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  • 1
    Electronic Resource
    Electronic Resource
    The seeds of rich galaxy clusters in the Universe (1998)
    [s.l.] : Macmillan Magazines Ltd.
    Nature 392 (1998), S. 359-361 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] The discovery of a population of young galaxies at a redshift when the Universe was about a tenth of its current age has shed new light on the question of when and how galaxies formed. Within the context of popular models, this is the population of primeval galaxies that built themselves up to ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/32837
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    Paper (German National Licenses)
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  • 2
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    Black hole accretion versus star formation rate: theory confronts observations (2015)
    Oxford University Press
    Details
    Publication Date: 2015-06-20
    Description: We use a suite of hydrodynamical simulations of galaxy mergers to compare star formation rate (SFR) and black hole accretion rate (BHAR) for galaxies before the interaction (‘stochastic’ phase), during the ‘merger’ proper, lasting ~0.2–0.3 Gyr, and in the ‘remnant’ phase. We calculate the bivariate distribution of SFR and BHAR and define the regions in the SFR–BHAR plane that the three phases occupy. No strong correlation between BHAR and galaxy-wide SFR is found. A possible exception are galaxies with the highest SFR and the highest BHAR. We also bin the data in the same way used in several observational studies, by either measuring the mean SFR for AGN in different luminosity bins, or the mean BHAR for galaxies in bins of SFR. We find that the apparent contradiction or SFR versus BHAR for observed samples of AGN and star-forming galaxies is actually caused by binning effects. The two types of samples use different projections of the full bivariate distribution, and the full information would lead to unambiguous interpretation. We also find that a galaxy can be classified as AGN-dominated up to 1.5 Gyr after the merger-driven starburst took place. Our study is consistent with the suggestion that most low-luminosity AGN hosts do not show morphological disturbances.
    Print ISSN: 1745-3925
    Electronic ISSN: 1745-3933
    Topics: Physics
    Published by Oxford University Press
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  • 3
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    Off the beaten path: a new approach to realistically model the orbital decay of supermassive black holes in galaxy formation simulations (2015)
    Oxford University Press
    Details
    Publication Date: 2015-06-11
    Description: We introduce a sub-grid force correction term to better model the dynamical friction experienced by a supermassive black hole (SMBH) as it orbits within its host galaxy. This new approach accurately follows an SMBH's orbital decay and drastically improves over commonly used ‘advection’ methods. The force correction introduced here naturally scales with the force resolution of the simulation and converges as resolution is increased. In controlled experiments, we show how the orbital decay of the SMBH closely follows analytical predictions when particle masses are significantly smaller than that of the SMBH. In a cosmological simulation of the assembly of a small galaxy, we show how our method allows for realistic black hole orbits. This approach overcomes the limitations of the advection scheme, where black holes are rapidly and artificially pushed towards the halo centre and then forced to merge, regardless of their orbits. We find that SMBHs from merging dwarf galaxies can spend significant time away from the centre of the remnant galaxy. Improving the modelling of SMBH orbital decay will help in making robust predictions of the growth, detectability and merger rates of SMBHs, especially at low galaxy masses or at high redshift.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 4
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    Milking the spherical cow - on aspherical dynamics in spherical coordinates (2015)
    Oxford University Press
    Details
    Publication Date: 2015-06-07
    Description: Galaxies and the dark matter haloes that host them are not spherically symmetric, yet spherical symmetry is a helpful simplifying approximation for idealized calculations and analysis of observational data. The assumption leads to an exact conservation of angular momentum for every particle, making the dynamics unrealistic. But how much does that inaccuracy matter in practice for analyses of stellar distribution functions, collisionless relaxation, or dark matter core-creation? We provide a general answer to this question for a wide class of aspherical systems; specifically, we consider distribution functions that are ‘maximally stable’, i.e. that do not evolve at first order when external potentials (which arise from baryons, large-scale tidal fields or infalling substructure) are applied. We show that a spherically symmetric analysis of such systems gives rise to the false conclusion that the density of particles in phase space is ergodic (a function of energy alone). Using this idea we are able to demonstrate that: (a) observational analyses that falsely assume spherical symmetry are made more accurate by imposing a strong prior preference for near-isotropic velocity dispersions in the centre of spheroids; (b) numerical simulations that use an idealized spherically symmetric setup can yield misleading results and should be avoided where possible; and (c) triaxial dark matter haloes (formed in collisionless cosmological simulations) nearly attain our maximally stable limit, but their evolution freezes out before reaching it.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 5
