Molecular dark matter in galaxies

Abstract

Clouds containing molecular dark matter in quantities relevant for star formation may exist in minihaloes of the type of cold dark matter included in many cosmological simulations or in the regions of some galaxies extending far beyond their currently known boundaries. We have systematically explored parameter space to identify conditions under which plane-parallel clouds contain sufficient column densities of molecular dark matter that they could be reservoirs for future star formation. Such clouds would be undetected or at least appear by current observational criteria to be uninteresting from the perspective of star formation. We use a time-dependent PDR code to produce theoretical models of the chemistry and emission arising in clouds for our chosen region of parameter space. We then select a subset of model clouds with levels of emission that are low enough to be undetectable or at least overlooked by current surveys. The existence of significant column densities of cold molecular dark matter requires that the background radiation field be several or more orders of magnitude weaker than that in the solar neighbourhood. Lower turbulent velocities and cosmic ray induced ionization rates than typically associated with molecular material within a kpc of the Sun are also required for the molecular matter to be dark. We find that there is a large region within the parameter space that results in clouds that might contain a significant mass of molecular gas whilst remaining effectively undetectable or at least not particularly noticeable in surveys. We note briefly conditions under which molecular dark matter may contain a dynamically interesting mass.

9 pages, 2 figures, accepted for publication in A&A; additional concluding paragraph added at proof stage

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Cited by in corpus (7)

• Effective destruction of CO by cosmic rays: implications for tracing H${}_2$ gas in the Universe
• UCLCHEM: A Gas-Grain Chemical Code
• Molecules in galaxies
• The Distribution and Origin of C${}_2$H in NGC 253 from ALCHEMI
• Energizing Star Formation: The Cosmic Ray Ionization Rate in NGC 253 Derived From ALCHEMI Measurements of H${}_3$O${}^{+}$ and SO
• The KOSMA-$\tau$ PDR Model -- I. Recent updates to the numerical model of photo-dissociated regions
• Testing PDR models against ISO fine structure line data for extragalactic sources