Masashi Chiba

Two stellar components in the halo of the Milky Way

The halo of the Milky Way provides unique elemental abundance and kinematic information on the first objects to form in the Universe, and this information can be used to tightly constrain models of galaxy formation and evolution. Although the halo was once considered a single component, evidence for its dichotomy has slowly emerged in recent years from inspection of small samples of halo objects. Here we show that the halo is indeed clearly divisible into two broadly overlapping structural components—an inner and an outer halo—that exhibit different spatial density profiles, stellar orbits and stellar metallicities (abundances of elements heavier than helium). The inner halo has a modest net prograde rotation, whereas the outer halo exhibits a net retrograde rotation and a peak metallicity one-third that of the inner halo. These properties indicate that the individual halo components probably formed in fundamentally different ways, through successive dissipational (inner) and dissipationless (outer) mergers and tidal disruption of proto-Galactic clumps.

Kinematics of Metal-poor Stars in the Galaxy. III. Formation of the Stellar Halo and Thick Disk as Revealed from a Large Sample of Nonkinematically Selected Stars

We present a detailed analysis of the space motions of 1203 solar-neighborhood stars with metal abundances [Fe/H] ≤ -0.6, on the basis of a catalog, of metal-poor stars selected without kinematic bias recently revised and supplemented by Beers et al. This sample, having available proper motions, radial velocities, and distance estimates for stars with a wide range of metal abundances, is by far the largest such catalog to be assembled to date. We show that the stars in our sample with [Fe/H] ≤-2.2, which likely represent a pure halo component, are characterized by a radially elongated velocity ellipsoid (σU, σV, σW) = (141 ± 11, 106 ± 9, 94 ± 8) km s-1 and small prograde rotation V = 30 to 50 km s-1, consistent with previous analysis of this sample by Beers and Sommer-Larsen based on radial velocity information alone. In contrast to the previous analysis, we find a decrease in V with increasing distance from the Galactic plane for stars that are likely to be members of the halo population (ΔV/Δ|Z| = -52 ± 6 km s-1 kpc-1), which may represent the signature of a dissipatively formed flattened inner halo. Unlike essentially all previous kinematically selected catalogs, the metal-poor stars in our sample exhibit a diverse distribution of orbital eccentricities, e, with no apparent correlation between [Fe/H] and e. This demonstrates, clearly and convincingly, that the evidence offered in 1962 by Eggen, Lynden-Bell, & Sandage for a rapid collapse of the Galaxy, an apparent correlation between the orbital eccentricity of halo stars with metallicity, is basically the result of their proper-motion selection bias. However, even in our nonkinematically selected sample, we have identified a small concentration of high-e stars at [Fe/H] ~ -1.7, which may originate, in part, from infalling gas during the early formation of the Galaxy. We find no evidence for an additional thick disk component for stellar abundances [Fe/H] ≤ -2.2. The kinematics of the intermediate-abundance stars close to the Galactic plane are, in part, affected by the presence of a rapidly rotating thick disk component with V 200 km s-1 (with a vertical velocity gradient on the order of ΔV/Δ|Z| = -30 ± 3 km s-1 kpc-1) and velocity ellipsoid (σU, σV, σW) = (46 ± 4, 50 ± 4, 35 ± 3) km s-1. The fraction of low-metallicity stars in the solar neighborhood that are members of the thick disk population is estimated as ~10% for -2.2 < [Fe/H] ≤ -1.7 and ~30% for -1.7 < [Fe/H] ≤ -1. We obtain an estimate of the radial scale length of the metal-weak thick disk of 4.5 ± 0.6 kpc. We also analyze the global kinematics of the stars constituting the halo component of the Galaxy. The outer part of the halo, which we take to be represented by local stars on orbits reaching more than 5 kpc from the Galactic plane, exhibits no systematic rotation. In particular, we show that previous suggestions of the presence of a counter-rotating high halo are not supported by our analysis. The density distribution of the outer halo is nearly spherical and exhibits a power-law profile that is accurately described as ρ ∝ R-3.55±0.13. The inner part of the halo is characterized by a prograde rotation and a highly flattened density distribution. We find no distinct boundary between the inner and outer halo. We confirm the clumping in angular-momentum phase space of a small number of local metal-poor stars noted in 1999 by Helmi et al. We also identify an additional elongated feature in angular-momentum phase space extending from the clump to regions with high azimuthal rotation. The number of members in the detected clump is not significantly increased from that reported by Helmi et al., even though the total number of the sample stars we consider is almost triple that of the previous investigation. We conclude that the fraction of halo stars that may have arisen from the precursor object of this clump may be smaller than 10% of the present Galactic halo, as previously suggested. The implications of our results for the formation of the Galaxy are discussed, in particular in the context of the currently favored cold dark matter theory of hierarchical galaxy formation.