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    Faint dwarfs as a test of DM models: WDM versus CDM (2015)
    Oxford University Press
    Details
    Publication Date: 2015-02-08
    Description: We use high-resolution Hydro+ N -Body cosmological simulations to compare the assembly and evolution of a small field dwarf (stellar mass ~10 6–7  M , total mass 10 10  M ) in -dominated cold dark matter (CDM) and 2 keV warm dark matter (WDM) cosmologies. We find that star formation (SF) in the WDM model is reduced and delayed by 1–2 Gyr relative to the CDM model, independently of the details of SF and feedback. Independent of the dark matter (DM) model, but proportionally to the SF efficiency, gas outflows lower the central mass density through ‘dynamical heating’, such that all realizations have circular velocities 〈20 km s –1 at 500 pc, in agreement with local kinematic constraints. As a result of dynamical heating, older stars are less centrally concentrated than younger stars, similar to stellar population gradients observed in nearby dwarf galaxies. Introducing an important diagnostic of SF and feedback models, we translate our simulations into artificial colour–magnitude diagrams and star formation histories (SFHs) in order to directly compare to available observations. The simulated galaxies formed most of their stars in many ~10 Myr long bursts. The CDM galaxy has a global SFH, H i abundance and Fe/H and alpha-elements distribution well matched to current observations of dwarf galaxies. These results highlight the importance of directly including ‘baryon physics’ in simulations when (1) comparing predictions of galaxy formation models with the kinematics and number density of local dwarf galaxies and (2) differentiating between CDM and non-standard models with different DM or power spectra.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 6
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    All about baryons: revisiting SIDM predictions at small halo masses (2015)
    Oxford University Press
    Details
    Publication Date: 2015-07-16
    Description: We use cosmological hydrodynamic simulations to consistently compare the assembly of dwarf galaxies in both dominated, cold dark matter (CDM) and self-interacting dark matter (SIDM) models. The SIDM model adopts a constant cross-section of 2 cm 2 g –1 , a relatively large value to maximize its effects. These are the first SIDM simulations that are combined with a description of stellar feedback that naturally drives potential fluctuations able to create dark matter (DM) cores. Remarkably, SIDM fails to significantly lower the central DM density within the central 500 pc at halo peak velocities V max  〈 30 km s –1 . This is due to the fact that the central regions of very low mass field haloes have relatively low central velocity dispersion and densities, leading to time-scales for SIDM collisions greater than a Hubble time. CDM haloes with V max  〈 30 km s –1  have inefficient star formation, and hence weak supernova feedback. At a fixed 2 cm 2 g –1 SIDM cross-section, the DM content of very low mass CDM and SIDM haloes differs by no more than a factor of 2 within 100–200 pc. At larger halo masses (~10 10 M ), the introduction of baryonic processes creates field dwarf galaxies with DM cores and central DM+baryon distributions that are effectively indistinguishable between CDM and SIDM. Both models are in broad agreement with observed Local Group field galaxies across the range of masses explored. To significantly differentiate SIDM from CDM at the scale of faint dwarf galaxies, a velocity-dependent cross-section that rapidly increases to values larger than 2 cm 2 g –1 for haloes with V max  〈 25–30 km s –1 needs to be introduced.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 7
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    Milking the spherical cow - on aspherical dynamics in spherical coordinates (2015)
    Oxford University Press
    Details
    Publication Date: 2015-06-12
