Kenji Bekki

Unequal-Mass Galaxy Mergers and the Creation of Cluster S0 Galaxies

It is a long-standing and remarkable problem as to when and how red S0 galaxies were formed in clusters of galaxies. We here propose that the major mechanism for the S0 creation is galaxy merging between two spirals with unequal mass. Our numerical simulations demonstrate that galaxy merging exhausts a large amount of the interstellar medium of two gas-rich spirals, owing to the moderately enhanced star formation, and subsequently transforms the two into one gas-poor S0 galaxy with a structure and kinematics strikingly similar to the observed ones. This secondary S0 formation with enhanced star formation explains the smaller fraction of the S0 population recently observed in some distant clusters of galaxies. Unequal-mass galaxy mergers thus provide an evolutionary link between a larger number of blue spirals observed in intermediate-redshift clusters and the red S0s prevalent in the present-day ones.

Group-Cluster Merging and the Formation of Starburst Galaxies

A significant fraction of clusters of galaxies are observed to have substructure, which implies that merging between clusters and subclusters is a rather common physical process of cluster formation. It still remains unclear how cluster merging affects the evolution of cluster member galaxies. We report the results of numerical simulations, which show the dynamical evolution of a gas-rich late-type spiral in a merger between a small group of galaxies and a cluster. The simulations demonstrate that time-dependent tidal gravitational field of the merging excites non-axisymmetric structure of the galaxy, subsequently drives efficient transfer of gas to the central region, and finally triggers a secondary starburst. This result provides not only a new mechanism of starbursts but also a close physical relationship between the emergence of starburst galaxies and the formation of substructure in clusters. We accordingly interpret post-starburst galaxies located near substructure of the Coma cluster as one observational example indicating the global tidal effects of group-cluster merging. Our numerical results furthermore suggest a causal link between the observed excess of blue galaxies in distant clusters and cluster virialization process through hierarchical merging of subclusters.

Formation of a Polar Ring Galaxy in a Galaxy Merger

We numerically investigate stellar and gas dynamics in star-forming and dissipative galaxy mergers between two disk galaxies with specific orbital configurations. We find that violent relaxation combined with gaseous dissipation in galaxy merging transforms two disk galaxies into one S0 galaxy with polar rings; both the central S0-like host and the polar ring component in a polar ring galaxy are originally disk galaxies. We also find that morphology of the developed polar rings reflects both the initial orbit configuration of galaxy merging and the initial mass ratio of the two merger progenitor disk galaxies. Based upon these results, we discuss the origin of the fundamental observational properties of polar ring galaxies, such as the prevalence of S0 galaxies among polar ring galaxies, the rarity of polar ring galaxies among S0 galaxies, the dichotomy between narrow polar rings and annular ones, the shapes of polar ring warps, and an appreciably larger amount of interstellar gas in the polar ring component.

Stellar Populations in Gas-rich Galaxy Mergers. II. Feedback Effects of Type Ia and Type II Supernovae

We numerically investigate the chemodynamical evolution of major disk-disk galaxy mergers in order to explore the origin of mass-dependent chemical, photometric, and spectroscopic properties observed in elliptical galaxies. We particularly investigate the dependence of the fundamental properties on the merger progenitor-disk mass (Md). The main results obtained in this study are the following five. (1) More massive (luminous) ellipticals formed by galaxy mergers between more massive spirals have larger metallicities (Z) and thus show redder colors. The typical metallicity ranges from ~1.0 solar abundance (Z~0.02) for ellipticals formed by mergers with Md=1010 M☉ to ~2.0 solar (Z~0.04) for those with Md=1012 M☉. (2) The absolute magnitude of negative metallicity gradients developed in galaxy mergers is more likely to be larger for massive ellipticals. The absolute magnitude of the metallicity gradient correlates with that of the age gradient in ellipticals in the sense that an elliptical with steeper negative metallicity gradient is more likely to show a steeper age gradient. (3) The radial color gradient is more likely to be larger for more massive ellipticals, which reflects the fact that the metallicity gradient is larger for more massive ellipticals. For example, the typical U-R color gradient (ΔU-R/ΔlogR) for 0.1≤R/Re≤1.0 is -0.13 for ellipticals with Md=1012 M☉ and -0.07 for those with Md=1010 M☉. (4) Both the Mg2 line index in the central parts of ellipticals (R≤0.1Re) and the radial Mg2 gradient (ΔMg2/ΔlogR) are more likely to be larger for massive ellipticals. ΔMg2/ΔlogR correlates reasonably well with the central Mg2 in ellipticals. For most of the present merger models, ellipticals show positive radial gradients of the Hβ line index. (5) Both M/LB and M/LK, where M, LB, and LK are the total stellar mass of galaxy mergers and the B- and K-band luminosities, respectively, depend on galactic mass in such a way that more massive ellipticals have larger M/LB and smaller M/LK. The essential reason for the mass dependence of the derived chemical, photometric, and spectroscopic properties of ellipticals is that galactic mass can largely determine the total amount of metal-enriched interstellar gas, the star formation histories of galaxy mergers, and the effectiveness of Type Ia and II supernova feedback, all of which greatly affect the chemodynamical evolution of galaxy mergers.

