arXiv subjects

Kenji Bekki

Publications and source records attributed to Kenji Bekki.

At least 163 records · Page 9Linked to original sources

arXiv2009

Origin of rotational kinematics in the globular cluster system of M31: A new clue to the bulge formation

We propose that the rotational kinematics of the globular cluster system (GCS) in M31 can result from a past major merger event that could have formed its bulge component. We numerically investigate kinematical properties of globular clusters (GCs) in remnants of galaxy mergers between two disks with GCs in both their disk and halo components. We find that the GCS formed during major merging can show strongly rotational kinematics with the maximum rotational velocities of 140 - 170 km/s for a certain range of orbital parameters of merging. We also find that a rotating stellar bar, which can be morphologically identified as a boxy bulge if seen edge-on, can be formed in models for which the GCSs show strongly rotational kinematics. We thus suggest that the observed rotational kinematics of GCs with different metallicities in M31 can be closely associated with the ancient major merger event. We discuss whether the formation of the rotating bulge/bar in M31 can be due to the ancient merger.

astro-ph.CO↗

arXiv2009

Dynamical friction of star clusters against disk field stars in galaxies: Implications on stellar nucleus formation and globular cluster luminosity functions

We numerically investigate orbital evolution of star clusters (SCs) under the influence of dynamical friction by field stars of their host disk galaxies embedded in dark matter halos. We find that SCs with masses larger than 2 * 10^5 M_sun can show significant orbital decay within less than 1 Gyr due to dynamical friction by disk field stars in galaxies with disk masses M_d less than 10^9 M_sun. We also find that orbital decay of SCs due to dynamical friction is more remarkable in disk galaxies with smaller M_d and higher mass-ratios of disks to dark matter halos. The half-number radii R_h, sc and mean masses within R_h,sc of the SC systems (SCSs) in low-mass disk galaxies with M_d < 10^9 M_sun are found to evolve significantly with time owing to dynamical friction of SCs. More massive SCs that can spiral-in to the central regions of disks can form multiple SC systems with smaller velocity dispersions so that they can merge with one another to form single stellar nuclei with their masses comparable to ~0.4% of their host disk masses. Based on these results, we suggest that luminosity functions (LFs) for more massive globular clusters (GCs) with masses larger than 2 * 10^5 M_sun can steepen owing to transformation of the more massive GCs into single stellar nuclei through GC merging in less luminous galaxies. We also suggest that the half-number radii of GC systems can evolve owing to dynamical friction only for galaxies with their total masses smaller than ~ 10^{10} M_sun.

astro-ph.CO↗

arXiv2009

Young Stellar Populations in the Collisional Ring Galaxy NGC 922

We studied the star cluster population properties in the nearby collisional ring galaxy NGC 922 using HST/WFPC2 photometry and population synthesis modeling. We found that 69% of the detected clusters are younger than 7 Myr, and that most of them are located in the ring or along the bar, consistent with the strong Halpha emission. The images also show a tidal plume pointing toward the companion. Its stellar age is consistent with pre-existing stars that were probably stripped off during the passage of the companion. We compared the star-forming complexes observed in NGC 922 with those of a distant ring galaxy from the GOODS field. It indicates very similar masses and sizes, suggesting similar origins. Finally, we found clusters that are excellent progenitor candidates for faint fuzzy clusters.

astro-ph.CO↗

arXiv2009

Ram pressure stripping of halo gas in disk galaxies: Implications on galactic star formation in different environments

We numerically investigate evolution of gaseous halos around disk galaxies in different environments ranging from small groups to rich clusters in order to understand galaxy evolution in these environments. Our simulations self-consistently incorporate effects of ram pressure of intergalactic medium (IGM) on disk and halo gas of galaxies and hydrodynamical interaction between disk and halo gas so that mass fractions of halos gas stripped by ram pressure of IGM (F_strip) can be better estimated. We mainly investigate how F_strip depends on total masses of their host environments (M_host}), galactic masses (M_gal), densities and temperature of IGM (T_IGM and rho_IGM, respectively), relative velocities between IGM and galaxies (V_r), and physical properties of disks (e.g., gas mass fraction). We find that typically 60-80% of halo gas can be efficiently stripped from Milky Way-type disk galaxies by ram pressure in clusters with M_host 10^{14} M_sun We also find that F_strip depends on M_host such that F_strip is higher for larger M_host. Furthermore it is found that F_strip can be higher in disk galaxies with smaller M_gal for a given environment. Our simulations demonstrate that the presence of disk gas can suppress ram pressure stripping of halo gas owing to hydrodynamical interaction between halo and disk gas. Ram pressure stripping of halo gas is found to be efficient (i.e., F_strip>0.5) even in small and/or compact groups, if rho_IGM ~ 10^5 M_sun/kpc^3 and V_r ~ 400 km/s. Based on the derived radial distributions of remaining halo gas after ram pressure stripping, we propose that truncation of star formation after halo gas stripping can occur outside-in in disk galaxies.

