The final task in bringing together the wide-spread radio data on HII regions is to construct a readily accessible catalog summarizing the basic information on each of the 1442 sources covered in the comprehensive data base of the Master Catalog. This Synthetic Catalog gives the best available data on flux density, angular diameter and, where available, the line velocity.
2.7 GHz was chosen as the base frequency for the Synthetic Catalog; it lies in the middle of the frequency range of those catalogs containing most of the data, namely 1.4, 2.7 and 5 GHz. Since there is not complete source coverage at any one frequency, we derive the flux density at 2.7 GHz from the observed frequencies for all the sources. We now describe how best-estimate values of the flux density, diameter and velocity with corresponding error estimates are derived from the Master Catalog.
We estimate the flux density at 2.7 GHz in four different ways:
We consider the data at 3.9, 14.7, 15 and 86 GHz less reliable for the following reasons: the 3.9 GHz measure may be significantly affected by the ellipticity of the antenna pattern; at 14.7 and 15 GHz the observing resolution is much higher than the typical reference catalogs resolution; at 86 GHz, in addition to the higher resolution, dust emission may contribute largely to the observed flux. Moreover, we prefer to interpolate between 1.4 and 5 GHz rather than extrapolate to 2.7 GHz directly from only one of these two frequencies because we have no information on the frequency location of the free-free knee.
In cases when multiple observations are available at the same frequency, a weighted mean flux density is computed, using the errors discussed below.
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Figure 1:
Results of the flux error estimate for: earlier Parkes survey at 2.7 GHz
(top left); Mezger ![]() ![]() ![]() |
For the majority of the
sources in the reference
catalogs, flux density errors are quoted and these are used in
deriving the flux density and its error in the Synthetic Catalog.
In the catalogs where no errors are given, an estimate of the error
is made by comparing the flux S* from a catalog without errors with
the flux S from a catalog (or some catalogs) giving errors.
In particular, we compute the relative (%) dispersion,
,
and try to fit the resulting distribution
in the
plane with
a constant,
, a linear,
,
and a quadratic,
,
dependence of
on S*.
In these equations the fluxes are in Jy.
Before fitting the
distribution, we remove the relatively small number of points
with a dispersion
130%.
Although we have considered also fits with
a constant, linear, and quadratic
dependence of
on
,
we prefer to use
the distributions recovered from the fit carried out
with linear variables, since they
give, as expected, typical values of
larger than
those obtained in the case of logarithmic variables and
so giving more conservative error estimates. Figure 1 shows the results
of the procedure we have described.
An example of the estimates of errors is given by the early Parkes
2.7 GHz catalogs. We compare the list of Parkes sources
to the reference catalogs at the same frequency - 2.7 GHz -
(namely, Altenhoff et al. 1970;
F
rst et al. 1987; Reich et al. 1986; Wendker 1970),
in order to retrieve the subset of Parkes sources whose flux
has been quoted with an estimated error. In particular, the comparison
retrieves 105 sources in common with the Altenhoff et al. (1970) catalog.
The scatter
in the flux density differences, quite well fitted by the law
,
is comparable to the errors of 10-30% of the
Altenhoff et al. catalog alone.
Similar comparisons have been carried out for the Mezger
Henderson (1967) data and for Wink et al. (1983) data.
However, as for the Mezger
Henderson data, the comparison
with other single catalogs does not provide a significant
statistical subset of sources. Therefore, we estimate the error
by considering simultaneously the sources from the Mezger
Henderson
catalog and from all the other catalogs at 5 GHz (which leads to
consider altogether 38 common sources).
For the Wink et al. (1983) catalog, we derive an error estimate
from the comparison with the Wink et al. (1982) catalog for
which we have 21 common sources.
After having estimated the errors on the flux density measures for all catalogs,
we compute the errors on the flux densities of the Synthetic Catalog at 2.7 GHz.
