Dr
Rhys Taylor
(Astronomical Institute of the Czech Academy of Sciences)
The "missing satellite problem" is a long-standing discrepancy between cosmological models of galaxy formation and observations. The models (e.g. Moore et al. 1999) have generally only used dark matter, which is computationally cheap and thought to dominate the mass on galaxy scales. While extremely successful at reproducing the large-scale distribution of galaxy structures, e.g. the "cosmic web" of filaments and voids, on smaller scales the missing satellite problem and other issues remain difficult to understand. In particular, models generally predict around a factor of ten more dark matter "halos" than observed galaxies.
An increasingly popular suggestion is that most of the dark matter halos found in simulations do exist in reality, but not all of them accumulate enough gas for star formation to occur. The conditions under which gas is converted into stars are poorly understood, but most galaxies are known to possess a gas disc significantly more extended than their stellar component (Broeils & Rhee 1999). Davies et al. 2004 suggested that it might be possible for a dark halo to accumulate enough gas to be detectable with radio surveys without triggering star formation. Whereas gaseous features produced in galaxy-galaxy encounters (tidal debris) generally have low spectral line widths, such "dark galaxies" would have higher widths characteristic of rotation.
Surveys such as AGES (Auld et al. 2006) use radio telescopes to search for neutral atomic hydrogen (HI) gas independently of optical emission. Taylor et al. 2012 discovered eight HI clouds with particularly intriguing properties : they have high spectral line widths (180 km/s), no optical counterparts, and are relatively isolated. Taylor et al. 2016,2017 showed that such objects are extremely difficult to produce in galaxy-galaxy encounters. While streaming motions can produce high line widths that could be mistaken for rotation, such features are highly transient - implying that tidal debris should be found close to its parent galaxy. In contrast, if the clouds are indeed rotating, this would imply a high dark matter content which would allow them to remain stable and move freely.
Burkhart & Loeb 2016 proposed a new model for the dark HI clouds in which their internal pressure is balanced by the confining pressure of a hot, thin, external medium. The internal pressure would have to be dynamic, not thermal, as the line width of the clouds would imply a temperature (>100,000 K) which is too high to sustain HI. We investigated this model on the Salomon cluster using 3D hydrodynamic simulations in FLASH. The HI is modelled as a Gaussian-density sphere with a turbulent velocity field, with properties that match the AGES clouds, embedded in a hot, thin external medium with properties based on X-ray observations. We found (Taylor, Wünsch & Palouš 2018) that such clouds evolve rapidly and resemble the observed clouds for < 100 Myr. This model would require us to have detected eight clouds in a very brief, unusual stage of their evolution, whereas in the dark galaxy hypothesis they are stable and long-lived.
References
Auld et al., 2006, MNRAS, 371, 1617
Broeils & Rhee, 1999, A&A, 324, 877
Burkhart & Loeb, 2016, ApJ, 824, 7
Davies et al., 2004, MNRAS, 349, 922
Moore et al., 1999, ApJ, 524, 19
Taylor et al., 2016, MNRAS, 461, 3001
Taylor et al., 2017, MNRAS, 467, 3648
Taylor, Wünsch & Palouš, 2018, 479, 377
Summary
Cosmological models often attempt to model the Universe using only computationally cheap dark matter, avoiding the complexities of hydrodynamics and star formation. While successful at reproducing the observed large-scale structures, they over-predict the number of detectable galaxies by a factor ten. One suggestion is that most of the simulated dark matter 'halos' do exist, but never accumulate enough gas to trigger star formation. Observations have actually detected several candidate 'dark galaxies' which contain gas but no detectable stars, but other interpretations have been proposed. We use 3D hydrodynamic FLASH models to investigate the idea that these gas clouds are supported against collapse by a balance of their internal, dynamic pressure with that of their surrounding medium, and require no dark matter halos. We find that such clouds rapidly destroy themselves and that no equilibrium state exists, unlike the dark galaxy hypothesis which readily explains their apparent longevity.
Dr
Rhys Taylor
(Astronomical Institute of the Czech Academy of Sciences)
Dr
Richard Wunsch
(Astronomical Institute, Czech Academy of Sciences)
