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

By M. Chiba, Timothy C. Beers. 2000

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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 ⟨Vϕ⟩ = 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 ⟨Vϕ⟩ with increasing distance from the Galactic plane for stars that are likely to be members of the halo population (Δ⟨Vϕ⟩/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 ⟨Vϕ⟩ ≃200 km s-1 (with a vertical velocity gradient on the order of Δ⟨Vϕ⟩/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.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/b17c44501a515d4856caa15be10007a27c1bc375

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Citations in the corpus

Cosmological simulations of the formation of the stellar haloes around disc galaxies (2011, background): “…(Harris 1976; Zinn 1985; Saha 1985; Preston, Shectman Beers 1991; Wetterer & McGraw 1996; Ivezić et al. 2000; Morrison et al. 2000; Yanny et al. 2000; Chiba & Beers 2000; Chen et al. 2001; Siegel et al. 2002; Vivas & Zinn 2006; Bell et al. 2008; Juŕıc et al. 2008; Smith et al. 2009; Sesar et al.…”
Dark matter in the Milky Way II. The HI gas distribution as a tracer of the gravitational potential (2007, methodology): “The modelparameters used for the stellar disk, in particular for the exponential radial scale length of the thick disk (HR = 7.5 kpc, Chiba & Beers, 2000), are highly uncertain.”
Deciphering the Last Major Invasion of the Milky Way (2002, methodology): “These data are compared with the predictions of a model with thin disk, thick disk and halo with standard stellar kinematics, resulting from the median of 11 random samplings of three Gaussians with means and dispersions given in Table 1 (based on Dehnen & Binney 1998 and Chiba & Beers 2000).”
A Two Micron All Sky Survey View of the Sagittarius Dwarf Galaxy. I. Morphology of the Sagittarius Core and Tidal Arms (2003, background): “…downward), came from a progenitor system that ‘‘ probably resembled the Fornax and Sagittarius dwarf spheroidal galaxies ’’ and that may have contributed 12% of all metal-poor halo stars outside the solar circle (a number that may be 3 times smaller according to a reanalysis by Chiba & Beers 2000).”
THE BROWN DWARF KINEMATICS PROJECT I. PROPER MOTIONS AND TANGENTIAL VELOCITIES FOR A LARGE SAMPLE OF LATE-TYPE M, L, AND T DWARFS (2008, unknown): “Soubiran et al. (2003) find (σU , σV , σW )=(39 ± 2, 20 ± 2, 20 ± 1) km s−1 for the thin disk and (σU , σV , σW )=(63 ± 6, 39 ± 4, 39 ± 4) km s−1 for the thick disk, and Chiba & Beers (2000) find (σU , σV , σW )=(141 ± 11, 106 ± 9, 94 ± 8) km s−1 for the halo portion of the Galaxy.”
Thin disc, thick disc and halo in a simulated galaxy (2012, background): “…shown that thinand thick-disc stars with similar iron abundances ([Fe/H]) differ in their abundances of α elements (e.g. Majewski 1993; Fuhrmann 1998; Chiba & Beers 2000; Bensby, Feltzing & Lundström 2003; Soubiran, Bienaymé & Siebert 2003; Reddy, Lambert & Allende Prieto 2006; Wyse et al. 2006;…”
Near-Field Cosmology with Extremely Metal-Poor Stars (2015, background): “The paradigm that stars in our Galaxy can tell us about conditions that existed during the first few billion years after the Big Bang, as set down by Freeman & Bland-Hawthorn (2002) 3 and described by them as “near-field” cosmology, is an idea that formed slowly, over some decades.”
KINEMATICAL AND CHEMICAL VERTICAL STRUCTURE OF THE GALACTIC THICK DISK. II. A LACK OF DARK MATTER IN THE SOLAR NEIGHBORHOOD, (2012, background): “Such a large scale length was proposed by some authors (Ratnatunga et al. 1989; Beers & Sommer-Larsen 1995; Chiba & Beers 2000; Larsen & Humphreys 2003; Chang et al. 2011), but it was always associated to a higher scale height, as shown5 in Figure 3.”
