An Analysis of IRAS Identified Hii Regions and their Radio Properties

Abstract

To try and confirm the types of object in the list of 2298 potential HII regions identified by Hughes \& MacLeod from the IRAS Point Source Catalog, we selected a sample of 82 for observing at the VLA. We selected half with values of Y = log(F$_{25}$/F$_{12}$) $\geq$ 0.8, and for control purposes, half with values of 0.3 $\leq$ log(F$_{25}$/F$_{12}$) $\leq$ 0.5. 78 radio sources were detected, and of all the objects, 72\% had at least one associated radio source. Most of the radio sources had diameters of $<$ 3\arcsec, which was the limit to the angular resolving power of the survey. Those with larger values of Y had significantly larger values of peak radio, integrated radio, and 100$μ$m flux densities than those with smaller Y. Also, they generally had associated masers, and thus were most likely young compact HII regions containing star forming regions. Those with smaller Y tended not to have associated maser activity and are probably older HII regions, or stars with high IR and ionizing radiation, such as T-Tauri type stars. However, all 2298 objects appear to have a strong galactic concentration. Some comparison is made with the selection of 1717 objects identified by Wood \& Churchwell as HII regions, but their selection criteria were somewhat different, and they actually form a subset of ours.