Kaplan & Pikelner (1970) provide an excellent review of early knowledge of the interstellar medium (ISM). Good recent reviews of the nature of the ISM are given by Bland-Hawthorn & Reynolds (2000), and Ferrière (2001). Galactic geometry is reviewed by most astronomy texts (e.g. Zeilik & Gregory, 1998), and in brief the main Galactic disc of gas and stars is roughly circular, ~0.5 kpc thick, with a diameter of ~50 kpc.
The ISM consists mainly of molecular hydrogen (H2), neutral atomic hydrogen (HI), and ionized hydrogen (HII). Table 2.1, taken from Reynolds (2002), shows the hydrogen exists mainly in warm neutral (HI) and warm or hot ionised (HII) phases, and helium accounts for 30% of the ISM’s mass, and 9% of its number density.
Ionised regions constitute two-thirds by volume of the ISM, and radio wave scattering by this ISM component causes several phenomena in addition to scintillation, including dispersion, temporal broadening of pulsar signals, and angular broadening of radio images (e.g. Lyne & Graham-Smith, 1998). Measurements of these effects form the basis for modelling the large scale distribution of ISM free electron density. Figure 2.7 shows the results of the model developed by Cordes & Lazio (2006a; 2006b).
H2Clouds HI Clouds Warm HI Warm HII Hot HII
Temperature (K) 15 120 8,000 8,000 106
Midplane density (cm-3) 200 25 0.3 0.15 0.002
Thickness of layer (pc) 150 200 1,000 2,000 6,000
Volume fraction (%) 0.1 2 35 20 43
Mass fraction (%) 18 30 30 20 2
Table 2.1 ISM components (after Reynolds, 2002).
Figure 2.7 Model distribution of the Galaxy’s ISM free electron density on a 30 x 30 kpc plane. The light patches are local ISM features, whose lower than average density enables the model to predict certain pulsar distances that are known independently. From Cordes & Lazio (2006a).
The average vertical electron density, ne, away from the Galactic Bulge can be described as (Reynolds, 1991): 3 cm pc 70 exp 015 . 0 pc 900 exp 025 . 0 z z ne
where the 900 pc and 70 pc scale heights account for electron density contributions from the diffuse warm HII,and the bulk effect of discrete hot HIIregions respectively. The equation is only a bulk description of the vertical electron density structure, and it is not valid near the Sun, which resides in a region of hot (106K) HIIgas known as the Local Bubble (e.g. Maíz- Apellániz, 2001). The Local ISM consists of the Local Bubble and its surrounding region, sometimes termed the Local Cavity, and soft X-ray data indicate it extends 50-100 pc in the Galactic Plane, and 200-300 pc perpendicular to the Plane (e.g. Snowden et al., 1998). The Sun is located outside the Galactic Bulge at z15 pc above the Galactic Plane, for which the above equation predicts ne 0.037 cm-3. The electron density distribution in the Local ISM is not well known, (Bhat et al., 1998), but the mean electron density is believed to be
ne 0.005 cm-3(Ferrière, 2001). However, patchiness probably characterises much of the Galaxy’s ISM, and is not likely to be the purely local effect suggested by Figure 2.7.
Cordes & Lazio (2006a; 2006b) incorporated four distinct local ISM features into their electron density model, as shown in Figure 2.8, in which the Sun is located at {X,Y} = {0 8.5} kpc. The features are a local hot bubble (LHB), the “Loop 1” component, a local superbubble (LSB), and a low density region (LDR). The Gum Nebula and Vela pulsar are modelled as spherical regions of enhanced electron density.
Figure 2.8 Features of the Local Bubble in the Cordes & Lazio (2006b) model. See
The distribution of neutral gas density in the Local ISM is better known than its electron density distribution, through surveys of NaI absorption. Figure 2.9 from Maíz-Apellániz (2001) shows a Galactic Plane map of neutral gas density within 150pc of the Sun (Figure 2.8 only shows the region within 1 kpc of the Sun). NaI absorption is mainly associated with cold (< 1,000 K) regions, so Figure 2.9 maps a different component of the ISM from that mapped by electron density distributions. In Figure 2.9, the darker shading denotes high gas density areas, while the red circle defines the region within 100 pc of the Sun. The hot Local Bubble’s extent is indicated by the solid / dashed line, dashed where uncertain. The local ISM is of central importance to understanding AGN radio signal variability due to interstellar scintillation. As discussed in Section 2.4, scattering material is usually expected to be within 200 pc of the Sun, and often within 50 pc. Fuhrmann et al. (2002) discuss attempts to detect evidence of scattering material in the local ISM along lines of sight to AGNs in which radio signal variability is observed. They report the discovery of molecular clouds 100 pc distant, very close to the lines of sight to the scintillators 0917+624 and 0954+658. They suggest that, if these clouds are associated with enveloping shells of ionized material, they are attractive candidates for the sites of scattering material.
Figure 2.9 Distribution of neutral gas density in the Local ISM. Dark shading
indicates high density gas. The red circle defines the region within 100 pc of the Sun (Maíz-Apellániz, 2001).
There is every reason to expect the local ISM to have plasma features able to cause radio signal scintillation. The Local Bubble is believed to be the result of several supernova explosions within the last 10 million years (Cox, 1998; Maíz-Apellániz, 2001), and it is a widely believed that supernovae cause ISM turbulence. In addition to shock waves, plasma structures may be produced by turbulent wakes from molecular clouds (Norman & Ferrara, 1996), and by elongation of plasma along magnetic field lines (Cordes & Lazio, 2001). Blazar radio variability due to interstellar scintillation has the potential to probe the local ISM’s electron density microstructure, with rapid scintillators associated with closer ISM scattering screens. Scintillation effects associated with pulsar radio variability are subtlety different, since pulsars are effectively point sources, while AGNs are extended sources. Bhat et al. (1998) examined the scintillation of 20 pulsars, and found scattering effects that they suggested may be explained by the solar neighbourhood being surrounded by a shell of much higher electron density fluctuations, embedded in the large scale ISM.