Unveiling Electron Density Profile in Nearby Galaxies using SDSS MaNGA

TL;DR

This study maps the electron density distribution in nearby galaxies using SDSS MaNGA data, revealing radial gradients that inform magnetic field research in galactic interstellar media.

Contribution

It introduces a novel method of estimating electron densities via [S II] emission line ratios in spatially resolved galaxy data, enhancing understanding of galactic electron distribution.

Findings

Electron densities decrease from inner to outer galactic disks.

Inner disk electron densities are approximately 50-100 cm$^{-3}$.

Outer disk electron densities drop to about 20-35 cm$^{-3}$.

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 collision-excited doublet emission line ratios of [S II] $\lambda\lambda$ 6716, 6731 $\r{A}$ 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 both the MaNGA Data Analysis Pipeline (DAP). We present a spatially resolved analysis of $66$ face-on galaxies (inclination, $i \leq 15^\circ$), 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) = $52.87 \pm 8.32$ cm$^{-3}$ and $99.39 \pm 24.37$ cm$^{-3}$, respectively, in the inner disk region (r/R$_e$ $\leq$ 1.5), which decreases to $n_e$(S II) = $20.92 \pm 4.2$ cm$^{-3}$ in SFGs and $34.64 \pm 11.24$ 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 $N_e \sim 10^{22}$ cm$^{-2}$ at $\sim$14 kpc in the disk outer region. We have also discussed the profiles obtained using [O II] $\lambda\lambda$ 3726, 3729 $\r{A}$ doublet. These electron density estimates at different radii provide valuable insights for resolving ambiguities in current and future studies of magnetic fields in galaxies.