Unveiling Electron Density Profile in Nearby Galaxies using SDSS MaNGA

TL;DR

This paper tackles the uncertainty in the spatial distribution of thermal electron densities ($n_e$) in galactic disks, a key factor for interpreting Faraday-rotation measurements of magnetic fields. It maps $n_e$ using collisionally excited [S II] 6716/6731 and [O II] 3726/3729 line ratios from SDSS MaNGA IFU data, analyzed with MaNGA DAP and pyPipe3D pipelines across 66 face-on galaxies (46 SFGs, 20 Non-SFGs). The authors find clear radial gradients: for SFGs, inner disk densities are around $n_e\approx52.9$ cm$^{-3}$ and drop to $\approx20.9$ cm$^{-3}$ in the outer disk; for Non-SFGs, inner densities are higher, around $n_e\approx99.4$ cm$^{-3}$, falling to $\approx34.6$ cm$^{-3}$ outward. Translating $n_e$ into electron column densities with a 1 kpc disk thickness yields $N_e$ values near $10^{22}$ cm$^{-2}$ at $\sim14$ kpc, highlighting substantial radial variation and informing RM-based magnetic-field studies; the work also compares S II with O II diagnostics and discusses pipeline-related systematics. These spatially resolved $n_e$ maps provide critical inputs for resolving ambiguities in current and future magnetic-field analyses in galaxies.

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] $λλ$ 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 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] $λλ$ 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.

Figures (12)

• Figure 1: Upper Panel: The figure presents the spatial distribution of DAP fluxes s$_1$ (left) and s$_2$(middle), along with the electron density $n_e$ (right), derived from s$_1$ and s$_2$, for the galaxy with "MaNGA ID 1-606221". Lower Panel: Similarly, the left and middle panel displays the spatial maps of fluxes o$_1$ and o$_2$, respectively, while the right panel shows the corresponding $n_e$, obtained using o$_1$ and o$_2$. The yellow annuli on both electron density maps represent the linear binning scheme.
• Figure 2: Left Panel: The figure illustrates the results from linear binning applied to individual galaxies, using DAP data. The left panel depicts scatter plots of $n_e$(S ii) for SFGs (blue dots) and Non-SFGs (brown crosses). Each data point corresponds to the $\bar{n}_{e_w}$ computed across different bins within individual galaxies. Right Panel: Similarly, the right panel represent the scatter plots for the case of non-linear binning.
• Figure 3: DAP Profiles:-Left panel: The figure shows the radial profiles of $n_e$(S ii) for SFGs. The data points represent the $\langle {n}_{e_{w^{\prime}}} \rangle$ obtained by binning the scatter plots in Fig. \ref{['fig:figure_2']} using the volume weight of each galaxy. Red dots and black diamonds depict the linear and non-linear binning cases, respectively. Right panel: Similarly, the right panel illustrate the corresponding radial profiles for Non-SFGs. The grey vertical lines in each subplot represent the demarcation at r/R$_e$=1.5. [Note: The linear and non-linear binning has been implemented on the $n_e$ maps of each galaxy, not on the scatter plots in Fig. \ref{['fig:figure_2']}.]
• Figure 4: Left Panel: The figure illustrates the results from linear binning applied to individual galaxies, using Pipe3D data. The left panel depict scatter plots of $n_e$(S ii) for SFGs (blue dots) and Non-SFGs (brown crosses). Each data point corresponds to the $\bar{n}_{e_w}$ computed across different bins within individual galaxies. Right Panel: Similarly, the right panel represent the scatter plots for the case of non-linear binning.
• Figure 5: Pipe3D Profiles:-Left panel: The figure shows the radial profiles of $n_e$(S ii) for SFGs. The data points represent the $\langle {n}_{e_{w^{\prime}}} \rangle$ obtained by binning the scatter plots in Fig. \ref{['fig:figure_5']} using the volume weight of each galaxy. Red dots and black diamonds depict the linear and non-linear binning cases, respectively. Right panel: Similarly, the right panel illustrate the corresponding radial profiles for Non-SFGs. The grey vertical lines in each subplot represent the demarcation at r/R$_e$=1.5. [Note: The linear and non-linear binning has been implemented on the $n_e$ maps of each galaxy, not on the scatter plots in Fig. \ref{['fig:figure_5']}].