arXiv subjects

Kenji Bekki

Publications and source records attributed to Kenji Bekki.

At least 73 records · Page 4Linked to original sources

arXiv2020

Formation of massive globular clusters with dark matter and its implication on dark matter annihilation

Recent observational studies of $γ$-ray emission from massive globular clusters (GCs) have revealed possible evidence of dark matter (DM) annihilation within GCs. It is, however, still controversial whether the emission comes from DM or from milli-second pulsars. We here present the new results of numerical simulations, which demonstrate that GCs with DM can originate from nucleated dwarfs orbiting the ancient MW. The simulated stripped nuclei (i.e., GCs) have the central DM densities ranging from 0.1 to several ${\rm M_\odot pc^{-3}}$, depending on the orbits and the masses of the host dwarf galaxies. However, GCs born outside the central regions of their hosts can have no/little DM after their hosts are destroyed and the GCs become the Galactic halo GCs. These results suggest that only GCs originating from stellar nuclei of dwarfs can possibly have DM. We further calculate the expected $γ$-ray emission from these simulated GCs and compare them to observations of $ω$ Cen. Given the large range of DM densities in the simulated GCs, we suggest that the recent possible detection of DM annihilation from GCs should be more carefully interpreted.

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arXiv2019

Formation of N-rich field stars in the high-density building blocks of the Galactic bulge

Recent observational studies of the Galactic bulge by APOGEE have revealed that about 1% of the bulge stars have rather high nitrogen abundances ([N/Fe]>0.5). We here numerically investigate in what physical conditions these N-rich stars (NRS) can be formed in spherical and disky stellar systems with stellar masses of 10^7-10^9 M_sun that are the bulge's building blocks. The principal results are as follows. A large fraction (>0.5) of new stars formed from interstellar medium polluted (ISM) by ejecta of asymptotic giant branch stars can have [N/Fe]>0.5 within stellar systems, if the gas mass fraction of ISM (f_g) is low (< 0.03). The mass fraction of NRS among all stars (f_nrs) can be higher than 1% within 0.5 Gyr timescale of star formation, if the mean stellar densities (rho_s) of the systems are higher than 0.1 M_sun/ pc^3. The [N/Fe] distributions depend on rho_s, f_g, and age distributions of their host stellar systems. NRS have compact and disky spatial distributions within their host systems and have rotational kinematics. Based on these results, we propose that the vast majority of the bulge's NRS originate not from globular clusters (GCs) but from its high-density building blocks. We suggest that NRS in the Galactic stellar halo have the same origin as those in the bulge. We also suggest that low-density dwarf spheroidal and gas-rich dwarfs are unlikely to form NRS. GCs are not only the formation sites of NRS.

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arXiv2019

A new formation model for omega Centauri: the crossroad of astrophysical processes

We investigate the formation processes of the Galactic globular cluster (GC) omega Cen with multiple stellar populations based on our original hydrodynamical simulations with chemical enrichment by Type II supernovae (SNe II), asymptotic giant branch (AGB) stars, and neutron star mergers (NSMs). The principal results are as follows. Multiple stellar populations with a wide range of [Fe/H] can be formed from rather massive and compact molecular cloud with a mass of 2 * 10^7 M_sun in the central region of its dwarf galaxy within less than a few hundred Myr. Gas ejected from SNe II and AGB stars can mix well to form new stars with higher He abundances (Y) and higher [Fe/H]. The He-rich stars are strongly concentrated in the GC's central region so that the GC can show a steep negative gradient of Y. Relative ratios of light elements to Fe show bimodal distributions for a given [Fe/H] owing to star formation from original gas and AGB ejecta. [La/Fe] and [Ba/Fe] can rapidly increase until [Fe/H]~-1.5 and then decrease owing to Fe ejection from SNe II. Although AGB ejecta can be almost fully retained in intra-cluster medium, NSM ejecta can be retained only partially. This difference in the retention capability is responsible for the observed unique [Eu/Fe]-[Fe/H] and [La/Eu]-[Fe/H] relations in omega Cen. Some observational results such as the [O/Na]$-$[Fe/H] relation and radial [Fe/H] gradient are yet to be well reproduced in the present model.

