Evangelia Ntormousi

Gravitational collapse creates large amounts of kinetic energy that could potentially seed turbulence. If such turbulence were also suitable to initiate dynamo action, the resulting magnetic field would further modify the dynamics, especially on small length scales. However, a small-scale dynamo is believed to require vortical turbulence, whereas the collapse produces mainly irrotational motions, which may not be efficient for dynamo action. Here, we study the efficiency of vorticity production during a turbulent collapse. We adopt a barotropic equation of state, where pressure and density gradients are parallel, and no magnetic field, so that vorticity can only be produced by viscosity and by vortex dynamo-type amplification. Using direct numerical simulations of gravitational collapse, we show that, for the parameter space accessible to our numerical resolution, these effects are related to the initial irrotational turbulence and are not induced by the collapse. Vorticity production along with the associated small-scale dynamo action are still expected to occur for sufficiently large Reynolds numbers, especially when baroclinic effects are allowed for, but some of the earlier numerical evidence in the literature using a barotropic equation of state is now found to be the result of subgrid-scale modeling and not reproduced in direct numerical simulations.

Magnetohydrodynamical (MHD) simulations of isothermal compressible turbulence report that the density distribution is well described by a lognormal with a variance proportional to the flow’s Mach number. The distribution of the magnetic field strength also has a lognormal component, but it includes long power-law-like tails. In this work, we use semianalytical arguments to predict the distributions of density and magnetic field strength in compressible turbulent flows. Specifically, in the Lagrangian description of the continuity and the induction equations, we modeled the velocity gradients of the turbulent flow as a simple random process, essentially turning these equations into stochastic differential equations. Integrating them led to a lognormal distribution for the density field and the strength of the magnetic field. The power-law tails in the magnetic field probability density function appeared when we introduced intermittent shocks due to sampling rare events. Gradually increasing the frequency of these events, essentially moving closer to a continuous process, led to lognormal-like distributions again. The asymmetry is connected to the relative abundance of slow and fast shocks. An overabundance of fast MHD shocks produces a high-value tail, while the contrary produces low-value tails. We propose that the appearance of power-law tails along lognormals in turbulent flows is the signature of the coexistence of continuous diffusion-like propagation combined with localized intermittent events.

We present first results from SIMPLIFI (Study of Interstellar Magnetic Polarization: a Legacy Investigation of Filaments), a SOFIA/HAWC+ 214 μ m polarimetric survey of Galactic molecular cloud filaments. We trace magnetic field morphology from the DR21 Main Ridge into surrounding subfilaments at ∼0.1 pc resolution, extending polarimetric detections for the first time beyond high-column-density regions probed by prior submillimeter observations. We compare the plane-of-sky orientations of the magnetic field, B ˆ pos , the projected gravitational acceleration, g pos , and the intensity gradient rotated by 90°, ∇ ˆ ⊥ I . The relative orientation of B ˆ pos and ∇ ˆ ⊥ I transitions from preferentially parallel in subfilaments to perpendicular in the DR21 Main Ridge at N (H ₂ ) ∼ 2 × 10 ²² cm ⁻² , consistent with thresholds seen with Planck and expected in clouds formed from strongly magnetized, sub-Alfvénic, magnetically subcritical gas ( M A ≲ 1 , M / Φ B < [ M / Φ B ] cr ). We find that the relative alignment between orientations shows region-to-region and pixel-to-pixel variations at fixed column density. Column density alone is thus not sufficient to encode changes in magnetic field structure. Theoretical models must account for additional drivers. Our central finding is that g pos and B ˆ pos remain aligned throughout the cloud regardless of column density or environment, unlike the environment-dependent behavior of B ˆ pos versus ∇ ˆ ⊥ I and g pos versus ∇ ˆ ⊥ I . This persistent alignment is consistent with magnetically guided accretion: subfilaments channel material along field lines at several 10 ⁻³ M _⊙ yr ⁻¹ , sufficient to assemble the Ridge within ∼10 ⁶ yr and sustain high-mass star formation. This framework also explains why observed radial velocities (≈2 km s ⁻¹ ) fall well below freefall expectations (≈8 km s ⁻¹ ): with the field nearly in the plane of the sky, only a small fraction of the accretion velocity projects along the line of sight.