THE CASE FOR THE DUAL HALO OF THE MILKY WAY (2011, unknown): “…papers include Hartwick (1987), Sommer-Larsen & Zhen (1990; using density profiles inferred from local kinematics), Preston et al. (1991), Zinn (1993), Kinman et al. (1994), and Chiba & Beers (2000) (using density profiles inferred from local kinematics), all of which reached similar conclusions.”
STRUCTURE AND KINEMATICS OF THE STELLAR HALOS AND THICK DISKS OF THE MILKY WAY BASED ON CALIBRATION STARS FROM SLOAN DIGITAL SKY SURVEY DR7 (2009, methodology): “1991; Chiba & Beers 2000).”
A Spectroscopic Study of the Ancient Milky Way: F- and G-Type Stars in the Third Data Release of the Sloan Digital Sky Survey (2005, background): “…of the Galaxy from a snapshot provided by star counts to a more dynamical perspective, allowing the study of not just the present structure of theMilkyWay, but to peer into its 804 formation and evolution (see, e.g., Chiba & Beers 2000, 2001; Freeman & Bland-Hawthorn 2002; Nordström et al. 2004).”
Galactic stellar haloes in the CDM model (2009, background): “A mildly oblate halo with c/a ∼ 0.6 is reported for the Milky Way, with an increase in flattening at smaller radii (<20 kpc; e.g. Chiba & Beers 2000; Bell et al. 2008; Carollo et al. 2007).”
THE MILKY WAY TOMOGRAPHY WITH SDSS. III. STELLAR KINEMATICS (2009, background): “Our analysis extends the early measurements of this gradient (e.g., Murray 1986; Majewski 1992; Chiba & Beers 2000) to vastly larger sky area and to a much larger distance range.”
Gas physics, disk fragmentation, and bulge formation in young galaxies (2003, background): “By contrast, the metallicity distribution of the Milky Way halo (Chiba & Beers, 2000) resembles more the distribution created in the cold moel B, and points to a faster settling of the gas to the disk in the early collapse of the Milky Way (see also SG03).”
The Milky Way Tomography with SDSS. I. Stellar Number Density Distribution (2005, methodology): “…field stars (Hartwick 1987; Sommer-Larsen & Zhen 1990; Allen et al. 1991; Preston et al. 1991; Kinman et al. 1994; Carney et al. 1996; Chiba & Beers 2000), globular clusters (Zinn 1993; Dinescu et al. 1999), and MS stars (Gilmore et al. 1985; Siegel et al. 2002) and are unlikely to…”
MAPPING THE STELLAR STRUCTURE OF THE MILKY WAY THICK DISK AND HALO USING SEGUE PHOTOMETRY (2009, unknown): “…a double exponential thin disk and thick disk (Bahcall & Soneira 1980; Gilmore & Reid 1983), and an axisymmetric power-law halo (Eggen et al. 1962; Chiba & Beers 2000; Jurić et al. 2008): ρ(R,Z) = ρthin,⊙ ( eR⊙/L1exp ( − R L1 − Z H1 ) + fthick,⊙e R⊙/L2exp ( − R L2 − Z H2 ) (2) + fhalo,⊙ ( R⊙…”
Co-formation of the disc and the stellar halo (2018, background): “In terms of the halo spin, the amount of prograde rotation at high |z| is consistent with the majority of both the past and recent studies of the stellar halo (see e.g. Chiba & Beers 2000; Deason et al. 2017).”
Observations of Thick Disks in the Hubble Space Telescope Ultra Deep Field (2006, background): “In the Milky Way, it has a vertical velocity dispersion of ∼ 35±3 km s−1 (Freeman 1987; Chiba & Beers 2000) and an exponential scale height between 600 pc (Chen et al. 2001) and 900 ± 300 (Buser et al. 1999; Ojha 2001; Kerber et al. 2001; Larsen & Humphreys 2003).”
Star Counts Redivivus. IV. Density Laws through Photometric Parallaxes (2002, background): “…1987; Sandage & Fouts 1987; Carney, Latham, & Laird 1989; Beers & Sommer-Larsen 1995; Guo 1995; Reid, Hawley, & Gizis 1995; Ojha et al. 1996; Chiba & Beers 2000), which implies a vertical scale height of around 1 kpc, with possibly as much as 10% of the local stars included in this high- W…”