Natures of a clump-origin bulge: a pseudobulge-like but old metal-rich bulge

Abstract

Bulges in spiral galaxies have been supposed to be classified into two types: classical bulges or pseudobulges. Classical bulges are thought to form by galactic merger with bursty star formation, whereas pseudobulges are suggested to form by secular evolution due to spiral arms and a barred structure funneling gas into the galactic centre. Noguchi (1998,1999) suggested another bulge formation scenario, `clump-origin bulge'. He demonstrated using a numerical simulation that a galactic disc suffers dynamical instability to form clumpy structures in the early stage of disc formation since the premature disc is expected to be highly gas-rich, then the clumps are sucked into the galactic centre by dynamical friction and merge into a single bulge at the centre. This bulge formation scenario, which is expected to happen only at the high-redshift, is different from the galactic merger and the secular evolution. Therefore, clump-origin bulges may have their own unique properties. We perform a high-resolution N-body/smoothed particle hydrodynamics (SPH) simulation for the formation of the clump-origin bulge in an isolated galaxy model and study dynamical and chemical properties of the clump-origin bulge. We find that the clump-origin bulge resembles pseudobulges in dynamical properties, a nearly exponential surface density profile, a barred boxy shape and a significant rotation. We also find that this bulge consists of old and metal-rich stars, displaying resemblance to classical bulges. These natures, old metal-rich population but pseudobulge-like structures, mean that the clump-origin bulge can not be simply classified into classical bulges nor pseudobulges. From these results, we discuss similarities of the clump-origin bulge to the Milky Way bulge.

13 pages, 10 figures, Accepted for publication in MNRAS

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