arXiv subjects

Kenji Bekki

Publications and source records attributed to Kenji Bekki.

At least 145 records · Page 8Linked to original sources

arXiv2011

Formation of massive globular clusters with heavy element abundance spread in the Galactic building blocks

A growing number of recent observations have revealed that the Galactic globular cluster (GC) omega Cen is not the only GC that shows abundance spread in heavy elements (e.g., Fe). In order to understand the origin of the Galactic GCs with heavy element abundance spread ("HEAS"), we investigate the formation processes of massive GCs (MGCs) with masses larger than 10^6 M_sun in gas-rich dwarf galaxies interacting and merging with the very young Galaxy. We find that massive and compact stellar clumps with masses larger than 10^6 M_sun, which can be regarded as progenitors of MGCs, can form from massive gas clumps that are developed through merging of gaseous regions initially at different regions and thus with different metallicities. Therefore it is inevitable that MGCs formed in dwarfs have HEAS. The abundance spread in each individual MGC depends on the radial metallicity gradient of the host dwarf such that it can be larger for the steeper metallicity gradient. For example, MGCs formed in a dwarf with a central metallicity of [Fe/H]=-1.1 and the radial gradient of -0.2 dex kpc^{-1} can have the abundance spread of Delta [Fe/H] ~.2. The simulated MGCs appear to be significantly flattened owing to their dissipative formation from gas disks of their host dwarfs. Based on these results, we discuss possibly diverse formation mechanisms for the Galactic GCs such as M22, M54, NGC 2419, omega Cen, and Terzan 5.

astro-ph.GA↗

arXiv2011

Stellar population gradients in the cores of nearby field E+A galaxies

We have selected a sample of local E+A galaxies from the Sloan Digital Sky Survey (SDSS) Data Release 7 for follow up integral field spectroscopy with the Wide Field Spectrograph (WiFeS) on the ANU 2.3-m telescope. The sample was selected using the Halpha line in place of the [OII]3727 line as the indicator of on-going star formation (or lack thereof). This allowed us to select a lower redshift sample of galaxies than available in the literature since the [OII]3727 falls off the blue end of the wavelength coverage in the SDSS for the very lowest redshift objects. This low redshift selection means that the galaxies have a large angular to physical scale which allows us to resolve the central ~1kpc region of the galaxies; the region where stellar population gradients are expected. Such observations have been difficult to make using other higher redshift samples because even at redshifts z~0.1 the angular to physical scale is similar to the resolution provided by ground based seeing. Our integral field spectroscopy has enabled us to make the first robust detections of Balmer line gradients in the centres of E+A galaxies. Six out of our sample of seven, and all the galaxies with regular morphologies, are observed to have compact and centrally-concentrated Balmer line absorption. This is evidence for compact young cores and stellar population gradients which are predicted from models of mergers and tidal interactions which funnel gas into the galaxy core. Given the generally isolated nature of our sample this argues for the galaxies being seen in the late stage of a merger where the progenitors have already coalesced.

astro-ph.CO↗

arXiv2011

On the origin of the stellar halo and multiple stellar populations in the globular cluster NGC 1851

We propose that the observed stellar halo around the globular cluster (GC) NGC 1851 is evidence for its formation in the central region of its defunct host dwarf galaxy. We numerically investigate the long-term dynamical evolution of a nucleated dwarf galaxy embedded in a massive dark matter halo under the strong tidal field of the Galaxy. The dwarf galaxy is assumed to have a stellar nucleus (or a nuclear star cluster) that could be the progenitor for NGC 1851. We find that although the dark matter halo and the stellar envelope of the host dwarf of NGC 1851 can be almost completely stripped during its orbital evolution around the Galaxy, a minor fraction of stars in the dwarf can remain trapped by the gravitational field of the nucleus. The stripped nucleus can be observed as NGC 1851 with no/little dark matter whereas stars around the nucleus can be observed as a diffuse stellar halo around NGC 1851. The simulated stellar halo has a symmetric distribution with a power-law density slope of ~ -2 and shows no tidal tails within ~200pc from NGC 1851. We show that two GCs can merge with each other to form a new nuclear GC embedded in field stars owing to the low stellar velocity dispersion of the host dwarf. This result makes no assumption on the ages and/or chemical abundances of the two merging GCs. Thus the observed stellar halo and characteristic multiple stellar populations in NGC 1851 suggest that NGC 1851 could have formed initially in the central region of an ancient dwarf galaxy.