astro-ph.CO↗

arXiv2009

Dark impact and galactic star formation: Origin of the Gould belt

The Milky Way has a giant stellar structure in the solar neighborhood, which has a size of $\sim 1$ kpc, a mass of $\sim 10^6 {\rm M}_{\odot}$, and a ring-like distribution of young stars. Fundamental physical properties of this local enigmatic structure, known as the Gould belt (GB), have not been reproduced by previously proposed models. We first show that the local enigmatic structure can be formed about 30 Myr ago as a result of a high-speed, oblique collision between a gas cloud with a mass of $\sim 10^6 {\rm M}_{\odot}$ and a dark matter clump with a mass of $\sim 10^7 {\rm M}_{\odot}$ based on numerical simulations of the collision. We find that strong dynamical impact of the clump transforms the flattened cloud into a ring-like stellar structure after induced star formation within the cloud. Our simulations furthermore demonstrate that the stellar structure is moderately elongated and significantly inclined with respect to the disk of the Milky Way owing to the strong tidal torque by the colliding clump. We thus suggest that the GB is one of stellar substructures formed from collisions between gas clouds and dark matter clumps predicted in the hierarchical clustering scenario of galaxy formation. We also suggest that collisions of dark matter clumps with their host galaxies can significantly change star formation histories for some of their gas clouds thus influence galactic global star formation histories to some extent.

astro-ph.GA↗

arXiv2009

Chemical Signature of a Major Merger in the Early Formation of Small Magellanic Cloud

The formation history of the Small Magellanic cloud (SMC) is unraveled based on the results of our new chemical evolution models constructed for the SMC, highlighting the observed anomaly in the age-metallicity relation for star clusters in the SMC. We first propose that evidence of a major merger is imprinted in the age-metallicity relation as a dip in [Fe/H]. Our models predict that the major merger with a mass ratio of 1:1 to 1:4 occurred at ~7.5 Gyr ago, with a good reproduction of the abundance distribution function of field stars in the SMC. Furthermore, our models predict a relatively large scatter in [Mg/Fe] for -1.4 < [Fe/H] <-1.1 as a reflection of a looping feature resulting from the temporally inverse progress of chemical enrichment, which can be tested against future observational results. Given that the observed velocity dispersion (~30 km/s) of the SMC is much smaller than that (~160 km/s) of the Galactic halo, our finding strongly implies that the predicted merger event happened in a small group environment that was far from the Galaxy and contained a number of small gas-rich dwarfs comparable to the SMC. This theoretical view is extensively discussed in the framework that considers a connection with the formation history of the Large Magellanic cloud.

astro-ph.GA↗

arXiv2009

Origin of structural and kinematical properties of the Small Magellanic Cloud

We investigate structural, kinematical, and chemical properties of stars and gas in the Small Magellanic Cloud (SMC) interacting with the Large Magellanic Cloud (LMC) and the Galaxy based on a series of self-consistent chemodynamical simulations. We adopt a new "dwarf spheroidal model" in which the SMC initially has both old stars with a spherical spatial distribution and an extended HI gas disk. We mainly investigate SMC's evolution for the last 3 Gyr within which the Magellanic stream (MS) and the Magellanic bridge (MB) can be formed as a result of the LMC-SMC-Galaxy interaction. Our principal results, which can be tested against observations, are as follows. The final spatial distribution of the old stars projected onto the sky is spherical even after the strong LMC-SMC-Galaxy interaction, whereas that of the new ones is significantly flattened and appears to form a bar structure. Old stars have the line-of-sight velocity dispersion (sigma) of ~ 30 km/s and slow rotation with the maximum rotational velocity (V) of less than slow rotation with the maximum rotational velocity (V) of less than 20 km/s and show asymmetry in the radial profiles. New stars have a smaller sigma than old ones and a significant amount of rotation (V/sigma >1). HI gas shows velocity dispersions of sigma = 10-40 km/s a high maximum rotational velocity (V ~ 50 km/s), and the spatial distribution similar to that of new stars. The new stars with ages younger than 3 Gyr show a negative metallicity gradient in the sense that more metal-rich stars are located in the inner regions of the SMC.

