The imprint of magnetic fields on absorption spectra from circumgalactic wind-cloud systems

Abstract

Context. Galactic winds determine how stellar feedback regulates the mass and metallicity of galaxies. Observational studies show that galactic winds are multi-phase and magnetised. In the local Universe, the dense phase is traced by emission and absorption lines, which reveal the presence of fast-moving clouds embedded in hot streams. Simulations of such streams indicate to us that magnetic fields can shield such clouds and help to delay their disruption, but observational effects are rarely discussed. Aims. Using a suite of 3D magnetohydrodynamical simulations, we studied the influence of two orientations of the magnetic field (aligned and transverse to the wind) on the cloud morphology, temperature and density structure, mixing fraction, ion kinematics, column densities, and absorption spectra. Methods. We numerically studied supersonic wind-cloud systems with radiative processes, and developed a framework to extract ion column density maps and synthetic absorption spectra. The framework relies on studying ion populations and creating down-the-barrel spectra via an interface that links our PLUTO simulations to TRIDENT using the yt-package infrastructure, CLOUDY, and STARBURST99. Results. We find that the transverse initial magnetic field makes the cloud asymmetric, shields and protects dense cold gas, and reduces mixing fractions compared to the aligned case. Ions can reach higher velocities in the transverse field case. The imprints of the initial orientation of the field on the synthetic spectra can be described as follow: (a) in the cold phase, we find no signature of C ¯II and Si ¯II when the field is aligned; (b) in the intermediate phase traced by C ¯IV and Si ¯IV, we find broader lines in the transverse case; and (c) in the warm phase, we find deeper lines for O ¯VI and N ¯V in the aligned case, but they are less sensitive overall to the field orientation. Conclusions. Magnetic fields significantly affect the absorption spectra of cold clouds. Intermediate ions are the most sensitive to the magnetic field orientation and can potentially yield information about magnetic field topology.