arXiv subjects

Kenji Bekki

Publications and source records attributed to Kenji Bekki.

At least 181 records · Page 10Linked to original sources

arXiv2007

The environments and clustering properties of 2dFGRS-selected starburst galaxies

We investigate the environments and clustering properties of starburst galaxies selected from the 2dF Galaxy Redshift Survey (2dFGRS) in order to determine which, if any, environmental factors play a role in triggering a starburst. We quantify the local environments, clustering properties and luminosity functions of our starburst galaxies and compare to random control samples. The starburst galaxies are also classified morphologically in terms of their broad Hubble type and evidence of tidal merger/interaction signatures. We find the starburst galaxies to be much less clustered on large (5-15 Mpc) scales compared to the overall 2dFGRS galaxy population. In terms of their environments, we find just over half of the starburst galaxies to reside in low to intermediate luminosity groups, and a further ~30 per cent residing in the outskirts and infall regions of rich clusters. Their luminosity functions also differ significantly from that of the overall 2dFGRS galaxy population, with the sense of the difference being critically dependent on the way their star formation rates are measured. In terms of pin-pointing what might trigger the starburst, it would appear that factors relating to their local environment are most germane. Specifically, we find clear evidence that the presence of a near neighbour of comparable luminosity/mass within 20 kpc is likely to be important in triggering a starburst. We also find that a significant fraction (20-30 per cent) of our starburst galaxies have morphologies indicative of either an ongoing or recent tidal interaction and/or merger. These findings notwithstanding, there remain a significant portion of starburst galaxies where local environmental influences are not in any obvious way playing a triggering role, leading us to conclude that starbursts can also be internally driven.

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arXiv2007

On the Origin of the Kinematical Differences Between the Stellar Halo and the Old Globular Cluster System in the Large Magellanic Cloud

We discuss structural and kinematical properties of the stellar halo and the old globular cluster system (GCS) in the Large Magellanic Cloud (LMC) based on numerical simulations of the LMC formation. We particularly discuss the observed possible GCS's rotational kinematics (V/sigma > 2) that appears to be significantly different from the stellar halo's one with a large velocity dispersion (~50 km/s). We consider that both halo field stars and old GCs can originate from low-mass subhalos virialized at high redshifts (z >6). We investigate the final dynamical properties of the two old components in the LMC's halo formed from merging of low-mass subhalos with field stars and GCs. We find that the GCS composed of old globular clusters (GCs) formed at high redshifts (z > 6) has little rotation (V/sigma ~0.4) and structure and kinematics similar to those of the stellar halo. This inconsistency between the simulated GCS's kinematics and the observed one is found to be seen in models with different parameters. This inconsistency therefore implies that if old, metal-poor GCs in the LMC have rotational kinematics, they are highly unlikely to originate from the low-mass subhalos that formed the stellar halo. We thus discuss a scenario in which the stellar halo was formed from low-mass subhalos with no/few GCs whereas the GCS was formed at the very early epoch of the LMC's disk formation via dissipative minor and major merging of gas-rich subhalos and gas infall. We suggest that there can be a threshold subhalo mass above which GCs can be formed within subhalos at high redshifts and thus that this threshold causes differences in physical properties between stellar halos and GCSs in less luminous galaxies like the LMC.

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arXiv2007

The Magellanic Squall: Gas Replenishment from the Small to Large Magellanic Cloud

We first show that a large amount of metal-poor gas is stripped from the Small Magellanic Cloud (SMC) and fallen into the Large Magellanic Cloud (LMC) during the tidal interaction between the SMC, the LMC, and the Galaxy over the last 2 Gyrs. We propose that this metal-poor gas can closely be associated with the origin of LMC's young and intermediate-age stars and star clusters with distinctively low-metallicities with [Fe/H] < -0.6. We numerically investigate whether gas initially in the outer part of the SMC's gas disk can be stripped during the LMC-SMC-Galaxy interaction and consequently can pass through the central region (R<7.5 kpc) of the LMC. We find that about 0.7 % and 18 % of the SMC's gas can pass through the central region of the LMC about 1.3 Gyr ago and 0.2 Gyr ago, respectively. The possible mean metallicity of the replenished gas from the SMC to LMC is about [Fe/H] = -0.9 to -1.0 for the two interacting phases. These results imply that the LMC can temporarily replenish gas supplies through the sporadic accretion and infall of metal-poor gas from the SMC. These furthermore imply that if these gas from the SMC can collide with gas in the LMC to form new stars in the LMC, the metallicities of the stars can be significantly lower than those of stars formed from gas initially within the LMC.