astro-ph↗

arXiv2009

The total mass and dark halo properties of the Small Magellanic Cloud

We discuss the total mass and dark halo properties of the Small Magellanic Cloud (SMC) for reasonable $V$-band stellar-to-mass-to-light ratios ($M_{\rm s}/L_{\rm V}$) based on the high-resolution neutral hydrogen (HI) observations of the SMC. We adopt both the Burkert and the NFW profiles for the dark matter halo of the SMC in order to model the radial profile of the observed rotation curve. We show that the mass ($M_{\rm dm}$) and the mean density ($ρ_{\rm dm}$) of the dark matter halo within the optical radius ($\sim 3$ kpc) of the SMC can be significantly low for $M_{\rm s}/L_{\rm V}=2-4$ reasonable for a galaxy dominated by older stellar populations. For example, the estimated $M_{\rm dm}$ and $ρ_{\rm dm}$ are $7.5 \times 10^7 {\rm M}_{\odot}$ and $6.7 \times 10^{-4} {\rm M}_{\odot}$ pc$^{-3}$, respectively, for $M_{\rm s}/L_{\rm V} = 3.8$. The models with lower $M_{\rm s}/L_{\rm V}$ ($<1.0$) can show higher $M_{\rm dm}$ and $ρ_{\rm dm}$ yet have difficulties in reproducing the inner rotation curve profile both for the Burkert and the NFW profiles. The Burkert profile with a larger core radius ($>1$ kpc) (thus a low density) and a large mass (${\rm M}_{\rm dm} > 3 \times 10^9 {\rm M}_{\odot}$) can be better fit to the observed slowly rising rotation curve than the NFW one for a reasonable $M_{\rm s}/L_{\rm V} \sim2$. This means that most of the dark halo mass can be initially located outside the optical radius in the SMC and thus that the dark halo would have already lost a significant fraction of its original mass due to the strong tidal interactions with the Galaxy and possibly with the Large Magellanic Cloud (LMC).

astro-ph↗

arXiv2008

Formation of the off-center bar in the Large Magellanic Cloud: A collision with a dark satellite ?

Recent observations on structural properties of the Large Magellanic Cloud (LMC) based on the Deep Near-Infrared Southern Sky Survey (DENIS) and Two Micron All-Sky Survey (2MASS) have revealed that the LMC has an off-center bar even in the older stellar populations. Previous dynamical models including tidal interaction between the LMC, the Small Magellanic Cloud (SMC), and the Galaxy, however, did not reproduce so well the older off-center bar. We here show that the off-center bar can be formed if the LMC with an already existing bar can collide with a low-mass Galactic subhalo as massive as 5 * 10^8 M_sun (corresponding roughly to a few % of the LMC mass). The simulated stellar distribution after the collision appears to show an ``off-center bar'', not because the center of the bar significantly deviates from the dynamical center of the LMC, but because the underlying stellar distribution of the disk is significantly asymmetric with respect to the center of the bar. We discuss whether off-center bars observed in Magellanic-type dwarf galaxies can be formed as a result of tidal interaction with low-mass halos with no or little visible matters.

astro-ph↗

arXiv2008

A possible common halo of the Magellanic Clouds

Recent observational and theoretical studies on the three-dimensional (3D) space motions of the Large and the Small Magellanic Clouds (LMC and SMC, respectively) have strongly suggested that the latest proper motion measurements of the Magellanic Clouds (MCs) are consistent with their orbital evolution models in which the MCs have arrived in the Galaxy quite recently for the first time. The suggested orbital models appear to be seriously inconsistent with the tidal interaction models in which the Magellanic Stream (MS) can be formed as a result of the mutual tidal interaction between the MCs and the Galaxy for the last ~2 Gyr. Based on orbital models of the MCs, we propose that if the MCs have a common diffuse dark halo with the mass larger than ~ 2 * 10^{10} M_sun, the MCs can not only have the present 3D velocities consistent with the latest proper motion measurements but also interact strongly with each other and with the Galaxy for the last 2 Gyr.These results imply that if the observed proper motions of the MCs are true ones of the centers of mass for the MCs, the common halo of the MCs would need to be considered in constructing self-consistent MS formation models. We discuss whether the origin of the possible common halo can be closely associated either with the past binary formation or with the MCs having been in a small group.

