The Angular Momentum Problem and the Formation of Bulgeless Galaxies

Abstract

The specific angular momentum of Cold Dark Matter (CDM) halos in a ΛCDM universe is investigated. Their dimensionless specific angular momentum $\lambda ' = \frac{j}{{\sqrt 2 {V_{vir}}{R_{vir}}}}$ with V _vir and R _vir the virial velocity and virial radius, respectively depends strongly on their merging histories. We investigate a set of simulations of the Cold Dark Matter models (ΛCDM) to explore the specific angular momentum content of halos formed through various merging histories. We show that halos with a quiet merging history, dominated by minor mergers and accretion until the present epoch, acquire by tidal torques, on average, only 2% per cent of the angular momentum required for their rotational support (λ′ = 0.02), whereas observational data for a sample of late-type bulgeless galaxies indicates that those galaxies reside in dark halos with exceptionally high values of λ′ ≈ 0.06 – 0.07. By minor mergers and accretion the specific angular momentum of dark halos is preserved or increases slowly with time, but not enough to spin up to the observed values for late-type dwarf galaxies. Feedback processes have been invoked to solve the problem that gas from looses a large fraction of its specific angular momentum during infall. Our results indicate that cosmological models of bulgeless galaxy formation have an even more severe problem as even without any angular momentum loss the specific angular momentum gained through smooth merging and accretion will be a factor of 3 smaller than observed.