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Faraday tomography of radio polarimetric data below 200 MHz from LOFAR are providing us with a new perspective on the diffuse and magnetized interstellar medium (ISM). Of particular interest is the discovery of Faraday-rotated synchrotron polarization associated with neutral gas, as traced by atomic hydrogen (HI) and dust. Here we present the first in-depth numerical study of these LOFAR results. We produce and analyze comprehensive synthetic observations of low-frequency synchrotron polarization from MHD simulations of colliding super shells in the multiphase ISM. We define five distinct gas phases over more than four orders of magnitude in gas temperature and density, ranging from hot, and warm, fully ionized gas to cold neutral medium. We focus on the contribution of each gas phase to synthetic observations of both rotation measure and synchrotron polarized intensity below 200 MHz. We also investigate the link between the latter and synthetic observations of optically thin HI gas. We find that, not only the fully ionized gas but also the warm partially ionized and neutral phases strongly contribute to the total rotation measure and polarized intensity. However, the contribution of each phase to the observables strongly depends on the choice of integration axis and the orientation of the mean magnetic field with respect to the shell-collision axis. Strong correlation between HI synthetic data and synchrotron polarized intensity, reminiscent of LOFAR results, is obtained with lines of sight perpendicular to the mean magnetic field direction. Our study suggests that multiphase modelling of MHD processes is needed in order to interpret observations of the radio sky at low frequency. This work is a first step toward understanding the complexity of low-frequency synchrotron emission that will be soon revolutionized by large-scale surveys with LOFAR and the SKA., Comment: 17 pages, 20 figures, accepted by A&A, in press

We use N-body simulations to examine whether a characteristic turnaround radius, as predicted from the spherical collapse model in a $\rm {\Lambda CDM}$ Universe, can be meaningfully identified for galaxy clusters, in the presence of full three-dimensional effects. We use The Dark Sky Simulations and Illustris-TNG dark-matter--only cosmological runs to calculate radial velocity profiles around collapsed structures, extending out to many times the virial radius $R_{200}$. There, the turnaround radius can be unambiguously identified as the largest non-expanding scale around a center of gravity. We find that: (a) Indeed, a single turnaround scale can meaningfully describe strongly non-spherical structures. (b) For halos of masses $M_{200}>10^{13}M_\odot$, the turnaround radius $R_{ta}$ scales with the enclosed mass $M_{ta}$ as $M_{ta}^{1/3}$, as predicted by the spherical collapse model. (c) The deviation of $R_{ta}$ in simulated halos from the spherical collapse model prediction is insensitive to halo asphericity. Rather, it is sensitive to the tidal forces due to massive neighbors when such are present. (d) Halos exhibit a characteristic average density within the turnaround scale. This characteristic density is dependent on cosmology and redshift. For the present cosmic epoch and for concordance cosmological parameters ($\Omega_m \sim 0.7$; $\Omega_\Lambda \sim 0.3$) turnaround structures exhibit an average matter density contrast with the background Universe of $\delta \sim 11$. Thus $R_{ta}$ is equivalent to $R_{11}$ -- in a way analogous to defining the "virial" radius as $R_{200}$ -- with the advantage that $R_{11}$ is shown in this work to correspond to a kinematically relevant scale in N-body simulations., Comment: Submitted to A&A, nine pages, 10 figures

Magnetic fields play a very important role in the evolution of galaxies through their direct impact on star formation and stellar feedback-induced turbulence. However, their co-evolution with these processes has still not been thoroughly investigated, and the possible effect of the initial conditions is largely unknown. This letter presents the first results from a series of high-resolution numerical models, aimed at deciphering the effect of the initial conditions and of stellar feedback on the evolution of the galactic magnetic field in isolated, Milky-Way-like galaxies. The models start with an ordered, either poloidal or toroidal, magnetic field of varying strength, and are evolved with and without supernova feedback. They include a dark matter halo, a stellar and a gaseous disk, as well as the appropriate cooling and heating processes for the interstellar medium. Independently of the initial conditions, the galaxies develop a turbulent velocity field and a random magnetic field component in under 15 Myrs. Supernova feedback is extremely efficient in building a random magnetic field component up to large galactic heights. However, a random magnetic field emerges even in runs without feedback, which points to an inherent instability of the ordered component. Supernova feedback greatly affects the velocity field of the galaxy up to large galactic heights, and helps restructure the magnetic field up to 10 kpc above the disk, independently of the initial magnetic field morphology. On the other hand, the initial morphology of the magnetic field can accelerate the development of a random component at large heights. These effects have important implications for the study of the magnetic field evolution in galaxy simulations., A&A Letters, accepted

Understanding the formation of binary and multiple stellar systems largely comes down to studying the circumstances for the fragmentation of a condensing core during the first stages of the collapse. However, the probability of fragmentation and the number of fragments seem to be determined to a large degree by the initial conditions. In this work we study the fate of the linear perturbations of a homogeneous gas sphere both analytically and numerically. In particular, we investigate the stability of the well-known homologous solution that describes the collapse of a uniform spherical cloud. The difficulty of the mathematical singularity in the perturbation equations is surpassed here by explicitly introducing a weak shock next to the sonic point. In parallel, we perform adaptive mesh refinement (AMR) numerical simulations of the linear stages of the collapse and compared the growth rates obtained by each method. With this combination of analytical and numerical tools, we explore the behavior of both spherically symmetric and non-axisymmetric perturbations. The numerical experiments provide the linear growth rates as a function of the core's initial virial parameter and as a function of the azimuthal wave number of the perturbation. The overlapping regime of the numerical experiments and the analytical predictions is the situation of a cold and large cloud, and in this regime the analytically calculated growth rates agree very well with the ones obtained from the simulations. The use of a weak shock as part of the perturbation allows us to find a physically acceptable solution to the equations for a continuous range of growth rates. The numerical simulations agree very well with the analytical prediction for the most unstable cores, while they impose a limit of a virial parameter of 0.1 for core fragmentation in the absence of rotation., Accepted by A&A