astro-ph.GA↗

arXiv2011

Formation of the Galactic bulge from a two-component stellar disk: Explaining cylindrical rotation and vertical metallicity gradient

Recent observational studies have revealed that the Galactic bulge has cylindrical rotation and a steeper vertical metallicity gradient. We adopt two representative models for the bulge formation and thereby investigate whether the two models can explain both the observed cylindrical rotation and vertical metallicity gradient in a self-consistent manner. One is the "pure disk scenario" (PDS) in which the bulge is formed from a pure thin stellar disk through spontaneous bar instability. The other is the "two-component disk scenario" (TCDS) in which the bulge is formed from a disk composed of thin and thick disks through bar instability. Our numerical simulations show that although PDS can reproduce the cylindrical rotation, it shows a rather flatter vertical metallicity gradient that is inconsistent with observations. The derived flatter metallicity gradient is due to the vertical mixing of stars with different initial metallicities by the stellar bar. This result implies that the bulge can not be simply formed from a pure thin stellar disk. On the other hand, the bulge formed from the two-component disk in TCDS can explain both the observed cylindrical rotation and vertical metallicity gradient of the Galactic bulge reasonably well. In TCDS, more metal-poor stars at higher |z| (vertical distance) which originate from the already dynamically hotter thick disk can not be strongly influenced by vertical mixing of the bar so that they can stay in situ for longer timescales and thus keep the lower metallicity at higher |z|. Consequently, the vertical metallicity gradient of the bulge composed of initially thin and thick disk stars can not be so flattened, even if the gradient of the thin disk can be flattened significantly by the bar in TCDS.

astro-ph.GA↗

arXiv2011

When was the Large Magellanic Cloud accreted onto the Galaxy ?

Using fully self-consistent N-body models for the dynamical evolution of the Large Magellanic Cloud (LMC) in the Galaxy, we show that if the LMC initially has an extended old stellar halo before its commencement of tidal interaction with the Galaxy, physical properties of the stars stripped from the LMC stellar halo can have fossil information as to when and where the LMC was accreted onto the Galaxy for the first time. If the epoch of the first LMC accretion onto the Galaxy from outside its viriral radius is more than ~4 Gyr ago (i.e., at least two pericenter passages), the stars stripped from the stellar halo of the LMC can form an irregular polar ring or a thick disk with a size of ~100 kpc and rotational kinematics. On the other hand, if the LMC was first accreted onto the Galaxy quite recently (~ 2 Gyr ago), the stripped stars form shorter leading and trailing stellar stream at R=50-120 kpc. Also distributions of the stripped stars in phase space between the two cases can be significantly different. The derived differences in structure and kinematics of the stripped stars therefore suggest that if we compare the observed three-dimensional (3D) distribution and kinematics of the outer Galactic stellar halo along the polar-axis, then we can give strong constraints on the past orbit of the LMC. We find that the orbital properties of the LMC in the successful formation models of the Magellanic Stream (MS) are consistent with those predicted from recent LambdaCDM cosmological simulations. We conclude that the LMC was accreted onto the Galaxy more than ~4 Gyr ago so that interaction between the LMC, the Small Magellanic Cloud (SMC), and the Galaxy could form the MS and its Leading Arms (LAs).

