Ultracompact H II Regions with Extended Emission: The Complete View

TL;DR

This study investigates the morphology and connection of ultracompact H II regions with surrounding extended emission using radio and infrared observations, revealing irregular shapes and potential impacts on infrared-excess understanding.

Contribution

It provides a comprehensive catalogue of ultracompact H II regions connected with extended emission and analyzes their morphology and infrared associations.

Findings

Most ultracompact H II regions have irregular, bubble-like shapes.

A direct spatial connection between ultracompact regions and extended emission is confirmed.

Extended emission influences the interpretation of infrared excess and ionizing photon estimates.

Abstract

In this paper we present the results of a morphological study performed on a sample of 28 ultracompact \hiirs~located near extended free-free emission, using radio continuum observations at 3.6~cm with the C and D VLA~configurations, with the aim of determining a direct connection between them. By using previously published observations in B and D VLA~configurations, we compiled a final catalogue of 21 ultracompact \hiirs ~directly connected with the surrounding extended emission. The observed morphology of most of the ultracompact \hiirs~in radio continuum emission is irregular (single or multi-peaked sources) and resembles a classical bubble structure in the Galactic plane with well-defined cometary arcs. Radio continuum images superimposed on colour composite \textit{Spitzer} images reinforce the assignations of direct connection by the spatial coincidence between the ultracompact components and regions of saturated 24~\micron~emission. We also find that the presence of extended emission may be crucial to understand the observed infrared-excess because an underestimation of ionizing Lyman photons was considered in previous works.