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Dark Matter (DM) halos with masses below $\sim10^{8}$ $M_{\odot}$, which would help to discriminate between DM models, may be detected through their gravitational effect on distant sources. The same applies to primordial black holes, considered as an alternative scenario to DM particle models. However, there is still no evidence for the existence of such objects. With the aim of finding compact objects in the mass range $\sim$ 10$^{6}$ -- 10$^{9}$$M_{\odot}$, we search for strong gravitational lenses on milli (mas)-arcseconds scales (< 150 mas). For our search, we used the Astrogeo VLBI FITS image database -- the largest publicly available database, containing multi-frequency VLBI data of 13828 individual sources. We used the citizen science approach to visually inspect all sources in all available frequencies in search for images with multiple compact components on mas-scales. At the final stage, sources were excluded based on the surface brightness preservation criterion. We obtained a sample of 40 sources that passed all steps and therefore are judged to be milli-arcsecond lens candidates. These sources are currently followed-up with on-going European VLBI Network (EVN) observations at 5 and 22 GHz. Based on spectral index measurements, we suggest that two of our candidates have a higher probability to be associated with gravitational lenses., 6 pages, 1 figure; accepted for publication

Herschel surveys of Galactic clouds support a paradigm for low-mass star formation in which dense filaments play a crucial role. The detailed fragmentation properties of star-forming filaments remain poorly understood, however, and the validity of the filament paradigm in the high-mass regime is still unclear. To investigate the density/velocity structure of the filament in the high-mass star-forming region NGC6334, we conducted ALMA observations in the 3mm continuum and the N2H+(1-0) line at ~3arcsec resolution. The filament was detected in both tracers. We identified 26 cores at 3mm and 5 velocity-coherent fiber-like features in N2H+ within the filament. The typical length of, and velocity difference between, the fiber-like features of the NGC6334 filament are reminiscent of the properties for the fibers of the low-mass star-forming filament B211/B213. Only 2 or 3 of the 5 velocity-coherent features are well aligned with the filament and may represent genuine, fiber sub-structures. The core mass distribution has a peak at ~10Msun. They can be divided into 7 groups of cores, closely associated with ArTeMiS clumps. The projected separation between cores and the projected spacing between clumps are roughly consistent with the effective Jeans length in the filament and a physical scale of about 4 times the filament width, respectively, suggesting a bimodal filament fragmentation process. Despite being one order of magnitude denser and more massive than the B211/B213 filament, the NGC6334 filament has a similar density/velocity structure. The difference is that the cores in NGC6334 appear to be an order of magnitude denser and more massive than the cores in Taurus. This suggests that dense filaments may evolve and fragment in a similar manner in low- and high-mass star-forming regions, and that the filament paradigm may hold in the intermediate-mass (if not high-mass) star formation regime., 21 pages, 18 figures, accepted for publication in A&A

The connection between the pre-stellar core mass function (CMF) and the stellar initial mass function (IMF) lies at the heart of all star formation theories. In this paper, we study the earliest phases of star formation with a series of high-resolution numerical simulations that include the formation of sinks. In particular, we focus on the transition from cores to sinks within a massive molecular filament. We compare the CMF and IMF between magnetized and unmagnetized simulations, and between different resolutions. We find that selecting cores based on their kinematic virial parameter excludes collapsing objects because they host large velocity dispersions. Selecting only the thermally unstable magnetized cores, we observe that their mass-to-flux ratio spans almost two orders of magnitude for a given mass. We also see that, when magnetic fields are included, the CMF peaks at higher core mass values with respect to pure hydrodynamical simulations. Nonetheless, all models produce sink mass functions with a high-mass slope consistent with Salpeter. Finally, we examine the effects of resolution and find that, in isothermal simulations, even models with very high dynamical range fail to converge in the mass function. Our main conclusion is that, although the resulting CMFs and IMFs have similar slopes in all simulations, the cores have slightly different sizes and kinematical properties when a magnetic field is included. However, a core selection based on the mass-to-flux ratio alone is not enough to alter the shape of the CMF, if we do not take thermal stability into account. Finally, we conclude that extreme care should be given to resolution issues when studying sink formation with an isothermal equation of state., Comment: Accepted for publication by A&A

