Shivam Burman

Most observational studies of galactic-scale magnetic fields using Faraday rotation rely on estimates of thermal electron densities in galaxies and their radial variations. However, the spatial distribution of electrons in the interstellar medium (ISM) is not clearly known. In this study, we propose and utilize collisionally-excited doublet emission line ratios of [S ii ] λλ 6716, 6731 Å to estimate the electron densities ( n _e ). To map their distribution in the galaxies, we employ IFU spectroscopic observations from the SDSS MaNGA survey, utilising data products from the MaNGA Data Analysis Pipeline (DAP). We present a spatially resolved analysis of 66 face-on galaxies (inclination, i ≤ 15°), including 46 star-forming galaxies (SFGs) and 20 Non-SFGs. Azimuthally averaged radial profiles of n _e are obtained. We found that both SFGs and Non-SFGs exhibit n _e gradients, with higher densities of n _e (S ii ) = 52.87 ± 8.32 cm ^-3 and 99.39 ± 24.37 cm ^-3 , respectively, in the inner disk region (r/R _e ≤ 1.5), which decreases to n _e (S ii ) = 20.92 ± 4.2 cm ^-3 in SFGs and 34.64 ± 11.24 cm ^-3 in Non-SFGs, in the outer disk region (r/R _e > 1.5). We have also analysed these sources with Pipe3D fluxes. We translated n _e to electron column densities ( N _e ) by assuming a typical disk of thickness 1 kpc and note that N _e ∼ 10 ²² cm ^-2 at ∼14 kpc in the disk outer region. We have also discussed the profiles obtained using [O ii ] λλ 3726, 3729 Å doublet. These electron density estimates at different radii provide valuable insights for resolving ambiguities in current and future studies of magnetic fields in galaxies.

Probing magnetic fields in high-redshift galactic systems is crucial to investigate galactic dynamics and evolution. Utilizing the rotation measure of the background quasars, we have developed a radial profile of the magnetic field in a typical high-z galaxy. We have compiled a catalog of 59 confirmed quasar sightlines, having one intervening Mg II absorber in the redshift range 0.372 ≤ z _abs ≤ 0.8. The presence of the foreground galaxy is ensured by comparing the photometric and spectroscopic redshifts within 3σ _z-photo and visual checks. These quasar line-of-sights (LoS) pass through various impact parameters (D) up to 160 kpc, covering the circumgalactic medium of a typical Milky-Way type galaxy. Utilizing the residual rotation measure (RRM) of these sightlines, we estimated the excess in RRM dispersion, σ _ex ^RRM. We found σ _ex ^RRM decreases with increasing D. We translated σ _ex ^RRM to average LoS magnetic field strength, 〈 B _∥ 〉 by considering a typical electron column density. Consequently, the decreasing trend is sustained in the magnetic field. In particular for sightlines with D ≤ 50 kpc and D> 50 kpc, 〈 B _∥ 〉 is found to be 2.39 ± 0.7 μG and 1.67 ± 0.38 μG, respectively. This suggests a clear indication of varying magnetic field from the disk to the circumgalactic medium. This work provides a methodology that, when applied to ongoing and future radio polarisation surveys such as LOFAR and SKA, promises to significantly enhance our understanding of magnetic field mapping in galactic systems.