2 The Master Catalog compilation

In this section we discuss the data processing which was applied to convert the information from the various reference catalogs into a uniform format for easy intercomparison in a Master Catalog.

2.1 Selection of the original catalogs

The Master Catalog was compiled from the 24 radio catalogs shown in Table 1. We follow the classification of HII regions by Lockman et al. (1996) and only include catalogs of diffuse and compact HII regions. Surveys of ultra-compact (UCHII) and extremely extended objects (EHE) will be considered in a forthcoming extension of the present list: they are bright in the infrared (UCHII) and in the optical (EHE) but relatively weak at radio frequencies.

The reason for following this route is driven by the general goals of the work. Planned and on-going CMB experiments typically have angular resolutions ranging from a few arcmin up to tens of arcmin in order to exploit the wealth of cosmological information encoded in the CMB anisotropies. Therefore, diffuse and compact HII regions - with a typical angular extension of a few arcmin - are the most important to be taken into account if we want to investigate the free-free emission as a contaminant of the CMB.

We have therefore considered single-dish medium-resolution observations, typically with beamwidths of a few arcmin, rather than interferometer or synthesis telescope surveys (Becker et al. 1994; Taylor et al. 1996) which usually achieve a resolution of a few arcsec. Also, single-dish low-resolution observations (Westerhout 1958; Wilson $\&$ Bolton 1960), whose typical angular resolution is ${\sim} 1^\circ$, have not been included in the sample of selected references in order to avoid problems arising from double-counting of the sources. Likewise we have not considered surveys simply oriented to the study of the morphology and spectral behaviour of the diffuse Galactic radiation at centimetric wavelengths (Reich et al. 1984; Reich et al. 1990; Fürst et al. 1990a; Fürst et al. 1990b; Duncan et al. 1995).

 

Table 1: List of the references (in alphabetical order) for the catalog. Listed $l_{\min}$ and $l_{\max}$ provide the longitude range spanned by each survey. The range in latitude is $\vert b\vert \leq 2^\circ {-}4^\circ$.
$^\dag$ Number of sources after subtraction of nonthermal objects. Notes: (a) the list of sources is retrieved by the intersection with the radio recombination line survey by Lockman et al. (1989); (b) early Parkes survey at 2.7 GHz; (c) Effelberg 100-m survey at 2.7 GHz; (d) Cygnus X; (e) nonuniform sky coverage; (f) Green Bank and Parkes-MIT-NRAO surveys cover almost the whole sky: HII regions have been identified from either optical or radio recombination line surveys along the disk.

Reference	$l_{\min}$	$l_{\max}$	$\nu$ (GHz)	HPBW (')	Number$^\dag$ of sources
Altenhoff et al. (1970)	$335^\circ$	$75^\circ$	1.4/2.7/5	10	325
Altenhoff et al. (1979)(a)	$2^\circ$	$60^\circ$	5	2.6	265
Beard (1966)(b)	$331^\circ$	$333^\circ$	2.7	7.4	13
Beard $\&$ Kerr (1969)(b)	$27^\circ$	$38^\circ$	2.7	7.4	34
Beard et al. (1969)(b)	$345^\circ$	$5^\circ$	2.7	7.4	72
Berlin et al. (1985)	$4^\circ$	$10^\circ$	3.9	$0.8\times18.3$	45
Caswell $\&$ Haynes (1987)	$190^\circ$	$40^\circ$	5	4.2	308
Day et al. (1969)(b)	$307^\circ$	$330^\circ$	2.7	8.2	109
Day et al. (1970)(b)	$37^\circ$	$47^\circ$	2.7	8.2	48
Downes et al. (1980)	$357^\circ$	$60^\circ$	5	2.6	169
Felli $\&$ Churchwell (1972)	(e)	(e)	1.4	10	80
Fürst et al. (1987)(c)	(e)	(e)	2.7	4.27	7
Goss $\&$ Day (1970)(b)	$6^\circ$	$26^\circ$	2.7	8	85
Kuchar $\&$ Clark (1997)	(f)	(f)	4.8	3.1/4.2	760
Mezger $\&$ Henderson (1967)	(e)	(f)	5	6.3	17
Reich et al. (1986)(c)	(e)	(e)	2.7	4.27	5
Reifenstein et al. (1970)	$348^\circ$	$80^\circ$	5	6.5	105
Thomas $\&$ Day (1969a)(b)	$288^\circ$	$307^\circ$	2.7	8.2	39
Thomas $\&$ Day (1969b)(b)	$334^\circ$	$345^\circ$	2.7	8.2	29
Wendker (1970)(d)	$76^\circ$	$84^\circ$	2.7	11	77
Wilson et al. (1970)	$282^\circ$	$346^\circ$	5	4	132
Wink et al. (1982)	(e)	(e)	5/15/86	2.6/1/1.3	112
Wink et al. (1983)	$359^\circ$	$50^\circ$	14.7	1	84

