In the last few years, prominent high-ionization nebular emission lines (i.e., O III], C III], C IV, He II) have been observed in the deep UV spectra of z ∼ 5 − 7 galaxies, indicating that extreme radiation fields characterize reionization-era systems. These lines have been linked to the leakage of Lyman continuum photons (necessary for reionization) both theoretically and observationally. Consequently, high-ionization UV emission lines present our best probe to detect and characterize the most distant galaxies that we will observe in the coming years, and are key to understanding the sources of reionization, yet the physics governing their production is poorly understood. Here we present recent high-resolution Hubble Space Telescope spectra of two nearby extreme UV emission-line galaxies, J104457 and J141851. We report the first observations of intense nebular He II and double-peaked, resonantly-scattered C IV emission, a combination that suggests these galaxies both produce and transmit a significant number of very high-energy ionizing photons (E > 47.89 eV) through relatively low column densities of high-ionization gas. This suggests that, in addition to photons at the Hionizing edge, the very hard ionizing photons that escape from these galaxies may provide a secondary source of ionization that is currently unconstrained observationally. Simultaneous radiative transfer models of Lyα and C IV are needed to understand how ionizing radiation is transmitted through both low-and high-ionization gas. Future rest-frame FUV observations of galaxies within the epoch of reionization using the James Webb Space Telescope (JWST) or extremely large telescopes (ELTs) will allow us to constrain the escape of helium-ionizing photons and provide an estimate for their contribution to the reionization budget.

We identify 709 arc-shaped mid-infrared nebula in 24 µm Spitzer Space Telescope or 22 µm Wide Field Infrared Explorer surveys of the Galactic Plane as probable dusty interstellar bowshocks powered by early-type stars. About 20% are visible at 8 µm or shorter mid-infrared wavelengths as well. The vast majority (660) have no previous identification in the literature. These extended infrared sources are strongly concentrated near Galactic mid-Plane with an angular scale height of ∼0.6 • . All host a symmetrically placed star implicated as the source of a stellar wind sweeping up interstellar material. These are candidate "runaway" stars potentially having high velocities in the reference frame of the local medium. Among the 286 objects with measured proper motions, we find an unambiguous excess having velocity vectors aligned with the infrared morphology -kinematic evidence that many of these are "runaway" stars with large peculiar motions responsible for the bowshock signature. We discuss a population of "in-situ" bowshocks (∼103 objects) that face giant H II regions where the relative motions between the star and ISM may be caused by bulk outflows from an overpressured bubble. We also identify ∼58 objects that face 8 µm bright-rimmed clouds and apparently constitute a sub-class of in-situ bowshocks where the stellar wind interacts with a photo-evaporative flow from an eroding molecular cloud interface (i.e., "PEF bowshocks"). Orientations of the arcuate nebulae exhibit a correlation over small angular scales, indicating that external influences such as H II regions are responsible for producing some bowshock nebulae. However, the vast majority of this sample appear to be isolated (499 objects) from obvious external influences.

Stellar population models produce radiation fields that ionize oxygen up to O+2, defining the limit of standard H ii region models (<54.9 eV). Yet, some extreme emission-line galaxies, or EELGs, have surprisingly strong emission originating from much higher ionization potentials. We present UV HST/COS and optical LBT/MODS spectra of two nearby EELGs that have very high-ionization emission lines (e.g., He ii λλ1640,4686 C iv λλ1548,1550, [Fe v]λ4227, [Ar iv]λλ4711,4740). We define a four-zone ionization model that is augmented by a very high-ionization zone, as characterized by He+2 (>54.4 eV). The four-zone model has little to no effect on the measured total nebular abundances, but does change the interpretation of other EELG properties: we measure steeper central ionization gradients; higher volume-averaged ionization parameters; and higher central T e , n e , and log U values. Traditional three-zone estimates of the ionization parameter can underestimate the average log U by up to 0.5 dex. Additionally, we find a model-independent dichotomy in the abundance patterns, where the α/H abundances are consistent but N/H, C/H, and Fe/H are relatively deficient, suggesting these EELGs are α/Fe-enriched by more than three times. However, there still is a high-energy ionizing photon production problem (HEIP3). Even for such α/Fe enrichment and very high log U s, photoionization models cannot reproduce the very high-ionization emission lines observed in EELGs.

Stellar feedback is needed to produce realistic giant molecular clouds and galaxies in simulations, but due to limited numerical resolution, feedback must be implemented using sub-grid models. Observational work is an important means to test and anchor these models, but limited studies have assessed the relative dynamical role of multiple feedback modes, particularly at the earliest stages of expansion when H II regions are still deeply embedded. In this paper, we use multiwavelength (radio, infrared, and X-ray) data to measure the pressures associated with direct radiation (P dir ), dust-processed radiation (P IR ), photoionization heating (P H II ), and shockheating from stellar winds (P X ) in a sample of 106 young, resolved H II regions with radii 0.5 pc to determine how stellar feedback drives their expansion. We find that the P IR dominates in 84% of the regions and that the median P dir and P H II are smaller than the median P IR by factors of ≈6 and ≈9, respectively. Based on the radial dependences of the pressure terms, we show that H II regions transition from P IR -dominated to P H II -dominated at radii of ∼3 pc. We find a median trapping factor of f trap ∼ 8 without any radial dependence for the sample, suggesting this value can be adopted in sub-grid feedback models. Moreover, we show that the total pressure is greater than the gravitational pressure in the majority of our sample, indicating that the feedback is sufficient to expel gas from the regions.