D. B. Zucker

Macquarie University, Sydney, New South Wales, Australia

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Publications (54)162.36 Total impact

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    ABSTRACT: We present the identification of potential members of nearby Galactic globular clusters using radial velocities from the RAdial Velocity Experiment Data Release 4 (RAVE-DR4) survey database. Our identifications are based on three globular clusters – NGC 3201, NGC 5139 (ω Cen) and NGC 362 – all of which are shown to have |RV| > 100 km s −1. The high radial velocity of cluster members compared to the bulk of surrounding disc stars enables us to identify members using their measured radial velocities, supplemented by proper motion information and location relative to the tidal radius of each cluster. The identification of globular cluster stars in RAVE DR4 data offers a unique opportunity to test the precision and accuracy of the stellar parameters determined with the currently available Stellar Parameter Pipelines (SPPs) used in the survey, as globular clusters are ideal testbeds for the validation of stellar atmospheric parameters, abundances, distances and ages. For both NGC 3201 and ω Cen, there is compelling evidence for numerous members (> 10) in the RAVE database; in the case of NGC 362 the evidence is more ambiguous, and there may be significant foreground and/or background contamination in our kinematically-selected sample. A comparison of the RAVE-derived stellar parameters and abundances with published values for each cluster and with BASTI isochrones for ages and metallicities from the literature reveals overall good agreement, with the exception of the apparent underestimation of surface gravities for giants, in particular for the most metal-poor stars. Moreover, if the selected members are part of the main body of each cluster our results would also suggest that the distances from Binney et al. (2014), where only isochrones more metal-rich than-0.9 dex were used, are typically underestimated by ∼ 40% with respect to the published distances for the clusters, while the distances from Zwitter et al. (2010) show stars ranging from 1 to ∼ 6.5 kpc – with indications of a trend toward higher distances at lower metallicities – for the three clusters analysed in this study.
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    ABSTRACT: We estimate the size and distribution of the parent populations for the 6 largest (at least 20 stars in the Solar neighborhood) chemical groups identified in the Chemical Tagging experiment by Mitschang et al. 2014. Stars in the abundance groups tend to lie near a boundary in angular momentum versus eccentricity space where the probability is highest for a star to be found in the Solar neighborhood and where orbits have apocenter approximately equal to the Sun's galactocentric radius. Assuming that the parent populations are uniformly distributed at all azimuthal angles in the Galaxy, we estimate that the parent populations of these abundance groups contain at least 200,000 members. The spread in angular momentum of the groups implies that the assumption of a uniform azimuthal distribution only fails for the two youngest groups and only for the highest angular momentum stars in them. The parent populations of three thin disk groups have narrow angular momentum distributions, but tails in the eccentricity and angular momentum distributions suggest that only a small fraction of stars have migrated and increased in eccentricity. In contrast, the parent populations of the thick disk groups exhibit both wide angular momentum and eccentricity distributions implying that both heating and radial migration has taken place.
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    ABSTRACT: We estimate the size and distribution of the parent populations for the 6 largest (at least 20 stars in the Solar neighborhood) chemical groups identified in the Chemical Tagging experiment by Mitschang et al.~2014. Stars in the abundance groups tend to lie near a boundary in angular momentum versus eccentricity space where the probability is highest for a star to be found in the Solar neighborhood and where orbits have apocenter approximately equal to the Sun's galactocentric radius. Assuming that the parent populations are uniformly distributed at all azimuthal angles in the Galaxy, we estimate that the parent populations of these abundance groups contain at least 200,000 members. The spread in angular momentum of the groups implies that the assumption of a uniform azimuthal distribution only fails for the two youngest groups and only for the highest angular momentum stars in them. The parent populations of three thin disk groups have narrow angular momentum distributions, but tails in the eccentricity and angular momentum distributions suggest that only a small fraction of stars have migrated and increased in eccentricity. In contrast, the parent populations of the thick disk groups exhibit both wide angular momentum and eccentricity distributions implying that both heating and radial migration has taken place.