astro-ph.CO↗

arXiv2011

Origin of chemical and dynamical properties of the Galactic thick disk

We adopt a scenario in which the Galactic thick disk was formed by minor merging between the first generation of the Galactic thin disk (FGTD) and a dwarf galaxy about 9 Gyr ago and thereby investigate chemical and dynamical properties of the Galactic thick disk. In this scenario, the dynamical properties of the thick disk have long been influenced both by the mass growth of the second generation of the Galactic thin disk (i.e., the present thin disk) and by its non-axisymmetric structures. On the other hand, the early star formation history and chemical evolution of the thin disk was influenced by the remaining gas of the thick disk. Based on N-body simulations and chemical evolution models, we investigate the radial metallicity gradient, structural and kinematical properties, and detailed chemical abundance patterns of the thick disk. Our numerical simulations show that the ancient minor merger event can significantly flatten the original radial metallicity gradient of the FGTD, in particular, in the outer part, and also can be responsible for migration of inner metal-rich stars into the outer part (R>10kpc). The simulations show that the central region of the thick disk can develop a bar due to dynamical effects of a separate bar in the thin disk. The simulated orbital eccentricity distributions in the thick disk for models with higher mass-ratios (~0.2) and lower orbital eccentricities (~ 0.5) of minor mergers are in good agreement with the corresponding observations. The simulated V_{phi}-|z| relation of the thick disk in models with low orbital inclination angles of mergers are also in good agreement with the latest observational results. Our Galactic chemical evolution models can explain both the observed metallicity distribution functions (MDFs) and correlations between [Mg/Fe] and [Fe/H] for the two disks in a self-consistent manner.

astro-ph.GA↗

arXiv2011

Transformation from spirals into S0s with bulge growth in groups of galaxies

Recent observations have revealed that the time evolution of the S0 number fraction at intermediate and high redshifts (0.2<z<0.8) is more dramatic in groups of galaxies than in clusters. In order to understand the origin of S0s in groups, we investigate numerically the morphological transformation of spirals into S0s through group-related physical processes. Our chemodynamical simulations show that spirals in group environments can be strongly influenced by repetitive slow encounters with group member galaxies so that those with thin disks and prominent spiral arm structures can be transformed into S0s with thick disks and without prominent spiral arm structure. Such tidal interactions can also trigger repetitive starbursts within the bulges of spirals and consequently increase significantly the masses of their bulges. Owing to rapid consumption of gas initially in spirals during the bulge growth, the S0s can become gas-poor. The S0s transformed from spirals in this way have young and metal-rich stellar populations in the inner regions of their bulges. The simulated S0s have lower maximum rotational velocities and flatter radial line-of-sight velocity dispersion profiles in comparison to their progenitor spirals. The formation processes of S0s due to tidal interactions depend not only on the masses and orbits of the progenitor spirals, but also on group mass. A significant fraction (10-30%) of stars and gas can be stripped during this spiral to S0 morphological transformation so that intragroup stars and gas can be formed. Based on these results, we discuss structures, kinematics, chemical properties, and the Tully-Fisher relation of S0s in groups.

astro-ph.CO↗

arXiv2011

Global Properties of `Ordinary' Early-type Galaxies: photometry and spectroscopy of stars and globular clusters in NGC 4494

We present a comprehensive analysis of the spatial, kinematic, and chemical properties of stars and globular clusters (GCs) in the `ordinary' elliptical galaxy NGC 4494 using data from the Keck and Subaru telescopes. We derive galaxy surface brightness and colour profiles out to large galactocentric radii. We compare the latter to metallicities derived using the near-infrared Calcium Triplet. We obtain stellar kinematics out to ~3.5 effective radii. The latter appear flattened or elongated beyond ~1.8 effective radii in contrast to the relatively round photometric isophotes. In fact, NGC 4494 may be a flattened galaxy, possibly even an S0, seen at an inclination of ~45 degrees. We publish a catalogue of 431 GC candidates brighter than i=24 based on the photometry, of which 109 are confirmed spectroscopically and 54 have measured spectroscopic metallicities. We also report the discovery of 3 spectroscopically confirmed ultra-compact dwarfs around NGC 4494 with measured metallicities of -0.4<[Fe/H]<-0.3. Based on their properties, we conclude that they are simply bright GCs. The metal-poor globular clusters are found to be rotating with similar amplitude as the galaxy stars, while the metal-rich globular clusters show marginal rotation. We supplement our analysis with available literature data and results. Using model predictions of galaxy formation, and a suite of merger simulations, we find that many of the observational properties of NGC 4494 may be explained by formation in a relatively recent gas-rich major merger. Complete studies of individual galaxies incorporating a range of observational avenues and methods such as the one presented here will be an invaluable tool for constraining the fine details of galaxy formation models, especially at large galactocentric radii.

