The Vertical Structure of Warm Ionised Gas in the Milky Way (0808.2550v1)

Abstract: We present a new joint analysis of pulsar dispersion measures and diffuse H-alpha emission in the Milky Way, which we use to derive the density, pressure and filling factor of the thick disk component of the warm ionised medium (WIM) as a function of height above the Galactic disk. By excluding sightlines at low Galactic latitude that are contaminated by HII regions and spiral arms, we find that the exponential scale-height of free electrons in the diffuse WIM is 1830 (+120, -250) pc, a factor of two larger than has been derived in previous studies. The corresponding inconsistent scale heights for dispersion measure and emission measure imply that the vertical profiles of mass and pressure in the WIM are decoupled, and that the filling factor of WIM clouds is a geometric response to the competing environmental influences of thermal and non-thermal processes. Extrapolating the properties of the thick-disk WIM to mid-plane, we infer a volume-averaged electron density 0.014 +- 0.001 cm^-3, produced by clouds of typical electron density 0.34 +- 0.06 cm^-3 with a volume filling factor 0.04 +- 0.01. As one moves off the plane, the filling factor increases to a maximum of ~30% at a height of approximately 1-1.5 kpc, before then declining to accommodate the increasing presence of hot, coronal gas. Since models for the WIM with a ~1 kpc scale-height have been widely used to estimate distances to radio pulsars, our revised parameters suggest that the distances to many high-latitude pulsars have been substantially underestimated.

• The paper refines the vertical structure of the warm ionised medium by using pulsar dispersion measures and Hα emissions to determine an exponential scale height of approximately 1830 pc.
• It finds that the mid-plane electron density is 0.014 cm⁻³, with distinct densities within clouds averaging 0.34 cm⁻³ and a filling factor peaking around 30% at 1–1.5 kpc.
• The study implies that pulsar distance estimates using previous models like NE2001 are underestimated, urging a recalibration of the Galactic electron distribution.

Overview of The Vertical Structure of Warm Ionised Gas in the Milky Way

The paper conducted by Gaensler et al. offers a comprehensive analysis of the vertical distribution of the warm ionised medium (WIM) in the Milky Way. Utilizing pulsar dispersion measures and diffuse H$\alpha$ emissions, the research redefines previously established parameters for the density, pressure, and filling factor of the WIM, significantly impacting our understanding of Galactic structure and pulsar distance estimation.

Gaensler et al. present evidence for a revised exponential scale height of free electrons in the WIM, estimating it at $1830^{+120}_{-250}$ pc. This finding is pivotal as it indicates a scale height approximately double that previously accepted by the scientific community. By excluding sightlines below $|b| < 40^\circ$ to minimize contamination from high-density regions, the paper isolates the truly diffuse WIM, presenting a more precise vertical profile. Consequently, this adjusted scale height suggests the distances to high-latitude pulsars, standardly derived using models like NE2001, have likely been underestimated.

Their analysis further indicates that while the mid-plane volume-averaged electron density is $0.014\pm0.001$ cm$^{-3}$, the typical electron density within WIM clouds is $0.34\pm0.06$ cm$^{-3}$ with a volume filling factor of $0.04\pm0.01$. This nuanced understanding reflects a systematic variation in the electron density profile across different vertical heights, showcasing a geometric adaptation influenced by thermal and non-thermal processes.

Methodology and Results

The team refines the WIM model by filtering the pulsar data, enabling a robust estimation of scale height through a Levenberg-Marquardt fit. Their approach addresses the complex interplay between electron densities (both within and outside of clouds) and emission measures, thus identifying a decoupling between mass and pressure profiles in the WIM. Moreover, their findings highlight that the filling factor increases, reaching around 30% at $z =$ 1–1.5 kpc, before decreasing, indicating a transition towards the predominance of coronal gas.

These results diverge significantly from earlier models such as NE2001, which posited a thicker WIM at lower $z$ heights, thus providing a potentially richer volumetric analysis of the Milky Way’s electron landscape.

Implications and Future Directions

This paper holds substantial implications for the techniques used to estimate pulsar distances, as the underestimation of WIM scale height directly affects our understanding of pulsar distribution, luminosity, and space velocities. The acknowledgment of a larger-scale WIM model supports a need for recalibration in pulsar astrophysics, possibly leading to enhanced pulsar population models.

The findings also invite further exploration into the correlation between WIM properties and extragalactic observations, suggesting a wider application of these results to spiral galaxies exhibiting similar WIM characteristics. Future research can expand on these results by integrating wider surveys and advanced modeling techniques.

The displacement of WHAM to the southern hemisphere, alongside improved pulsar detection surveys, promises to refine our understanding of WIM distribution further. Moreover, incorporating these corrections into new models for the Galactic magnetic field and cosmic ray distribution will likely yield richer insights into the energetic dynamics between the Milky Way’s disk and halo.

In summary, Gaensler et al.’s work provides a refined structural model essential for appreciating the complexities of the Galactic ISM, setting the stage for future endeavors into the vast distributed characteristics of the Galactic halo and beyond.