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    ABSTRACT: The GALAH survey is a large high-resolution spectroscopic survey using the newly commissioned HERMES spectrograph on the Anglo-Australian Telescope. The HERMES spectrograph provides high-resolution (R ~28,000) spectra in four passbands for 392 stars simultaneously over a 2 degree field of view. The goal of the survey is to unravel the formation and evolutionary history of the Milky Way, using fossil remnants of ancient star formation events which have been disrupted and are now dispersed throughout the Galaxy. Chemical tagging seeks to identify such dispersed remnants solely from their common and unique chemical signatures; these groups are unidentifiable from their spatial, photometric or kinematic properties. To carry out chemical tagging, the GALAH survey will acquire spectra for a million stars down to V~14. The HERMES spectra of FGK stars contain absorption lines from 29 elements including light proton-capture elements, alpha-elements, odd-Z elements, iron-peak elements and n-capture elements from the light and heavy s-process and the r-process. This paper describes the motivation and planned execution of the GALAH survey, and presents some results on the first-light performance of HERMES.
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    ABSTRACT: We show that a combination of infrared photometry from WISE and 2MASS surveys can yield highly pure samples of M giant stars. We take advantage of the new WISE$\cap$2MASS M giant selection to trace the Sagittarius trailing tail behind the Galactic disk in the direction of the anti-centre. The M giant candidates selected via broad-band photometry are confirmed spectroscopically using AAOmega on the AAT in 3 fields around the extremity of the Sgr trailing tail in the Southern Galactic hemisphere. We demonstrate that at the Sgr longitude $\tilde \Lambda_{\odot} = 204^{\circ}$, the line-of-sight velocity of the trailing tail starts to deviate from the track of the Law & Majewski (2010) model, confirming the prediction of Belokurov et al. (2014). This discovery serves to substantiate the measurement of low differential orbital precession of the Sgr stream which in turn may imply diminished dark matter content within 100 kpc.
    Monthly Notices of the Royal Astronomical Society 10/2014; 446(3). DOI:10.1093/mnras/stu2263 · 5.23 Impact Factor
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    ABSTRACT: The early science results from the new generation of high-resolution stellar spectroscopic surveys, such as GALAH and the Gaia-ESO survey, will represent major milestones in the quest to chemically tag the Galaxy. Yet this technique to reconstruct dispersed coeval stellar groups has remained largely untested until recently. We build on previous work that developed an empirical chemical tagging probability function, which describes the likelihood that two field stars are conatal, that is, they were formed in the same cluster environment. In this work we perform the first ever blind chemical tagging experiment, i.e., tagging stars with no known or otherwise discernable associations, on a sample of 714 disc field stars with a number of high quality high resolution homogeneous metal abundance measurements. We present evidence that chemical tagging of field stars does identify coeval groups of stars, yet these groups may not represent distinct formation sites, e.g. as in dissolved open clusters, as previously thought. Our results point to several important conclusions, among them that group finding will be limited strictly to chemical abundance space, e.g. stellar ages, kinematics, colors, temperature and surface gravity do not enhance the detectability of groups. We also demonstrate that in addition to its role in probing the chemical enrichment and kinematic history of the Galactic disc, chemical tagging represents a powerful new stellar age determination technique.
    Monthly Notices of the Royal Astronomical Society 12/2013; 438(4). DOI:10.1093/mnras/stt2320 · 5.23 Impact Factor
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    ABSTRACT: We have developed a method to identify planetary nebula (PN) candidates in imaging data of the Sloan Digital Sky Survey (SDSS). This method exploits the SDSS' five-band sampling of emission lines in PN spectra, which results in a color signature distinct from that of other sources. Selection criteria based on this signature can be applied to nearby galaxies in which PNe appear as point sources. We applied these criteria to the whole area of M31 as scanned by the SDSS, selecting 167 PN candidates that are located in the outer regions of M31. The spectra of 80 selected candidates were then observed with the 2.2m telescope at Calar Alto Observatory. These observations and cross-checks with literature data show that our method has a selection rate efficiency of about 90%, but the efficiency is different for the different groups of PNe candidates. In the outer regions of M31, PNe trace different well-known morphological features like the Northern Spur, the NGC205 Loop, the G1 Clump, etc. In general, the distribution of PNe in the outer region 8<R<20 kpc along the minor axis shows the "extended disk" - a rotationally supported low surface brightness structure with an exponential scale length of 3.21+/-0.14 kpc and a total mass of ~10^10 M_{\sun}, which is equivalent to the mass of M33. We report the discovery of three PN candidates with projected locations in the center of Andromeda NE, a very low surface brightness giant stellar structure in the outer halo of M31. Two of the PNe were spectroscopically confirmed as genuine PNe. These two PNe are located at projected distances along the major axis of ~48 Kpc and ~41 Kpc from the center of M31 and are the most distant PNe in M31 found up to now.