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arXiv2019

The VMC Survey -- XXXIV. Morphology of Stellar Populations in the Magellanic Clouds

The Magellanic Clouds are nearby dwarf irregular galaxies whose morphologies show different properties when traced by different stellar populations, making them an important laboratory for studying galaxy morphologies. We study the morphology of the Magellanic Clouds using data from the VISTA survey of the Magellanic Clouds system (VMC). We used about $10$ and $2.5$ million sources across an area of $\sim105$ deg$^2$ and $\sim42$ deg$^2$ towards the Large and Small Magellanic Cloud (LMC and SMC), respectively. We estimated median ages of stellar populations occupying different regions of the near-infrared ($J-K_\mathrm{s}, K_\mathrm{s}$) colour-magnitude diagram. Morphological maps were produced and detailed features in the central regions were characterised for the first time with bins corresponding to a spatial resolution of $0.13$ kpc (LMC) and $0.16$ kpc (SMC). In the LMC, we find that main sequence stars show coherent structures that grow with age and trace the multiple spiral arms of the galaxy, star forming regions become dimmer as we progress in age, while supergiant stars are centrally concentrated. Intermediate-age stars, despite tracing a regular and symmetrical morphology, show central clumps and hints of spiral arms. In the SMC, young main sequence stars depict a broken bar. Intermediate-age populations show signatures of elongation towards the Magellanic Bridge that can be attributed to the LMC-SMC interaction $\sim200$ Myr ago. They also show irregular central features suggesting that the inner SMC has also been influenced by tidal interactions.

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arXiv2019

Star formation within globular clusters:discrete multiple bursts and top-light mass functions

The observed discrete multiple stellar populations and internal abundance spreads in r- and s-process elements within globular clusters (GCs) have been suggested to be explained self-consistently by discrete star formation events over a longer timescale (10^8 yr). We here investigate whether such star formation is really possible within GCs using numerical simulations that include effects of dynamical interaction between individual stars and the accumulated gas ("star-gas interaction") on star formation. The principal results are as follows. Small gas clouds with densities larger than $10^{10}$ atoms cm^{-3} corresponding to first stellar cores can be developed from gas without turbulence. Consequently, new stars can be formed from the gas with high star formation efficiencies (>0.5) in a bursty manner. However, star formation can be suppressed when the gas mass fractions within GCs (f_g) are less than a threshold value (f_g, th). This f_g, th is larger for GCs with lower masses and larger gas disks. Star-gas interaction and gravitational potentials of GCs can combine to suppress the formation of massive stars (i.e., "top-light" stellar initial mass function). Formation of He-rich stars directly from gas of massive AGB stars is possible in massive GCs due to low f_g, th (<0.01). Short bursty star formation only for f_g>f_g, th can be partly responsible for discrete multiple star formation events within GCs.

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arXiv2019

Complex distribution and velocity field of molecular gas in NGC 1316 as revealed by Morita Array of ALMA

We present the results of $^{12}$CO($J$=1-0) mosaicing observations of the cD galaxy NGC 1316 at kpc-resolution performed with the Morita Array of the Atacama Large Millimeter/submillimeter Array (ALMA). We reveal the detailed distribution of the molecular gas in the central region for the first time: a shell structure in the northwest, a barely resolved blob in the southeast of the center and some clumps between them. The total molecular gas mass obtained with a standard Milky-Way CO-to-H$_2$ conversion factor is $(5.62 \pm 0.53)\times10^8$ M$_\odot$, which is consistent with previous studies. The disturbed velocity field of the molecular gas suggests that the molecular gas is injected very recently ($<1$ Gyr) if it has an external origin and is in the process of settling into a rotating disk. Assuming that a low-mass gas-rich galaxy has accreted, the gas-to-dust ratio and H$_2$-to-HI ratio are unusually low ($\sim 28$) and high ($\sim 5.6$), respectively. To explain these ratios, additional processes should be taken into accounts such as an effective dust formation and conversion from atomic to molecular gas during the interaction. We also discuss the interaction between the nuclear jet and the molecular gas.

