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  • 1
    Electronic Resource
    Electronic Resource
    Tidal disruption of the Magellanic Clouds by the Milky Way (1998)
    [s.l.] : Macmillan Magazines Ltd.
    Nature 394 (1998), S. 752-754 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Interactions between galaxies are common, and influence physical properties such as the global morphology and star-formation rate (Hubble type). Galaxies can interact in many different ways: they can merge together; they can pass through each other, with gas being stripped from the smaller of ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/29466
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    Paper (German National Licenses)
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  • 2
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    Chemical separation of disc components using RAVE (2016)
    Oxford University Press
    Details
    Publication Date: 2016-07-29
    Description: We present evidence from the RAdial Velocity Experiment (RAVE) survey of chemically separated, kinematically distinct disc components in the solar neighbourhood. We apply probabilistic chemical selection criteria to separate our sample into α-low (‘thin disc’) and α-high (‘thick disc’) sequences. Using newly derived distances, which will be utilized in the upcoming RAVE DR5, we explore the kinematic trends as a function of metallicity for each of the disc components. For our α-low disc, we find a negative trend in the mean rotational velocity ( V ) as a function of iron abundance ([Fe/H]). We measure a positive gradient V /[Fe/H] for the α-high disc, consistent with results from high-resolution surveys. We also find differences between the α-low and α-high discs in all three components of velocity dispersion. We discuss the implications of an α-low, metal-rich population originating from the inner Galaxy, where the orbits of these stars have been significantly altered by radial mixing mechanisms in order to bring them into the solar neighbourhood. The probabilistic separation we propose can be extended to other data sets for which the accuracy in [α/Fe] is not sufficient to disentangle the chemical disc components a priori. For such data sets which will also have significant overlap with Gaia DR1, we can therefore make full use of the improved parallax and proper motion data as it becomes available to investigate kinematic trends in these chemical disc components.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 3
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    The rich are different: evidence from the RAVE survey for stellar radial migration (2015)
    Oxford University Press
    Details
    Publication Date: 2015-01-26
    Description: Using the RAdial Velocity Experiment fourth data release (RAVE DR4), and a new metallicity calibration that will be also taken into account in the future RAVE DR5, we investigate the existence and the properties of supersolar metallicity stars ([ M /H]  +0.1 dex) in the sample, and in particular in the solar neighbourhood. We find that RAVE is rich in supersolar metallicity stars, and that the local metallicity distribution function declines remarkably slowly up to +0.4 dex. Our results show that the kinematics and height distributions of the supersolar metallicity stars are identical to those of the [ M /H]  0 thin-disc giants that we presume were locally manufactured. The eccentricities of the supersolar metallicity stars indicate that half of them are on a roughly circular orbit ( e  ≤ 0.15), so under the assumption that the metallicity of the interstellar medium at a given radius never decreases with time, they must have increased their angular momenta by scattering at corotation resonances of spiral arms from regions far inside the solar annulus. The likelihood that a star will migrate radially does not seem to decrease significantly with increasing amplitude of vertical oscillations within range of oscillation amplitudes encountered in the disc.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 4
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    The role of gas infall in the evolution of disc galaxies (2016)
    Oxford University Press
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    Publication Date: 2016-08-13
    Description: Spiral galaxies are thought to acquire their gas through a protracted infall phase resulting in the inside-out growth of their associated discs. For field spirals, this infall occurs in the lower density environments of the cosmic web. The overall infall rate, as well as the galactocentric radius at which this infall is incorporated into the star-forming disc, plays a pivotal role in shaping the characteristics observed today. Indeed, characterizing the functional form of this spatio-temporal infall in situ is exceedingly difficult, and one is forced to constrain these forms using the present day state of galaxies with model or simulation predictions. We present the infall rates used as input to a grid of chemical evolution models spanning the mass spectrum of discs observed today. We provide a systematic comparison with alternate analytical infall schemes in the literature, including a first comparison with cosmological simulations. Identifying the degeneracies associated with the adopted infall rate prescriptions in galaxy models is an important step in the development of a consistent picture of disc galaxy formation and evolution.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 5