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    ABSTRACT: Observations were carried out using the Fibre Large Array Multi Element Spectrograph (FLAMES) multi-object fibre system feeding the Ultraviolet and Visual Echelle Spectrograph (UVES; R~45000) at the Very Large Telescope (VLT) as a part of service programme 072.B-0331B over nine nights in 2004. (2 data files).
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    ABSTRACT: Wrapping around the Milky Way, the Sagittarius (Sgr) stream is the most dominant substructure in the halo. Its progenitor, the Sgr dwarf galaxy, has been assumed to be a non-rotating, pressure-supported dwarf spheroidal galaxy. However, to date, no such model for the interaction of Sgr with the Milky Way has been able to reproduce all of the observational features of the stream. We have obtained spectra of over 21,000 stars in the stream and the core of Sgr, providing the most comprehensive spectral observations ever taken for this system. Our analysis demonstrates that Sgr is unlikely to have originated as a disk galaxy. Using careful selection criteria, we have identified several hundred likely members of the Sgr stream. From the most probable members identified, we characterize the chemical and dynamical nature and, through comparison to theory, provide a new mapping of this extended system of Sgr stars.
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    ABSTRACT: The first generation of large-scale chemical tagging surveys, in particular the HERMES/GALAH million star survey, promises to vastly expand our understanding of the chemical and dynamical evolution of the Galaxy. This, however, is contingent on our ability to confidently perform chemical tagging on such a large data-set. Chemical homogeneity has been observed across a range of elements within several Galactic open clusters, yet the level to which this is the case globally, and particularly in comparison to the scatter across clusters themselves, is not well understood. The patterns of elements in coeval cluster members, occupying a complex chemical abundance space, are rooted in the evolution, ultimately the nature of the very late stages, of early generations of stars. The current astrophysical models of such stages are not yet sufficient to explain all observations, combining with our significant gaps in the understanding of star formation, makes this a difficult arena to tackle theoretically. Here, we describe a robust pair-wise metric used to gauge the chemical difference between two stellar components. This metric is then applied to a database of high-resolution literature abundance sources to derive a function describing the probability that two stars are of common evolutionary origin. With this cluster probability function, it will be possible to report a confidence, grounded in empirical observational evidence, with which clusters are detected, independent of the group finding methods. This formulation is also used to probe the role of chemical dimensionality, and that of individual chemical species, on the ability of chemical tagging to differentiate coeval groups of stars.
    Monthly Notices of the Royal Astronomical Society 10/2012; 428(3). DOI:10.1093/mnras/sts194 · 5.23 Impact Factor
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    ABSTRACT: KOALA, the Kilofibre Optimised Astronomical Lenslet Array, is a wide-field, high efficiency integral field unit being designed for use with the bench mounted AAOmega spectrograph on the AAT. KOALA will have 1000 fibres in a rectangular array with a selectable field of view of either 1390 or 430 sq. arcseconds with a spatial sampling of 1.25" or 0.7" respectively. To achieve this KOALA will use a telecentric double lenslet array with interchangeable fore-optics. The IFU will feed AAOmega via a 31m fibre run. The efficiency of KOALA is expected to be ≍ 52% at 3700A and ≍ 66% at 6563°Å with a throughput of > 52% over the entire wavelength range.