Context. In studies of the interstellar medium (ISM), defining an ordered and a random velocity or magnetic field extends beyond the scope of simple thought experiments. It often plays a crucial role in interpreting the dynamics of turbulence, both in simulations and in observations. Aims. We investigate how the choice of an averaging scale affects the measurement and characterization of magnetic and kinetic turbulence in Milky Way-sized disk galaxies with different initial magnetic morphologies. Methods. We analyzed two magnetohydrodynamic (MHD) simulations of isolated disk galaxies: one initialized with a toroidal magnetic field (model T) and the other with a random field (model R). Using a spherical filtering method, we decomposed the magnetic and velocity fields into mean and fluctuating components, while varying the averaging scale. We examined their energy ratios and power spectra as functions of time and the averaging radius. Results. Both models develop ordered and turbulent magnetic structures, whose relative strengths vary strongly with the averaging radius. The power spectra of the velocity and magnetic mean fields steepen with an increasing smoothing scale, tracing the transition from coherent to turbulent regimes. The turbulent kinetic energy dominates over the magnetic counterpart, although the latter remains dynamically significant. Importantly, we find that these ratios depend more strongly on the averaging radius than on the initial conditions of the magnetic field. Conclusions. The characterization of turbulence as strong or weak, meaning whether or not fluctuations dominate over the mean depends sensitively on the chosen averaging scale, rather than being an intrinsic property of the system. The strong dependence of the turbulent fractions on the averaging scale has direct implications for magnetic field estimates obtained from observational methods such as the Davis-Chandrasekhar-Fermi technique. A careful consideration of scale is therefore essential when interpreting magnetic and kinetic turbulence in galaxies.

In interstellar medium studies, separating ordered and random velocity or magnetic fields is essential for interpreting turbulence in both simulations and observations. We investigate how the choice of averaging scale affects the measurement and characterization of magnetic and kinetic turbulence in Milky Way-sized disk galaxies with different initial magnetic morphologies. We analyze two magnetohydrodynamic simulations of isolated disk galaxies, one initialized with a toroidal magnetic field (Model T) and the other with a random field (Model R). Using spherical filtering, we decompose the magnetic and velocity fields into mean and fluctuating components while varying the averaging scale, and examine their energy ratios and power spectra as functions of time and averaging radius. Both models develop ordered and turbulent magnetic structures, whose relative strengths vary strongly with the averaging radius. The power spectra of the velocity and magnetic mean fields steepen with increasing smoothing scale, tracing the transition from coherent to turbulent regimes. The turbulent kinetic energy dominates over the magnetic counterpart, though the latter remains dynamically significant. Very importantly, we find that these ratios depend more strongly on the averaging radius than on the initial conditions of the magnetic field. The characterization of turbulence as strong or weak, meaning whether or not fluctuations dominate over the mean depends sensitively on the chosen averaging scale, rather than being an intrinsic property of the system. The strong dependence of the turbulence fractions on the averaging scale has direct implications for magnetic field estimates obtained from observational methods such as the Davis-Chandrasekhar-Fermi technique emphasizing the need for careful scale selection when interpreting magnetic and kinetic turbulence in galaxies.

It has been suggested that, if the free-fall time of star-forming clouds is shorter than the lifetime ( ∼ 3 Myr) of massive stars exploding as supernovae (SN), a large fraction of the cloud gas can be converted into stars during an allegedly `feedback-free’ phase. Here, we show that radiation pressure from Ly α photons produced in the pre-SN phase can instead erase feedback-free conditions, and severely limit the star formation efficiency (SFE). We find that, for a constant star formation rate, all clouds with gas surface density have ϵ * < 0.08 . Higher SFE values can only be reached if Ly α -driven shells fragment and form stars. While advanced RHD simulations are required to establish the importance of this effect, adopting an optimistic guess, we find that the SFE increases with cloud surface density, rising from ϵ * = 0.023 at Σ g = 37 M ⊙ p c − 2 to ϵ * = 0.27 at Σ g = 1.7 × 10 5 M ⊙ p c − 2 . Given the optimistic assumptions adopted, these numbers should be regarded as upper limits. We conclude that Ly α radiation pressure strongly limits the SFE, even at solar metallicities, erasing the possibility that a feedback-free star formation mode with ϵ * ≳ 0.4 exists in the pre-SN phase. This conclusion remains valid even when other effects such as dust destruction of Ly α photons, presence of HII regions, velocity gradients, atomic recoil, and turbulence are considered.

The question of whether a dynamo can be triggered by gravitational collapse is of great interest, especially for the early Universe. Here, we employ supercomoving coordinates to study the magnetic field amplification from decaying turbulence during gravitational collapse. We perform 3D simulations and show that for large magnetic Reynolds numbers, there can be exponential growth of the comoving magnetic field with conformal time before the decay of turbulence impedes further amplification. The collapse dynamics only affect the nonlinear feedback from the Lorentz force, which diminishes more rapidly for shorter collapse times, allowing nearly kinematic continued growth. We confirm that helical turbulence is more efficient in driving dynamo action than nonhelical turbulence, but this difference decreases for larger collapse times. We also show that for nearly irrotational flows, dynamo amplification is still possible, but it is always associated with a growth of vorticity—even if it still remains very small. In nonmagnetic runs, the growth of vorticity is associated with viscosity and grows with the Mach number. In the presence of magnetic fields, vorticity emerges from the curl of the Lorentz force. During a limited time interval, an exponential growth of the comoving magnetic field with conformal time is interpreted as clear evidence of dynamo action.

