TY - JOUR A1 - Hani, Maan H. A1 - Ellison, Sara L. A1 - Sparre, Martin A1 - Grand, Robert J. J. A1 - Pakmor, Rüdiger A1 - Gómez, Facundo A. A1 - Springel, Volker T1 - The diversity of the circumgalactic medium around z=0 Milky Way-mass galaxies from the Auriga simulations JF - Monthly notices of the Royal Astronomical Society N2 - Galaxies are surrounded by massive gas reservoirs ( i.e. the circumgalactic medium; CGM) which play a key role in their evolution. The properties of the CGM, which are dependent on a variety of internal and environmental factors, are often inferred from absorption line surveys which rely on a limited number of single lines-of-sight. In this work we present an analysis of 28 galaxy haloes selected from the Auriga project, a cosmological magneto-hydrodynamical zoom-in simulation suite of isolated MilkyWay-mass galaxies, to understand the impact of CGM diversity on observational studies. Although the Auriga haloes are selected to populate a narrow range in halo mass, our work demonstrates that the CGM of L-star galaxies is extremely diverse: column densities of commonly observed species span similar to 3-4 dex and their covering fractions range from similar to 5 to 90 per cent. Despite this diversity, we identify the following correlations: 1) the covering fractions ( CF) of hydrogen and metals of the Auriga haloes positively correlate with stellar mass, 2) the CF of H I, C IV, and Si II anticorrelate with active galactic nucleus luminosity due to ionization effects, and 3) the CF of H I, C IV, and Si II positively correlate with galaxy disc fraction due to outflows populating the CGM with cool and dense gas. The Auriga sample demonstrates striking diversity within the CGM of L-star galaxies, which poses a challenge for observations reconstructing CGM characteristics from limited samples, and also indicates that long-term merger assembly history and recent star formation are not the dominant sculptors of the CGM. KW - methods: numerical KW - galaxies: evolution KW - galaxies: haloes Y1 - 2019 U6 - https://doi.org/10.1093/mnras/stz1708 SN - 0035-8711 SN - 1365-2966 VL - 488 IS - 1 SP - 135 EP - 152 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Werhahn, Maria A1 - Pfrommer, Christoph A1 - Girichidis, Philipp A1 - Puchwein, Ewald A1 - Pakmor, Rüdiger T1 - Cosmic rays and non-thermal emission in simulated galaxies BT - I. Electron and proton spectra compared to Voyager-1 data JF - Monthly notices of the Royal Astronomical Society N2 - Current-day cosmic ray (CR) propagation studies use static Milky Way models and fit parametrized source distributions to data. Instead, we use three-dimensional magnetohydrodynamic (MHD) simulations of isolated galaxies with the moving-mesh code arepo that self-consistently accounts for hydrodynamic effects of CR protons. In post-processing, we calculate their steady-state spectra, taking into account all relevant loss processes. We show that this steady-state assumption is well justified in the disc and generally for regions that emit non-thermal radio and gamma rays. Additionally, we model the spectra of primary electrons, accelerated by supernova remnants, and secondary electrons and positrons produced in hadronic CR proton interactions with the gas. We find that proton spectra above 10 GeV only weakly depend on galactic radius, while they acquire a radial dependence at lower energies due to Coulomb interactions. Radiative losses steepen the spectra of primary CR electrons in the central galactic regions, while diffusive losses dominate in the outskirts. Secondary electrons exhibit a steeper spectrum than primaries because they originate from the transported steeper CR proton spectra. Consistent with Voyager-1 and AMS-02 data, our models (i) show a turnover of proton spectra below GeV energies due to Coulomb interactions so that electrons start to dominate the total particle spectra and (ii) match the shape of the positron fraction up to 10 GeV. We conclude that our steady-state CR modelling in MHD CR galaxy simulations is sufficiently realistic to capture the dominant transport effects shaping their spectra, arguing for a full MHD