When an extrapolation with
a spectral index
from a measure at a single frequency different
from 2.7 GHz is applied, we use the standard error propagation rules.
No error on
is included in the error propagation,
for simplicity.
Table 3 summarizes the errors quoted in the original catalogs and those estimated in this work.
We discuss here the assignment of an angular diameter with its
associated error for every source in the Synthetic Catalog. The
published diameter data are not as comprehensive as the flux
density data described in Sect. 3.1; nevertheless we will assign
a value of the diameter and an error for each source. For 42%
of the sources (derived from the Parkes catalogs and those of
Mezger
Henderson 1967; Wendker 1970; Wink et al. 1982;
Wink et al. 1983; Berlin et al. 1985) there are no listed
diameter errors. For 14% of the sources no diameter is
given; for these we derive an indicative diameter.
We assign a diameter for sources in the Synthetic Catalog as follows:
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Figure 2: Correlation between flux densities at 2.7 GHz and angular diameters observed at the same frequency. The solid line represents the best-fit for the plotted distribution. |
Values at 14.7 and 15 GHz are preferably excluded because of the much higher resolution at these frequencies than the typical reference catalog resolution. In each case, when multiple observations are available at the same frequency, a weighted mean is computed, using the errors discussed below.
Finally, we consider the assignment of a diameter to the
14% of HII regions in the Synthetic Catalog which have
no quoted diameter in the reference catalogs.
It is possible to derive a first-order indicative
diameter by noting that the flux density and the diameter of HII regions are
weakly correlated, as shown in Fig. 2 which includes
all the sources at 2.7 GHz with measured diameters.
The best fit to the data gives
as indicative diameter
with a typical error
of a factor of 2-3. Each source with an indicative diameter is
annotated in the Synthetic Catalog; such diameter data clearly should not
be used in astrophysical analysis of the catalog.
Following the same strategy as in Sect. 3.1,
where a catalog has no quoted diameter errors we estimate
an overall error for that catalog by comparing
the observed diameters of that catalog with another
catalog having an adequate number of sources in common.
However, with respect to the flux density case, we can now
relax the strict requirement of comparing only different datasets
at the same frequency.
As shown in Fig. 3, there is no significant frequency dependence in the
measured diameters at 2.7 GHz (the Parkes survey)
and at 5 GHz (Altenhoff et al. 1970; Mezger
Henderson 1967;
Reifenstein et al. 1970).
On the other hand, provided that an adequate number of sources in common
is retrieved, we prefer to consider in the comparison
surveys at the same frequency and with similar
angular resolution.
Therefore, let
(
)
be the source diameter measure
in the catalog without quoted errors (with quoted errors).
We compute the relative (%) dispersion
of the diameter measures,
,
and try to fit the resulting distribution
in the
plane
again with a constant, linear, and quadratic,
dependence of
on
(or of
on
;
in this case the fit error estimates are again less conservative) and after
removing the points with a dispersion
130%.
In these equations
the angular diameters are in arcmin.
![]() |
Figure 3:
Diameter measures at 2.7 GHz (y-axis) versus
diameter measures and at 5 GHz (x-axis).
It is evident the good agreement between the
two datasets. The best fit function is:
![]() ![]() |
![]() |
Figure 4:
Results of the diameter error estimate for: Altenhoff et al.
(1970)a (top left); Altenhoff et al. (1970)b (top right); earlier Parkes survey
at 2.7 GHz
(left, second row); Caswell ![]() ![]() ![]() ![]() ![]() |
Figure 4 shows the results of the error estimates.