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arXiv2007

Massive stars and globular cluster formation

We first present chemodynamical simulations to investigate how stellar winds of massive stars influence early dynamical and chemical evolution of forming globular clusters (GCs). In our numerical models, GCs form in turbulent,high-density giant molecular clouds (GMCs), which are embedded in a massive dark matter halo at high redshifts. We show how high-density, compact stellar systems are formed from GMCs influenced both by physical processes associated with star formation and by tidal fields of their host halos. We also show that chemical pollution of GC-forming GMCs by stellar winds from massive stars can result in star-to-star abundance inhomogeneities among light elements (e.g., C, N, and O) of stars in GCs. The present model with a canonical initial mass function (IMF) also shows a C-N anticorrelation that stars with smaller [C/Fe] have larger [N/Fe] in a GC. Although these results imply that ``self-pollution'' of GC-forming GMCs by stellar winds from massive stars can cause abundance inhomogeneities of GCs, the present models with different parameters and canonical IMFs can not show N-rich stars with [N/Fe] ~ 0.8 observed in some GCs (e.g., NGC 6752). We discuss this apparent failure in the context of massive star formation preceding low-mass one within GC-forming GMCs (``bimodal star formation scenario''). We also show that although almost all stars (~97%) show normal He abundances (Y) of ~0.24 some stars later formed in GMCs can have Y as high as ~0.3 in some models. The number fraction of He-rich stars with Y >0.26 is however found to be small (~10^-3) for most models.

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arXiv2007

Dynamical Influences of the Last Magellanic Interaction On the Magellanic Clouds

We investigate the present distributions of gas and young stars in the Large and Small Magellanic Clouds (LMC and SMC) based on fully self-consistent numerical simulations of the Clouds for the last ~ 0.8 Gyr. Our principal results, which can be tested against observations, are as follows. The last dynamical and hydrodynamical interaction between the Clouds about ~ 0.2 Gyr ago can form the apparently off-center bar and peculiar HI spirals of the LMC. The present spatial distributions of young stars with ages less than 20 Myr in the LMC can be significantly asymmetric and clumpy owing to the interaction. A small but non-negligible fraction of stellar and gaseous components can be transferred from the SMC into the LMC during the interaction to form diffuse halo components around the LMC. The burst of star formation in the SMC can be synchronized with that of the LMC about 0.2 Gyr ago in some models. New stars can form from gas in the SMC's tidal tails, one of which can be observed as the Magellanic Bridge (MB). The metallicity distribution function of new stars in the MB has a peak of [Fe/H] ~ -0.8, which is significantly smaller than the stellar metallicity of the SMC. Based on these results, we discuss the origin of 30 Doradus, the southern molecular ridge of the LMC, the globular cluster ESO 121-SC03, metal-poor inter-Cloud stars within the MB, and giant HI holes of the LMC.

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arXiv2006

Triggered star formation in the Magellanic Clouds

We discuss how tidal interaction between the Large Magellanic Cloud (LMC), the Small Magellanic Cloud (SMC), and the Galaxy triggers galaxy-wide star formation in the Clouds for the last ~0.2 Gyr based on our chemodynamical simulations on the Clouds. Our simulations demonstrate that the tidal interaction induces the formation of asymmetric spiral arms with high gas densities and consequently triggers star formation within the arms in the LMC. Star formation rate in the present LMC is significantly enhanced just above the eastern edge of the LMC's stellar bar owing to the tidal interaction. The location of the enhanced star formation is very similar to the observed location of 30 Doradus, which suggests that the formation of 30 Doradus is closely associated with the last Magellanic collision about 0.2 Gyr ago. The tidal interaction can dramatically compress gas initially within the outer part of the SMC so that new stars can be formed from the gas to become intergalactic young stars in the inter-Cloud region (e.g., the Magellanic Bridge). The metallicity distribution function of the newly formed stars in the Magellanic Bridge has a peak of [Fe/H] ~ -0.8, which is significantly lower than the stellar metallicity of the SMC.