astro-ph↗

arXiv2008

Self-consistent simulations of star cluster formation from gas clouds under the influence of galaxy-scale tidal fields

We present the first results of a project aimed at following the formation and long-term dynamical evolution of star clusters within the potential of a host galaxy. Here we focus on a model evolved within a simplified potential representing the Large Magellanic Cloud. This demonstrates for the first time the self-consistent formation of a bound star cluster from a giant molecular cloud. The model cluster reproduces the density profiles and structural characteristics of observed star clusters.

astro-ph↗

arXiv2008

Formation of a giant HI bridge between M31 and M33 from their tidal interaction

Recent observations have discovered a giant HI bridge that appears to connect between the outer halo regions of M31 and M33. We propose that this HI bridge can be formed as a result of the past interaction between M31 and M33 based on test particle simulations with different orbits of M31 and M33 for the last ~ 9 Gyr. We show that strong tidal interaction between M31 and M33 about 4-8 Gyr ago can strip HI gas from M33 to form HI streams around M31 which can be observed as a HI bridge if they are projected onto the sky. We show that the number fraction of models reproducing well the observed HI distribution of the bridge is only ~0.01% (i.e., ~10 among ~10^5 models) and thus suggest that the observed structure and kinematics of the HI bridge can give some constraints on the past orbits of M31 and M33. We suggest that the observed outer HI warp in M33 could be fossil evidence for the past M31-M33 interaction. We also suggest that some of high velocity clouds (HVCs) recently found in M31 could be HI gas originating from M33. We briefly discuss other possible scenarios for the formation of the HI bridge.

astro-ph↗

arXiv2008

Origin of dwarf irregular galaxies with outer early-type structures

Recent observations have reported that some gas-rich dwarf irregular (dIrr) galaxies appear to have spherical distributions in the outer underlying old and intermediate-age stellar populations (e.g., NGC 6822). These observations imply that some dIrr's have two distinct (or ``two-component'') structures, i.e., inner disky and outer spherical ones, though the number fraction of dIrr's with such structures remains observationally unclear. We discuss how such two distinct structures are formed during dIrr formation based on observations and simulations. Our numerical simulations show that the remnants of mergers between two gas-rich dIrr's with initially extended gas disks can have both extended spheroids composed of older stellar populations and disks composed mostly of gas and young stars. The simulated remnants with two distinct structures can be still identified as dIrr's owing to the presence of star-forming regions. The structural properties of outer spherical structures in dIrr's formed from dIrr-dIrr merging depend on initial conditions of merging, which suggests that outer structures in dIrr's can be diverse. We also discuss other possible physical mechanisms for the formation of outer spherical structures composed of older stars in dIrr's.

astro-ph↗

arXiv2008

Formation of blue compact dwarf galaxies from merging and interacting gas-rich dwarfs

We present the results of numerical simulations which show the formation of blue compact dwarf (BCD) galaxies from merging between very gas-rich dwarfs with extended HI gas disks. We show that dwarf-dwarf merging can trigger central starbursts and form massive compact cores dominated by young stellar populations. We also show that the pre-existing old stellar components in merger precursor dwarfs can become diffuse low surface brightness components after merging. The compact cores dominated by younger stellar populations and embedded in more diffusely distributed older ones can be morphologically classified as BCDs. Since new stars can be formed from gas transferred from the outer part of the extended gas disks of merger precursors, new stars can be very metal-poor ([Fe/H] < -1). Owing to very high gaseous pressure exceeding 10^5 k_B (where k_B is the Boltzmann constant) during merging, compact star clusters can be formed in forming BCDs. The BCDs formed from merging can still have extended HI gas disks surrounding their blue compact cores. We discuss whether tidal interaction of gas-rich dwarfs without merging can also form BCDs.

astro-ph↗

arXiv2008

Formation of the Small Magellanic Cloud: ancient major merger as a solution to the kinematical differences between old stars and HI gas