In particular, we consider the comparison between the following
datasets (the first one is that for which we are estimating the errors):
Altenhoff et al. (1970)a vs. Reifenstein et al. (1970) (48 common
sources);
Altenhoff et al. (1970)b vs. Kuchar
Clark (1997) (63 common sources);
Caswell
Haynes (1987) vs. Wilson et al. (1970) (79 common sources);
early Parkes 2.7 GHz Survey vs. Reifenstein et al. (1970) (31 common sources);
Felli
Churchwell (1972) vs. Kuchar
Clark (1997) (21 common sources);
Wendker (1970) vs. Reifenstein et al. (1970) (8 common sources);
Wink et al. (1983) vs. Downes (1980) (59 common sources); Wink et al. (1982) vs. Altenhoff et al. (1979) (53 common sources).
As for the Altenhoff et al. (1970) catalog, the data have been
split because the diameters have been
measured in two different ways: they have been either taken from a survey
of Galactic sources made at 5 GHz with the 140-ft
antenna - beamwidth 6'
(W. J. Altenhoff, P. G. Mezger and J. Schraml, private communication) -
or they have been measured directly from contour maps.
We will annotate the set of sources for which
the size was measured in the 5 GHz survey Altenhoff et al. (1970)a and the remaining sources Altenhoff et al. (1970)b.
For the Berlin et al. (1985) catalog, since none of the comparisons
with other catalogs retrieves
a significant number of common sources, we derive the error from the
mean of the dispersions
given by each comparison.
Table 3 summarizes the estimated and quoted errors used
in calculating the diameters in the Synthetic Catalog.
The Synthetic Catalog summarizes the known radio frequency information on 1442 Galactic HII regions. It contains the position, flux density, diameter data for each HII region, supplemented by velocity data where available. To those HII region with no published diameter data, an indicative diameter is given (marked by **) on the basis of the flux-size correlation in Fig. 2. For sake of clarity, the first ten lines of the Synthetic Catalog are reported in Table 2. The line velocity value that we quote in Col. 9 is the weighted average of the available data (see Sect. 2.4 for details). Although the original measures can be, for the same source, at different frequencies, the weighted average of these data is a meaningful quantity and provides a useful first-sight kinematic indication. Since the line velocity is an effect of the Galactic rotation motion, it does not strongly depend on the frequency of observation.
N | l | b | RA | DEC | S |
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Notes |
1 | 0.1 | 0.0 | 17 45 51.3 | -28 51 08 | 230.0 | 24.1 | 5.9 | 0.5 | -27.4 | 4.0 | W24 |
2 | 0.2 | -0.1 | 17 46 29.0 | -28 49 07 | 209.4 | 10.5 | 10.7 | 0.5 | 24.5 | 3.5 | |
3 | 0.2 | -0.0 | 17 46 05.6 | -28 46 00 | 177.6 | 38.1 | 9.2 | 0.5 | -12.7 | 3.5 | W24 |
4 | 0.3 | -0.5 | 17 48 17.0 | -28 56 25 | 2.5 | 0.7 | 2.7 | 1.7 | 20.0 | 4.9 | C, S |
5 | 0.4 | -0.8 | 17 49 41.7 | -29 00 33 | 8.0 | 2.6 | 7.0 | 2.3 | 20.0 | 4.9 | C, S |
6 | 0.4 | -0.5 | 17 48 31.1 | -28 51 17 | 4.1 | 1.0 | 3.9 | 1.8 | 24.0 | 4.9 | C, S |
7 | 0.5 | -0.7 | 17 49 32.2 | -28 52 19 | 2.9 | 0.9 | 2.3 | 1.4 | 17.5 | 4.9 | C |
8 | 0.5 | -0.1 | 17 47 11.6 | -28 33 44 | 28.3 | 4.0 | 3.3 | 1.4 | 45.8 | 5.0 | S, X |
9 | 0.5 | 0.0 | 17 46 48.2 | -28 30 37 | 40.3 | 8.6 | 4.8 | 0.3 | 47.1 | 2.0 | |
10 | 0.6 | -0.9 | 17 50 33.3 | -28 53 19 | 2.5 | 0.8 | 2.4 | 1.3 | 15.0 | 4.9 | S, S21, RCW142 |
Copyright ESO 2003