MPA :: Current Research Highlight

The standard paradigm for disk galaxy formation states that disk galaxies form when gas cools and condenses within a dark matter halo. Only about 20 percent of the available baryons in dark matter halos surrounding typical present-day spiral galaxies are locked up in stars. It follows that there should be a large reservoir of baryons located outside galaxies and theoretical models predict that this gas should currently be cooling and accreting to form disks. However, clear observational evidence of gas accretion and on-going disk formation has been lacking so far. Hot X-ray emitting gas has been detected around the Milky Way and other luminous spirals, and we also know that clouds of neutral hydrogen surround our Galaxy. However, estimates of the rate at which this gas accretes onto our Galaxy yield values that are too low to explain the star formation rates in galaxies like the Milky Way, which currently forms stars with a few solar masses per year on average. One possibility to explain this discrepancy is that the gas accretion is not continuous, but episodic.

Galaxies such as our own Milky Way consist of 10 percent gas and 90 percent stars. Only a minority of disk galaxies of the same total mass contain up to a factor of 10 times more gas. Nevertheless, the galaxies observed as part of the Bluedisk project were chosen to be among the most gas-rich systems in the nearby Universe. This is because previous work by the same group had revealed that such galaxies had outer disks with very blue colours, indicating active on-going formation of stars in these regions. These observations provide indirect evidence that the disks in such galaxies may be experiencing a period of renewed growth fuelled by a recent gas accretion episode.

In order to understand the gas accretion process in more detail, an international team of astronomers led by Guinevere Kauffmann and Jing Wang from the MPA, and Gyula Jozsa and Paolo Serra from ASTRON, The Netherlands used the Westerbork Synthesis Radio Telescope (WSRT, see Figure 1) to map hydrogen in a sample of 25 very gas-rich galaxies, along with a similar-sized sample of "control" galaxies with similar masses, sizes and redshifts. Figure 2 shows examples of such maps. The observations took place from December 2011 to May 2012.

One of the main results so far is that these gas-rich galaxies indeed have very large neutral hydrogen disks that extend to much larger radii than the stellar disk. In the most extreme cases, these disks even have diameters as large as 100 kiloparsecs and thus are a factor of 3-4 larger than the stellar disk. The disks of the gas-rich galaxies are also significantly clumpier than those of normal spirals (see top-left panel of Fig. 2).

Remarkably, these enormously gas-rich galaxies have the same gas mass versus size relation as normal spiral galaxies, i.e. the gas is spread over a larger size. There is no evidence that these large gas disks are strongly out of equilibrium, because they are not lopsided or warped. In fact, the centre of the hydrogen distribution in the gas-rich galaxies corresponds more closely with the centre of the optical light than in normal spirals.

These results seem to argue against a recent major interaction, which might have been responsible for bringing in the gas. The MPA/ASTRON team suggest that the excess gas must be accreted with a broad range of angular momenta and in a relatively well-ordered way. Possibly, this "order" results from the gas initially being in equilibrium with the surrounding dark matter halo, but these questions need to be investigated in more detail by comparing the observations with hydrodynamical simulations of disk formation in a cosmological context.

The MPA/ASTRON team hopes that the results of the Bluedisk project will motivate preparations for further, future large-scale neutral hydrogen surveys with wide-field instruments like Apertif. These will obtain a wealth of data of similar quality for samples of tens of thousands of nearby galaxies.

Jing Wang and Guinevere Kauffmann

The Bluedisk project: searching for clues about how disk galaxies form

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