    Proceedings of SPIE - The International Society for Optical Engineering 09/2012; DOI:10.1117/12.925812 · 0.20 Impact Factor
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    ABSTRACT: The Sagittarius dwarf galaxy and its vast associated stellar stream represent the most dramatic example of a satellite accreting onto the Milky Way in the past few billion years. We present an in-depth characterisation of the Sagittarius core using spectra of over 7,000 red giant stars taken with AAOmega on the AAT, and show the first results of stream and core data together. Future observations with HERMES will yield detailed abundances to compare with models of chemical enrichment.
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    ABSTRACT: We present a new abundance analysis of the intermediate age Galactic open cluster NGC 3680, based on high resolution, high signal-to-noise VLT/UVES spectroscopic data. Several element abundances are presented for this cluster for the first time, but most notably we derive abundances for the light and heavy s-process elements Y, Ba, La, and Nd. The serendipitous measurement of the rare-earth r-process element Gd is also reported. This cluster exhibits a significant enhancement of Na in giants as compared to dwarfs, which may be a proxy for an O to Na anti-correlation as observed in Galactic globular clusters but not open clusters. We also observe a step-like enhancement of heavy s-process elements towards higher atomic number, contrary to expectations from AGB nucleosynthesis models, suggesting that the r-process played a significant role in the generation of both La and Nd in this cluster
    Monthly Notices of the Royal Astronomical Society 03/2012; 422(4). DOI:10.1111/j.1365-2966.2012.20866.x · 5.23 Impact Factor
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    ABSTRACT: We present a new approach for identifying the Tip of the Red Giant Branch (TRGB) which, as we show, works robustly even on sparsely populated targets. Moreover, the approach is highly adaptable to the available data for the stellar population under study, with prior information readily incorporable into the algorithm. The uncertainty in the derived distances is also made tangible and easily calculable from posterior probability distributions. We provide an outline of the development of the algorithm and present the results of tests designed to characterize its capabilities and limitations. We then apply the new algorithm to three M31 satellites: Andromeda I, Andromeda II and the fainter Andromeda XXIII, using data from the Pan-Andromeda Archaeological Survey (PAndAS), and derive their distances as $731^{(+ 5) + 18}_{(- 4) - 17}$ kpc, $634^{(+ 2) + 15}_{(- 2) - 14}$ kpc and $733^{(+ 13)+ 23}_{(- 11) - 22}$ kpc respectively, where the errors appearing in parentheses are the components intrinsic to the method, while the larger values give the errors after accounting for additional sources of error. These results agree well with the best distance determinations in the literature and provide the smallest uncertainties to date. This paper is an introduction to the workings and capabilities of our new approach in its basic form, while a follow-up paper shall make full use of the method's ability to incorporate priors and use the resulting algorithm to systematically obtain distances to all of M31's satellites identifiable in the PAndAS survey area.
    The Astrophysical Journal 07/2011; 740(2). DOI:10.1088/0004-637X/740/2/69; · 6.28 Impact Factor
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    ABSTRACT: Wrapping around the Milky Way, the Sagittarius (Sgr) stream is one of the most dramatic examples of a stellar tidal stream currently known. Its progenitor, the Sgr dwarf galaxy, has been assumed to be a non-rotating, pressure-supported dwarf spheroidal galaxy. However, to date, no such model for the interaction of Sgr with the Milky Way has been able to reproduce all of the observational features of the stream. Recent theoretical models proposing that the progenitor was a rotating disc galaxy may provide a solution; in particular, the bifurcation of the leading tail of the Sgr stream detected in the SDSS survey naturally arises in models where the Sgr disc is misaligned with respect to the orbital plane. If Sgr was in fact a disc galaxy, these models predict that the core should still show residual internal rotation with a measurable amplitude ( 20 km/s). We have obtained spectra of over 2000 stars near the core of Sgr with AAOmega on the AAT in order to test the disc-galaxy hypothesis, and we present here our preliminary results, along with a comparison to model predictions.
