Bulgeless Galaxies and Their Angular Momentum Problem

Abstract

The specific angular momentum of cold dark matter (CDM) halos in a ΛCDM universe is investigated. Their dimensionless specific angular momentum λ' = j/(VvirRvir) with Vvir and Rvir the virial velocity and virial radius, respectively, depends strongly on their merging histories. We investigate a set of ΛCDM simulations and explore the specific angular momentum content of halos formed through various merging histories. Halos with a quiet merging history, dominated by minor mergers and accretion until the present epoch, acquire by tidal torques on average only 2%-3% of the angular momentum required for their rotational support (λ' = 0.02). This is in conflict with observational data for a sample of late-type bulgeless galaxies that indicates that those galaxies reside in dark halos with exceptionally high values of λ' ≈ 0.06-0.07. Minor mergers and accretion preserve or slowly increase the specific angular momentum of dark halos with time. This mechanism, however, is not efficient enough in order to explain the observed spin values for late-type dwarf galaxies. Energetic feedback processes have been invoked to solve the problem that gas loses a large fraction of its specific angular momentum during infall. Under the assumption that dark halos hosting bulgeless galaxies acquire their mass via quiescent accretion, our results indicate yet another serious problem: the specific angular momentum gained during the formation of these objects is not large enough to explain their observed rotational properties, even if no angular momentum would be lost during gas infall.