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arXiv2006

On spatial distributions of old globular clusters in clusters of galaxies

We investigate structural properties of old, metal-poor globular clusters (GCs) formed at high redshifts (z>6) and located inside and outside virialized galaxy-scale halos in clusters of galaxies with the total masses of M_CL based on high-resolution cosmological simulations with models of GC formation. We mainly derive the parameter dependences of physical properties of intracluster GCs (ICGCs) based on the results of 14 models. Our principle results are summarized as follows. (1) The projected radial density profiles (Sigma_GC) of ICGCs in clusters with different M_CL can be diverse, though ICGCs have inhomogeneous, asymmetric, and somewhat elongated distributions in most models. If Sigma_GC (R) ~ R^alpha, alpha ranges from -1.5 to -2.5 for GCs in clusters. (2) Although total number of GCs within the central 0.05 Mpc (N_GC,0.05) and 0.2 Mpc (N_GC,0.2) are diverse in different clusters, they can depend weakly on M_CL in such a way that both N_GC,0.05 and N_GC,0.2 are likely to be larger for clusters with larger M_CL. (3) Total number of GCs per cluster masses (specific frequency of GCs for clusters of galaxies) are more likely to be larger in more massive clusters, mainly because a larger number of earlier virialized objects can be located in more massive clusters. (4) Spatial distributions of old GCs in clusters can depend on the truncation epoch of GC formation (z_trun) such that they can be steeper and more compact in the models with higher z_trun. (5) The mean metallicity of ICGCs in a cluster can be smaller than that of GCs within the cluster member galaxy-scale halos by ~ 0.3 in [Fe/H]. Metallicity distribution functions (MDFs) of ICGCs show peak values around [Fe/H] ~ -1.6 and do not have remarkable bimodality.

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arXiv2006

Numerical simulations on the relative importance of starbursts and AGN in ultra-luminous infrared galaxies

We investigate the relative importance of starbursts and AGN in nuclear activities of ultra-luminous infrared galaxies (ULIRGs) based on chemodynamical simulations combined with spectrophotometric synthesis codes. We numerically investigate both the gas accretion rates (m_acc) onto super massive black holes (SMBHs) and the star formation rates (m_sf) in ULIRGs formed by gas-rich galaxy mergers and thereby discuss what powers ULIRGs. Our principal results, which can be tested against observations, are as follows. (1) ULIRGs powered by AGN can be formed by major merging between luminous, gas-rich disk galaxies with prominent bulges containing SMBHs, owing to the efficient gas fuelling m_acc > 1 M_sun/yr of the SMBH. AGN in these ULIRGs can be surrounded by compact poststarburst stellar populations (e.g., A-type stars). (2) ULIRGs powered by starbursts with m_sf ~ 100 M_sun/yr can be formed by merging between gas-rich disk galaxies with small bulges having the bulge-to-disk-ratio (f_b) as small as 0.1. (3) The relative importance of starbursts and AGN can depend on physical properties of merger progenitor disks, such as f_b, gas mass fraction, and total masses. For example, more massive galaxy mergers are more likely to become AGN-dominated ULIRGs. (4) For most models, major mergers can become ULIRGs, powered either by starbursts or by AGN, only when the two bulges finally merge. Interacting disk galaxies can become ULIRGs with well separated two cores (> 20kpc) at their pericenter when they are very massive and have small bulges. (5) Irrespective of the choice of model, interacting/merging galaxies show the highest accretion rates onto the central SMBHs, and the resultant rapid growth of the SMBHs occur when their star formation rates are very high.