Probing Dark Matter Substructure in Lens Galaxies

We investigate the eUects of numerous dark matter subhalos in a galaxy-sized halo on the events of strong lensing to assess their presence as expected from the cold dark matter scenario. Lens galaxies are represented by a smooth ellipsoid in an external shear —eld and additional cold dark matter subhalos taken from Monte Carlo realizations that accord with recent N-body results. We also consider other possible perturbers, globular clusters and luminous dwarf satellites, for comparison. We then apply the models to the particular lens systems with four images, B1422]231 and PG 1115]080, for which smooth lens models are unable to simultaneously reproduce both the positions of the images and their radio —ux ratios or dust-free optical —ux ratios. We show that the perturbations by both globular clusters and dwarf satellites are too small to change the —ux ratios, whereas cold dark matter subhalos are the most likely perturbers to reproduce the observed —ux ratios in a statistically signi—cant manner. This result suggests to us the presence of numerous subhalos in lens galaxies, which is consistent with the results of cosmological N-body simulations.

The mass of the Milky Way: Limits from a newly assembled set of halo objects

We set new limits on the mass of the Milky Way, making use of the latest kinematic information for Galactic satellites and halo objects. Our sample consists of 11 satellite galaxies, 137 globular clusters, and 413 field horizontal-branch (FHB) stars up to distances of 10 kpc from the Sun. Roughly half of the objects in this sample have measured proper motions, permitting the use of their full space motions in our analysis. In order to bind these sample objects to the Galaxy, their rest-frame velocities must be lower than their escape velocities at their estimated distances. This constraint enables us to show that the mass estimate of the Galaxy is largely aected by several high-velocity objects (Leo I, Pal 3, Draco, and a few FHB stars), not by a single object alone (such as Leo I), as has often been the case in past analyses. We also find that a gravitational potential that gives rise to a declining rotation curve is insucient to bind many of our sample objects to the Galaxy; a possible lower limit on the mass of the Galaxy is about 2:2 10 12 M. To be more quantitative, we adopt a Bayesian likelihood approach to reproduce the observed distribution of the current positions and motions of the sample, in a prescribed Galactic potential that yields a flat rotation curve. This method enables a search for the most likely total mass of the Galaxy, without undue influence in the final result arising from the presence or absence of Leo I, provided that both radial velocities and proper motions are used. Although the best mass estimate depends somewhat on the model assumptions, such as the unknown prior probabilities for the model parameters, the resultant systematic change in the mass estimate is confined to a relatively narrow range of a few times 10 11 M, owing to our consideration of many FHB stars. The most likely total mass derived from this method is 2:5 +0:5 1:0 10 12 M

Kinematics of Metal-poor Stars in the Galaxy. II. Proper Motions for a Large Nonkinematically Selected Sample

We present a revised catalog of 2106 Galactic stars, selected without kinematic bias and with available radial velocities, distance estimates, and metal abundances in the range -4.0 ≤ [Fe/H] ≤ 0.0. This update of the 1995 Beers & Sommer-Larsen catalog includes newly derived homogeneous photometric distance estimates, revised radial velocities for a number of stars with recently obtained high-resolution spectra, and refined metallicities for stars originally identified in the HK objective-prism survey (which account for nearly half of the catalog) based on a recent recalibration. A subset of 1258 stars in this catalog have available proper motions based on measurements obtained with the Hipparcos astrometry satellite or taken from the updated Astrographic Catalogue (second epoch positions from either the Hubble Space Telescope Guide Star Catalog or the Tycho Catalogue), the Yale/San Juan Southern Proper Motion Catalog 2.0, and the Lick Northern Proper Motion Catalog. Our present catalog includes 388 RR Lyrae variables (182 of which are newly added), 38 variables of other types, and 1680 nonvariables, with distances in the range 0.1 to 40 kpc.