astro-ph.CO↗

arXiv2011

The Effect of Drag from the Galactic Hot Halo on the Magellanic Stream and Leading Arm

We study the effect of drag induced by the Galactic hot halo on the two neutral hydrogen (HI) cloud complexes associated with the Large and Small Magellanic Clouds: the Magellanic Stream (MS) and the Leading Arm (LA). In particular, we adopt the numerical models of previous studies and re-simulate the tidal formation of the MS and LA with the inclusion of a drag term. We find that the drag has three effects which, although model-dependent, may bring the tidal formation scenario into better agreement with observations: correcting the LA kinematics, reproducing the MS column density gradient, and enhancing the formation of MS bifurcation. We furthermore propose a two-stage mechanism by which the bifurcation forms. In general, the inclusion of drag has a variety of both positive and negative effects on the global properties of the MS and LA, including their on-sky positions, kinematics, radial distances, and column densities. We also provide an argument which suggests that ram pressure stripping and tidal stripping are mutually exclusive candidates for the formation of the MS and LA.

astro-ph.GA↗

arXiv2011

On interpretation of recent proper motion data for the Large Magellanic Cloud

Recent observational studies using the Hubble Space Telescope (HST) have derived the center-of-mass proper motion (CMPM) of the Large Magellanic Cloud (LMC). Although these studies carefully treated both rotation and perspective effects in deriving the proper motion for each of the sampled fields, they did not consider the effects of local random motion in the derivation. This means that the average PM of the fields (i.e., the observed CMPM) could significantly deviate from the true CMPM, because the effect of local random motion can not be close to zero in making the average PM for the small number of the fields (~10). We discuss how significantly the observationally derived CMPM can deviate from the true CMPM by applying the same method as used in the observations for a dynamical model of the LMC with a known true CMPM. We find that the deviation can be as large as ~ 50 km/s (~0.21 mas/yr), if the LMC has a thick disk and a maximum circular velocity of ~120$ km/s. We also find that the deviation depends both on the total number of the sampled fields and on structure and kinematics of the LMC. We therefore suggest that there is a possibility that the observed CMPM of the LMC deviates from the true one to some extent. We also show that a simple mean of PM for a large number of the LMC fields (~1000) can be much closer to the true CMPM.

astro-ph.CO↗

arXiv2011

Constraining the orbital history of the Magellanic Clouds: A new bound scenario suggested by the tidal origin of the Magellanic Stream

Bound orbits have traditionally been assigned to the Large and Small Magellanic Clouds (LMC and SMC, respectively) in order to provide a formation scenario for the Magellanic Stream (MS) and its Leading Arm (LA), two prominent neutral hydrogen (HI) features connected to the LMC and SMC. However, Hubble Space Telescope (HST) measurements of the proper motions of the LMC and SMC have challenged the plausibility of bound orbits, causing the origin of the MS to re-emerge as a contested issue. We present a new tidal model in which structures resembling the bifurcated MS and elongated LA are able to form in a bound orbit consistent with the HST proper motions. The LMC and SMC have remained bound to each other only recently in our model despite being separately bound to the Milky Way for more than 5 Gyr. We find that the MS and LA are able to form as a consequence of LMC-dominated tidal stripping during the recent dynamical coupling of the LMC and SMC. Our orbital model depends on our assumption that the Milky Way has a constant circular velocity of V_cir=250 km/s up to 160 kpc, which implies a massive isothermal halo that is not completely rejected by observations.

astro-ph.GA↗

arXiv2010

Secondary star formation within massive star clusters: Origin of multiple stellar populations in globular clusters

