5 Conclusions

  The unbiased radio continuum survey of the ISOGAL field at $l=+45^{\circ}$ has resolved the structure of five thermal extended complexes and discovered 21 additional small diameter sources, six of which are candidate HII regions.

Comparison with the IRAS PSC shows that all 5 of the extended thermal sources have corresponding FIR emission and that the colors of these sources satisfy the WC89 color criteria for UCHII. Our sources, however, are not UCHII regions, but are more evolved extended HII regions. This result is consistent with the results of earlier single dish surveys (Codella et al. 1994) and of a recent survey for extended emission around IRAS-selected UCHII regions (Kurtz et al. 1999b).

We conclude that UCHII counts based on IRAS selected samples are overestimated by a large factor, consequently the estimated lifetime of the UCHII phase may be substantially reduced, removing the so-called lifetime problem for UCHII regions.

The percentage of IRAS sources associated with HII regions is $\sim$10% in our field, which seems to be a general property of IRAS sources in the galactic plane.

Acknowledgements

Support from CNR-NATO Advanced Fellowship program and from NASA's Origins of Solar Systems program (through grant NAGW-4030) is gratefully acknowledged.