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
Further reading:
Wang, J.; Kauffmann G.; J?zsa, G. I. G.; Serra, P. et al.,
"The Bluedisks project, a study of unusually HI-rich galaxies: I. HI Sizes and Morphology",
2013, submitted to MNRAS
(arXiv:1303.3538)
Related Links:
Bluedisk webpage
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