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arXiv2019

Classifying the formation processes of S0 galaxies using Convolutional Neural Networks

Numerous studies have demonstrated the ability of Convolutional Neural Networks (CNNs) to classify large numbers of galaxies in a manner which mimics the expertise of astronomers. Such classifications are not always physically motivated, however, such as categorising galaxies by their morphological types. In this work, we consider the use of CNNs to classify simulated S0 galaxies based on fundamental physical properties. In particular, we undertake two investigations: (1) the classification of simulated S0 galaxies into three distinct evolutionary paths (isolated, tidal interaction in a group halo, and Spiral-Spiral merger), and (2) the prediction of the mass ratio for the S0s formed via mergers. To train the CNNs, we first run several hundred N-body simulations to model the formation of S0s under idealised conditions; and then we build our training datasets by creating images of stellar density and two dimensional kinematic maps for each simulated S0. Our trained networks have remarkable accuracies exceeding 99% when classifying the S0 formation pathway. For the case of predicting merger mass ratios, the mean predictions are consistent with the true values to within roughly one standard deviation across the full range of our data. Our work demonstrates the potential of CNNs to classify galaxies by the fundamental physical properties which drive their evolution.

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arXiv2019

Automated Simulations of Galaxy Morphology Evolution using Deep Learning and Particle Swarm Optimisation

The formation of Hoag-type galaxies with central spheroidal galaxies and outer stellar rings has yet to be understood in astronomy. We consider that these unique objects were formed from the past interaction between elliptical galaxies and gas-rich dwarf galaxies. We have modelled this potential formation process through simulation. These numerical simulations are a means of investigating this formation hypothesis, however the parameter space to be explored for these simulations is vast. Through the application of machine learning and computational science, we implement a new two-fold method to find the best model parameters for stellar rings in the simulations. First, test particle simulations are run to find a possible range of parameters for which stellar rings can be formed around elliptical galaxies (i.e. Hoag-type galaxies). A novel combination of particle swarm optimisation and Siamese neural networks has been implemented to perform the search over the parameter space and test the level of consistency between observations and simulations for numerous models. Upon the success of this initial step, we subsequently run full chemodynamical simulations for the derived range of model parameters in order to verify the output of the test particle simulations. We successfully find parameter sets at which stellar rings can be formed from the interaction between a gas-rich dwarf galaxy and a central elliptical galaxy. This is evidence that supports our hypothesis about the formation process of Hoag-type galaxies. In addition, this suggests that our new two-fold method has been successfully implemented in this problem search-space and can be investigated further in future applications. ~

astro-ph.IM↗

arXiv2018

The Neutral Hydrogen Properties of Galaxies in Gas-rich Groups

We present an analysis of the integrated neutral hydrogen (HI) properties for 27 galaxies within nine low mass, gas-rich, late-type dominated groups which we denote "Choirs". We find that majority of the central Choir galaxies have average HI content: they have a normal gas-mass fraction with respect to isolated galaxies of the same stellar mass. In contrast, we find more satellite galaxies with a lower gas-mass fraction than isolated galaxies of the same stellar mass. A likely reason for the lower gas content in these galaxies is tidal stripping. Both the specific star formation rate and the star formation efficiency of the central group galaxies are similar to galaxies in isolation. The Choir satellite galaxies have similar specific star formation rate as galaxies in isolation, therefore satellites that exhibit a higher star formation efficiency simply owe it to their lower gas-mass fractions. We find that the most HI massive galaxies have the largest HI discs and fall neatly onto the HI size-mass relation, while outliers are galaxies that are experiencing interactions. We find that high specific angular momentum could be a reason for galaxies to retain the large fraction of HI gas in their discs. This shows that for the Choir groups with no evidence of interactions, as well as those with traces of minor mergers, the internal galaxy properties dominate over the effects of residing in a group. The probed galaxy properties strengthen evidence that the Choir groups represent the early stages of group assembly.