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    ABSTRACT: We present the first spectroscopic analysis of the faint M31 satellite galaxies, AndXI and AndXIII, and a reanalysis of existing spectroscopic data for two further faint companions, And IX and AndXII. By combining data obtained using the DEIMOS spectrograph mounted on the Keck II telescope with deep photometry from the Suprime-Cam instrument on Subaru, we have calculated global properties for the dwarfs, such as systemic velocities, metallicites and half-light radii.We find each dwarf to be very metal poor ([Fe/H] -2 both photometrically and spectroscopically, from their stacked spectrum), and as such, they continue to follow the luminosity-metallicity relationship established with brighter dwarfs. We are unable to resolve a dispersion for And XI due to small sample size and low S/N, but we set a one sigma upper limit of sigma-v <5 km/s. For And IX, And XII and And XIII we resolve velocity dispersions of v=4.5 (+3.4,-3.2), 2.6(+5.1,-2.6) and 9.7(+8.9,-4.5) km/s, and derive masses within the half light radii of 6.2(+5.3,-5.1)x10^6 Msun, 2.4 (+6.5,-2.4)x10^6 Msun and 1.1(+1.4,-0.7)x10^7 Msun respectively. We discuss each satellite in the context of the Mateo relations for dwarf spheroidal galaxies, and the Universal halo profiles established for Milky Way dwarfs (Walker et al. 2009). For both galaxies, this sees them fall below the Universal halo profiles of Walker et al. (2009). When combined with the findings of McConnachie & Irwin (2006a), which reveal that the M31 satellites are twice as extended (in terms of both half-light and tidal radii) as their Milky Way counterparts, these results suggest that the satellite population of the Andromeda system could inhabit halos that are significantly different from those of the Milky Way in terms of their central densities (abridged).
    Monthly Notices of the Royal Astronomical Society 10/2010; 407(4):2411-2433. DOI:10.1111/j.1365-2966.2010.17069.x · 5.23 Impact Factor
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    ABSTRACT: Context: Dwarf spheroidal (dSph) galaxies are the least luminous, least massive galaxies known. Recently, the number of observed galaxies in this class has greatly increased thanks to large surveys. Determining their properties, such as mass, luminosity and metallicity, provides key information in our understanding of galaxy formation and evolution. Aims: Our aim is to provide as clean and as complete a sample as possible of red giant branch stars that are members of the Hercules dSph galaxy. With this sample we explore the velocity dispersion and the metallicity of the system. Methods: Strömgren photometry and multi-fibre spectroscopy are combined to provide information about the evolutionary state of the stars (via the Strömgren c1 index) and their radial velocities. Based on this information we have selected a clean sample of red giant branch stars, and show that foreground contamination by Milky Way dwarf stars can greatly distort the results. Results: Our final sample consists of 28 red giant branch stars in the Hercules dSph galaxy. Based on these stars we find a mean photometric metallicity of -2.35 ± 0.31 dex which is consistent with previous studies. We find evidence for an abundance spread. Using those stars for which we have determined radial velocities we find a systemic velocity of 45.20 ± 1.09 { km s-1} with a dispersion of 3.72 km s-1, this is lower than values found in the literature. Furthermore we identify the horizontal branch and estimate the mean magnitude of the horizontal branch of the Hercules dSph galaxy to be V_0=21.17 ± 0.05, which corresponds to a distance of 147+8-7 kpc. Conclusions: When studying sparsely populated and/or heavily foreground contaminated dSph galaxies it is necessary to include knowledge about the evolutionary stage of the stars. This can be done in several ways. Here we have explored the power of the c1 index in Strömgren photometry. This index is able to clearly identify red giant branch stars redder than the horizontal branch, enabling a separation of red giant branch dSph stars and foreground dwarf stars. Additionally, this index is also capable of correctly identifying both red and blue horizontal branch stars. We have shown that a proper cleaning of the sample results in a smaller value for the velocity dispersion of the system. This has implications for galaxy properties derived from such velocity dispersions. Based on observations made with the INT telescope operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias. Based on spectra obtained with the VLT-U2 telescope ESO Proposal number 079.B-0447(A).
