Masafumi Noguchi

Early Evolution of Disk Galaxies: Formation of Bulges in Clumpy Young Galactic Disks

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 ~109 M?, are able to spiral toward the galactic center within a few 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.

Effects of gas on the global stability of galactic disks - Radial flows

We study numerically the effect of gas on the global stability of a two-component self-gravitating galactic disk embedded in a live halo. The stars are evolved by using a 3D collisionless N-body code, and the gas is represented by an ensemble of finite size inelastic particles. The gravitational interaction of stars and gas is calculated using a TREE method. We find that the evolution of the gaseous distribution in the globally unstable disks can be described by two different regimes. When the gas mass fraction is less than about 10 percent, the gas is channeled toward the galactic center by a growing stellar bar. For higher gas fractions, the gas becomes highly inhomogeneous, and the bar instability in the disk is heavily damped. The gas falls toward the inner kpc due to dynamical friction. Domains of both regimes depend on the efficiency of dissipation in the gas. We also discuss the relevance of the Jeans instability and give an empirical criterion for the global bar instability in a two-component self-gravitating disk.

Dynamical Properties of Tidally-Induced Galactic Bars

We simulated a series of fully three-dimensional N-body models of tidal encounters between a disk galaxy and a perturbing galaxy to investigate the dynamical properties of tidally induced galactic bars, especially in connection with the resonance structure, which we have little knowledge about. We also calculated another set of N-body models on isolated galaxies with bar-unstable disks to make a comparison between galactic bars of different origins. To reveal the resonances in a highly nonaxisymmetric potential for which the ordinary method of deriving resonances using the Ω ± κ/2 curves gives unreliable results, we employed the analysis based on the families of periodic orbits in a barred potential. It is found from our simulations that the tidally induced bars sometimes rotate quite slowly and have inner Lindblad resonances (ILR) near the bar ends, whereas the spontaneously formed bars have no ILR and end near corotation because of their fast rotation. Since the difference in resonance structures affects the kinematics of the interstellar gas, these peculiar bars terminated by ILRs may give us a new way of creating the vast morphological and kinematical variety observed in the real barred galaxies, which may not be explained solely by the spontaneous bars. Slow rotation of the tidal bar is caused by two major factors. First, the small mass fraction of the disk in which a tidal bar is created leads to a small pattern speed. Second, the angular momentum transfer from the inner disk to the perturber at the time when the perturber passes the pericenter serves to reduce the pattern speed. The former effect is observed most clearly in the light disk models in which only 10% of the total mass of the galaxy is ascribed to the disk component. On the other hand, massive disk models in which the disk is stabilized by large random motions in disk stars are quite sensitive to the second process. This complicated behavior of tidal bars can be understood naturally by recognizing two regimes of tidal bar formation. When the tidal perturbation is relatively weak, it works only as a trigger of bar formation, and the bar properties are determined largely by the internal structure of the target galaxy. On the other hand, a sufficiently strong tidal perturbation washes out the intrinsic property of the target galaxy and imposes on the bar a common characteristic determined by the parameters of the tidal encounter.

Clumpy star-forming regions as the origin of the peculiar morphology of high-redshift galaxies

Many high-redshift galaxies have peculiar morphologies and photometric properties. 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.

FROM DIVERSITY TO DICHOTOMY, AND QUENCHING: MILKY-WAY-LIKE AND MASSIVE-GALAXY PROGENITORS AT 0.5 < z < 3.0

Using the HST/WFC3 and ACS multi-band imaging data taken in CANDELS and 3D-HST, we study the general properties and the diversity of the progenitors of the Milky Way (MWs) and local massive galaxy (MGs) at 0.5 2.5 kpc) radius since z ~ 2. Although the radial mass profiles evolve in distinct ways, the formation and quenching of the central dense region (or bulge) ahead of the outer disk formation are found to be common for both systems. The sudden reddening of bulge at z ~ 1.6 and z ~ 2.4 for MWs and MGs, respectively, suggests the formation of bulge and would give a clue to the different gas accretion histories and quenching. A new approach to evaluate the morphological diversity is conducted by using the average surface density profile and its dispersion. The variety of the radial mass profiles for MGs peaks at higher redshift (z > 2.8), and then rapidly converges to more uniform shape at z < 1.5, while that for MWs remains in the outer region over the redshift. Compared with the observed star formation rates and color profiles, the evolution of variety is consistently explained by the star formation activities.

The formation of solar-neighbourhood stars in two generations separated by 5 billion years

Abundance of chemical elements in the stars provides important clues regarding galaxy formation. The most powerful diagnostics is the relative abundance of α-elements (O, Mg, Si, S, Ca, and Ti) with respect to iron (Fe), [α/Fe], each of which is produced by different kinds of supernovae. The existence of two distinct groups of stars in the solar neighbourhood, one with high [α/Fe] and another with low [α/Fe], suggests that the stars in the solar vicinity have two different origins. However, the specific mechanism of the realization of this bimodality is unknown. Here, we show that the cold flow hypothesis recently proposed for the accretion process of primordial gas onto forming galaxies predicts two episodes of star formation separated by a hiatus 6-7 Gyr ago and naturally explains the observed chemical bimodality. We found that the first phase of star formation that forms high [α/Fe] stars is caused by the ‘genuine’ cold flow, in which unheated primordial gas accretes to the galactic disk in a freefall fashion. The second episode of star formation that forms low [α/Fe] stars is sustained by much slower gas accretion as the once-heated gas gradually cools by radiation. The cold flow hypothesis can also explain the large-scale variation in the abundance pattern observed in the Milky Way galaxy in terms of the spatial variation of gas accretion history.

Bar Formation by Galaxy-Galaxy Interactions

Bar formation process in interacting galaxies is reviewed on the basis of recent numerical simulations. I focus especially on the possible kinematical difference between tidally-induced bars and spontaneous bars: the former spans a wider range in the rotation rate, depending upon the parameters of the interaction as well as the internal structure of the host (i.e. perturbed) galaxy. I also discuss possible observational methods for discriminating between and weighing these two classes of galactic bars, including rotation rate measurements and environment statistics.

Gas flow in a two component galactic disk

Numerical simulations of two-component (stars + gas) self-gravitating galactic disks show that the interstellar gas can significantly affect the dynamical evolution of the disk even if its mass fraction (relative to the total galaxy mass) is as low as several percent. Aided by efficient energy dissipation, the gas becomes gravitationally unstable on local scale and forms massive clumps. Gravitational scattering of stars by these clumps leads to suppression of bar instability usually seen in heavy stellar disks. In this case, gas inflow towards the galactic center is driven by dynamical friction which gas clumps suffer instead of bar forcing.