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Subject: methods: numerical ×

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    On the absence of backsplash analogues to NGC 3109 in the ΛcDM framework
    (Oxford : Oxford Univ. Press, 2021) Banik, Indranil; Haslbauer, Moritz; Pawlowski, Marcel S; Famaey, Benoit; Kroupa, Pavel
    The dwarf galaxy NGC 3109 is receding 105 km s-1 faster than expected in a Lambda cold dark matter (ΛCDM) timing argument analysis of the Local Group and external galaxy groups within 8 Mpc. If this few-body model accurately represents long-range interactions in ΛCDM, this high velocity suggests that NGC 3109 is a backsplash galaxy that was once within the virial radius of the Milky Way and was slingshot out of it. Here, we use the Illustris TNG300 cosmological hydrodynamical simulation and its merger tree to identify backsplash galaxies. We find that backsplashers as massive (≥4.0 × 1010 M⊙) and distant (≥1.2 Mpc) as NGC 3109 are extremely rare, with none having also gained energy during the interaction with their previous host. This is likely due to dynamical friction. Since we identified 13 225 host galaxies similar to the Milky Way or M31, we conclude that postulating NGC 3109 to be a backsplash galaxy causes >3.96σ tension with the expected distribution of backsplashers in ΛCDM. We show that the dark matter only version of TNG300 yields much the same result, demonstrating its robustness to how the baryonic physics is modelled. If instead NGC 3109 is not a backsplasher, consistency with ΛCDM would require the 3D timing argument analysis to be off by 105 km s-1 for this rather isolated dwarf, which we argue is unlikely. We discuss a possible alternative scenario for NGC 3109 and the Local Group satellite planes in the context of MOND, where the Milky Way and M31 had a past close flyby 7-10 Gyr ago.
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    Astraeus - VI. Hierarchical assembly of AGN and their large-scale effect during the Epoch of Reionization
    (Oxford : Oxford Univ. Press, 2022) Trebitsch, Maxime; Hutter, Anne; Dayal, Pratika; Gottlöber, Stefan; Legrand, Laurent; Yepes, Gustavo
    In this work, the sixth of a series, we use the seminumerical rAdiative tranSfer coupling of galaxy formaTion and Reionization in N-body dark-matter simUlationS (ASTRAEUS) framework to investigate the nature of the sources that reionized the Universe. We extend ASTRAEUS, which already couples a galaxy formation semi-analytical model with a detailed seminumerical reionization scheme, to include a model for black-hole formation, growth, and the production of ionizing radiation from associated active galactic nuclei (AGNs). We calibrate our fiducial AGN model to reproduce the bolometric luminosity function at z ≃ 5, and explore the role of the resulting AGN population in reionizing the Universe. We find that in all the models yielding a reasonable AGN luminosity function, galaxies dominate overwhelmingly the ionizing budget during the Epoch of Reionization, with AGN accounting for 1-10 per cent of the ionizing budget at z = 6 and starting to play a role only below z ≾ 5.
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    The Effect of Toroidal Magnetic Fields on Solar Oscillation Frequencies
    (London : Institute of Physics Publ., 2018) Kiefer, René; Roth, Markus
    Solar oscillation frequencies change with the level of magnetic activity. Localizing subsurface magnetic field concentrations in the Sun with helioseismology will help us to understand the solar dynamo. Because the magnetic fields are not considered in standard solar models, adding them to the basic equations of stellar structure changes the eigenfunctions and eigenfrequencies. We use quasi-degenerate perturbation theory to calculate the effect of toroidal magnetic fields on solar oscillation mean multiplet frequencies for six field configurations. In our calculations, we consider both the direct effect of the magnetic field, which describes the coupling of modes, and the indirect effect, which accounts for changes in stellar structure due to the magnetic field. We limit our calculations to self-coupling of modes. We find that the magnetic field affects the multiplet frequencies in a way that depends on the location and the geometry of the field inside the Sun. Comparing our theoretical results with observed shifts, we find that strong tachocline fields cannot be responsible for the observed frequency shifts of p modes over the solar cycle. We also find that part of the surface effect in helioseismic oscillation frequencies might be attributed to magnetic fields in the outer layers of the Sun. The theory presented here is also applicable to models of solar-like stars and their oscillation frequencies.