arXiv subjects

Masashi Chiba

Publications and source records attributed to Masashi Chiba.

At least 91 records · Page 5Linked to original sources

arXiv2010

Structure and Kinematics of the Stellar Halos and Thick Disks of the Milky Way Based on Calibration Stars from SDSS DR7

The structure and kinematics of the recognized stellar components of the Milky Way are explored, based on well-determined atmospheric parameters and kinematic quantities for 32360 calibration stars from the Sloan Digital Sky Survey (SDSS) and its first extension, (SDSS-II), which included the sub-survey SEGUE: Sloan Extension for Galactic Understanding and Exploration. Full space motions for a sub-sample of 16920 stars, exploring a local volume within 4 kpc of the Sun, are used to derive velocity ellipsoids for the inner- and outer-halo components of the Galaxy, as well as for the canonical thick-disk and proposed metal-weak thick-disk populations. We first examine the question of whether the data require the presence of at least a two-component halo in order to account for the rotational behavior of likely halo stars in the local volume, and whether more than two components are needed. We also address the question of whether the proposed metal-weak thick disk is kinematically and chemically distinct from the canonical thick disk. In addition, we consider the fractions of each component required to understand the nature of the observed kinematic behavior of the stellar populations of the Galaxy as a function of distance from the plane. Scale lengths and scale heights for the thick-disk and metal-weak thick-disk components are determined. Spatial density profiles for the inner- and outer-halo populations are inferred from a Jeans Theorem analysis. The full set of calibration stars (including those outside the local volume) is used to test for the expected changes in the observed stellar metallicity distribution function with distance above the Galactic plane in-situ, due to the changing contributions from the underlying stellar populations. [abridged]

astro-ph.GA↗

arXiv2009

Chemical Compositions of Kinematically Selected Outer Halo Stars

Chemical abundances of 26 metal-poor dwarfs and giants are determined from high-resolution and high signal-to-noise ratio spectra obtained with Subaru/HDS. The sample is selected so that most of the objects have outer-halo kinematics. Self-consistent atmospheric parameters were determined by an iterative procedure based on spectroscopic analysis. Abundances of 13 elements, including $α$-elements (Mg, Si, Ca, Ti), odd-Z light elements (Na, Sc), iron-peak elements (Cr, Mn, Fe, Ni, Zn) and neutron-capture elements (Y, Ba), are determined by two independent data reduction and LTE analysis procedures, confirming the consistency of the stellar parameters and abundances results. We find a decreasing trend of [$α$/Fe] with increasing [Fe/H] for the range of $-3.5 <$ [Fe/H]$ < -1$, as found by Stephens and Boesgaard (2002). [Zn/Fe] values of most objects in our sample are slightly lower than the bulk of halo stars previously studied. These results are discussed as possible chemical properties of the outer halo in the Galaxy.

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arXiv2009

On the Size Evolution of a Galactic Disk in Hierarchical Merging of Cold Dark Matter Halos

We investigate the dynamical effects of dark matter subhalos on the structure and evolution of a galactic disk, using semi-analytic method that includes approximated and empirical relations as achieved in detailed numerical simulations of the cold dark matter model. We calculate the upper limit for the size of a galactic disk at a specific redshift $z$, based on the orbital properties of subhalos characterized by their pericentric distances from the center of a host halo. We find that this possibly largest size of a disk as determined by the smallest pericentric distances of subhalos shows the characteristic properties, which are basically in agreement with an observed galactic disk at low and high $z$. Namely, it is found that a massive disk can have a larger size than a less massive one, because of its stability against the destruction effect of subhalos. Also, with fixed mass, the size of a galactic disk at low $z$ can be larger than that at high $z$, reflecting the orbital evolution of subhalos with respect to a host halo. These results suggest that the presence and structure of a galactic disk may be dynamically limited by the interaction with dark matter substructures, especially at high $z$.

astro-ph.CO↗

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.

