Secular evolution of late-type disc galaxies: formation of bulges and the origin of bar dichotomy

Abstract

Origins of galactic bulges and bars remain elusive, although they constitute fundamental components of disc galaxies. This paper proposes that the secular evolution process driven by the interstellar gas in galactic discs is closely associated with the formation of bulges and bars, and tries to explain the observed variation of these components along the Hubble morphological sequence. The ample interstellar medium serves as a coolant which makes the galactic disc gravitationally unstable and induces fragmentation of the disc. The resulting clumps spiral in to the disc centre because of dynamical friction. Efficiency of this process is governed by the gas-richness of the galactic disc, which is in turn controlled by two parameters: (i) the rapidity of the infall of the halo primordial gas to the disc plane and (ii) the amount of total accreted matter. When these two parameters are appropriately related to the mass and density of the galaxy and the effect of a star formation threshold in the disc is introduced, the gas-driven evolution through disc clumping leads to the appearance of two evolution regimes along the Hubble sequence, separated by the intermediate Hubble type for which the gas-driven mass concentration is minimal. Spiral galaxies of relatively early Hubble type (characterized by large mass and density) experience a rapid clump accumulation to the disc centre in their early evolution phase, which is identified with the formation of a bulge. On the other hand, the evolution of late-type spirals having small mass and density is strongly influenced by the existence of the star formation threshold. The disc in these galaxies becomes gas-rich in a relatively late epoch and experiences a prolonged clump-driven mass accumulation. I argue that this process leads to the formation of not a bulge but a short bar embedded in the galactic disc. The present scenario provides a natural explanation for the well-known bar dichotomy, namely that galactic bars are divided into two major groups on morphological grounds and each group is associated with different Hubble types.