Fossil Signatures of Ancient Accretion Events in the Halo

Johnston, Kathryn V.; Hernquist, Lars; Bolte, Michael

doi:10.1086/177418

Cited by 273 publications

(307 citation statements)

References 35 publications

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“…Interestingly, the amorphous high-redshift galaxies would evolve into structures similar to dwarf and irregular galaxies if isolated. If such galaxies are responsible for the modern-day halo, then dynamical arguments (Johnston, Hernquist, & Bolte 1996) suggest that the disrupted structures should retain some coherence within the phase space of the halo. Helmi et al (1999) claim to have detected such structure.…”

Section: The Iron Peak Elementsmentioning

confidence: 99%

Abundances from High-Resolution Spectra of Kinematically Interesting Halo Stars

Stephens

Boesgaard

2002

The Astronomical Journal

145

176

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There are stars in the halo of the Galaxy whose orbital properties are distinctly different from the majority of the halo population. One suggestion to account for these kinematically peculiar stars is that they have been gravitationally subsumed by the Milky Way through the breakup of nearby dwarf galaxies. One test of this hypothesis lies in determining the chemical composition of the deviant halo stars relative to that of normal ('' native '') halo field stars. The possibly accreted stars are predicted to have different chemical enrichments due to a slower rate of star formation in nearby low-mass spheroidal or irregular galaxies. We have obtained echelle spectra of metal-poor halo dwarfs with unusual orbital properties: from the outer halo, from the '' high '' halo, and on retrograde orbits. Our spectra are at high spectral resolution (35,000-48,000) and high signal-to-noise ratios (median value 140), primarily from the Keck I 10 m telescope with HIRES (53 stars), but also from the echelle spectrometer of the KPNO Mayall 4 m Telescope (three stars). The spectra cover a large spectral range: $4000-6800 Å . The strengths of approximately 9000 lines have been measured to determine the stellar parameters (T eff , log g, [Fe/H], and ) and the abundances of 10 elements, including Na, -fusion products (Mg, Si, Ca, Ti), iron peak elements (Cr, Fe, Ni), and neutron capture elements (Y, Ba). A model atmosphere was computed for each star, which was used to determine the composition under the assumption of LTE. The comparison of the abundances with the kinematic properties revealed no special trends, except that the ratio of the mean of the -fusion elements to Fe is weakly correlated with stellar apogalactica in the sense that the outermost stars have lower ratios of [ /Fe] than those with smaller apogalactica. The hallmark of an accreted halo star is presumed to be a deficiency (compared with normal stars) of the -elements (e.g., O, Mg, Si, Ca, Ti) with respect to Fe, a consequence of sporadic bursts of star formation within the dwarf galaxies. However [Fe/H]. The kinematically peculiar stars in our sample must have had their origins in localized star-forming regions far removed from the Galactic center. They do not carry the chemical signature of an accreted population. It seems clear that the chemical enrichment in our sample comes primarily from Type II supernovae with only a small component from Type Ia supernovae. Our stars would have formed at most $1 Gyr after the beginning of a star formation episode in the remote halo. The general conclusion extracted from these data is that the formation of the nascent Milky Way was not dominated by the late accretion of dwarf galaxies like the ones that currently orbit the Galaxy. However, the assimilation of fragments early in the evolution of the Galaxy is a natural by-product of hierarchical models of structure formation and can explain many properties of the halo population.

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Section: The Iron Peak Elementsmentioning

confidence: 99%

Abundances from High-Resolution Spectra of Kinematically Interesting Halo Stars

Stephens

Boesgaard

2002

The Astronomical Journal

145

176

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“…These integrations were made by D. Dinescu, and full details of the integration routine may be found in Dinescu, Girard, & van Altena (1999). The orbits were integrated in Galactic potential models given by Johnston, Spergel, & Hernquist (1995) and (1990). The two Paczynski potentials yield similar orbital parameters, and only the orbital parameters from the (1990) potential are Paczynski shown in Table 3.…”

Section: Hipparcos Field Starsmentioning

confidence: 99%

The Relative Age of the Thin and Thick Galactic Disks

Liu

Chaboyer

2000

ApJ

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We determine the relative ages of the open cluster NGC 188 and selected Hipparcos Ðeld stars by isochrone Ðtting and compare them to the age of the thick-disk globular cluster 47 Tuc. The best-Ðt age for NGC 188 was determined to be 6.5^1.0 Gyr. The solar-metallicity Hipparcos Ðeld stars yielded a slightly older thin-disk age, 7.5^0.7 Gyr. Two slightly metal-poor ([Fe/H] \ [0.22) Ðeld stars whose kinematic and orbital parameters indicate that they are members of the thin disk were found to have an age of 9