Formation and evolution of the Magellanic Clouds – I. Origin of structural, kinematic and chemical properties of the Large Magellanic Cloud

We investigate the dynamical and chemical evolution of the Large Magellanic Cloud (LMC) interacting with the Galaxy and the Small Magellanic Cloud (SMC) based on a series of self-consistent chemodynamical simulations. Our numerical models are aimed at explaining the entire properties of the LMC, i.e. the observed structure and kinematics of its stellar halo and disc components as well as the populations of the field stars and star clusters. The main results of the present simulations are as follows. Tidal interaction between the Clouds and the Galaxy during the last 9 Gyr has transformed the initially thin, non-barred LMC disc into three different components: central bar, thick disc and kinematically hot stellar halo. The central bar is composed of both old field stars and newly formed ones, with the two fractions being equal in its innermost part. The final thick disc has central velocity dispersion of ∼30 km s−1 and shows rotationally supported kinematics with Vm/σ0∼ 2.3. The stellar halo is formed during the interaction, and consists mainly of old stars originating from the outer part of the initially thin LMC disc. The outer halo shows velocity dispersion of ∼40 km s−1 at a distance of 7.5 kpc from the LMC centre and has a somewhat inhomogeneous distribution of stars. The stellar halo contains relatively young, metal-rich stars with a mass fraction of 2 per cent. Repetitive interaction between the Clouds and the Galaxy has moderately enhanced the star formation rate to ∼0.4 M⊙ yr−1 in the LMC disc. Most of the new stars (∼90 per cent) are formed within the central 3 kpc of the disc, in particular, within the central bar for the last 9 Gyr. Consequently, the half-mass radius is different by a factor of 2.3 between old field stars and newly formed ones. Efficient globular cluster formation does not occur until the LMC starts interacting violently and closely with the SMC (∼3 Gyr ago). The newly formed globular cluster system has a disc-like distribution with rotational kinematics, and its mean metallicity is ∼1.2 higher than that of new field stars because of pre-enrichment by the formation of field stars prior to cluster formation. The LMC evolution depends on its initial mass and orbit with respect to the Galaxy and the SMC. In particular, the epoch of the bar and thick disc formation and the mass fraction of the stellar halo depend on the initial mass of the LMC. Based on these results, we discuss the entire formation history of the LMC, the possible fossil records of past interaction between the Clouds and the Galaxy, and the star formation history of the SMC for the past several Gyr.

Extragalactic science, cosmology, and Galactic archaeology with the Subaru Prime Focus Spectrograph

The Subaru Prime Focus Spectrograph (PFS) is a massively-multiplexed fiber-fed optical and near-infrared 3-arm spectrograph (N_fiber=2400, 380<lambda<1260nm, 1.3 degree diameter FoV), offering unique opportunities in survey astronomy. Here we summarize the science case feasible for a survey of Subaru 300 nights. We describe plans to constrain the nature of dark energy via a survey of emission line galaxies spanning a comoving volume of 9.3 (Gpc/h)^3 in the redshift range 0.8<z<2.4. In each of 6 redshift bins, the cosmological distances will be measured to 3% precision via BAO, and redshift-space distortions will be used to constrain structure growth to 6% precision. In the GA program, radial velocities and chemical abundances of stars in the Milky Way and M31 will be used to infer the past assembly histories of spiral galaxies and the structure of their dark matter halos. Data will be secured for 10^6 stars in the Galactic thick-disk, halo and tidal streams as faint as V~22, including stars with V < 20 to complement the goals of the Gaia mission. A medium-resolution mode with R = 5000 to be implemented in the red arm will allow the measurement of multiple alpha-element abundances and more precise velocities for Galactic stars, elucidating the detailed chemo-dynamical structure and evolution of each of the main stellar components of the Milky Way Galaxy and of its dwarf spheroidal galaxies. For the extragalactic program, our simulations suggest the wide avelength range will be powerful in probing the galaxy population and its clustering over a wide redshift range. We propose to conduct a color-selected survey of 1<z<2 galaxies and AGN over 16 deg^2 to J~23.4, yielding a fair sample of galaxies with stellar masses above ~10^{10}Ms at z~2. A two-tiered survey of higher redshift LBGs and LAEs will quantify the properties of early systems close to the reionization epoch.