Stellar Populations in Gas-rich Galaxy Mergers. I. Dependence on Star Formation History

We investigate the nature of stellar populations of major galaxy mergers between late-type spirals with considerably abundant interstellar medium by performing numerical simulations designed to solve both the dynamical and chemical evolution of the mergers in a self-consistent manner. We particularly consider that the star formation history of galaxy mergers is a crucial determinant of the nature of stellar populations of merger remnants, and therefore we investigate how the difference in star formation history between galaxy mergers affects the chemical evolution of galaxy mergers. We found that the rapidity of star formation, which is defined as the ratio of the dynamical timescale to the timescale of gas consumption by star formation, is the most important determinant for a number of fundamental characteristics of stellar populations of merger remnants. The main results obtained in this study are the following. 1. A galaxy merger with more rapid star formation becomes elliptical with larger mean metallicity. This is primarily because, in the merger with more rapid star formation, a smaller amount of metal-enriched gas is tidally stripped away during merging, and, consequently, a larger amount of the gas can be converted to stellar components. This demonstrates that the cause of the color-magnitude relation of elliptical galaxies can be closely associated with the details of merging dynamics that depend on the rapidity of star formation in galaxy mergers. 2. A negative metallicity gradient fitted reasonably well by a power law can be reproduced by a dissipative galaxy merger with star formation. The magnitude of the metallicity gradient is larger for an elliptical galaxy formed by a galaxy merger with less rapid star formation. 3. The absolute magnitude of the metallicity gradient correlates with that of the age gradient in galaxy mergers in the sense that a merger remnant with a steeper negative metallicity gradient is more likely to show a steeper age gradient. 4. The outer part of a stellar population is both older and less metal-enriched than the nucleus in an elliptical galaxy formed by a galaxy merger with less rapid star formation. Moreover, the metallicity of the outer part of the gaseous component for some models with less rapid star formation is appreciably smaller than the stellar metallicity. This result implies that the origin of metal-poor hot gaseous X-ray halos in real elliptical galaxies can essentially be ascribed to the dynamics of dissipative galaxy merging. 5. Irrespective of the rapidity of star formation, the epoch of galaxy merging affects both the mean stellar metallicity and the mean stellar age of merger remnants: later galaxy mergers are more likely to become ellipticals with both younger and more metal-enriched stellar populations. This result reflects the fact that in the later mergers, a larger amount of more metal-enriched interstellar gas is preferentially converted into stars during the later period of star formation triggered by galaxy merging. These five results clearly demonstrate that even the chemical evolution of elliptical galaxies can be strongly affected by the details of dynamical evolution of galaxy merging, which are furthermore determined by the rapidity of star formation of galaxy mergers. In particular, tidal stripping of interstellar gas and the total amount of gaseous dissipation during galaxy merging are demonstrated to play vital roles in determining a number of chemical properties of merger remnants. Based on these results, we adopt a specific assumption of luminosity dependence on rapidity of star formation and thereby discuss how successfully the present merger model can reproduce a number of fundamental chemical, photometric, and spectroscopic characteristics of elliptical galaxies.

Starbursts in multiple galaxy mergers

We numerically investigate stellar and gaseous dynamical evolution of mergers among five identical late-type disk galaxies with the special emphasis on star formation history and chemical evolution of multiple galaxy mergers. We found that multiple encounter and merging can trigger repetitive massive starbursts (typically ~100 M☉ yr-1) owing to the strong tidal disturbance and the resultant gaseous dissipation during merging. The magnitude of the starburst is found to depend on initial virial ratio (i.e., the ratio of total kinematical energy to total potential energy) such that the maximum star formation rate is larger for the merger with smaller virial ratio. Furthermore, we found that the time interval between the epochs of the triggered starbursts is longer for the merger with the larger virial ratio. The remnant of a multiple galaxy merger with massive starbursts is found to have a metal-poor gaseous halo that is formed by tidal stripping during the merging. We accordingly suggest that a metal-poor gaseous halo in a field elliptical galaxy is a fossil record of the past multiple merging events for the galaxy.

