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Subsections

4 Statistical properties of the Synthetic Catalog

The Synthetic Catalog combines the observational data from 24 catalogs with various flux density limits and different angular resolutions. The Kuchar $\&$ Clark (1997) catalog, for example, reaches to a sensitivity of 30 mJy but is not sensitive to sources with diameters greater than $\sim $10$^{\prime}$ due to its beamswitching strategy.

Other surveys (e.g. Altenhoff et al. 1970) are sensitive to a wide range of angular scales but include only sources stronger than 1 Jy. We now consider the properties of the sources listed in the Synthetic Catalog in the light of these selection criteria.

4.1 Completeness and flux density limit

A useful way of investigating the completeness of a catalog is to plot the integral count N(>S) as a function of flux density S. Figure 5 shows the integral count for the 1442 sources included in the Synthetic Catalog and for the five contributing catalogs containing more than 200 HII regions. It is evident that the Synthetic Catalog is losing sources fainter than $\sim $1 Jy. The data in the Synthetic Catalog for S<1 Jy are mainly contributed by the Kuchar $\&$ Clark (1997) catalog. Some 50% of the sources at 1 Jy are contributed by this catalog, with a higher fraction at lower flux densities.

The absence of sources at lower flux densities is largely the consequence of source confusion along the Galactic plane. For example, in the region | l $~\vert ~ < ~ 60^\circ$ there are 1200 Synthetic Catalog sources within the Galactic plane (| b $~\vert ~ < ~ 2^\circ$); in a 10$^{\prime}$ beam (or a 10$^{\prime}$ diameter source) there is one HII region per 15 beam area which represents a significant confusion level.

It is clearly difficult to resolve into individual sources the structure seen in the central regions of the Galaxy. The majority, $\sim $150, of the weaker (< 1 Jy) sources catalogued by Kuchar $\&$ Clark are in the less complex regions of the Galaxy such as the anticentre region (quadrants 2 and 3) - 73 sources - and Galactic latitudes greater than 1$^\circ $ (| b $~\vert ~ > ~ 1^\circ$) in quadrants 1 and 4 - 19 sources -.

  \begin{figure}
\par\includegraphics[width=6.8cm,height=5.5cm]{aah3866f5.eps}\end{figure} Figure 5: Cumulative counts, N(>S), for the Synthetic Catalog (solid line) and 5 contributing catalogs: Kuchar $\&$ Clark (1997) (dash-dot line), early Parkes 2.7 GHz catalogs (dashed line), Caswell $\&$ Haynes (1987) (long-dash line), Altenhoff et al. (1970) (dotted line), Altenhoff et al. (1979) (three dots-dashes).


  \begin{figure}
\par\includegraphics[width=6.7cm,height=5cm, clip]{aah3866f6a.eps}\par\includegraphics[width=6.5cm,height=5cm, clip]{aah3866f6b.eps}\end{figure} Figure 6: Top panel: Galactic latitude distribution of the HII regions of the Synthetic catalog. The mean Galactic latitude is is ${b}=0.05^\circ $. Bottom panel: Galactic longitude distribution.

4.2 Spatial distribution

The Galactic latitude and longitude distribution of the HII regions in the Synthetic Catalog are shown in Fig. 6. A striking feature is the narrow distribution in Galactic latitude where the full width at half power is 0.95$^\circ $ when averaged over all longitudes; the mean Galactic latitude is ${b}=0.05^\circ $. This narrow distribution will reflect the distribution of the O and B stars responsible for the ionization.

The main concentration in longitude is at | ${\it l}$ | < 60$^\circ $ where the line of sight cuts the spiral arms internal to the local arm in which the Sun lies. There are also discernable peaks at ${\it l}\sim\pm 80^\circ$ associated with the local arm; the peak at ${\it l}\sim 80^\circ$ is the Cygnus X region where Wendker (1970) identified 77 HII regions in his survey. There is a clear deficit of sources in the anticentre region of the Galaxy ( ${l}=90^\circ$-270$^\circ $). Although there is a less complete coverage in this sector of the Galactic plane as indicated in Table 1, the large deficit of sources is real. The optical study of the distribution of HII regions in the outer Galaxy by Fich $\&$ Blitz (1984) indicate that they are limited to R < 20 Kpc.


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