2.2 Identification and removal of SNRs

The selected catalogs may contain in principle also some Galactic sources, different from HII regions, that we need to exclude from our Master Catalog compilation. Therefore, supernova remnants (SNRs) of each selected catalog have been identified by comparing the coordinates of the catalog sources with those of the catalog of Galactic supernova remnants by Green (2000). The identification has been performed taking into account the average position uncertainty of the selected catalogs of HII regions and of the Green Catalog (both ${\sim} 1'$).

2.3 Quoted flux densities and angular diameters

We provide the flux densities integrated over each source, S, and the source angular diameter, $\Theta_{\rm HII}$. In a few references (Wink et al. 1982 at 86 GHz; Wink et al. 1983) the flux density is given only as a peak value. We correct this to a true value by assuming that the source has a Gaussian profile and using the observed diameter (see Rohlfs 1990).

The source angular diameter is given in terms of observed diameter in a few references (Altenhoff et al. 1979; Downes et al. 1980; Wink et al. 1983). We derive a true diameter again assuming a Gaussian source profile. For source dimensions significantly smaller than the beamwidth, the references give upper limit diameters. The Master Catalogs includes these derived flux densities and diameters for completeness.

2.4 Other relevant data

In addition to the basic data on flux density and diameter, for each source there are available further relevant data which make the catalog more useful.

We include the notes from Kuchar $\&$ Clark (1997) on the environment of each source, indicating whether the source is complex or if it has a strong source nearby. Again, following Kuchar & Clark (1997) we give radio counterparts in other catalogs other than those referred to in Sect. 2.1. Optical counterparts of the unobscured sources are obtained from the identification given in the Catalog by Mársalková (1974).

We include available radio recombination line data because they provide important information on kinematics, distances and electron density for each HII region. The data are given by Downes et al. (1980), Caswell $\&$ Haynes (1987), Kuchar $\&$ Clark (1997), Reifenstein et al. (1970), Wilson et al. (1970), Wink et al. (1982) and Wink et al. (1983) in the source catalogs listed in Sect. 2.1 and are complemented by the recombination line data of Lockman (1989).

2.5 The Master Catalog

All sources observed at least by one of the surveys of Table 1 and recognized as Galactic HII regions were included in the Master Catalog. 1442 sources are listed. All relevant information from each of the reference catalogs is retained. Data for individual HII regions are available from the references at 8 frequencies (1.4, 2.7, 3.9, 4.8, 5., 14.7, 15 and 86 GHz). This large data base is presented in a readily accessible form as 11 sub-catalogs:

sub-catalog 1 coordinates (epoch 2000) and remarks
sub-catalog 2 / 3 flux density (Jy) / 1-$\sigma$ ($\%$) error
sub-catalog 4 / 5 diameter (arcmin) / 1-$\sigma$ ($\%$) error
sub-catalog 6 / 7 line velocity (km s^-1) / 1-$\sigma$ error (km s^-1)
sub-catalog 8 / 9 line width (km s^-1) / 1-$\sigma$ error (km s^-1)
sub-catalog 10 / 11 line temperature (K) / 1-$\sigma$ error (K).

The sub-catalogs give, for each source, an identification number along with the position in Galactic and celestial coordinates. Apart from sub-catalog 1, each sub-catalog has 37 columns. The columns are in order of increasing frequency of observation; at each frequency the columns are in alphabetical order of the references in Table 1. We note that for sub-catalog 6-11 (the radio recombination line data) the observations may refer to frequencies other than those in the main continuum catalog (in this case, the line frequency is reported).

Appendix I gives a detailed description of the content of each sub-catalog.