The Vertical Structure of Warm Ionised Gas in the Milky Way

By B. Gaensler, G. Madsen, S. Chatterjee, S. Sydney, U. Wisconsin, Madison, H. CfA. Publications Astronomical Society of Australia, 2008

225 citations recorded by the source corpus.

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 1830–250 +120 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.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/89b9dd105426b891ccdec314a240e9f79a385cc6

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Citations in the corpus

Galactic synchrotron emission with cosmic ray propagation models (2013, methodology): “The thermal electron distribution is usually based on the NE2001 model (Cordes & Lazio 2002), which is known be an inadequate approximation (Gaensler et al. 2008).”
A NEW ELECTRON-DENSITY MODEL FOR ESTIMATION OF PULSAR AND FRB DISTANCES (2016, methodology): “For NE2001 and TC93, the rms deviations mp are about 60% and 89% respectively, corresponding to 95% confidence limits of approximately 120% and 180% for the two models.”
The H i Distribution of the Milky Way (2009, background): “This value matches, within the uncertainties, the expectations for a WIM scale height of hz(R ) ∼ 2 kpc, close to the most recent determination of hz(R ) = 1830+120−250 pc by Gaensler et al. (2008).”
The Sydney-AAO Multi-object Integral field spectrograph (2011, methodology): “The Reynolds Layer in our Galaxy, recently mapped by the WHAM Hα survey telescope (Madsen, Haffner & Reynolds 2006), is a good example of this phenomenon (see also Gaensler et al. 2008).”
STARS, GAS, AND DARK MATTER IN THE SOLAR NEIGHBORHOOD (2015, background): “Subsequently, Gaensler et al. (2008) used pulsars with known distances to show that the scale height of the ionized gas is considerably larger, about 1.8 kpc. Recently, Schnitzeler (2012) has reviewed these and other models for the electron distribution and concluded that the model that best fits…”
Probing Galactic Halos with Fast Radio Bursts (2019, methodology): “To estimate the contribution from the Galactic halo, one then requires an estimate to DMLMC from the Galactic ISM.”
Cosmological Magnetic Fields: Their Generation, Evolution and Observation (2013, methodology): “Taking into account the remaining large uncertainties in modeling the free electron distribution (Cordes and Lazio 2002; Gaensler et al 2008), as well as in the degeneracy of model parameters of the Galactic magnetic field (Jansson and Farrar 2012a,b; Pshirkov et al 2011), it is not clear whether…”
CHIME/FRB Discovery of Eight New Repeating Fast Radio Burst Sources (2019, methodology): “Using the work of Gaensler et al. (2008), we adopt a lower limit f >∼ 0.04 since f is thought to increase with distance from the Plane.”
ENVIRONMENTAL DEPENDENCE OF THE KENNICUTT–SCHMIDT RELATION IN GALAXIES (2010, background): “…ionized gas is present both inside the disk and at large distances (up to ∼ 2 − 4 kpc) from the midplane both in the Milky Way (Reynolds 1989, 1991; Gaensler et al. 2008) and other nearby galaxies (e.g., Hoopes et al. 1999; Collins et al. 2000; Rossa & Dettmar 2003, see Haffner et al. 2009…”
DEVELOPMENT OF THE MODEL OF GALACTIC INTERSTELLAR EMISSION FOR STANDARD POINT-SOURCE ANALYSIS OF FERMI LARGE AREA TELESCOPE DATA (2016, methodology): “Based on Gaensler et al. (2008), we used an exponential scale height of 1 kpc to build a simple +NH map which also failed to improve the fit to the LAT data.”
THE GALACTIC POSITION DEPENDENCE OF FAST RADIO BURSTS AND THE DISCOVERY OF FRB011025 (2014, background): “Subsequent publications have argued that the scale height of the “thick disk” in ne2001 should be larger (Gaensler et al. 2008; Schnitzeler 2012).”
REGULATION OF STAR FORMATION RATES IN MULTIPHASE GALACTIC DISKS: A THERMAL/DYNAMICAL EQUILIBRIUM MODEL (2010, unknown): “We have also neglected the contribution from diffuse warm ionized gas, which has a low mean density and large scale height compared to that of the neutral gas (e.g. Gaensler et al. 2008).”
H i IN LOCAL GROUP DWARF GALAXIES AND STRIPPING BY THE GALACTIC HALO (2009, background): “…the distance limit where the dwarfs have HI, or ∼270 kpc. Observations indicate densities on the order of 10−3 cm−3 are extremely unlikely at this distance (Gaensler et al. 2008; Sembach et al. 2003; Putman et al. 2004; Peek et al. 2007), as do simulations and calculations of the hot halo density…”
THE FIRST FERMI LAT SUPERNOVA REMNANT CATALOG (2015, background): “…to allow for possible CR spectral 14http://fermi.gsfc.nasa.gov/ssc/data/access/lat/Model_details/Pass7_galactic.html 15Using the H II model of Gaensler et al. (2008a). variations among the annuli for all H I and CO maps and to allow for spectral variations in the electron distribution for…”
A Nearby Repeating Fast Radio Burst in the Direction of M81 (2021, methodology): “To estimate the WIM-dominated Milky Way disk DM, we used a range of free electron density (Ne) values of the WIM from the literature (Reynolds et al. 1995; Reynolds 2004; Gaensler et al. 2008; Velusamy et al. 2012; Ocker et al. 2020), Ne≈ 0.2–0.9 cm −3, and computed DM= EM/Ne∼ 10–40 pc cm −3.”
Constraints on the distribution and energetics of fast radio bursts using cosmological hydrodynamic simulations (2014, methodology): “We note that the NE2001 model is known not to give reliable estimates for pulsar distances or DMs at high Galactic latitudes,|b| & 40◦, as discussed extensively by Gaensler et al. (2008).”
The warm ionized medium in spiral galaxies (2009, background): “Data also suggest that the filling fraction increases from f ∼ 0.1 at the midplane to f > 0.3– 0.4 at |z| = 1000 pc (Berkhuijsen et al., 2006; Gaensler et al., 2008; Kulkarni and Heiles, 1987; Reynolds, 1991b).”
DERIVING THE GLOBAL STRUCTURE OF THE GALACTIC MAGNETIC FIELD FROM FARADAY ROTATION MEASURES OF EXTRAGALACTIC SOURCES (2011, background): “Our fits favor the increased vertical scale of the electron density distribution as proposed by Gaensler et al. (2008).”