Formation of Boxy and Disky Elliptical Galaxies in Early Dissipative Mergers

We numerically study the dynamical evolution of mergers of gas-rich galaxies, with special emphasis on the effects of star formation upon the appearance of the isophotal shape of the remnant. We find that the rapidity of gas consumption by star formation greatly affects the isophotal shapes of merger remnants. Mergers with gradual star formation are more likely to form elliptical galaxies with disky isophotes, and those with rapid star formation are more likely to form ellipticals that appear to be both boxy and disky depending on the viewing angle of the observer. We furthermore find that both the radial density profile and the compactness of the core of the merger remnant depend on the rapidity of gas consumption by star formation. Our numerical results imply that the isophotal shapes clearly reflect a difference in the history of star formation of elliptical galaxies at the epoch of their formation.

Formation of Polar Ring S0 Galaxies in Dissipative Galaxy Mergers

We numerically investigate stellar and gasdynamics of galaxy mergers between gas-rich, late-type spirals in order to explore the origin of polar ring S0 galaxies. We found that dissipative galaxy merging with a particular orbit configuration transforms two late-type spirals into one S0 galaxy with polar rings. The formation process of polar ring S0 galaxies is described as follows. A spiral galaxy intruding from the polar axis of the other victim galaxy excites the outwardly propagating density wave in the gaseous component of the victim galaxy. The subsequent gaseous dissipation and star formation dramatically transform the victim galaxy into polar rings. The intruding galaxy, on the other hand, is inevitably transformed into a rapidly rotating and spindly S0 galaxy, owing to the violent gravitational interaction of galaxy merging. This numerical result implies that dissipative galaxy merging between two gas-rich spirals is a new, promising candidate that can explain not only the formation of the central S0-like host but also the formation of a gas-rich polar ring component in a polar ring S0 galaxy.

The Fundamental Plane and Merger Scenario. I. Star Formation History of Galaxy Mergers and Origin of the Fundamental Plane

We perform numerical simulations of galaxy mergers between star-forming and gas-rich spirals in order to explore the origin of the fundamental plane (FP) of elliptical galaxies. We consider, in particular, that the origin of the slope of the FP is essentially due to the nonhomology in the structure and kinematics of elliptical galaxies and, accordingly, investigate structural and kinematic properties of elliptical galaxies formed by dissipative galaxy merging with star formation. We found that the rapidity of star formation, which is defined as the ratio of dynamical timescale of a merger progenitor to the timescale of gas consumption by star formation, is a key determinant for nonhomology parameters, such as the density profile of the stellar component, the relative importance of global rotation in kinematics, and the ratio of total dynamical mass to luminous mass, in merger remnants. We furthermore found that this result does not depend so strongly on initial intrinsic spins of progenitor disks and orbital energy and angular momentum of mergers. These results strongly suggest that the structural and kinematic nonhomology observed in elliptical galaxies can be closely associated with the difference in star formation history between elliptical galaxies formed by dissipative galaxy merging. Based on these results, we discuss a close physical relation between the origin of the FP and the star formation history of elliptical galaxies.

Spectrophotometric Evolution of Spiral Galaxies with Truncated Star Formation: An Evolutionary Link between Spirals and S0s in Distant Clusters

A one-zone chemospectrophotometric model is used to investigate the time evolution of disk galaxies whose star formation is truncated and to determine the dependence of this evolution on the previous star formation history and the truncation epoch. Truncated spirals show red colors ~1 Gyr after truncation and evolve spectrally from an e(b) type, down through the e(a), a+k, and k+a classes, to finally become a k type. The exact behavior in this phase depends on the truncation epoch and the star formation history prior to truncation. For example, earlier type disks show redder colors and do not show a+k-type spectra after truncation of star formation. We also discuss a possible evolutionary link between the k-type galaxies with spiral morphology found in distant clusters and present-day S0s by investigating whether truncated spirals reproduce the infrared color-magnitude relation of Coma galaxies. We suggest that only less luminous, later-type disk galaxies whose star formation is truncated at intermediate and high redshifts can reproduce the red I-K colors observed for S0s in the Coma cluster.