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    The sensitivity of harassment to orbit: mass loss from early-type dwarfs in galaxy clusters (2015)
    Oxford University Press
    Details
    Publication Date: 2015-10-15
    Description: We conduct a comprehensive numerical study of the orbital dependence of harassment on early-type dwarfs consisting of 168 different orbits within a realistic, Virgo-like cluster, varying in eccentricity and pericentre distance. We find harassment is only effective at stripping stars or truncating their stellar discs for orbits that enter deep into the cluster core. Comparing to the orbital distribution in cosmological simulations, we find that the majority of the orbits (more than three quarters) result in no stellar mass loss. We also study the effects on the radial profiles of the globular cluster systems of early-type dwarfs. We find these are significantly altered only if harassment is very strong. This suggests that perhaps most early-type dwarfs in clusters such as Virgo have not suffered any tidal stripping of stars or globular clusters due to harassment, as these components are safely embedded deep within their dark matter halo. We demonstrate that this result is actually consistent with an earlier study of harassment of dwarf galaxies, despite the apparent contradiction. Those few dwarf models that do suffer stellar stripping are found out to the virial radius of the cluster at redshift = 0, which mixes them in with less strongly harassed galaxies. However when placed on phase-space diagrams, strongly harassed galaxies are found offset to lower velocities compared to weakly harassed galaxies. This remains true in a cosmological simulation, even when haloes have a wide range of masses and concentrations. Thus phase-space diagrams may be a useful tool for determining the relative likelihood that galaxies have been strongly or weakly harassed.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 6
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    Tracking Cluster Debris (TraCD) - I. Dissolution of clusters and searching for the solar cradle (2015)
    Oxford University Press
    Details
    Publication Date: 2015-04-16
    Description: The capability to reconstruct dissolved stellar systems in dynamical and chemical space is a key factor in improving our understanding of the evolution of the Milky Way. Here we concentrate on the dynamical aspect and given that a significant portion of the stars in the Milky Way have been born in stellar associations or clusters that have lived a few Myr up to several Gyr, we further restrict our attention to the evolution of star clusters. We have carried out our simulations in two steps: (1) we create a simulation of dissolution and mixing processes which yields a close fit to the present-day Milky Way dynamics and (2) we have evolved three sets of stellar clusters with masses of 400, 1000 and 15 000 M to dissolution. The birth location of these sets was 4, 6, 8 and 10 kpc for the 400 and 1000 M clusters and 4, 6, 8, 10 and 12 kpc for the 15 000 M . We have focused our efforts on studying the state of the escapers from these clusters after 4.5 Gyr of evolution with particular attention to stars that reach the solar annulus, i.e. 7.5 ≤  R gc  ≤ 8.5 kpc. We give results for solar twins and siblings over a wide range of radii and cluster masses for two dissolution mechanisms. From kinematics alone, we conclude that the Sun was ~50 per cent more likely to have been born near its current Galactocentric radius, rather than have migrated (radially) ~2 kpc since birth. We conclude our analysis by calculating magnitudes and colours of our single stars for comparison with the samples that the Gaia , Gaia -ESO and GALAH-AAO surveys will obtain. In terms of reconstructing dissolved star clusters, we find that on short time-scales we cannot rely on kinematic evolution alone and thus it will be necessary to extend our study to include information on chemical space.
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    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 7
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    Identification of globular cluster stars in RAVE data - I. Application to stellar parameter calibration (2015)
    Oxford University Press
    Details
    Publication Date: 2015-06-07
    Description: 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 data base. Our identifications are based on three globular clusters – NGC 3201, NGC 5139 ( 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 used in the survey, as globular clusters are ideal test-beds for the validation of stellar atmospheric parameters, abundances, distances and ages. For both NGC 3201 and Cen, there is compelling evidence for numerous members (〉10) in the RAVE data base; 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., where only isochrones more metal rich than –0.9 dex were used, are typically underestimated by ~40 per cent with respect to the published distances for the clusters, while the distances from Zwitter et al. show stars ranging from 1 to ~6.5 kpc – with indications of a trend towards higher distances at lower metallicities – for the three clusters analysed in this study.