treatment to accurately model CR transport in the future. KW - astroparticle physics KW - MHD KW - methods: numerical KW - cosmic rays KW - local KW - interstellar matter Y1 - 2021 U6 - https://doi.org/10.1093/mnras/stab1324 SN - 0035-8711 SN - 1365-2966 VL - 505 IS - 3 SP - 3273 EP - 3294 PB - Oxford University Press CY - Oxford ER - TY - JOUR A1 - Whittingham, Joseph A1 - Sparre, Martin A1 - Pfrommer, Christoph A1 - Pakmor, Rüdiger T1 - The impact of magnetic fields on cosmological galaxy mergers BT - I. Reshaping gas and stellar discs JF - Monthly notices of the Royal Astronomical Society N2 - Mergers play an important role in galaxy evolution. In particular, major mergers are able to have a transformative effect on galaxy morphology. In this paper, we investigate the role of magnetic fields in gas-rich major mergers. To this end, we run a series of high-resolution magnetohydrodynamic (MHD) zoom-in simulations with the moving-mesh code arepo and compare the outcome with hydrodynamic simulations run from the same initial conditions. This is the first time that the effect of magnetic fields in major mergers has been investigated in a cosmologically consistent manner. In contrast to previous non-cosmological simulations, we find that the inclusion of magnetic fields has a substantial impact on the production of the merger remnant. Whilst magnetic fields do not strongly affect global properties, such as the star formation history, they are able to significantly influence structural properties. Indeed, MHD simulations consistently form remnants with extended discs and well-developed spiral structure, whilst hydrodynamic simulations form more compact remnants that display distinctive ring morphology. We support this work with a resolution study and show that whilst global properties are broadly converged across resolution and physics models, morphological differences only develop given sufficient resolution. We argue that this is due to the more efficient excitement of a small-scale dynamo in higher resolution simulations, resulting in a more strongly amplified field that is better able to influence gas dynamics. KW - MHD KW - methods: numerical KW - galaxies: interactions KW - galaxies: magnetic KW - fields Y1 - 2021 U6 - https://doi.org/10.1093/mnras/stab1425 SN - 0035-8711 SN - 1365-2966 VL - 506 IS - 1 SP - 229 EP - 255 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Thomas, Timon A1 - Pfrommer, Christoph A1 - Pakmor, Rüdiger T1 - A finite volume method for two-moment cosmic ray hydrodynamics on a moving mesh JF - Monthly notices of the Royal Astronomical Society N2 - We present a new numerical algorithm to solve the recently derived equations of two-moment cosmic ray hydrodynamics (CRHD). The algorithm is implemented as a module in the moving mesh AREPO code. Therein, the anisotropic transport of cosmic rays (CRs) along magnetic field lines is discretized using a path-conservative finite volume method on the unstructured time-dependent Voronoi mesh of AREPO. The interaction of CRs and gyroresonant Alfven waves is described by short time-scale source terms in the CRHD equations. We employ a custom-made semi-implicit adaptive time stepping source term integrator to accurately integrate this interaction on the small light-crossing time of the anisotropic transport step. Both the transport and the source term integration step are separated from the evolution of the magnetohydrodynamical equations using an operator split approach. The new algorithm is tested with a variety of test problems, including shock tubes, a perpendicular magnetized discontinuity, the hydrodynamic response to a CR overpressure, CR acceleration of a warm cloud, and a CR blast wave, which demonstrate that the coupling between CR and magnetohydrodynamics is robust and accurate. We demonstrate the numerical convergence of the presented scheme using new linear and non-linear analytic solutions. KW - hydrodynamics KW - MHD KW - methods: numerical KW - cosmic rays Y1 - 2021 U6 - https://doi.org/10.1093/mnras/stab397 SN - 0035-8711 SN - 1365-2966 VL - 503 IS - 2 SP - 2242 EP - 2264 PB - Oxford University Press CY - Oxford ER -