The Vertical Structure of Warm Ionised Gas in the Milky Way

Abstract

We present a new joint analysis of pulsar dispersion measures and diffuse Hα 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 ≈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.

A New Electron-Density Model for Estimation of Pulsar and FRB Distances

“…This is just over half of the perpendicular DM for the NE2001 model, but comparable to those for the TC93, Gaensler et al (2008), and Savage & Wakker (2009) models. Correspondingly, the YMW16 mid-plane density is about one third the NE2001 value and the scale height is about 70% larger than the NE2001 value.…”

“…Further evidence came from observations of diffuse Hα and other recombination lines (Reynolds et al 1973;Haffner et al 1999;Madsen et al 2006), pulsar DMs (Manchester & Taylor 1981;Nordgren et al 1992;Cordes & Lazio 2002;Gaensler et al 2008;Savage & Wakker 2009;Schnitzeler 2012), and interstellar scattering (Readhead & Duffett-Smith 1975;Cordes et al 1991;Cordes & Lazio 2002), all suggesting mid-plane electron densities of 0.02-0.03 cm −3 and scale heights of ∼1000 pc with no strong dependence of either mid-plane n e or scale height on Galactocentric radius out to the edge of the Galactic disk. Haffner et al (1999) used observations of diffuse Hα from the Perseus arm region, about 2.5 kpc from the Sun at Galactic longitudes  125 - 150 , corresponding toR 10.5 kpc, to estimate a scale height H n e for the WIM in this region of 1000±100 pc.…”

“…The lines show the variation of DM^for the thick-disk component of several different models of the Galactic n e distribution. For SW09 (Savage & Wakker 2009) and GM08 (Gaensler et al 2008), the line is based on fits of just the thick disk to DMs and distances for high-latitude pulsars, but for TC93, NE2001, and YMW16, the line represents the thick-disk component resulting from a global fit to the distance data. The thick-disk DM components for the YMW16 model are shown by open circles for the high-latitude pulsars.…”

“…For the TC93, N2001, Gaensler et al (2008), and Savage & Wakker (2009) fits, DM^is, respectively, 16.5 cm −3 pc, 33.0 cm −3 pc, 25.6 cm −3 pc, and 21.9 cm −3 pc. Gaensler et al (2008) also estimated a mid-plane volume filling factor ( f v ) for the thick-disk electrons of 0.014, rising to around 0.3 atz 1000 pc. 8 Based on accurate parallax measurements using the VLBA, Chatterjee et al (2009) also showed that the NE2001 model underpredicts distances for several high-latitude pulsars by a factor of two, but also overpredicts distances for several relatively local pulsars.…”

“…To address this, Gaensler et al (2008) product is equal to the "perpendicular DM" (DM^), i.e., the DM integrated to infinity perpendicular to the Galactic disk. For the TC93, N2001, Gaensler et al (2008), and Savage & Wakker (2009) fits, DM^is, respectively, 16.5 cm −3 pc, 33.0 cm −3 pc, 25.6 cm −3 pc, and 21.9 cm −3 pc. Gaensler et al (2008) also estimated a mid-plane volume filling factor ( f v ) for the thick-disk electrons of 0.014, rising to around 0.3 atz 1000 pc.…”