We numerically investigate whether and how gaseous ejecta from AGB stars can be converted into new stars within originally massive star clusters (MSCs) in order to understand the origin of multiple stellar populations in globular clusters (GCs). We adopt a scenario in which (i) MSCs with masses of M_s can be formed from high-mass, high-density giant molecular clouds (GMCs) in their host galactic building blocks embedded in dark matter halos at high redshifts and (ii) their evolution therefore can be significantly influenced by M_s, their initial locations, and physical properties of their hosts. Our 3D hydrodynamical simulations show that gaseous ejecta from AGB stars can be retained within MSCs and consequently converted into new stars very efficiently in the central regions of MSCs, only if M_s exceed a threshold mass (M_th) of ~10^6 M_sun. The new stars can correspond to the ``second generation (SG)'' of stars with higher Na and lower O abundances observed in GCs. Star formation efficiencies during the formation of SG stars within MSCs with M_s > M_th can be rather high (0.3-0.9) so that very compact new clusters within original MSCs can be formed. M_s should be as large as 10^6-10^7 M_sun to explain the observed large fraction of SG stars in the present ordinary Galactic GCs, because new stars can consist of only 1-4% among all stars for the standard IMF. Nuclear MSCs are found to retain much more effectively the AGB ejecta and convert more efficiently the gas into new stars owing to much deeper gravitational potential of their hosts. We suggest that both M_s and their locations within their hosts can determine whether abundance spread can be seen only in light elements or even in heavy ones.

astro-ph.GA↗

arXiv2010

Rotation and multiple stellar population in globular clusters

We investigate structure and kinematics of the second generation of stars (SG) formed from gaseous ejecta of the first generation of stars (FG) in forming globular clusters (GCs). We consider that SG can be formed from gaseous ejecta from AGB stars of FG with the initial total mass of 10^6-10^8 M_sun to explain the present masses of the Galactic GCs. Our 3D hydrodynamical simulations with star formation show that SG formed in the central regions of FG can have a significant amount of rotation (V/sigma ~0.8-2.5). The rotational amplitude of SG can depend strongly on the initial kinematics of FG. We thus propose that some GCs composed of FG and SG had a significant amount of rotation when they were formed. We also suggest that although later long-term (~10 Gyr) dynamical evolution of stars can smooth out the initial structural and kinematical differences between FG and SG to a large extent, initial flattened structures and rotational kinematics of SG can be imprinted on shapes and internal rotation of the present GCs. We discuss these results in terms of internal rotation observed in the Galactic GCs.

astro-ph.CO↗

arXiv2010

Origin of the unusually low nitrogen abundances in young populations of the Large Magellanic Cloud

It is a longstanding problem that HII regions and very young stellar populations in the Large Magellanic Cloud (LMC) have the nitrogen abundances ([N/H]) by a factor of ~7 lower than the solar value. We here discuss a new scenario in which the observed unusually low nitrogen abundances can be closely associated with recent collision and subsequent accretion of HI high velocity clouds (HVCs) that surround the Galaxy and have low nitrogen abundances. We show that if the observed low [N/H] is limited to very young stars with ages less than ~10^7 yr, then the collision/accretion rate of the HVCs onto the LMC needs to be ~ 0.2 M_sun/yr (corresponding to the total HVC mass of 10^6-10^7 M_sun) to dilute the original interstellar medium (ISM) before star formation. The required accretion rate means that even if the typical mass of HVCs accreted onto the LMC is ~ 10^7 M_sun, the Galaxy needs to have ~2500 massive HVCs within the LMC's orbital radius with respect to the Galactic center. The required rather large number of massive HVCs drives us to suggest that the HVCs are not likely to efficiently dilute the ISM of the LMC and consequently lower the [N/H]. We thus suggest the transfer of gas with low [N/H] from the Small Magellanic Cloud (SMC) to the LMC as a promising scenario that can explain the observed low [N/H].

astro-ph.CO↗

arXiv2010

Origin of optically passive spiral galaxies with dusty star-forming regions: Outside-in truncation of star formation?