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arXiv2018

Constraining the three-dimensional orbits of galaxies under ram pressure stripping with convolutional neural networks

Ram pressure stripping (RPS) of gas from disk galaxies has long been considered to play vital roles in galaxy evolution within groups and clusters. For a given density of intracluster medium (ICM) and a given velocity of a disk galaxy, RPS can be controlled by two angles (theta and phi) that define the angular relationship between the direction vector of the galaxy's three-dimensional (3D) motion within its host cluster and the galaxy's spin vector. We here propose a new method in which convolutional neutral networks (CNNs) are used to constrain theta and phi of disk galaxies under RPS. We first train a CNN by using ~10^5 synthesized images of gaseous distributions of the galaxies from numerous RPS models with different theta and phi. We then apply the trained CNN to a new test RPS model to predict theta and phi. The similarity between the correct and predicted theta and $ϕ$ is measured by cosine similarity (cos-Theta) with cos-Theta =1 being perfectly accurate prediction. We show that the average cos-Theta among test models is ~0.95, which means that theta and phi can be constrained well by applying the CNN to the spatial distributions of their gas. This result suggests that if the ICM is in hydrostatic equilibrium (thus not moving), the 3D orbit of a disk galaxy within its host cluster can be constrained by the spatial distribution of the gas being stripped by RPS. We discuss the applications of the method to HI surveys such as WALLABY and SKA projects.

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arXiv2018

The VMC Survey - XXXII. Pre-main sequence populations in the Large Magellanic Cloud

Detailed studies of intermediate/low mass pre-main sequence (PMS) stars outside the Galaxy have so far been conducted only for small targeted regions harbouring known star formation complexes. The VISTA Survey of the Magellanic Clouds (VMC) provides an opportunity to study PMS populations down to solar masses on a galaxy-wide scale. Our goal is to use near-infrared data from the VMC survey to identify and characterise PMS populations down to ~1 M_sun across the Magellanic Clouds. We present our colour-magnitude diagram method, and apply it to a ~1.5 deg^2 pilot field located in the Large Magellanic Cloud. The pilot field is divided into equally-sized grid elements. We compare the stellar population in every element with the population in nearby control fields by creating K_s/(Y-K_s) Hess diagrams; the observed density excesses over the local field population are used to classify the stellar populations. Our analysis recovers all known star formation complexes in this pilot field (N44, N51, N148 and N138) and for the first time reveals their true spatial extent. In total, around 2260 PMS candidates with ages $\lesssim$ 10 Myr are found in the pilot field. PMS structures, identified as areas with a significant density excess of PMS candidates, display a power-law distribution of the number of members with a slope of -0.86+-0.12. We find a clustering of the young stellar populations along ridges and filaments where dust emission in the far-infrared (FIR) (70 micron - 500 micron) is bright. Regions with young populations lacking massive stars show a lesser degree of clustering and are usually located in the outskirts of the star formation complexes. At short FIR wavelengths (70 micron, 100 micron) we report a strong dust emission increase in regions hosting young massive stars, which is less pronounced in regions populated only by less massive ($\lesssim$ 4 M_sun) PMS stars.

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arXiv2018

Blue straggler stars beyond the Milky Way. III. Detection of evolved blue straggler candidates in Large Magellanic Cloud clusters

We analyze {\sl Hubble Space Telescope} observations of nine Large Magellanic Cloud star clusters with ages of 1--2 Gyr to search for evolved counterparts of blue straggler stars. Near the red clump regions in the clusters' color--magnitude diagrams, we find branches of evolved stars that are much brighter than normal evolved stars. We examine the effects of photometric artifacts, differential reddening, and field contamination. We conclude that these bright evolved stars cannot be explained by any of these effects. Our statistical tests show that the contributions of photometric uncertainties and crowding effects, as well as that owing to differential reddening, to these bright evolved stars are insufficient to fully explain the presence of these bright evolved stars. Based on isochrone fitting, we also ruled out the possibility that these bright evolved stars could be reproduced by an internal chemical abundance spread. The spatial distributions of the bright evolved stars exhibit clear concentrations that cannot be explained by homogeneously distributed field stars. This is further confirmed based on Monte Carlo-based tests. By comparing our observations with stellar evolution models, we find that the masses of most of bright evolved stars do not exceed twice the average mass of normal evolved stars. We suggest that these bright evolved stars are, in fact, evolved blue straggler stars.

