VLA observations of nine extended green objects in the Milky Way: ubiquitous weak, compact continuum emission, and multi-epoch emission from methanol, water, and ammonia masers

We have observed a sample of nine Extended Green Objects (EGOs) at 1.3 and 5 cm with the Very Large Array (VLA) with subarcsecond resolution and ∼7–14 μJy beam⁻¹-sensitivities in order to characterize centimeter continuum emission as it first appears in these massive protoclusters. We find an EGO-associated continuum emission—within 1'' of the extended 4.5 μm emission—in every field, which is typically faint (order 10¹–10² μJy) and compact (unresolved at 0''.3–0''.5). The derived spectral indices of our 36 total detections are consistent with a wide array of physical processes, including both non-thermal (19% of detections) and thermal free–free processes (e.g., ionized jets and compact H ii regions, 78% of sample) and warm dust (1 source). We also find an EGO-associated 6.7 GHz CH₃OH and 22 GHz H₂O maser emission in 100% of the sample and a NH₃ (3,3) masers in ∼45%; we do not detect any NH₃ (6,6) masers at ∼5.6 mJy beam⁻¹ sensitivity. We find statistically-significant correlations between L_radio and L_bol at two physical scales and three frequencies, consistent with thermal emission from ionized jets, but no correlation between L_H20 and L_radio for our sample. From these data, we conclude that EGOs likely host multiple different centimeter continuum-producing processes simultaneously. Additionally, at our ∼1000 au resolution, we find that all EGOs except G18.89−0.47 contain 1 ∼ 2 massive sources based on the presence of CH₃OH maser groups, which is consistent with our previous work suggesting that these are typical massive protoclusters, in which only one to a few of the young stellar objects are massive.