The G305 star-forming complex: A wide-area radio survey of ultracompact Hii regions

We present wide-area radio continuum 5.5 and 8.8GHz (5.5 and 3.4cm) Australia Telescope Compact Array observations of the complex and rich massive star-forming region G305. The aim of this study is to perform an un-targeted survey of the region in search of the compact radio emission associated with ultracompact (UC) Hii regions. Observations presented here encompass the entire complex and have a maximum resolution of ? 1.5 × 1.4 arcsec and sensitivity of ?0.07mJybeam -1. By applying a data reduction method that emphasizes small-scale structure, we are able to detect 71 compact radio sources distributed throughout the observed field. To explore the nature of these compact radio sources we compare to mid-infrared data and in this way identify 56 background sources, eight stellar radio sources, a single bright-rimmed cloud and six candidate UCHii regions. The physical properties of these candidate UCHii regions are determined and reveal that five candidates have peak properties consistent with known UCHii regions with source radii ranging from 0.04 to 0.1pc, emission measures from 2.56 to 10.3×10 -6pc cm -6 and electron densities of 0.34-1.03 × 10 4cm -3. We comment on these sites of recent massive star formation within G305 and by comparing to other star formation tracers (masers, NH 3 and young stellar objects) build a picture of the star formation history of the region. Using these results we estimate a lower limit to the star formation rate for the region of ?0.003M ?yr -1.