A new idea is proposed for the origin of bulges in spiral galaxies. Numerical simulations of protogalactic collapse suggest strongly that galactic bulges have been assembled from massive clumps formed in galactic disks in their early evolutionary phase. These clumps result from the gravitational instability of the gas-rich disks of young galaxies. Owing to dynamical frictions, those massive clumps, individual masses of which can be as large as D109 are able to spiral toward the galactic center within a few M _ , Gyr. Inward transport of disk matter by this process leads to the formation of a galactic bulge. A simple analytical model has been constructed in which the clumpy evolution of a disk galaxy is controlled by two parameters : the timescale with which the primordial gas in the halo accretes onto the disk plane (i.e., the collapse timescale) and the initial mass fraction of the gas relative to the galaxy total mass. Under plausible assumptions for the variation of these parameters among spiral galaxies, the clumpy evolution model can explain an observed trend in which the bulge-to-disk ratio increases as the total mass or the internal density of the galaxy increases. This success suggests that the clumpy evolution of the galactic disk constitutes an important ingredient of disk galaxy evolution. Star formation in primeval disk galaxies takes place mostly in the clumps. The resulting knotty appearance of these systems may explain the peculiar morphology observed in a number of high-redshift galaxies.

An extensive set of N-body simulations has been carried out on the gravitational interaction of the Small Magellanic Cloud (SMC) with the Galaxy and the Large Magellanic Cloud (LMC). The SMC is assumed to have been a barred galaxy with a disc-to-halo mass ratio of unity before interaction and modelled by a large number of self-gravitating particles, whereas the Galaxy and LMC have been represented by rigid spherical potentials. Our more advanced numerical treatment has enabled us to obtain the most integrated and systematic understanding to date of numerous morphological and kinematical features observed in the Magellanic system (excluding the LMC), which have been dealt with more or less separately in previous studies. The best model we have found succeeded in reproducing the Magellanic Stream (MS) as a tidal plume created by the SMC-LMC-Galaxy close encounter 1.5 Gyr ago. At the same time, we see the formation of a leading counterpart to the Magellanic Stream (the leading arm), on the opposite side of the Magellanic Clouds to the Stream, which mimicks the overall distribution of several neutral hydrogen clumps observed in the corresponding region of the sky. A close encounter with the LMC 0.2 Gyr ago created another tidal tail and bridge system, which constitutes the inter-Cloud region in our model. The elongation of the SMC bar along the line-of-sight direction suggested by Cepheid observations has been partially reproduced, alongside its projected appearance on the sky. The model successfully explains some major trends in the kinematics of young populations in the SMC bar and older populations in the 'halo' of the SMC, as well as the overall velocity pattern for the gas, young stars, and carbon stars in the inter-Cloud region.

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Many high-redshift galaxies have peculiar morphologies and photometric properties 1-5 . It is not clear whether these peculiarities originate in galaxy-galaxy interactions (or mergers) or are intrinsic to the galaxies, a natural consequence of the star formation process in primeval systems. Here I report the results of numerical simulations of protogalaxy evolution, which show that the gas-rich disk of a young galaxy becomes gravitationally unstable and fragments into massive clumps of sub-galactic size. Most of the stars are formed in these discrete clumps, thereby providing a natural explanation for the peculiar morphology of high-redshift galaxies. The dynamical evolution of these young systems is dominated by the clumps and ultimately leads to structures resembling present-day galaxies, with a spheroidal bulge and an exponential disk. I interpret the differences between the Hubble types of galaxies as resulting from different timescales of disk formation. Finally, the model provides a causal link between the emergence of quasar activity and the dynamical evolution of the host galaxy.The protogalaxy was modelled as an ensemble of numerous gas clouds and dark-matter particles distributed uniformly in a spherical volume. The initial radius of the galaxy is 15 kpc and the total mass of the galaxy is 1:5 ϫ 10 11 M ᭪ , where M ᭪ is the solar mass. A uniform (that is, solid-body) rotation with an angular frequency Ω ¼ 13:9 km s Ϫ 1 kpc Ϫ 1 was given so as to bring the system into nearly centrifugal equilibrium. Inelastic collisions were introduced between gas clouds to simulate the dissipative nature of the gas, whereas the dark-matter particles are assumed to move in a collisionless manner in the galaxy gravitational field. Every pair of two overlapping gas clouds are made to collide with the same restitution coefficient of 0.01, though the outcome of a cloud-cloud collision may in fact be more complicated, depending on the impact parameter and relative velocity 6 . The star formation process is incorporated into this simulation by converting each gas cloud into a stellar particle with a probability proportional to m g (r g ) 1/2 , where m g is the mass of that cloud and r g is the smoothed-out gas density in its vicinity. A threshold gas density of 0.1M ᭪ pc −3 was specified, below which no star formation is allowed. Stellar particles are treated as collisionless. Star formation is expected to affect the kinematics of the interstellar medium through the energy input from stellar winds and supernova explosions. This process is mimicked by making a newly born stellar particle give velocity boosts of ϳ10 km s −1 to nearby gas clouds. This value of the boost is the one which balances energy dissipation by cloud-cloud collisions in the numerical model for a present-day spiral galaxy 7 . Initially, the masses of the gas component and the dark halo are identical, and there is no star.As seen in Fig. 1, the galaxy collapses nearly perpendicularly to the galactic plane within ϳ1 Gyr, and produces a rotating disk. The most ...

In order to better understand the dynamical evolution and star formation history of the Magellanic system, realistic N‐body simulations of the tidal distortion of the Small Magellanic Cloud (SMC) as a result of the Galaxy and the Large Magellanic Cloud have been carried out, taking into account gas dynamics and star formation processes explicitly. The best model succeeds in reproducing the observed structural, kinematic and star formation properties of the SMC, including other related tidal features in the Magellanic system, without resorting to the ram pressure model. The best‐fitting simulation reproduced a gas stream with almost no stars and the observed H i gas fraction, for which the morphology and velocity field agree quite well with those of the Magellanic stream, a result of adopting an initial SMC model that has a compact stellar disc embedded in an extended gaseous disc. This implies that the existence of a purely gaseous Magellanic stream does not pose serious problems to a tidal model of formation. Also, in this best model, the central and south‐east side (wing region) of the SMC contained an excess of young stars, as is observed. Comparison with a reference simulation of isolated evolution demonstrated that the acceleration of star formation activity in these regions may be a direct result of the last interaction between the Magellanic Clouds roughly 0.2 Gyr ago, which formed the inter‐cloud region. The large extent in depth of the SMC implied by the spatial distribution of Cepheids, and the line‐of‐sight velocity pattern in H i around the SMC is also reproduced. Finally, the dependences of these results on the numerical parameters that specify the SMC mass model and interstellar gas processes are discussed.

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