Context. High-redshift (z ∼ 2 − 3) galaxies accrete circumgalactic gas through cold streams. Recent high-resolution magnetohydrodynamic (MHD) simulations of these streams show a significant amplification of the intergalactic magnetic field in the shear layer around them. Aims. For this work we estimated the magnetisation of high-redshift galaxies that would result purely due to the accretion of already magnetised gas from cold streams. Methods. We used the mass inflow rates and saturated magnetic field values from cold stream simulations as input to a simple analytic model that calculates the galactic magnetic field purely from mass accretion. Results. Our model predicts average magnetic field strengths that exceed μG values at z ∼ 2 − 3 for inflow rates above 0.1 M_⊙ yr⁻¹. For high inflow rates, our model results are consistent with the recent detection of a strong magnetic field in z ≳ 2.6 galaxies. Conclusions. Within the assumptions of our simple model, magnetised cold streams emerge as a viable mechanism for seeding a dynamically important galactic magnetic field.

Context. Magnetic turbulence is classified as weak or strong based on the relative amplitude of the magnetic field fluctuations compared to the mean field. These two classes have different energy transport properties. Aims. The purpose of this study is to analyze turbulence in the interstellar medium (ISM) based on this classification. Specifically, we examined the ISM of simulated galaxies to detect evidence of strong magnetic turbulence and provide statistics on the associated magnetic coherent structures (MCoSs), such as current sheets, that arise in this context. Methods. We analyzed magnetohydrodynamic galaxy simulations with different initial magnetic field structures (either completely ordered or completely random) and recorded statistics on the magnetic field fluctuations (δB/B₀) and the MCoSs, which are defined here as regions where the current density surpasses a certain threshold. We also studied the MCoS sizes and kinematics. Results. The magnetic field disturbances in both models follow a log-normal distribution, peaking at values close to unity; this distribution turns into a power law at large values (δB/B₀ > 1), which is consistent with strong magnetic turbulence The current densities are widely distributed, with non-power-law deviations from a log-normal at the largest values. These deviating values of the current density define MCoSs. We find that, in both models, MCoSs are fractally distributed in space, with a typical volume-filling factor of about 10%, and tend to coincide with peaks of star formation density. Their fractal dimension is close to unity on sub-kiloparsec scales, and between 2 and 3 on larger scales. These values are consistent with MCoSs having a sheet-like or filament-like morphology. Conclusions. Our work challenges the prevailing paradigm of weak magnetic turbulence in the ISM by demonstrating that strong magnetic disturbances can occur even when the initial magnetic field is completely ordered. This strong magnetic turbulence arises self-consistently from differential rotation and supernova feedback. Our findings provide a foundation for a magnetic turbulence description of the galactic ISM that includes strong fluctuations of the magnetic field.

The study of gas-phase metallicity and its spatial distribution at high redshift is crucial to understand the processes that shaped the growth and evolution of galaxies in the early Universe. Here we study the spatially resolved metallicity in three systems at z ∼ 6 − 8, namely A2744-YD4, BDF-3299, and COSMOS24108, with JWST NIRSpec IFU low-resolution (R ∼ 100) spectroscopic observations. These are among the highest-z sources in which metallicity gradients have been probed so far. Each of these systems hosts several spatial components in the process of merging within a few kiloparsecs, identified from the rest-frame UV and optical stellar continuum and ionised gas emission line maps. The sources have heterogeneous properties, with stellar masses log(M_*/M_⊙) ∼7.6–9.3, star formation rates (SFRs) ∼1–15 M_⊙ yr⁻¹, and gas-phase metallicities 12+log(O/H) ∼7.7–8.3, which exhibit a large scatter within each system. Their properties are generally consistent with those of the highest-redshift samples to date (z ∼ 3 − 10), though the sources in A2744-YD4 and COSMOS24108 are at the high end of the mass-metallicity relation (MZR) defined by the z ∼ 3 − 10 sources. Moreover, the targets in this work follow the predicted slope of the MZR at z ∼ 6 − 8 from most cosmological simulations. The gas-phase metallicity gradients are consistent with being flat in the main sources of each system. Flat metallicity gradients are thought to arise from gas mixing processes on galaxy scales, such as mergers or galactic outflows and supernova winds driven by intense stellar feedback, which wash out any gradient formed in the galaxy. The existence of flat gradients at z ∼ 6 − 8 sets also important constraints on future cosmological simulations and chemical evolution models, whose predictions on the cosmic evolution of metallicity gradients often differ significantly, especially at high redshift, but are mostly limited to z ≲ 3 so far.