astro-ph.SR↗

arXiv2018

Formation of globular clusters with multiple stellar populations from massive gas clumps in high-z gas-rich dwarf galaxies

One of the currently favored scenarios for the formation of globular clusters (GCs) with multiple stellar populations is that an initial massive stellar system forms (`first generation', FG), subsequently giving rise to gaseous ejecta which is converted into a second generation (SG) of stars to form a GC. We investigate, for the first time, the sequential formation processes of both FG and SG stars from star-forming massive gas clumps in gas-rich dwarf disk galaxies. We adopt a novel approach to resolve the two-stage formation of GCs in hydrodynamical simulations of dwarf galaxies.In the new simulations, new gas particles that are much less massive than their parent star particle are generated around each new star particle when the new star enters into the asymptotic giant branch (AGB) phase. Furthermore, much finer maximum time step width (<10^5 yr) and smaller softening length (<2 pc) are adopted for such AGB gas particles to properly resolve the ejection of gas from AGB stars and AGB feedback effects. Therefore, secondary star formation from AGB ejecta can be properly investigated in galaxy-scale simulations. An FG stellar system can first form from a massive gas clump developing due to gravitational instability within its host gas-rich dwarf galaxy. Initially the FG stellar system is not a single massive cluster, but instead is composed of several irregular stellar clumps (or filaments) with a total mass larger than 10^6 Msun. While the FG system is dynamically relaxing, gaseous ejecta from AGB stars can be gravitationally trapped by the FG system and subsequently converted into new stars to form a compact SG stellar system within the FG system. Interestingly, about 40% of AGB ejecta is from stars that do not belong to the FG system (`external gas accretion'). The mass-density relation for SG stellar systems can be approximated as rho_SG ~ M_SG^1.5.

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arXiv2018

A new model for the multiple stellar populations within Terzan 5

Recent observational studies have demonstrated that the complex stellar system Terzan 5 (Ter 5) harbours multiple populations of stars. Several models have attempted to interpret the large age difference of several Gyrs between the dominant populations, but none have been universally accepted. We propose a new scenario whereby a collision between a metal-poor Ter 5 and a giant molecular cloud (GMC) serves as a catalyst for the generation of a super-solar population of stars. Using numerical simulations of this new "GC-GMC" collision scenario we demonstrate that, within a time frame of several Gyrs, our synthetic Ter 5 was capable of interacting with a metal-rich GMC in the central region of the Galaxy. As a consequence of this, our simulated globular cluster (GC) is able to capture enough gas from the colliding GMC to form a new population of metal-rich stars. Furthermore, the younger population created from the high-density regions of the captured gas is shown to have a stronger central mass concentration than the older metal-poor one, which is consistent with observations. A chemical link between Ter 5 and the bulge population of the Milky Way has long been observed and these simulations finally provide evidence for their similarities. Our model rationalises the 5 Gyrs of quiescence observed between the two dominant populations of Ter 5 and justifies the existence of the young generation. We discuss the advantages and disadvantages of the new scenario in the context of the observed physical properties of Ter 5.

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arXiv2018

The VMC survey - XXXI. The spatially resolved star formation history of the main body of the Small Magellanic Cloud

We recover the spatially resolved star formation history across the entire main body and Wing of the Small Magellanic Cloud (SMC), using fourteen deep tile images from the VISTA survey of the Magellanic Clouds (VMC), in the YJKs filters. The analysis is performed on 168 subregions of size 0.143 deg2, covering a total contiguous area of 23.57 deg2. We apply a colour-magnitude diagram (CMD) reconstruction method that returns the best-fitting star formation rate SFR(t), age--metallicity relation, distance and mean reddening, together with their confidence intervals, for each subregion. With respect to previous analyses, we use a far larger set of VMC data, updated stellar models, and fit the two available CMDs (Y-Ks versus Ks and J-Ks versus Ks) independently. The results allow us to derive a more complete and more reliable picture of how the mean distances, extinction values, star formation rate, and metallicities vary across the SMC, and provide a better description of the populations that form its Bar and Wing. We conclude that the SMC has formed a total mass of (5.31+-0.05)x10^8 Msun in stars over its lifetime. About two thirds of this mass is expected to be still locked in stars and stellar remnants. 50 per cent of the mass was formed prior to an age of 6.3 Gyr, and 80 per cent was formed between 8 and 3.5 Gyr ago. We also illustrate the likely distribution of stellar ages and metallicities in different parts of the CMD, to aid the interpretation of data from future astrometric and spectroscopic surveys of the SMC.

