Is dark matter in spiral galaxies cold gas? II. Fractal models and star non-formation
Abstract
In a companion paper (Paper I) we have proposed a new candidate to account for the dark matter around spiral galaxies: cold H_2_ gas in a fractal structure, supported by rotation, and concomitant with the HI disc. We have shown that this hypothesis is compatible with dynamical and observational constraints about disc galaxies, and explains several conspiracies and paradoxes, since the dark matter is then in a form of fresh gas able to produce stars. In this paper we attempt to describe the physical conditions leading to a fractal state of cold gas in outer galaxy discs. Gas cloud models taking into account the recently disclosed fractal structure of cold gas are set up, showing that large errors in the classical gas mass determination based on smooth cloud models can easily follow if the gas is in reality fractal. Indeed the range of possible column densities is then much larger, including 5 or more decades of surface densities, instead of 2 for smooth cloud models. Thus fractal clouds must present both optically thin and optically thick clumps in any single wavelength observations. The observed fractal dimension of the cold ISM suggests that mass underestimates by a factor 10 or more are typical. Due to its low temperature (around 3K), and its condensed fractal structure, together with its low metallicity, the outer gas would be almost invisible for usual detectors. We consider the paradox of the persistence of cold Jeans unstable gas in outer discs, far from important heating sources, yet not forming stars or Jupiters. Following Rees (1976), we determine the smallest clump distribution that can persist in a collisional and almost isothermal fragmenting cold gas. At 3K these elementary cloudlets are predicted to have a radius of about 30AU, and have a mass of the order of a Jupiter. Their average density and column density are 10^9^cm^-3^ and 10^24^cm^-2^. They are gravitationally bound, and their line of sight thermal width is about 0.1kms^-1^. Their frequent collisions prevent them from forming Jupiters or stars and the near isothermality of the fractal nearly suppresses energy dissipation. At higher temperature, especially above H_2_ dissociation, the collision rate in the fractal decreases, favouring star formation. It turns out that the smallest density condensations, called "clumpuscules" offer favourable conditions for containing H_2_ in both vapour and solid phases. However it is unknown whether enough condensations sites such as dust exist in the outer discs to permit the freezing of H_2_. It is expected that the large sublimation energy prevents much H_2_ to become solid, but a small amount of H_2_ ice grains is a crucial factor for a good coupling between gas and the 3K background. Many of the general arguments presented here about fractals can be applied to other inhomogeneous structures, such as the hot gas in galaxy clusters. The clumpuscules presented here might be the form of matter in which cooling flows in clusters seem to disappear.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- May 1994
- DOI:
- 10.48550/arXiv.astro-ph/9311044
- arXiv:
- arXiv:astro-ph/9311044
- Bibcode:
- 1994A&A...285...94P
- Keywords:
-
- DARK MATTER;
- GALAXIES:ISM;
- ISM: STRUCTURE;
- STARS: FORMATION;
- Astrophysics
- E-Print:
- 25 pages, uuencoded compressed postscript (figures available by ftp, see abstract), OBSGE-DM-II