Probing ISM Models with H observations

H from the galactic DIG

The thick ( kpc) Reynolds layer of diffuse ionized gas (DIG) discovered in the Galaxy and in external ones poses some of the most challenging problems for our understanding of the large scale structure of the Galactic ISM (Reynolds 1995). One of the best ways to study this component is represented by spectroscopic observations of emission lines like , Å) and Å). The different excitation conditions found in a given galaxy as a function of height above the plane pose a relevant question concerning the amount of light originating in the disk (where the most obvious ionization sources are located) and light scattered back into the line of sight by dust. This aspect has been investigated recently by Ferrara etal. (1996), using Monte Carlo simulations to calculate the radiation transfer of line emission, produced both by HII regions in the disk and in the diffuse ionized gas (DIG), through the dust layer of the galaxy NGC891.

  
Figure 5: z-axis cuts through the center of NCG891 for (left column) luminosity profiles corresponding to contributions of HII regions and DIG component, (middle) ratio of the two components, and right) profiles of the linear polarization degree. Plots refer to the cases of x=0.7, 0.6, 0.5 (from top to bottom) where x is the ratio between luminosity from HII regions to the total (DIG + HII regions) one.

The amount of light originating in the HII regions of the disk and scattered by EGD can be then compared with the emission produced by recombinations in the DIG. The cuts of photometric and polarimetric maps along the z-axis show that scattered light from HII regions is still 10% of that of the DIG at  pc (Fig. 5), whereas the degree of linear polarization is small (<1%). This could explain the observed behavior of emission line ratios as a function of height (Ferrara etal. 1996).

© Copyright Astronomical Society of Australia 1997