Recent observations have revealed that red, optically--passive spiral galaxies with little or no optical emission lines, harbour significant amounts of dust-obscured star formation. We propose that these observational results can be explained if the spatial distributions of the cold gas and star-forming regions in these spiral galaxies are significantly more compact than those in blue star-forming spirals. Our numerical simulations show that if the sizes of star-forming regions in spiral galaxies with disk sizes of R_d are ~ 0.3R_d, such galaxies appear to have lower star formation rates as well as higher degrees of dust extinction. This is mainly because star formation in these spirals occurs only in the inner regions where both the gas densities and metallicities are higher, and hence the dust extinction is also significantly higher. We discuss whether star formation occurring preferentially in the inner regions of spirals is closely associated with the stripping of halo and disk gas via some sort of environmental effect. We suggest that the "outside-in truncation of star formation" is the key to a better understanding of apparently optically--passive spirals with dusty star-forming regions.

astro-ph.CO↗

arXiv2010

Synchronized formation of starburst and poststarburst galaxies in merging clusters of galaxies

We propose that synchronized triggering of star formation in gas-rich galaxies is possible during major mergers of cluster of galaxies, based on new numerical simulations of the time evolution of the physical properties of the intracluster medium (ICM) during such a merger event. Our numerical simulations show that the external pressure of the ICM in which cluster member galaxies are embedded, can increase significantly during cluster merging. As such, efficient star formation can be triggered in gas-rich members as a result of the strong compression of their cold gas by the increased pressure. We also suggest that these star-forming galaxies can subsequently be transformed into poststarburst galaxies, with their spatial distribution within the cluster being different to the rest of its population. We discuss whether this possible merger-induced enhancement in the number of star-forming and post-star-forming cluster galaxies is consistent with the observed evolution of galaxies in merging clusters.

astro-ph.CO↗

arXiv2010

On the transition from nuclear-cluster to black-hole dominated galaxy cores

Giant elliptical galaxies, believed to be built from the merger of lesser galaxies, are known to house a massive black hole at their center rather than a compact star cluster. If low- and intermediate-mass galaxies do indeed partake in the hierarchical merger scenario, then one needs to explain why their dense nuclear star clusters are not preserved in merger events. A valuable clue may the recent revelation that nuclear star clusters and massive black holes frequently co-exist in intermediate mass bulges and elliptical galaxies. In an effort to understand the physical mechanism responsible for the disappearance of nuclear star clusters, we have numerically investigated the evolution of merging star clusters with seed black holes. Using black holes that are 1-5% of their host nuclear cluster mass, we reveal how their binary coalescence during a merger dynamically heats the newly wed star cluster, expanding it, significantly lowering its central stellar density, and thus making it susceptible to tidal destruction during galaxy merging. Moreover, this mechanism provides a pathway to explain the observed reduction in the nucleus-to-galaxy stellar mass ratio as one proceeds from dwarf to giant elliptical galaxies.

astro-ph.CO↗

arXiv2010

The Star Cluster Population of the Collisional Ring Galaxy NGC 922

We present a detailed study of the star cluster population detected in the galaxy NGC922, one of the closest collisional ring galaxies known to date, using HST/WFPC2 UBVI photometry, population synthesis models, and N-body/SPH simulations.We find that 69% of the clusters are younger than 7Myr, and that most of them are located in the ring or along the bar, consistent with the strong Halpha emission. The cluster luminosity function slope of 2.1-2.3 for NGC922 is in agreement with those of young clusters in nearby galaxies. Models of the cluster age distribution match the observations best when cluster disruption is considered. We also find clusters with ages (>50Myr) and masses (>10^5 Msun) that are excellent progenitors for faint fuzzy clusters. The images also show a tidal plume pointing toward the companion. Its stellar age from our analysis is consistent with pre-existing stars that were stripped off during the passage of the companion. Finally, a comparison of the star-forming complexes observed in NGC922 with those of a distant ring galaxy from the GOODS field indicates very similar masses and sizes, suggesting similar origins.

astro-ph.CO↗