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arXiv2006

Stellar halos and elliptical galaxy formation: Origin of dynamical properties of the planetary nebular systems

Recent spectroscopic observations of planetary nebulae (PNe) in several elliptical galaxies have revealed structural and kinematical properties of the outer stellar halo regions. In order to elucidate the origin of the properties of these planetary nebula systems (PNSs), we consider the merger scenario in which an elliptical galaxy is formed by merging of spiral galaxies. Using numerical simulations, we particularly investigate radial profiles of projected PNe number densities, rotational velocities, and velocity dispersions of PNSs extending to the outer halo regions of elliptical galaxies formed from major and unequal-mass merging. We find that the radial profiles of the project number densities can be fitted to the power-law and the mean number density in the outer halos of the ellipticals can be more than an order of magnitude higher than that of the original spiral's halo. The PNSs are found to show a significant amount of rotation (V/sigma >0.5) in the outer halo regions ($R$ $>$ $5R_{\rm e}$) of the ellipticals. Two-dimensional velocity fields of PNSs are derived from the simulations and their dependences on model parameters of galaxy merging are discussed in detail. We compare the simulated kinematics of PNSs with that of the PNS observed in NGC 5128 and thereby discuss advantages and disadvantages of the merger model in explaining the observed kinematics of the PNS. We also find that the kinematics of PNSs in elliptical galaxies are quite diverse depending on the orbital configurations of galaxy merging, the mass ratio of merger progenitor spirals, and the viewing angle of the galaxies. This variation translates directly into possible biases by a factor of more than two in observational mass estimation (abridged). ~ ~

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arXiv2006

The U-shaped distribution of globular cluster specific frequencies in a biased globular cluster formation scenario

Using high-resolution numerical simulations, we investigate mass- and luminosity-normalized specific frequencies (T_N and S_N, respectively) of globular cluster systems (GCSs) in order to understand the origin of the observed U-shaped relation between S_N and V-band magnitude (M_V) of their host galaxies. We adopt a biased GC formation scenario in which GC formation is truncated in galaxy halos that are virialized at a later redshift, z_trun. T_N is derived for galaxies with GCs today and converted into S_N for reasonable galaxy mass-to-light-ratios (M/L). We find that T_N depends on halo mass (M_h) in the sense that T_N can be larger in more massive halos with M_h > 10^9 M_sun, if z_trun is as high as 15. We however find that the dependence is too weak to explain the observed S_N-M_V relation and the wide range of S_N in low-mass early-type galaxies with -20.5 < M_V < -16.0 mag for a reasonable constant M/L. The M_V-dependence of S_N for the low-mass galaxies can be well reproduced, if the mass-to-light-ratio M_h/L_V \propto M_h^α, where αis as steep as -1. Based on these results, we propose that the origin of the observed U-shaped S_N-M_V relation of GCSs can be understood in terms of the bimodality in the dependence of M_h/L_V on M_h of their host galaxies. We also suggest that the observed large dispersionin S_N in low-mass galaxies is due partly to the large dispersion in T_N.

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arXiv2006

Imprint of galaxy formation and evolution on globular cluster properties

We discuss the origin of physical properties of globular cluster systems (GCSs) in galaxies in terms of galaxy formation and evolution processes. Based on numerical simulations of dynamical evolution of GCSs in galaxies, we particularly discuss (1) the origin of radial density profiles of GCSs, (2) kinematics of GCSs in elliptical galaxies, (3) transformation from nucleated dwarf galaxies into GCs (e.g., omega Centauri), and (4) the origin of GCSs in the Large Magellanic Cloud (LMC).

