Environment matters: stronger magnetic fields in satellite galaxies

TL;DR

This paper shows that satellite galaxies experience stronger magnetic fields than isolated dwarfs of similar mass or SFR, particularly after their first close passage to a host. Using high-resolution cosmological zoom-ins with cosmic rays in the Auriga model, the authors demonstrate that enhanced turbulent driving during infall facilitates a resolution-insensitive amplification of magnetic fields, with pericentre distance correlating with the level of amplification. The findings have important implications for interpreting radio synchrotron and gamma-ray emissions from dwarfs and satellites, and they highlight environmental effects as a critical factor in modeling galaxy magnetic fields in large cosmological simulations.

Abstract

Magnetic fields are ubiquitous in the universe and an important component of the interstellar medium. It is crucial to accurately model and understand their properties in different environments and across all mass ranges of galaxies to interpret observables related to magnetic fields correctly. However, the assessment of the role of magnetic fields in galaxy evolution is often hampered by limited numerical resolution in cosmological simulations, in particular for satellite galaxies. To this end, we study the magnetic fields in high-resolution cosmological zoom simulations of disk galaxies (with $M_{200}\approx10^{10}$ to $10^{13}\,\mathrm{M}_\odot$) and their satellites within the Auriga galaxy formation model including cosmic rays. We find significantly higher magnetic field strengths in satellite galaxies compared to isolated dwarfs with a similar mass or star-formation rate, in particular after they had their first close encounter with their host galaxy. These are stronger on average by factors of 2-8 when compared at the same total mass, with a large scatter, ranging up to factors of $\sim$15. While this result is ubiquitous and independent of resolution in the satellites that are past their first infall, there seems to be a wide range of amplification mechanisms acting together. Our result highlights the importance of considering the environment of dwarf galaxies when interpreting their magnetic field properties as well as related observables such as their gamma-ray and radio emission, the latter being particularly relevant for future observations such as with the SKA observatory.

Figures (7)

• Figure 1: Maps of an example satellite at the time of its infall into the host halo (first row; $z=0.39,\ t_\mathrm{look}=4.31$ Gyr), at the apocentre of its orbit (second row; $z=0.14,\ t_\mathrm{look}=1.85$ Gyr), and an isolated dwarf with a similar stellar mass (third row; $z=0$). We show, from left to right, the stellar light projection (the red, green and blue colours in the stellar light projections represent the K-, B- and U-band luminosity of stars, respectively), a thin projection (of thickness 3 kpc) of the magnetic field strength and a slice of the metallicity, as well as the projected gas surface density in two different sized boxes. The orientation of the magnetic field in the second column is indicated by a relief created by the line integral convolution method 1993CabralLeedom. The first row shows maps rotated such that the satellite moves in the positive $x$-direction, while the second and third row show the galaxies face-on. The satellite exhibits a stronger magnetic field at apocentre than the isolated dwarf.
• Figure 2: Average magnetic field strength (within a sphere with a radius of 20 kpc) of the satellite galaxies (circles) in comparison to central galaxies (star symbols) as a function of total mass (left-hand panel) and SFR (right-hand panel) at $z=0$ of the CR-MHD simulations. The solid lines represent the binned averages of the three types of galaxies shown here. We find systematically higher magnetic field strengths in the satellites compared to central galaxies at a fixed mass or SFR, except for the satellites that are still on their first approach to their host galaxy (open circles). The colours in the left-hand panel show the pericentres of the satellites in units of $R_{200}$ of their host halo, indicating a trend of larger magnetic field amplification for closer encounters with the host. All galaxies in the right-hand panel are colour-coded by their stellar mass, indicating a similar trend of stronger magnetic fields also as a function of stellar mass.
• Figure 3: Left-hand panel: Time evolution of kinetic and magnetic energies of the highest stellar mass satellite of halo 1e12-h12 at different resolution levels ('high res': $m_\mathrm{gas}=6\times10^3$; 'medium res': $m_\mathrm{gas}=5\times10^4$; 'low res': $m_\mathrm{gas}=4\times10^5$). The energies are integrated within a sphere with a radius of 20 kpc around the centre of the satellite. The grey lines show the distance $D$ to the central galaxy in units of $R_{200}(t)$ (right-hand $y$-axis). During the first infall, starting at $t_\mathrm{look}\sim5$ Gyr, the magnetic energy significantly increases for all resolutions and converges towards similar values at late times, where it remains enhanced compared to before infall. The light blue vertical lines on top of the $x$-axis indicate the times of the velocity structure functions shown in the right-hand panel. Right-hand panel: Second order velocity structure function (see Eq. \ref{['eq:SF2']}) of the high-res simulation before and during the infall of the same satellite, indicating a larger turbulent driving scale during infall than before infall. The theoretical scaling for Kolmogorov turbulence 1941aKolmogorov is indicated by the dotted line.
• Figure 4: Total magnetic energy as a function of turbulent kinetic energy for the central galaxies (star symbols) and satellites (circles), colour-coded by their SFR. The dashed grey line shows equal energies, while the dotted grey line shows where $E_B$ is 10 per cent of $E_\mathrm{turb}$. While the magnetic energies of the highly star-forming central galaxies saturate at $\sim 20$ per cent of their turbulent energy, the isolated dwarfs only reach a few per cent. In contrast, the satellite galaxies yield much higher ratios of magnetic to kinetic energies than the isolated dwarfs at the same SFR.
• Figure 5: Magnetic field strength for different resolution levels as a function of total mass for satellites and central galaxies. The resolution levels 3, 4 and 5 correspond to 'high res' ($m_\mathrm{gas}=6\times10^3\,\mathrm{M_\odot}$), 'medium res' ($m_\mathrm{gas}=5\times10^4\,\mathrm{M_\odot}$) and 'low res' ($m_\mathrm{gas}=4\times10^5\,\mathrm{M_\odot}$) in Fig. \ref{['fig:vel_structure_function_Energy_evolution']}, respectively. While without CRs (left-hand panel), $m_\mathrm{gas}=10^2\,\mathrm{M_\odot}$ ('level 1') is required for convergence for isolated dwarfs with total masses of order $10^{10}\,\mathrm{M_\odot}$, including CRs (right-hand panel) leads to convergence already at lower resolution.