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“…The V component of XTE J1118+480 implies low rotation (V φ ∼ 122 km s −1 ) about the Galactic centre, contrary to the Galactic disk population, which in the solar neighborhood is in rapid rotation of ∼220 km s −1 . Figure 2 shows the orbit of the black hole binary about the Galactic centre derived from the mean values (U = -105±16, V = -98±16, W = -21±10) km sec −1 in the lsr frame, a distance of 1.85±0.36 kpc from the Sun, and the standard galactic gravitational potential that includes the disk, bulge and halo components 15 . A change of 10% in any of the free parameters of the standard galactic gravitational potential 15 results in changes of less than 5% in any of the orbital parameters of XTE J1118+480.…”

mentioning

confidence: 99%

A high-velocity black hole on a Galactic-halo orbit in the solar neighbourhood

Mirabel

Dhawan

Mignani

et al. 2001

Nature

152

161

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Only a few of the dozen or so stellar-mass black holes have been observed away from the plane of the Galaxy 1 . Those few could have been ejected from the plane as a result of a "kick" received during a supernova explosion, or they could be remnants of the population of massive stars formed in the early stages of evolution of the Galaxy. Determining their orbital motion should help to distinguish between these options. Here we report the transverse motion (in the plane of the sky) for the black hole Xray nova XTE J1118+480 (refs 2-5), from which we derive a large space velocity. This X-ray binary has an eccentric orbit around the Galactic Centre, like most objects in the halo of the Galaxy, such as ancient stars and globular clusters. The properties of the system suggest that its age is comparable to or greater than the age of the Galactic disk. Only an extraordinary "kick" from a supernova could have launched the black hole into an orbit like this from a birth place in the disk of the Galaxy.The high galactic latitude (l = 157.7 • , b = +62.3 • ) X-ray nova XTE J1118+480 was discovered 2 with the RXTE All-Sky Monitor on 2000 March 29. It exhibited slow outbursts that lasted ∼7 months with a peak X-ray luminosity of 4 x 10 35 (D/kpc) 2 erg s −1 and energy spectra typical of black hole binaries in low/hard state 6 . From observations of the ∼19 mag optical counterpart 3 in quiescence a mass function for the compact object f(M) ∼ 6.0±0.4 M ⊙ and an average distance of ∼ 1.85±0.36 kpc were determined 4,5 . For ∼100 days the source exhibited a steady and slowly variable unresolved radio counterpart 7,8 with persistent inverted radio spectrum, which is interpreted as optically thick emission from a compact, powerful synchrotron jet 8,9 , that could have a size ≤0.03 AU 9 . 1To measure the transverse motion on the plane of the sky of XTE J1118+480 we carried out observations of the radio counterpart with the Very Long Baseline Array (VLBA) at 15.4 GHz (λ2 cm) and 8.4 GHz (λ3.6 cm) on 2000 May 4 and July 24; afterwards it faded below detection. In both epochs the source was unresolved by the synthesized beams of 1×0.6 milli arc sec (mas) and 2×1 mas respectively, which correspond to physical dimensions smaller than ∼0.7 D/kpc AU, and a brightness temperature ≥10 9 K. Because of the high galactic latitude and low column of interstellar gas along the line of sight (N H ∼ 10 20 cm −2 ) 10 , the VLBA images are relatively unaffected by Galactic electron scattering, allowing us to determine in each epoch the position of the compact, unresolved radio source with relative errors ≤0.35 mas.The observations were done in cycles that included the target, the strong calibrator 3C273, a primary calibrator (Cal. 1) used as phase-reference source, and a second extragalactic calibrator (Cal.2). Their high elevation and good weather ensured that the phase connection was successful. The epoch, source, frequency, bandwidth, position, and flux for the VLBA observations of XTE J1118+480 and the two extragalactic sources used as references ...

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Fossil Signatures of Ancient Accretion Events in the Halo

Cited by 273 publications

References 35 publications

Abundances from High-Resolution Spectra of Kinematically Interesting Halo Stars

Abundances from High-Resolution Spectra of Kinematically Interesting Halo Stars

The Relative Age of the Thin and Thick Galactic Disks

A high-velocity black hole on a Galactic-halo orbit in the solar neighbourhood

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