    Description: Galaxies and the dark matter haloes that host them are not spherically symmetric, yet spherical symmetry is a helpful simplifying approximation for idealized calculations and analysis of observational data. The assumption leads to an exact conservation of angular momentum for every particle, making the dynamics unrealistic. But how much does that inaccuracy matter in practice for analyses of stellar distribution functions, collisionless relaxation, or dark matter core-creation? We provide a general answer to this question for a wide class of aspherical systems; specifically, we consider distribution functions that are ‘maximally stable’, i.e. that do not evolve at first order when external potentials (which arise from baryons, large-scale tidal fields or infalling substructure) are applied. We show that a spherically symmetric analysis of such systems gives rise to the false conclusion that the density of particles in phase space is ergodic (a function of energy alone). Using this idea we are able to demonstrate that: (a) observational analyses that falsely assume spherical symmetry are made more accurate by imposing a strong prior preference for near-isotropic velocity dispersions in the centre of spheroids; (b) numerical simulations that use an idealized spherically symmetric setup can yield misleading results and should be avoided where possible; and (c) triaxial dark matter haloes (formed in collisionless cosmological simulations) nearly attain our maximally stable limit, but their evolution freezes out before reaching it.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 8
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    Cold dark matter: Controversies on small scales [Colloquium Paper] (2015)
    National Academy of Sciences
    Details
    Publication Date: 2015-10-07
    Description: The cold dark matter (CDM) cosmological model has been remarkably successful in explaining cosmic structure over an enormous span of redshift, but it has faced persistent challenges from observations that probe the innermost regions of dark matter halos and the properties of the Milky Way’s dwarf galaxy satellites. We review...
    Keywords: Dark Matter Universe: On the Threshold of Discovery Sackler Colloquium
    Print ISSN: 0027-8424
    Electronic ISSN: 1091-6490
    Topics: Biology , Medicine , Natural Sciences in General
    Published by National Academy of Sciences
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  • 9
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    Simulating disc galaxy bulges that are consistent with observed scaling relations (2014)
    Oxford University Press
    Details
    Publication Date: 2014-04-02
    Description: We present a detailed comparison between the photometric properties of the bulges of two simulated galaxies and those of a uniform sample of observed galaxies. This analysis shows that the simulated galaxies have bulges with realistic surface brightnesses for their sizes and magnitude. These two field disc galaxies have rotational velocities ~100 km s –1 and were integrated to a redshift of zero in a fully cosmological cold dark matter context as part of high-resolution smoothed particle hydrodynamic simulations. We performed bulge–disc decompositions of the galaxies using artificial observations, in order to conduct a fair comparison to observations. We also dynamically decomposed the galaxies and compared the star formation histories of the bulges to those of the entire galaxies. These star formation histories showed that the bulges were primarily formed before z = 1 and during periods of rapid star formation. Both galaxies have large amounts of early star formation, which is likely related to the relatively high bulge-to-disc ratios also measured for them. Unlike almost all previous cosmological simulations, the realistically concentrated bulges of these galaxies do not lead to unphysically high rotational velocities, causing them to naturally lie along the observed Tully–Fisher relation.
    Print ISSN: 1745-3925
    Electronic ISSN: 1745-3933
    Topics: Physics
    Published by Oxford University Press
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  • 10
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    Effects of inclination on measuring velocity dispersion and implications for black holes (2014)
    Oxford University Press
    Details
    Publication Date: 2014-10-22
    Description: The relation of central black hole mass and stellar spheroid velocity dispersion (the M – relation) is one of the best-known and tightest correlations linking black holes and their host galaxies. There has been much scrutiny concerning the difficulty of obtaining accurate black hole measurements, and rightly so; however, it has been taken for granted that measurements of velocity dispersion are essentially straightforward. We examine five disc galaxies from cosmological SPH simulations and find that line-of-sight effects due to galaxy orientation can affect the measured los by 30 per cent, and consequently black hole mass predictions by up to 1.0 dex. Face-on orientations correspond to systematically lower velocity dispersion measurements, while more edge-on orientations give higher velocity dispersions, due to contamination by disc stars when measuring line-of-sight quantities. We caution observers that the uncertainty of velocity dispersion measurements is at least 20 km s –1 , and can be much larger for moderate inclinations. This effect may account for some of the scatter in the locally measured M – relation, particularly at the low-mass end. We provide a method for correcting observed los values for inclination effects based on observable quantities.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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