The Warm Ionized Gas Thin Layer

Among the phases of the interstellar gas, the warm ionized is the least known. Its spatial distribution is characterized by a thin and a thick layer where the former corresponds to the ionized gas in the proximity of the plane and the latter to the gas which is found at larger distances from the plane. The thin layer comprises discrete HII regions and diffuse gas. Despite the important role assigned to the thin layer by current theoretical models, so far our knowledge of it has been based either on oversimplified analytical calculations or on the scattered observational information coming from pulsar dispersion and scattering measures which largely suffer from undersampling. In this Thesis, after an overview of the current knowledge of the Galactic interstellar medium and of its constituents, and particularly of the warm ionized medium, we focus on our main goal which is to contribute to build a realistic picture of the thin layer by exploiting the information widely spread in the literature. This work has proceeded along two main lines: the systematic investigation of the thin-layer source-component, i.e. HII regions, and the study of the interplay between this source-component and the diffuse gas. Collecting radio data on Galactic HII regions from 24 published works, we have built a self-consistent data base comprising 1442 sources. This work has resulted in the construction of a Master Catalog (which consists in 11 sub-catalogs storing the original information from the source references) and of a Synthetic Catalog (which is a readily accessible version of the Master Catalog at 2. 7 GHz). This represents the most extensive list of compact HII regions to date. We have then exploited this data-base to investigate the distribution of the warm ionized gas, as well as for applications in the field of Cosmic Microwave Background studies (calibration, beam-shape and pointing reconstruction, stray light, understanding of the Galactic foregrounds). The kinematic information contained in the catalog has allowed us to study the spatial distribution of 550 objects taken from the Synthetic Catalog. For each source, a galactocentric distance has been derived using the Fich et al. [106] rotation model. vVe have found a highly significant correlation between luminosities and linear diameters, which was exploited to resolve the solar-distance degeneracy. vVe have then been able to estimate the scale height of the HII region distribution. Within the solar circle, we have obtained an azimuthally-averaged thickness, Omega_z around 52 pc which is comparable to the estimated scale height of the OB star distribution but narrower than that of the diffuse HII and HI. The analysis of the z-clistribution has retrieved other important results: a confirmation of the presence of the warp; evidence of an increasing width of the distribution as a function of the galactocentric radius; a confirmation of the electron temperature gradient with galactocentric radius; a spiral-like structure in agreement with the model by Taylor & Cordes [311]. The relationship between HII regions and the diffuse gas in the thin layer has been addressed by considering data at 408 MHz (Haslam et al. [150]) and at 5 GHz (Haynes et al. [151]) for the region of the Galactic Plane in the coordinate range 20° < l < 30°, -1.5° < b < +1.5°. After adjusting the zero levels of the two surveys, we have applied a component separation technique to the observed radio continuum by exploiting the spectral dependence of the two kinds of emissions which overlap at these frequencies, namely free-free and synchrotron radiation. The synchrotron spectral index has been kept fixed during this phase of the analysis. The distribution of spectral indices for the total Galactic emission has been recovered across the considered region. Evidence of a flattening of the spectral index in proximity of the plane has been found. This has been interpreted as the effect of intense thermal emission due to HII regions. A comparison between the latitude distribution of the free-free and that of the HII regions located in this coordinate range has shown clearly the presence of diffuse gas in the thin layer. A preliminary estimate of the contribution of HII regions to the total emission budget has yielded a value of order of 11 % which, again, supports the hypothesis of a dominant role of the diffuse ionized gas in the thin layer. Finally, we want to point out that the analysis we have described has relied on radio data which, so far, have found limited applications in this context. The continuum free-free emission has long been considered (as in the case of experiments dedicated to the Cosmic Microwave Radiation) as a source of disturbance, rather than a source of information or, in other words, as a foreground to remove. This is mostly due to the fact that this emission is partly masked by intense synchrotron radiation produced by relativistic electrons accelerated in the Galactic magnetic field. However such data, when properly handled, have the unique advantage with respect to other tracers of providing access to the distribution of Galactic warm ionized gas without suffering from undersampling or extinction. The same advantages also characterize radio recombination lines. So far, due to the limited instrumental capabilities, it was not possible to exploit fully the wealth of information contained in these data. Yet, the next generation of experiments will open up a new window on the warm ionized gas, allowing its systematic, unbiased exploration throughout the Galaxy.