Why Are There Dwarf Spheroidal Galaxies?

Abstract

There appears to exist a conflict between the standard structure formation theory and the existence of the faint dwarf spheroidal galaxies in the Local Group. Theoretical considerations suggest that a cold dark matter universe has been a very hostile place for the formation of small galaxies. In particular, gas would not have been able to cool and condense to form stars inside dark matter halos with a velocity dispersion <10km/s since before the cosmological reionization epoch. In other words, one should not expect to see any major star formation activities in dwarf galaxies with a velocity dispersion <10km/s during the past 12Gyr, according to the current structure formation theory. The real universe, on the other hand, shows that all dwarf spheroidal galaxies in the Local Group have velocity dispersions <10km/s and extended and recent star formation activities are quite common in them. This apparent conflict between cold dark matter models and local observations can be resolved, if one allows the cold dark matter particles to decay relatively recently. The resolution comes about in that the dwarf spheroidal galaxies with a velocity dispersion of \~10km/s seen today were born, in this picture, in a small fraction of halos with a velocity dispersion of ~20km/s, where gas can be retained and is able to cool and contract to form stars. The presently observed lower velocity dispersion ~10km/s of these dwarf spheroidal galaxies is a consequence of the decay of one half of the dark matter in and subsequent expansion of the halos since redshift z~2.

4 ApJ pages, submitted to ApJ Letters

References in corpus (12)

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• The Discovery of a Luminous z=5.80 Quasar from the Sloan Digital Sky Survey
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Cited by in corpus (12)

• Particle decays during the cosmic dark ages
• The Next Generation Virgo Cluster Survey. XXIII. Fundamentals of nuclear star clusters over seven decades in galaxy mass
• Model-Independent Bound on the Dark Matter Lifetime
• The Carina Project: II. Stellar Populations
• Extended Star Formation in Dwarf Spheroidal Galaxies: the Cases of Draco, Sextans, and Ursa Minor
• Cosmological Constraints on Invisible Decay of Dark Matter
• Unstable Cold Dark Matter and the Cuspy Halo Problem in Dwarf Galaxies
• Annihilation vs. Decay: Constraining dark matter properties from a gamma-ray detection
• Decaying Superheavy Dark Matter and Subgalactic Structure of the Universe
• Decaying Dark Matter and the Deficit of Dwarf Haloes
• The Dark Halo of NGC 5963 as a Constraint on Dark Matter Self-Interaction at the Low Velocity Regime
• Secular evolution of galaxies and galaxy clusters in decaying dark matter cosmology