Unveiling electron density profile in nearby galaxies using SDSS MaNGA

Abstract 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.