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arXiv2006

Dissipative transformation of non-nucleated dwarf galaxies into nucleated systems

Recent photometric observations by the {\it Hubble Space Telescope (HST)} have revealed the physical properties of stellar galactic nuclei in nucleated dwarf galaxies in the Virgo cluster of galaxies. In order to elucidate the formation processes of nucleated dwarfs, we numerically investigate gas dynamics, star formation, and chemical evolution within the central 1 kpc of gas disks embedded within the galactic stellar components of non-nucleated dwarfs. We find that high density, compact stellar systems can be formed in the central regions of dwarfs as a result of dissipative, repeated merging of massive stellar and gaseous clumps developed from nuclear gaseous spiral arms as a result of local gravitational instability. The central stellar components are found to have stellar masses which are typically $~$5% of their host dwarfs and show very flattened shapes, rotational kinematics, and central velocity dispersions significantly smaller than those of their host dwarfs. We also find that more massive dwarfs can develop more massive, more metal-rich, and higher density stellar systems in their central regions, because star formation and chemical enrichment proceed more efficiently owing to the less dramatic suppression of star formation by supernovae feedback effects in more massive dwarfs. Based on these results, we suggest that gas-rich, non-nucleated dwarfs can be transformed into nucleated ones as a result of dissipative gas dynamics in their central regions. We discuss the origin of the observed correlations between physical properties of stellar galactic nuclei and those of their host galaxies.

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arXiv2005

Primordial pollution of globular clusters within their host dwarfs embedded in dark matter halos at high redshifts

Recent observational studies have revealed star-to-star abundance inhomogeneity among light elements (e.g., C, N, O, Na, and Al) of stars on the main sequence in the Galactic globular clusters (GCs). One of promising interpretations for this result is that the observed abundance inhomogeneity is due to the second generation of stars formed from ejecta of the first generation of evolved stars (e.g., AGB stars) within GCs. However it remains unclear whether and how this primordial pollution can occur within GCs. We here propose a new scenario in which primordial pollution of GCs is highly likely to occur if GCs are located in the central regions of high redshift dark matter subhalos that can host low-mass dwarf galaxies. In this scenario, gas ejected from the first generation of stars of GCs can be effectively trapped in the deep gravitational potential of their host halos and consequently can be consumed for the formation of the second generation of stars without losing a significant amount of gas by ram pressure stripping of interstellar and intergalactic medium. During merging of these halos with the proto-Galaxy, the halos are completely destroyed owing to the strong tidal field of the Galaxy. The self-polluted GCs located initially in the central regions of the halos can survive from tidal destruction owing to their compactness and finally become the Galactic halo GCs. In this scenario, ejecta of field stars surrounding the central GCs can be also converted into stars within their host dwarfs and finally become the second generation of stars of GCs. We also discuss the origin of the difference in the degree of abundance inhomogeneity between different GCs, such as $ω$ Centauri and NGC 6752, in terms of the difference in physical properties between host halos from which GC originate.

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arXiv2005

Impact of dark matter subhalos on extended HI disks of galaxies: Possible formation of HI fine structures and stars

Recent observations have discovered star formation activities in the extreme outer regions of disk galaxies. However it remains unclear what physical mechanisms are responsible for triggering star formation in such low-density gaseous environments of galaxies. In order to understand the origin of these outer star-forming regions, we numerically investigate how the impact of dark matter subhalos orbiting a gas-rich disk galaxy embedded in a massive dark matter halo influences the dynamical evolution of outer HI gas disk of the galaxy. We find that if the masses of the subhalos ($M_{\rm sb}$) in a galaxy with an extended HI gas disk are as large as $10^{-3} \times M_{\rm h}$, where $M_{\rm h}$ is the total mass of the galaxy's dark halo, local fine structures can be formed in the extended HI disk. We also find that the gas densities of some apparently filamentary structures can exceed a threshold gas density for star formation and thus be likely to be converted into new stars in the outer part of the HI disk in some models with larger $M_{\rm sb}$. These results thus imply that the impact of dark matter subhalos (``dark impact'') can be important for better understanding the origin of recent star formation discovered in the extreme outer regions of disk galaxies. We also suggest that characteristic morphologies of local gaseous structures formed by the dark impact can indirectly prove the existence of dark matter subhalos in galaxies. We discuss the origin of giant HI holes observed in some gas-rich galaxies (e.g., NGC 6822) in the context of the dark impact.

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arXiv2005

The origin of the double main sequence in Omega Centauri: Helium enrichment due to gas fueling from its ancient host galaxy ?

