SCOTCH − III. Complete search for hypercompact H ii regions in the fourth quadrant

We present high-frequency (18–24 GHz) radio continuum observations towards 335 methanol masers, excellent signposts for young, embedded high-mass protostars. These complete the search for hypercompact H ii (HC H ii) regions towards young high-mass star-forming clumps within the fourth quadrant of the galactic plane. HC H ii regions are the earliest observable signatures of radio continuum emission from high-mass stars ionizing their surroundings, though their rarity and short lifetimes make them challenging to study. We have observed methanol maser sites at 20 arcsec resolution and identified 121 discrete high-frequency radio sources. Of these, 42 compact sources are embedded in dense clumps and coincide with methanol masers, making them as excellent HC H ii region candidates. These sources were followed up at higher resolution (0.5 arcsec) for confirmation. We constructed spectral energy distributions across 5–24 GHz to determine their physical properties, fitting either a simple H ii region model or a power law as needed. This analysis identified 20 HC H ii regions, 9 intermediate objects, 3 ultracompact (UC)  H ii regions, and 3 radio jet candidates. Combining these results with previous findings, the Search for Clandestine Optically Thick Compact H ii Regions (SCOTCH) survey has identified 33 HC H ii regions, 15 intermediate objects, 9 UC H ii regions, and 4 radio jet candidates, tripling the known number of HC H ii regions. Eleven of these sources remain optically thick at 24 GHz. This survey provides a valuable sample of the youngest H ii regions and insights into early massive star formation.