THE CASE FOR THE DUAL HALO OF THE MILKY WAY

Carollo et al. have recently resolved the stellar population of the Milky Way halo into at least two distinct components, an inner halo and an outer halo. This result has been criticized by Schonrich et al., who claim that the retrograde signature associated with the outer halo is due to the adoption of faulty distances. We refute this claim, and demonstrate that the Schonrich et al. photometric distances are themselves flawed because they adopted an incorrect main-sequence absolute magnitude relationship from the work of Ivezic et al. When compared to the recommended relation from Ivezic et al., which is tied to a Milky Way globular cluster distance scale and accounts for age and metallicity effects, the relation adopted by Schonrich et al. yields up to 18% shorter distances for stars near the main-sequence turnoff (TO). Use of the correct relationship yields agreement between the distances assigned by Carollo et al. and Ivezic et al. for low-metallicity dwarfs to within 6%-10%. Schonrich et al. also point out that intermediate-gravity stars (3.5 ≤log g < 4.0) with colors redder than the TO region are likely misclassified, with which we concur. We implement a new procedure to reassign luminosity classifications for the TO stars that require it. New derivations of the rotational behavior demonstrate that the retrograde signature and high velocity dispersion of the outer-halo population remain. We summarize additional lines of evidence for a dual halo, including a test of the retrograde signature based on proper motions alone, and conclude that the preponderance of evidence strongly rejects the single-halo interpretation.

FORMATION OF THE GALACTIC STELLAR HALO. I. STRUCTURE AND KINEMATICS

We perform numerical simulations for the formation of the Galactic stellar halo, based on the currently favored cold dark matter theory of galaxy formation. Our numerical models, taking into account both dynamical and chemical evolution processes in a consistent manner, are aimed at explaining the observed structure and kinematics of the stellar halo in the context of hierarchical galaxy formation. The main results of the present simulations are summarized as follows: (1) Basic physical processes involved in the formation of the stellar halo, composed of metal-deficient stars with [Fe/H] ≤ -1.0, are described by both dissipative and dissipationless merging of subgalactic clumps and their resultant tidal disruption in the course of gravitational contraction of the Galaxy at high redshift (z > 1). (2) The simulated halo has a density profile similar to the observed power-law form of ρ(r) ~ r-3.5 and also has a metallicity distribution similar to the observations. The halo shows virtually no radial gradient for stellar ages and only a small gradient for metallicities. (3) The dual nature of the halo, i.e., its inner flattened and outer spherical density distribution, is reproduced, at least qualitatively, by the present model. The outer spherical halo is formed via essentially dissipationless merging of small subgalactic clumps, whereas the inner flattened one is formed via three different mechanisms, i.e., dissipative merging between larger, more massive clumps, adiabatic contraction due to the growing Galactic disk, and gaseous accretion onto the equatorial plane. (4) For the simulated metal-poor stars with [Fe/H] ≤ -1.0, there is no strong correlation between metal abundances and orbital eccentricities, in good agreement with the recent observations. Moreover, the observed fraction of the low-eccentricity stars is reproduced correctly for [Fe/H] ≤ -1.6 and approximately for the intermediate-abundance range of -1.6 -2.2. The stars at smaller distance from the disk plane appear to show systematically larger . Based on these results, we discuss how early processes of dissipationless and dissipative merging of subgalactic clumps can reproduce plausibly and consistently the recent observational results on the Galactic stellar halo. We also present a possible scenario for the formation of the entire Galaxy structure, including bulge and disk components, in conjunction with halo formation.

New Limits on a Cosmological Constant from Statistics of Gravitational Lensing

We present new limits on cosmological parameters from the statistics of gravitational lensing, based on the recently revised knowledge of the luminosity function and internal dynamics of E/S0 galaxies that are essential in lensing high-redshift QSOs. We find that the lens models using updated Schechter parameters for such galaxies, derived from the recent redshift surveys combined with morphological classification, are found to give smaller lensing probabilities than were calculated earlier. Inconsistent adoption of these parameters from a mixture of various galaxy surveys gives rise to systematic biases in the results. We also show that less compact dwarf-type galaxies that largely dominate the faint part of the Schechter form luminosity function contribute little to lensing probabilities, so that earlier lens models overestimate incidents of small separation lenses. Applications of the lens models to the existing lens surveys indicate that reproduction of both the lensing probability of optical sources and the image separations of optical and radio lenses is significantly improved in the revised lens models. The likelihood analyses allow us to conclude that a flat universe with ?0=0.3 and ?0+?0=1 is preferable, and a matter-dominated flat universe with ?0=0 is ruled out at the 98% confidence level. These new limits are unaffected by the inclusion of uncertainties in the lens properties.