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    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 8
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    Identification of globular cluster stars in RAVE data - I. Application to stellar parameter calibration (2015)
    Oxford University Press
    Details
    Publication Date: 2015-06-12
    Description: 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 data base. Our identifications are based on three globular clusters – NGC 3201, NGC 5139 ( 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 used in the survey, as globular clusters are ideal test-beds for the validation of stellar atmospheric parameters, abundances, distances and ages. For both NGC 3201 and Cen, there is compelling evidence for numerous members (〉10) in the RAVE data base; 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., where only isochrones more metal rich than –0.9 dex were used, are typically underestimated by ~40 per cent with respect to the published distances for the clusters, while the distances from Zwitter et al. show stars ranging from 1 to ~6.5 kpc – with indications of a trend towards higher distances at lower metallicities – for the three clusters analysed in this study.
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    Topics: Physics
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  • 9
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    The [{alpha}/Fe] ratios of very metal-poor stars within the integrated galactic initial mass function theory (2013)
    Oxford University Press
    Details
    Publication Date: 2013-12-06
    Description: The aim of this paper is to quantify the amplitude of the predicted plateau in [α/Fe] ratios associated with the most metal-poor stars of a galaxy. We assume that the initial mass function (IMF) in galaxies is steeper if the star formation rate (SFR) is low – as per the integrated galactic initial mass function (IGIMF) theory. A variant of the theory, in which the IGIMF depends upon the metallicity of the parent galaxy, is also considered. The IGIMF theory predicts low [α/Fe] plateaus in dwarf galaxies, characterized by small SFRs. The [α/Fe] plateau is up to 0.7 dex lower than the corresponding plateau of the Milky Way. For a universal IMF one should expect instead that the [α/Fe] plateau is the same for all the galaxies, irrespective of their masses or SFRs. Assuming a strong dependence of the IMF on the metallicity of the parent galaxy, dwarf galaxies can show values of the [α/Fe] plateau similar to those of the Milky Way, and almost independent of the SFR. The [Mg/Fe] ratios of the most metal-poor stars in dwarf galaxies satellites of the Milky Way can be reproduced either if we consider metallicity-dependent IMFs or if the early SFRs of these galaxies were larger than we presently think. Present and future observations of dwarf galaxies can help disentangle between these different IGIMF formulations.
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    Topics: Physics
    Published by Oxford University Press
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  • 10
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    Chemodynamics of a simulated disc galaxy: initial mass functions and Type Ia supernova progenitors (2014)
    Oxford University Press
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    Publication Date: 2014-09-19
    Description: We trace the formation and advection of several elements within a cosmological adaptive mesh refinement simulation of an L * galaxy. We use nine realizations of the same initial conditions with different stellar initial mass functions (IMFs), mass limits for Type II and Type Ia supernovae (SNII, SNIa) and stellar lifetimes to constrain these subgrid phenomena. Our code includes self-gravity, hydrodynamics, star formation, radiative cooling and feedback from multiple sources within a cosmological framework. Under our assumptions of nucleosynthesis we find that SNII with progenitor masses of up to 100 M are required to match low-metallicity gas oxygen abundances. Tardy SNIa are necessary to reproduce the classical chemical evolution ‘knee’ in [O/Fe]–[Fe/H]: more prompt SNIa delayed time distributions do not reproduce this feature. Within our framework of hydrodynamical mixing of metals and galaxy mergers we find that chemical evolution is sensitive to the shape of the IMF and that there exists a degeneracy with the mass range of SNII. We look at the abundance plane and present the properties of different regions of the plot, noting the distinct chemical properties of satellites and a series of nested discs that have greater velocity dispersions are more α-rich and metal poor with age.
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    Topics: Physics
    Published by Oxford University Press
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