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arXiv2009

Subaru Mid-infrared Imaging of the Quadruple Lenses. II. Unveiling Lens Structure of MG0414+0534 and Q2237+030

We present mid-infrared imaging at 11.7 $μ$m for the quadruple lens systems, MG0414+0534 and Q2237+030, using the cooled mid-infrared camera and spectrometer (COMICS) attached on the Subaru telescope. MG0414+0534 is characterized by a bright pair of lensed images (A1, A2) and their optical flux ratio A2/A1 deviates significantly from the prediction of a smooth lens model. Q2237+030 is comprised of four lensed images, which are significantly affected by microlensing in a foreground lensing galaxy. Our mid-infrared observations of these lensed images have revealed that the mid-infrared flux ratio for A2/A1 of MG0414+0534 is nearly unity (0.90 +- 0.04). We find that this flux ratio is systematically small, at 4 to 5 sigma level, compared with the prediction of a best smooth lens model (1.09) represented by a singular isothermal ellipsoid and external shear, thereby suggesting the presence of substructures to explain our observational result. In contrast, for Q2237+030, our high signal-to-noise observation indicates that the mid-infrared flux ratios between all the four images of Q2237+030 are virtually consistent with the prediction of a smooth lens model. Based on the size estimate of the dust torus surrounding the nuclei of these QSOs, we set limits on the mass of a substructure in these lens systems, which can cause anomalies in the flux ratios. For MG0414+0534, since the required mass of a substructure inside its Einstein radius is > 360 M_sun, millilensing by a CDM substructure is most likely. If it is modeled as a singular isothermal sphere, the mass inside radius of 100 pc is given as > 1.0x10^5 M_sun. For Q2237+030, there is no significant evidence of millilensing, so the reported anomalous flux ratios in shorter wavelengths are entirely caused by microlensing by stars (abridged).

astro-ph.CO↗

arXiv2008

The Extended Star Formation History of the Andromeda Spheroid at 35 Kpc on the Minor Axis

Using the HST ACS, we have obtained deep optical images reaching well below the oldest main sequence turnoff in fields on the southeast minor-axis of the Andromeda Galaxy, 35 kpc from the nucleus. These data probe the star formation history in the extended halo of Andromeda -- that region beyond 30 kpc that appears both chemically and morphologically distinct from the metal-rich, highly-disturbed inner spheroid. The present data, together with our previous data for fields at 11 and 21 kpc, do not show a simple trend toward older ages and lower metallicities, as one might expect for populations further removed from the obvious disturbances of the inner spheroid. Specifically, the mean ages and [Fe/H] values at 11 kpc, 21 kpc, and 35 kpc are 9.7 Gyr and -0.65, 11.0 Gyr and -0.87, and 10.5 Gyr and -0.98, respectively. In the best-fit model of the 35 kpc population, one third of the stars are younger than 10 Gyr, while only ~10% of the stars are truly ancient and metal-poor. The extended halo thus exhibits clear evidence of its hierarchical assembly, and the contribution from any classical halo formed via early monolithic collapse must be small.

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↗

arXiv2008

Dark Matter Substructure in Lensing Galaxies

To set useful limits on the abundance of small-scale dark matter halos (subhalos) in a galaxy scale, we have carried out mid-infrared imaging and integral-field spectroscopy for a sample of quadruple lens systems showing anomalous flux ratios. These observations using Subaru have been successful for distinguishing millilensing by subhalos from microlensing by stars. Current status for our lensing analysis of dark matter substructure is reported.

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arXiv2008

Lens Mapping of Dark Matter Substructure with VSOP-2

Hierarchical clustering models of cold dark matter (CDM) predict that about 5% - 10% of a galaxy-sized halo with mass ~ 10^12 solar masses (M_sun) resides in substructures (CDM subhalos) with masses <= 10^8 M_sun. To directly identify such substructures, we propose to observe radio continuum emission from multiply imaged QSOs using VSOP-2 with a high angular resolution.

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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.

