The G305 star-forming complex : a wide-area radio survey of ultracompact H ii regions

We present wide-area radio continuum 5.5 and 8.8 GHz (5.5 and 3.4 cm) 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) H ii regions. Observations presented here encompass the entire complex and have a maximum resolution of similar to 1.5 x 1.4 arcsec and sensitivity of similar to 0.07 mJy beam-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 UC H ii regions. The physical properties of these candidate UC H ii regions are determined and reveal that five candidates have peak properties consistent with known UC H ii regions with source radii ranging from 0.04 to 0.1 pc, emission measures from 2.56 to 10.3 x 10-6 pc cm-6 and electron densities of 0.341.03 x 104 cm-3. We comment on these sites of recent massive star formation within G305 and by comparing to other star formation tracers (masers, NH3 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 similar to 0.003 M circle dot yr-1.