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

Stellar winds and supernova (SN) explosions of massive stars ("stellar feedback") create bubbles in the interstellar medium (ISM) and insert newly produced heavy elements and kinetic energy into their surroundings, possibly driving turbulence. Most of this energy is thermalized and immediately removed from the ISM by radiative cooling. The rest is available for driving ISM dynamics. In this work we estimate the amount of feedback energy retained as kinetic energy when the bubble walls have decelerated to the sound speed of the ambient medium. We show that the feedback of the most massive star outweighs the feedback from less massive stars. For a giant molecular cloud (GMC) mass of 1e5 solar masses (as e.g. found in the Orion GMCs) and a star formation efficiency of 8% the initial mass function predicts a most massive star of approximately 60 solar masses. For this stellar evolution model we test the dependence of the retained kinetic energy of the cold GMC gas on the inclusion of stellar winds. In our model winds insert 2.34 times the energy of a SN and create stellar wind bubbles serving as pressure reservoirs. We find that during the pressure driven phases of the bubble evolution radiative losses peak near the contact discontinuity (CD), and thus, the retained energy depends critically on the scales of the mixing processes across the CD. Taking into account the winds of massive stars increases the amount of kinetic energy deposited in the cold ISM from 0.1% to a few percent of the feedback energy., The manuscript MN-15-2374-MJ.R2 has been accepted for publication in Monthly Notices of the Royal Astronomical Society Main Journal on 2015 November 16

Large-scale shocks formed by clustered feedback of young OB stars are considered an important source of mechanical energy for the ISM and a trigger of molecular cloud formation. Their interaction sites are locations where kinetic energy and magnetic fields are redistributed between ISM phases. In this work we study the effect of the magnetic field on the expansion and fragmentation of supershells and look for the signatures of supershell collisions on dense structures and on the kinetic and magnetic energy distribution of the ISM. We performed a series of high-resolution, three-dimensional simulations of colliding supershells. The shocks are created by time-dependent feedback and evolve in a diffuse turbulent environment that is either unmagnetized or has different initial magnetic field configurations. In the hydrodynamical situation, the expansion law of the superbubbles is consistent with the radius-time relation that is theoretically predicted for wind-blown bubbles. The supershells fragment over their entire surface into small dense clumps that carry more than half of the total kinetic energy in the volume. However, this is not the case when a magnetic field is introduced, either in the direction of the collision or perpendicular to the collision. In magnetized situations, the shell surfaces are more stable to dynamical instabilities. When the magnetic field opposes the collision, the expansion law of the supershells also becomes significantly flatter than in the hydrodynamical case. Although a two-phase medium arises in all cases, in the MHD simulations the cold phase is limited to lower densities., Comment: to appear in A&A Section 6: Interstellar and circumstellar matter

We present results from numerical simulations of expanding and colliding supershells. These large-scale spherical shocks, created by the combined feedback from several OB stars, are unstable to a number of hydrodynamical instabilities, so they quickly fragment into cold and highly structured clumps. A collision between two large shells can organize these small clumps into very filamentary structures, of tens of parsecs length and less than a parsec thick. In simulations where the flow of stellar material is followed with a tracer quantity, cold structures practically do not contain any enriched material from the OB associations at the time of their creation. In this context then, the clumps are created almost exclusively out of diffuse ISM material, containing almost no wind or supernova matter. Although the mechanism presented here is possibly not the only route for filament creation, this predicted property may help identify regions of sequential star formation.

The filamentary structure of the molecular interstellar medium and the potential link of this morphology to star formation have been brought into focus recently by high resolution observational surveys. An especially puzzling matter is that local interstellar filaments appear to have the same thickness, independent of their column density. This requires a theoretical understanding of their formation process and the physics that governs their evolution. In this work we explore a scenario in which filaments are dissipative structures of the large-scale interstellar turbulence cascade and ion-neutral friction (also called ambipolar diffusion) is affecting their sizes by preventing small-scale compressions. We employ high-resolution, 3D MHD simulations, performed with the grid code RAMSES, to investigate non-ideal MHD turbulence as a filament formation mechanism. We focus the analysis on the mass and thickness distributions of the resulting filamentary structures. Simulations of both driven and decaying MHD turbulence show that the morphologies of the density and the magnetic field are different when ambipolar diffusion is included in the models. In particular, the densest structures are broader and more massive as an effect of ion-neutral friction and the power spectra of both the velocity and the density steepen at a smaller wavenumber. The comparison between ideal and non-ideal MHD simulations shows that ambipolar diffusion causes a shift of the filament thickness distribution towards higher values. However, none of the distributions exhibit the pronounced peak found in the observed local filaments. Limitations in dynamical range and the absence of self-gravity in these numerical experiments do not allow us to conclude at this time whether this is due to the different filament selection or due to the physics inherent of the filament formation., A&A accepted

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

Numerical models of disc galaxy formation predict the existence of extended, hot ( T~10^6 K) gas haloes around present day spirals. The X-ray luminosity of these haloes is predicted to increase strongly with galaxy mass. However, searches for their X-ray emission have not been successful so far. We calculate the all sky O VII column density distributions for the haloes of three Milky Way like disc galaxies, resulting from cosmological high-resolution, N-body/gasdynamical simulations. We perform calculations both including the disc gas and without it, so the disc contribution to the column density is quantified. It is found that the column densities estimated for Milky Way-like galaxies are just below the observational upper limit, making a test of the hot halo paradigm likely within observational reach.