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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.

astro-ph↗

arXiv2007

A Subaru/Suprime-Cam Survey of M31's spheroid along the South-East minor axis

We have used Suprime-Cam on the Subaru Telescope to conduct a V- and I-band imaging survey of fields sampling the spheroid of the Andromeda galaxy along its south-east minor axis. Our photometric data are deep enough to resolve stars down to the red clump. Based on a large and reliable sample of red giant stars available from this deep wide-field imager, we have derived metallicity distributions vs. radius and a surface brightness profile over projected distances of R=23-66 kpc from the galaxy's center. The metallicity distributions across this region shows a clear high mean metallicity and a broad distribution ([Fe/H] ~ -0.6 +/- 0.5), and indicates no metallicity gradient within our observed range. The surface brightness profile at R>40 kpc is found to be flatter than previously thought. It is conceivable that this part of the halo samples as yet unidentified, metal-rich substructure.

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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

Dynamical Effects of CDM Subhalos on a Galactic Disk

We investigate the dynamical interaction between a galactic disk and surrounding numerous dark subhalos as expected for a galaxy-sized halo in the cold dark matter (CDM) models. Our particular interest is to what extent accretion events of subhalos into a disk are allowed in light of the observed thinness of a disk. Several models of subhalos are considered in terms of their internal density distribution, mass function, and spatial and velocity distributions. Based on a series of N-body simulations, we find that the disk thickening quantified by the change of its scale height, Delta z_d, depends strongly on the individual mass of an interacting subhalo M_{sub}. This is described by the relation, Delta z_d / R_d = 8 Sum_{j=1}^N (M_{sub,j}/M_d)**2, where R_d is a disk scale length, M_d is a disk mass, and N is the total number of accretion events of subhalos inside a disk region (< 3R_d). Using this relation, we find that an observed thin disk has not ever interacted with subhalos with the total mass of more than 15% disk mass. Also, a less massive disk with smaller circular velocity V_c is more affected by subhalos than a disk with larger V_c, in agreement with the observation. Further implications of our results for the origin of a thick disk component are also discussed.

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arXiv2005

Direct Detection of Intermediate Mass Compact Objects via Submillilensing

A galaxy-sized halo may contain a large number of intermediate mass (10^{2-4} solar mass) compact objects (IMCOs), which can be intermediate mass black holes (IMBHs) or the CDM subhalos. We propose to directly detect the IMBHs by observing multiply imaged QSO-galaxy lens systems with a high angular resolution (~ 0.03mas), which would be achieved by the next-VLBI space missions. The silhouette of the IMBHs would appear as an either monopole-like or dipole-like variation at the scale of the Einstein radius against the QSO jets. As a byproduct, we can also directly detect the 10^{4-5} solar mass CDM subhalos. From a measurement of the local distortion in the surface brightness of the QSO jet, we can make a distinction between a point mass (corresponding to an IMBH) and an extended structure (corresponding to a CDM subhalo). It would be a unique probe of the IMCOs whose nature has been under the veil of mistery.

astro-ph↗

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

Extended Source Effects in Substructure Lensing

We investigate the extended source size effects on gravitational lensing in which a lens consists of a smooth potential and small mass clumps (``substructure lensing''). We first consider a lens model that consists of a clump modeled as a singular isothermal sphere (SIS) and a primary lens modeled as an external background shear and convergence. For this simple model, we derive analytic formulae for (de)magnification of circularly symmetric top-hat sources with three types of parity for their lensed images, namely, positive, negative, and doubly negative parities. Provided that the source size is sufficiently larger than the Einstein radius of the SIS, we find that in the positive (doubly negative) parity case, an extended source is always magnified (demagnified) in comparison with the unperturbed macrolens system, whereas in the negative parity case, the (de)magnification effect, which depends on the sign of convergence minus unity is weaker than those in other parities. It is shown that a measurement of the distortion pattern in a multiply lensed image enables us to break the degeneracy between the lensing effects of clump mass and those of clump distance if lensing parameters of the relevant macrolens model are determined from the position and flux of multiple images. We also show that an actual density profile of a clump can be directly measured by analyzing the ``fine structure'' in a multiply lensed image within the Einstein radius of the clump.

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