The Extension of the Transition Temperature Plasma into the Lower Galactic Halo

Abstract

Column densities for H I, Al III, Si IV, C IV, and O VI toward 109 stars and 30 extragalactic objects have been assembled to study the extensions of these species away from the Galactic plane into the Galactic halo. H I and Al III mostly trace the warm neutral and warm ionized medium, respectively, while Si IV, C IV and O VI trace a combination of warm photoionized and collisionally ionized plasmas. The much larger object sample compared to previous studies allows us to consider and correct for the effects of the sample bias that has affected earlier but smaller surveys of the gas distributions. We find Si IV and C IV have similar exponential scale heights of 3.2(+1.0, -0.6) and 3.6(+1.0, -0.8) kpc. The scale height of O VI is marginally smaller with h = 2.6±0.6 kpc. The transition temperature gas is ~3 times more extended than the warm ionized medium traced by Al III with h = 0.90(+0.62, -0.33) kpc and ~12 times more extended than the warm neutral medium traced by H I with h = 0.24 ±0.06 kpc. There is a factor of 2 decrease in the dispersion of the log of the column density ratios for transition temperature gas for lines of sight in the Galactic disk compared to extragalactic lines of sight through the entire halo. The observations are compared to the predictions of the various models for the production of the transition temperature gas in the halo.

A disk-dominated and clumpy circumgalactic medium of the Milky Way seen in X-ray emission

“…The midplane density of the disc-like component at the solar circle is (1.3 − 2.7) × 10 −3 cm −3 , which is similar to values found in magnetohydrodynamic simulations of stellar feedback at the solar circle 29 . The exponential scale height of 1.6-2.5 kpc is similar to values measured for the soft X-ray profiles of edge-on spiral galaxies 2 , but smaller than the scale height of transition temperature gas 20 . For a scale height of 2 kpc at |b| = 30 • , the ∼ 6 • angular scale of the EM fluctuations corresponds to a spatial scale of ∼ 400 pc, comparable to the size of superbubbles found within the Milky Way.…”

“…Two different fitting techniques were used to account for the large intrinsic EM variations. The first is use of a 'patchiness parameter' originally introduced by 45 that has been used previously in the literature for similar anlyses 20,46 . The intrinsic scatter is modeled as the patchiness parameter (σ p ), which is combined with the measurement uncertainty (σ m ) to obtain the uncertainty adopted in the model fitting σ 2 f = σ 2 p + σ 2 m .…”

“…We estimated the EM for each line of sight for each model by integrating 260 kpc along the line of sight in 10 4 steps. To account for intrinsic EM variations, a 'patchiness parameter' (σ p ) is combined with the measurement uncertainty (σ m ) to obtain the uncertainty adopted in the model fitting 20 , σ 2 f = σ 2 p + σ 2 m . The patchiness parameter was fixed to σ p = 3.4 × 10 −3 cm −6 pc to give a reduced χ 2 ≈ 1 for our final model fit.…”

New constraints on modelling the random magnetic field of the MW

“…The use of the NE2001 model is still under debate. Besides the study of [32] another study of [74] suggests a modification of the scale height, namely 1.4 kpc, and a corresponding mid-plane density of about 0.016 cm −3 . [75] revisit the results of their 2008 GMF model with improvements to NE2001, i.e.…”