Shigeki Inoue

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

Bulges in spiral galaxies have been supposed to be classified into classical bulges or pseudo-bulges. Classical bulges are thought to form by galactic merger with bursty star formation, whereas pseudo-bulges are suggested to form by secular evolution due to spiral arms and a barred structure funnelling gas into the galactic centre. Noguchi 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 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 pseudo-bulges 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 pseudo-bulge-like structures, mean that the clump-origin bulge cannot be simply classified into classical bulges or pseudo-bulges. From these results, we discuss similarities of the clump-origin bulge to the Milky Way bulge. Combined with a result of Elmegreen et al., this pseudo-bulge-like clump-origin bulge could be inferred to form in clump clusters with a relatively low surface density.

The test for suppressed dynamical friction in a constant density core of dwarf galaxies

The dynamical friction problem is a long-standing dilemma about globular clusters (hereafter GCs) belonging to dwarf galaxies. GCs are strongly affected by dynamical friction in dwarf galaxies, and are presumed to fall into the galactic centre. But, GCs do exist in dwarf galaxies generally. A solution of the problem has been proposed. If dwarf galaxies have a core dark matter halo which has constant density distribution in its centre, the effect of dynamical friction will be weakened considerably, and GCs should be able to survive beyond the age of the Universe. Then, the solution argued that, in a cored dark halo, interaction between the halo and the GC constructs a new equilibrium state, in which a part of the halo rotates along with the GC (corotating state). The equilibrium state can suppress the dynamical friction in the core region. In this study, I tested whether the solution is reasonable and reconsidered why a constant density, core halo suppresses dynamical friction, by means of N-body simulations. As a result, I conclude that the true mechanism of suppressed dynamical friction is not the corotating state, although a core halo can actually suppress dynamical friction on GCs significantly.

Corrective effect of many-body interactions in dynamical friction

Dynamical friction is a fundamental and important phenomenon in astrophysics. The Chandrasekhar formula is a well-known analytical estimation of the effect. However, current astrophysicists have realized that the formula is not correct in some cases because of several approximations used in the formulation and/or complex non-linearities in the real Universe. For example, it has been indicated that dynamical friction does not work in cored density profiles (constant density in the central region) despite the Chandrasekhar formula predicting drag force even for constant densities. In the first half of this paper, I show by N-body simulations that many-body interactions are also important in actual dynamical friction though derivation of the Chandrasekhar formula is based on the assumption of two-body interaction. In the simulation, the many-body interactions are caused by a very small number of field particles corotating with a perturber. However, the contribution from the many-body interactions accounts for a non-negligible fraction of the actual dynamical friction. In the second half of the paper, I discuss why the cored profiles suppress the dynamical friction. One possible explanation is that the corrective effect of the many-body interactions drives orbital motion of the perturber. The cessation of dynamical friction by this corrective effect would be feasible even in shallow cusp density profiles although the shallow cusp may evolve into a constant density.

Kinematic imprint of clumpy disk formation on halo objects

∼ 10 Gyr), clumpy disk formation can thus be presumed to take place in a pre-existing halo system. Aims. Giant clumps orbit in the same direction in a premature disk and are so massive that they may be expected to interact gravitationally with halo objects and exercise influence on the kine matic state of the halo. Accordingly, I scrutinize the possi bility that the clumps leave a kinematic imprint of the clumpy disk formation on a halo system. Methods. I perform a restricted N-body calculation with a toy model to study the kinematic infl uence on a halo by orbital motions of clumps and the dependence of the results on masses (mass loss), number, and orbital radii of the clumps. Results. I show that halo objects can catch clump motions and acquire disky rotation in a dynamical friction time scale of the clumps, ∼ 0.5 Gyr. The influence of clumps is limited within a region aroun d the disk, while the halo system shows vertical gradients of net rotation velocity and orbital eccentricity. The signifi cance of the kinematic influence strongly depends on the clum p masses; the lower limit of postulated clump mass would be∼ 5× 10 8 M⊙. The result also depends on whether the clumps are subjected to rapid mass loss or not, which is an open question under debate in recent studies. The existence of such massive clumps is not unrealistic. I therefore suggest that the imprints of past clumpy disk formation could remain in current galactic halos.