Recent observational studies of $ω$ Centauri by {\it Hubble Space Telescope} have discovered a double main sequence in the color magnitude diagrams (CMDs) of its stellar populations. The stellar population with the blue main sequence (bMS) is observationally suggested to have a helium abundance much larger, by $ΔY\sim 0.12$, than that of the red main sequence (rMS). By using somewhat idealized models in which stars of the bMS are formed from gas ejected from those of the rMS, we quantitatively investigate whether the helium overabundance of the bMS can result from self-enrichment from massive AGB stars, from mass loss of very massive young stars, or from type II supernovae within $ω$ Cen. We show that as long as the helium enrichment is due to ejecta from the rMS formed earlier than the bMS, none of the above three enrichment scenarios can explain the observed properties of the bMS self-consistently for reasonable IMFs. The common, serious problem in all cases is that the observed number fraction of the bMS can not be explained without assuming unusually top-heavy IMFs. This failure of the self-enrichment scenarios implies that most of the helium-enriched gas necessary for the formation of the bMS originated from other external sources. We thus suggest a new scenario that most of the second generation of stars (i.e., the bMS) in $ω$ Cen could be formed from gas ejected from field stellar populations that surrounded $ω$ Cen when it was a nucleus of an ancient dwarf galaxy.

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arXiv2005

Galaxy Groups: Proceedings from a Swinburne University Workshop

We present the proceedings from a 2-day workshop held at Swinburne University on the 24th-25th of May 2005. The workshop participants highlighted current Australian research on both theoretical and observational aspects of galaxy groups. These proceedings include short 1-page summaries of a number of the talks presented at the workshop. The talks presented ranged from reconciling N-body simulations with observations, to the HI content of galaxies in groups and the existence of ``dark galaxies''. The formation and existence of ultra-compact dwarfs in groups, and a new supergroup in Eridanus were also discussed.

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arXiv2005

Formation of intracluster globular clusters

We first present the results of numerical simulations on formation processes and physical properties of old globular clusters (GCs) located within clusters of galaxies (``intracluster GCs'') and in between clusters of galaxies (``intercluster GCs''). Our high-resolution cosmological simulations with models of GC formation at high redshifts ($z>6$) show that about 30 % of all GCs in a rich cluster can be ragarded as intracluster GCs that can freely drift being trapped by gravitational potential of the cluster rather than by the cluster member galaxies. The radial surface density profiles of the simulated intracluster GCs are highly likely to be flatter than those of GCs within cluster member galaxies. We also find that about 1% of all GCs formed before $z>6$ are not located within any virialized halos and can be regarded as ``intercluster'' (or ``intergalactic'') GCs. We discuss the dependences of physical properties of intracluster and intercluster GCs on the initial density profiles of GCs within low-mass dark matter halos at high redshifts ($z>6$).

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arXiv2005

On the structure of globular cluster systems in elliptical galaxies

It has long been known that the radial density profiles of globular cluster systems (GCSs) in elliptical galaxies vary with the total luminosities of their host galaxies. In order to elucidate the origin of this structural non-homology in GCSs, we numerically investigate the structural properties of GCSs in elliptical galaxies formed from a sequence of major dissipationless galaxy merging. We find that the radial density profiles of GCSs in elliptical galaxies become progressively flatter as the galaxies experience more major merger events. The density profiles of GCSs in ellipticals are well described as power-laws with slopes ($α_{\rm gc}$) ranging from -2.0 to -1.0. They are flatter than, and linearly proportional to, the slopes ($α_{\rm s}$) of the stellar density profiles of their host galaxies. We also find that the GCS core radii ($r_{\rm c}$) of the density profiles are larger in ellipticals that experienced more mergers. By applying a reasonable scaling relation between luminosities and sizes of galaxies to the simulation results, we show that $α_{\rm gc} \approx -0.36 M_{\rm V}-9.2$, $r_{\rm c} \approx -1.85 M_{\rm V}$, and $α_{\rm gc} \approx 0.93 α_{\rm s}$, where $M_{\rm V}$ is the total $V$-band absolute magnitude of a galaxy.

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