    Astronomy and Astrophysics 11/2009; 506(3):1147-1168. DOI:10.1051/0004-6361/200912718 · 4.48 Impact Factor
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    ABSTRACT: We present an analysis of the substructure revealed by RRLyraes in Sloan Digital Sky Survey Stripe 82, which covers in declination on the celestial equator over the right ascension range to . We use the new public archive of light-motion curves in Stripe 82, published by Bramich et al. in 2008, to identify a set of high-quality RRLyrae candidates. Period estimates are determined to high accuracy using a string-length method. A subset of 178 RRLyraes with spectrally derived metallicities are employed to derive metallicity-period-amplitude relations, which are then used, together with archive magnitude data and light-curve Fourier decomposition, to estimate metallicities and hence distances for the entire sample. The RRLyraes lie 5-115kpc from the Galactic Centre, with distance estimates accurate to ~8 per cent. The RRLyraes are further divided into subsets of 316 RRab types and 91 RRc types based on their period, colour and metallicity. We fit a smooth density law to the distribution as a simple representation of the data. For Galactocentric radii 5-25kpc the number density of RRLyraes falls as r-2.4, but beyond 25kpc, the number density falls much more steeply, as r-4.5. However, we stress that in practice the density distribution is not smooth, but dominated by clumps and substructure. Samples of 55 and 237 RRLyraes associated with the Sagittarius Stream and the Hercules-Aquila Cloud, respectively, are identified. Hence, ~70 per cent of the RRLyraes in Stripe 82 belong to known substructure, and the sharp break in the density law reflects the fact that the dominant substructure in Stripe 82 - the Hercules-Aquila Cloud and the Sagittarius Stream - lie within 40kpc. In fact, almost 60 per cent of all the RRLyraes in Stripe 82 are associated with the Hercules-Aquila Cloud alone, which emphasizes the cloud's pre-eminence. Additionally, evidence of a new and distant substructure - the Pisces Overdensity - is found, consisting of 28 faint RRLyraes centred on Galactic coordinates (l ~ 80°, b ~ -55°), with distances of ~80kpc. The total stellar mass in the Pisces Overdensity is ~104Msolar and its metallicity is [Fe/H] ~ -1.5.
    Monthly Notices of the Royal Astronomical Society 10/2009; 398(4):1757-1770. DOI:10.1111/j.1365-2966.2009.15242.x · 5.23 Impact Factor
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    ABSTRACT: We construct a new sample of ~1700 solar neighbourhood halo subdwarfs from the Sloan Digital Sky Survey, selected using a reduced proper motion diagram. Radial velocities come from the SDSS spectra and proper motions from the light-motion curve catalogue of Bramich et al. (2008). Using a photometric parallax relation to estimate distances gives us the full phase-space coordinates. Typical velocity errors are in the range 30-50 km/s. This halo sample is one of the largest constructed to-date and the disc contamination is at a level of < 1 per cent. This enables us to calculate the halo velocity dispersion to excellent accuracy. We find that the velocity dispersion tensor is aligned in spherical polar coordinates and that (sigma_r, sigma_phi, sigma_theta) = (143 \pm 2, 82 \pm 2, 77 \pm 2) km/s. The stellar halo exhibits no net rotation, although the distribution of v_phi shows tentative evidence for asymmetry. The kinematics are consistent with a mildly flattened stellar density falling with distance like r^{-3.75}. Using the full phase-space coordinates, we look for signs of kinematic substructure in the stellar halo. We find evidence for four discrete overdensities localised in angular momentum and suggest that they may be possible accretion remnants. The most prominent is the solar neighbourhood stream previously identified by Helmi et al. (1999), but the remaining three are new. One of these overdensities is potentially associated with a group of four globular clusters (NGC5466, NGC6934, M2 and M13) and raises the possibility that these could have been accreted as part of a much larger progenitor. Comment: 16 pages, 13 figures, MNRAS (in press). Revised following referee's comments; using new and improved parallax relation. Results and conclusions unchanged
    Monthly Notices of the Royal Astronomical Society 04/2009; DOI:10.1111/j.1365-2966.2009.15391.x · 5.23 Impact Factor