Blitz 1982)
do exist but they are severely limited by the
dust obscuration effects mentioned above.
At radio wavelengths,
the most comprehensive catalog of diffuse brightness nebulae
is the COBE-HII Catalog at 2.7 GHz (Witebsky 1978a,b).
It includes
909 objects and has provided a reliable tool for estimating the
contribution
of HII regions to Galactic free-free in the analysis of the
COBE data.
More recently, a list of 760
objects with angular diameter less than 10' has been compiled by Kuchar
Clark 1997 from the Green Bank (Condon et al. 1989)
and Parkes-MIT-NRAO (Condon et al. 1993; Tasker et al. 1994) radio surveys at 4.85 GHz.
These catalogs can now be combined and updated with other recently published data. A strong motivation supporting this work is its relevance to cosmic microwave background (CMB) studies. Since the successful results of the COBE satellite (Smoot et al. 1992), increasing efforts have been devoted to investigating the cosmological information contained in the CMB anisotropies. Recent balloon experiments such as BOOMERANG (de Bernardis et al. 2000) and MAXIMA-1 (Hanany et al. 2000) as well as ground interferometer experiments as DASI (Leitch et al. 2002), CBI (Mason et al. 2001) and VSA (Scott et al. 2002) have achieved remarkable results but, at the same time, have shown the necessity to improve our knowledge of all the relevant astrophysical foregrounds which may affect the recovery of the CMB angular power spectrum and, ultimately, of the cosmological parameters. In particular, the free-free emission is the least known among the Galactic foregrounds. However, much of the free-free emission at low Galactic latitudes is produced by individual bright HII regions. Because of their strong emission, they may represent a potential source of contamination for the new generation of nearly full-sky coverage CMB anisotropy space missions at high sensitivity and resolution. These sources are also good candidates for receiver and antenna pattern calibration.
By collecting the information from 24 previously published lists and catalogs, we present in this paper a comprehensive catalog (Master Catalog) of 1442 Galactic HII regions. For each object, we quote the original fluxes and diameters as well as the available information on radio recombination line velocities, line widths and line temperatures and the errors on all these quantitities.
By exploiting these data we produce a Synthetic Catalog of fluxes and diameters (with corresponding errors) at 2.7 GHz, motivated by the extensive although not complete information available at this frequency, widely spread among the different catalogs, and by its relevance for both detailed studies on Galactic HII regions and the extrapolation up to millimetric wavelengths. We discuss also some possible applications of this Synthetic Catalog to CMB studies. The paper is organized as follows. Section 2 presents the stages of the Master Catalog compilation (e.g., the selection criteria for the reference papers and the removal of non-thermal sources) and briefly summarizes the content of the final compilation, while an accurate and full description of the catalog content is given in Appendix I. Section 3 describes the production of the Synthetic Catalog at 2.7 GHz. Section 4 concerns the completeness and flux limit of the catalog and the spatial distribution of the listed sources. Perspectives of possible applications of this Synthetic Catalog to CMB studies are presented in Sect. 5. Finally, we draw our main conclusions in Sect. 6.
Copyright ESO 2003