Electron Density Structure of the Local Galactic Disk

Abstract

Pulsar dispersion measures (DMs) have been used to model the electron density of the interstellar medium (ISM) in the Galactic disk as a plane-parallel medium, despite significant scatter in the DM-distance distribution and strong evidence for inhomogeneities in the ISM. We use a sample of pulsars with independent distance measurements to evaluate a model of the local ISM in the thick disk of the Galaxy that incorporates turbulent fluctuations, clumps, and voids in the electron density. The latter two components are required because $\sim 1/3$ of the lines of sight are discrepant from a strictly plane parallel model. A likelihood analysis for smooth components of the model yields a scale height $z_0=1.57_{-0.14}^{+0.15}$ kpc and a mid-plane density $n_0=0.015\pm 0.001$ cm${}^{-3}$. The scatter in the DM-distance distribution is dominated by clumps and voids but receives significant contributions from a broad spectrum of density fluctuations, such as a Kolmogorov spectrum. The model is used to identify lines of sight with outlier values of DM. Three of these pulsars, J1614$-$2230, J1623$-$0908, and J1643$-$1224, lie behind known HII regions, and the electron density model is combined with H$\alpha$ intensity data to constrain the filling factors and other substructure properties of the HII regions (Sh 2-7 and Sh 2-27). Several pulsars also exhibit enhanced DM fluctuations that are likely caused by their lines of sight intersecting the superbubble GSH 238+00+09.

17 pages, 4 figures, 3 tables, accepted to ApJ

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