hydrogen in a sample of 25 very gas-rich galaxies as well as a similar-sized sample of “control” galaxies with similar masses, sizes and distances, but normal gas content. Their main tools were the Westerbork Synthesis Radio Telescope (WSRT) as well as elaborate computer simulations (see Research Highlight May 2013).

There have been many efforts over the past three decades to map the distribution of cold, atomic gas in galaxies using radio synthesis telescopes. The first analyses showed that the atomic gas exhibits a wide variety of detailed features. These can be attributed to irregularities in the galaxy such as spiral arms, rings, bars, warps etc. Studies of larger samples revealed basic scaling relations that provide hints of the mechanisms regulating the evolution of galaxies.

In contrast to the stellar surface density, which peaks in the centre of the galaxy and drops steeply with radius, the radial distribution of the atomic gas often flattens or even declines near the centre of the galaxy. In the outer regions, the gas disks usually extend to a larger distance from the centre than the stellar disks, and are well-fit by exponential functions.

Thanks to improvements in the WSRT instrumentation and data analysis, the observations by the Bluedisk team reached significantly lower column densities than previous surveys, i.e. they were able to map the gas in regions where the gas has low density. This sample is thus also well-suited for direct comparison with theoretical models. Observers and theorists worked together closely to improve their understanding of the radial distribution of the cold, atomic gas and to find a physical explanation for its structure.

The study revealed an interesting observational phenomenon: in the outer regions of all the galaxies, the gas exhibits a homogeneous surface density profile (if the sizes of the gas disks are properly scaled). This profile is well-fit by an exponential function with a universal scale-length. This universal profile appears to hold for all galaxies, irrespective of their stellar properties, gas masses, sizes, or morphologies (for an example see figure 1). This is remarkable, because the gas-rich galaxies contain on average 10 times more gas than the control sample.

In addition, the team found surprising agreement between their universal profile and results from simulations, both for smoothed-particle hydrodynamical simulations and for semi-analytic models of disk galaxy formation (see figure 2). It remains something of a mystery why the agreement with the smoothed-particle hydrodynamical simulations is quite so good.

In the semi-analytic models, the universal shape of the outer radial profiles is a direct consequence of the assumption that infalling gas is always distributed exponentially. However, there are observational indications that the atomic gas could be accreted in the form of "rings". Therefore, more work is underway on the theoretical side using smoothed particle hydrodynamical simulations to try and understand how gas settles onto the simulated galaxies in more detail (see figure 3).

Jing Wang & Guinevere Kauffmann

Further reading:

Wang, J.; Fu, J., Aumer, M., Kauffmann, G., et al., "An observational and theoretical view of radial distribution of HI gas in galaxies", 2014, submitted to MNRAS http://arxiv.org/abs/1401.8164

Related Links:

The Bluedisk project