This work aims at improving the current understanding of the interaction between H ii regions and turbulent molecular clouds. We propose a new method to determine the age of a large sample of OB associations by investigating the development of their associated H ii regions in the surrounding turbulent medium. Using analytical solutions, one-dimensional (1D), and three-dimensional (3D) simulations, we constrained the expansion of the ionized bubble depending on the turbulent level of the parent molecular cloud. A grid of 1D simulations was then computed in order to build isochrone curves for H ii regions in a pressure-size diagram. This grid of models allowed to date large sample of OB associations and was used on the H ii Region Discovery Survey (HRDS). Analytical solutions and numerical simulations showed that the expansion of H ii regions is slowed down by the turbulence up to the point where the pressure of the ionized gas is in a quasi-equilibrium with the turbulent ram pressure. Based on this result, we built a grid of 1D models of the expansion of H ii regions in a profile based on Larson laws. The 3D turbulence is taken into account by an effective 1D temperature profile. The ages estimated by the isochrones of this grid agree well with literature values of well-known regions such as Rosette, RCW 36, RCW 79, and M16. We thus propose that this method can be used to give ages of young OB associations through the Galaxy such as the HRDS survey and also in nearby extra-galactic sources., Comment: Accepted in A&A

The expansion and collision of two wind-blown superbubbles is investigated numerically. Our models go beyond previous simulations of molecular cloud formation from converging gas flows by exploring this process with realistic flow parameters, sizes and timescales. The superbubbles are blown by time-dependent winds and supernova explosions, calculated from population synthesis models. They expand into a uniform or turbulent diffuse medium. We find that dense, cold gas clumps and filaments form naturally in the compressed collision zone of the two superbubbles. Their shapes resemble the elongated, irregular structure of observed cold, molecular gas filaments and clumps. At the end of the simulations, between 65 and 80 percent of the total gas mass in our simulation box is contained in these structures. The clumps are found in a variety of physical states, ranging from pressure equilibrium with the surrounding medium to highly under-pressured clumps with large irregular internal motions and structures which are rotationally supported., Submitted to ApJ

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

Dense, star-forming gas is believed to form at the stagnation points of large-scale interstellar medium flows, but observational examples of this process in action are rare. We here present a giant molecular cloud (GMC) sandwiched between two colliding Milky Way supershells, which we argue shows strong evidence of having formed from material accumulated at the collision zone. Combining 12CO, 13CO, and C18O(J = 1-0) data with new high-resolution, three-dimensional hydrodynamical simulations of colliding supershells, we discuss the origin and nature of the GMC (G288.5+1.5), favoring a scenario in which the cloud was partially seeded by pre-existing denser material, but assembled into its current form by the action of the shells. This assembly includes the production of some new molecular gas. The GMC is well interpreted as non-self-gravitating, despite its high mass (), and is likely pressure confined by the colliding flows, implying that self-gravity was not a necessary ingredient for its formation. Much of the molecular gas is relatively diffuse, and the cloud as a whole shows little evidence of star formation activity, supporting a scenario in which it is young and recently formed. Drip-like formations along its lower edge may be explained by fluid dynamical instabilities in the cooled gas.

The magnetic fields of spiral galaxies are so strong that they cannot be primordial. Their typical values are over one billion times higher than any value predicted for the early Universe. Explaining this immense growth and incorporating it in galaxy evolution theories is one of the long-standing challenges in astrophysics. So far, the most successful theory for the sustained growth of the galactic magnetic field is the alpha-omega dynamo. This theory predicts a characteristic dipolar or quadrupolar morphology for the galactic magnetic field, which has been observed in external galaxies. However, so far, there has been no direct demonstration of a mean-field dynamo operating in direct, multi-physics simulations of spiral galaxies. We do so in this work. We employ numerical models of isolated, star-forming spiral galaxies that include a magnetized gaseous disk, a dark matter halo, stars, and stellar feedback. Naturally, the resulting magnetic field has a complex morphology that includes a strong random component. Using a smoothing of the magnetic field on small scales, we are able to separate the mean from the turbulent component and analyze them individually. We find that a mean-field dynamo naturally occurs as a result of the dynamical evolution of the galaxy and amplifies the magnetic field by an order of magnitude over half a Gyr. Despite the highly dynamical nature of these models, the morphology of the mean component of the field is identical to analytical predictions. This result underlines the importance of the mean-field dynamo in galactic evolution. Moreover, by demonstrating the natural growth of the magnetic field in a complex galactic environment, it brings us a step closer to understanding the cosmic origin of magnetic fields., Accepted for publication in Astronomy & Astrophysics