arXiv subjects

A. Konstantinou

Publications and source records attributed to A. Konstantinou.

3 recordsLinked to original sources

arXiv2026

What does it mean to take the mean? The effect of the averaging scale on the characterization of interstellar turbulence

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.

astro-ph.GA↗

arXiv2024

Time evolution of the galactic $B- ρ$ relation: the impact of the magnetic field morphology

One of the most frequently used indicators to characterize the magnetic field's influence on star formation is the relation between magnetic field strength and gas density ($B-ρ$ relation), usually expressed as $B \propto ρ^κ$. The value of $κ$ is an indication of the dynamical importance of the magnetic field during gas compression. Investigating the global magnetic field's impact on this relation and its evolution, we conduct MHD simulations of Milky-Way-like galaxies including gravity, star formation, and supernova feedback along with non-equilibrium chemistry up to $H_2$ formation fueling star formation. Two initial magnetic field morphologies are studied: one completely ordered (toroidal) and the other completely random. In these models, we study the dynamical importance of the magnetic field through the plasma $β$ and the $B-ρ$ relation. For both magnetic morphologies, low-density regions are thermally supported, while high-density regions are magnetically dominated. Equipartition is reached earlier and at lower densities in the toroidal model. However, the $B-ρ$ relation is not unique even within the same galaxy, as it consistently includes two different branches for a given density, with $κ$ ranging from about 0.2 to 0.8. The mean value of $κ$ for each model also displays significant variations over time, which supersede the differences between the two models. While our findings suggest that the magnetic field morphology does influence the galactic $B-ρ$ relation, its impact is transient, since time-averaged differences between the models fall within the large temporal scatter. The context and time-dependent nature of the $B-ρ$ relation underscore the need for comprehensive research and observations to understand the intricate role of magnetic fields in star formation processes across diverse galactic environments.

astro-ph.GA↗

arXiv2022

Polarization power spectra and dust cloud morphology

In the framework of studies of the CMB polarization and its Galactic foregrounds, the angular power spectra of thermal dust polarization maps have revealed an intriguing E/B asymmetry and a positive TE correlation. In interpretation studies of these observations, magnetized ISM dust clouds have been treated as filamentary structures only; however, sheet-like shapes are also supported by observational and theoretical evidence. In this work, we study the influence of cloud shape and its connection to the local magnetic field on angular power spectra of thermal dust polarization maps. We simulate realistic filament-like and sheet-like interstellar clouds, and generate synthetic maps of their thermal dust polarized emission using the software $Asterion$. We compute their polarization power spectra in multipole range $\ell \in [100,500]$ and quantify the E/B power asymmetry through the $R_{EB}$ ratio, and the correlation coefficient $r^{TE}$ between T and E modes. We quantify the dependence of $R_{EB}$ and $r^{TE}$ values on the offset angle (between longest cloud axis and magnetic field) and inclination angle (between line-of-sight and magnetic field) for both cloud shapes embedded either in a regular or a turbulent magnetic field. We find that both cloud shapes cover the same regions of the ($R_{EB}$, $r^{TE}$) parameter space. The dependence on inclination and offset angles are similar for both shapes although sheet-like structures generally show larger scatter. In addition to the known dependence on the offset angle, we find a strong dependence of $R_{EB}$ and $r^{TE}$ on the inclination angle. The fact that filament-like and sheet-like structures may lead to polarization power spectra with similar ($R_{EB}$, $r^{TE}$) values complicates their interpretation. In future analyses, this degeneracy should be accounted for as well as the connection to the magnetic field geometry.

astro-ph.GA↗