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arXiv2018

Formation of ultra-compact dwarf galaxies from supergiant molecular clouds

The origin of ultra-compact dwarf galaxies (UCDs) is not yet clear. One possible formation path of UCDs is the threshing of a nucleated elliptical dwarf galaxy (dE, N), however, it remains unclear how such massive nuclear stellar systems were formed in dwarf galaxies. To better establish the early history of UCDs, we investigate the formation of UCD progenitor clusters from super giant molecular clouds (SGMCs), using hydrodynamical simulations. In this study we focus on SGMCs with masses $10^{7} - 10^{8} \rm M_{\odot}$ that can form massive star clusters that display physical properties similar to UCDs. We find that the clusters have extended star formation histories with two phases, producing multiple distinct stellar populations, and that the star formation rate is dependent on the feedback effects of SNe and AGB stars. The later generations of stars formed in these clusters are more compact, leading to a clearly nested structure, and these stars will be more He-rich than those of the first generation, leading to a slight colour gradient. The simulated clusters demonstrate scaling relations between $R_{\rm eff}$ and $M$ and $σ_{v}$ and $M$ consistent with those observed in UCDs and strongly consistent with those of the original SGMC. We discuss whether SGMCs such as these can be formed through merging of self-gravitating molecular clouds in galaxies at high-z.

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arXiv2018

An HI study of the collisional ring galaxy NGC 922

We present new atomic hydrogen (HI) observations of the collisional ring galaxy NGC 922 obtained using the Australia Telescope Compact Array. Our observations reveal for the first time the vast extent of the HI disc of this galaxy. The HI morphology and kinematics of NGC 922 show that this galaxy is not the product of a simple drop-through interaction, but has a more complex interaction history. The integrated HI flux density of NGC 922 from our observations is 24.7 Jy km s$^{-1}$, which is within the error of the flux value obtained using the $64$-m Parkes radio telescope. This flux density translates to a total HI mass of $1.1*10^{10}$ M$_{\circ}$ and corresponds to an HI to total mass fraction (M$_{HI}$/M$_{tot}$) of approximately $0.11$. The gaseous structures of NGC 922 are more extended to the north and include an HI tail that has a projected physical length of $8$ kpc. Gas warps are also evident in the velocity field of NGC 922 and are more prominent on the approaching and the western side of the disc. In comparison with a large sample of star-forming galaxies in the local Universe, NGC 922 possesses a high gas fraction relative to galaxies with a similar stellar mass of ~$10^{10.4}$ M$_{\circ}$, and exhibits a high specific star formation rate.

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arXiv2018

The VMC Survey. XXIX. Turbulence-Controlled Hierarchical Star Formation in the Small Magellanic Cloud

In this paper we report a clustering analysis of upper main-sequence stars in the Small Magellanic Cloud, using data from the VMC survey (the VISTA near-infrared YJKs survey of the Magellanic system). Young stellar structures are identified as surface overdensities on a range of significance levels. They are found to be organized in a hierarchical pattern, such that larger structures at lower significance levels contain smaller ones at higher significance levels. They have very irregular morphologies, with a perimeter-area dimension of 1.44 +/- 0.02 for their projected boundaries. They have a power-law mass-size relation, power-law size/mass distributions, and a lognormal surface density distribution. We derive a projected fractal dimension of 1.48 +/- 0.03 from the mass-size relation, or of 1.4 +/- 0.1 from the size distribution, reflecting significant lumpiness of the young stellar structures. These properties are remarkably similar to those of a turbulent interstellar medium (ISM), supporting a scenario of hierarchical star formation regulated by supersonic turbulence.

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