Recent observations of the Small Magellanic Cloud (SMC) have revealed that the HI gas shows a significant amount of rotation (V_c 60 km/s), while no or little rotation is evident for the old stellar populations. We suggest that this unique kinematical difference between these components in the SMC can be caused by a major merger event which occurred in the early stage of the SMC formation. Our simulations show that dissipative dwarf-dwarf merging can transform two gas-rich dwarf irregulars into a new dwarf, which consists of a spheroidal stellar component and a rotating extended HI disk. The remnant of this dwarf-dwarf merging shows significantly different kinematics between stars and gas, in the sense that a gas disk rotates rapidly while a stellar component shows little rotation. We thus suggest that the simulated dwarf having a dynamically hot spheroid and an extended gas disk finally evolves into the present SMC after efficient stripping of the outer gas via tidal fields of the Galaxy and the Large Magellanic Cloud. We also suggest that spatial distributions and kinematics of RGB and AGB stars with different ages in the possible spheroidal component of the SMC can provide valuable information on whether and when a past major merger event really occurred in the SMC.

astro-ph↗

arXiv2007

The Magellanic impact: Collision between the outer Galactic HI disk and the leading arms of the Magellanic stream

We show that collisions between the outer Galactic HI disk and the leading arms (LAs) of the Magellanic stream (MS) can create giant HI holes and chimney-like structures in the disk. Based on the results of our N-body simulations on the last 2.5 Gyr evolution of the Large and Small Magellanic Clouds (LMC and SMC, respectively) interacting with the Galaxy, we investigate when and where the LAs can pass through the Galactic plane after the MS formation. We then investigate hydrodynamical interaction between LAs and the Galactic HI disk (``the Magellanic impact'') by using our new hydrodynamical simulations with somewhat idealized models of the LAs. We find that about 1-3% of the initial gas mass of the SMC, which consists of the LAs, can pass through the outer part (R=20-35 kpc) of the Galactic HI disk about 0.2 Gyr ago. We also find that the Magellanic impact can push out some fraction (~1%) of the outer Galactic HI disk to form 1-10 kpc-scale HI holes and chimney-like bridges between the LAs and the disk.

astro-ph↗

arXiv2007

Evolution of the Small Magellanic Cloud

Based on the results of N-body simulations on the last 2.5 Gyr evolution of the Large and Small Magellanic Clouds (LMC and SMC, respectively) interacting with the Galaxy, we firstly show when and where the leading arms (LAs) of the Magellanic stream (MS) can pass through the Galactic plane after the MS formation. We secondly show collisions between the outer Galactic HI disk and the LAs of the MS can create giant HI holes and chimney-like structures in the disk about 0.2 Gyr ago. We thirdly show that a large amount of metal-poor gas is stripped from the SMC and transfered to the LMC during the tidal interaction between the Clouds and the Galaxy about 0.2 and 1.3 Gyr ago. We thus 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.

astro-ph↗

arXiv2007

Origin of lower velocity dispersions of ultra-compact dwarf galaxy populations in clusters of galaxies

Recent observations have revealed that velocity dispersions of ``ultra-compact dwarf'' (UCD) galaxies are significantly smaller than those of other galaxy populations in the Fornax and the Virgo clusters of galaxies. In order to understand the origin of the observed lower velocity dispersions of UCDs, we numerically investigate line-of-sight velocity dispersion (sigma_los) of galaxy populations with variously different orbits in clusters of galaxies with the total masses of M_cl. We particularly investigate radial velocity dispersion profiles (sigma_los(R)) and velocity dispersions within the central 200 kpc of a cluster model (sigma_m) for galaxies with different pericenter distances (r_p) and orbital eccentricities (e) in the model with M_cl = 7.0 x 10^13 M_sun reasonable for the Fornax cluster. We find that sigma_los(R) and sigma_m of galaxies with smaller r_p are steeper and smaller, respectively, for a given initial e distribution of galaxies. For example, we find that sigma_m is ~ 260 km/s for galaxies with r_p <50 kpc and ~ 336 km/s for all galaxies in the model with the mean e of 0.6. These results imply that the observed lower velocity dispersion of UCD population is consistent with the UCDs having significantly smaller r_p than other galaxy populations in the Fornax. We discuss these results in the context of the ``galaxy threshing'' scenario in which UCDs originate from nuclei of nucleated dwarf galaxies. We suggest that the observed differences in kinematical properties between UCDs and other dwarf galaxy populations in clusters of galaxies can be understood in terms of the differences in orbital properties between UCDs and the dwarf populations.

astro-ph↗