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
10
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.
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 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
there are 1200 Synthetic Catalog sources within the Galactic plane
(| b
);
in a 10
beam (or a 10
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, 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
(| b
)
in quadrants 1 and 4 - 19 sources -.
![]() |
Figure 5:
Cumulative counts, N(>S), for the Synthetic Catalog
(solid line) and 5 contributing catalogs: Kuchar ![]() ![]() |
![]() |
Figure 6:
Top panel: Galactic latitude distribution
of the HII regions of the Synthetic catalog.
The mean Galactic latitude is is
![]() |
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
when averaged over all longitudes; the mean Galactic latitude
is
.
This narrow distribution will reflect the distribution
of the O and B stars responsible for the ionization.
The main concentration in longitude is at |
| < 60
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
associated with the local
arm; the peak at
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 (
-270
). 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.
Copyright ESO 2003