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Register a new userScience with an ngVLA: Radio Recombination Lines from HII Regions
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The ngVLA will create a Galaxy-wide, volume-limited sample of HII regions; solve some long standing problems in the physics of HII regions; and provide an extinction-free star formation tracer in nearby galaxies.
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Most massive galaxies are now thought to go through an Active Galactic Nucleus (AGN) phase one or more times. Yet, the cause of triggering and the variations in the intrinsic and observed properties of AGN population are still poorly understood. Young, compact radio sources associated with accreting supermassive black holes (SMBHs) represent an important phase in the life cycles of jetted AGN for understanding AGN triggering and duty cycles. The superb sensitivity and resolution of the ngVLA, coupled with its broad frequency coverage, will provide exciting new insights into our understanding of the life cycles of radio AGN and their impact on galaxy evolution. The high spatial resolution of the ngVLA will enable resolved mapping of young radio AGN on sub-kiloparsec scales over a wide range of redshifts. With broad continuum coverage from 1 to 116 GHz, the ngVLA will excel at estimating ages of sources as old as $30-40$ Myr at $z sim 1$. In combination with lower-frequency ($ u < 1$ GHz) instruments such as ngLOBO and the Square Kilometer Array, the ngVLA will robustly characterize the spectral energy distributions of young radio AGN.
We present a detailed characterization of the population of compact radio-continuum sources in W51 A using subarcsecond VLA and ALMA observations. We analyzed their 2-cm continuum, the recombination lines (RLs) H77$alpha$ and H30$alpha$, and the lines of $rm H_{2}CO(3_{0,3}-2_{0,2})$, $rm H_{2}CO(3_{2,1}-2_{2,0})$, and $rm SO(6_{5}-5_{4})$. We derive diameters for 10/20 sources in the range $D sim 10^{-3}$ to $sim 10^{-2}$ pc, thus placing them in the regime of hypercompact HII regions (HC HIIs). Their continuum-derived electron densities are in the range $n_{rm e} sim 10^4$ to $10^5$ cm$^{-3}$, lower than typically considered for HC HIIs. We combined the RL measurements and independently derived $n_{rm e}$, finding the same range of values but significant offsets for individual measurements between the two methods. We found that most of the sources in our sample are ionized by early B-type stars, and a comparison of $n_{rm e}$ vs $D$ shows that they follow the inverse relation previously derived for ultracompact (UC) and compact HIIs. When determined, the ionized-gas kinematics is always (7/7) indicative of outflow. Similarly, 5 and 3 out of the 8 HC HIIs still embedded in a compact core show evidence for expansion and infall motions in the molecular gas, respectively. We hypothesize that there could be two different types of $hypercompact$ ($D< 0.05$ pc) HII regions: those that essentially are smaller, expanding UC HIIs; and those that are also $hyperdense$ ($n_{rm e} > 10^6$ cm$^{-3}$), probably associated with O-type stars in a specific stage of their formation or early life.
Radio continuum observations have proven to be a workhorse in our understanding of the star formation process (i.e., stellar birth and death) from galaxies both in the nearby universe and out to the highest redshifts. In this article we focus on how the ngVLA will transform our understanding of star formation by enabling one to map and decompose the radio continuum emission from large, heterogeneous samples of nearby galaxies on $gtrsim 10$,pc scales to conduct a proper accounting of the energetic processes powering it. At the discussed sensitivity and angular resolution, the ngVLA will simultaneously be able to create maps of current star formation activity at $sim$100,pc scales, as well as detect and characterize (e.g., size, spectral shape, density, etc.) discrete H{sc ii} regions and supernova remnants on 10,pc scales in galaxies out to the distance of the Virgo cluster. Their properties can then be used to see how they relate to the local and global ISM and star formation conditions. Such investigations are essential for understanding the astrophysics of high-$z$ measurements of galaxies, allowing for proper modeling of galaxy formation and evolution.
Extraterrestrial amino acids, the chemical building blocks of the biopolymers that comprise life as we know it on Earth are present in meteoritic samples. More recently, glycine (NH$_2$CH$_2$COOH), the simplest amino acid, was detected by the Rosetta mission in comet 67P. Despite these exciting discoveries, our understanding of the chemical and physical pathways to the formation of (pre)biotic molecules is woefully incomplete. This is largely because our knowledge of chemical inventories during the different stages of star and planet formation is incomplete. It is therefore imperative to solidify our accounting of the chemical inventories, especially of critical yet low-abundance species, in key regions and to use this knowledge to inform, expand, and constrain chemical models of these reactions. This is followed naturally by a requirement to understand the spatial distribution and temporal evolution of this inventory. Here, we briefly outline a handful of particularly-impactful use cases in which the ngVLA will drive the field forward.
Planets assemble in the midplanes of protoplanetary disks. The compositions of dust and gas in the disk midplane region determine the compositions of nascent planets, including their chemical hospitality to life. In this context, the distributions of volatile organic material across the planet and comet forming zones is of special interest. These are difficult to access in the disk midplane at IR and even millimeter wavelengths due to dust opacity, which can veil the midplane, low intrinsic molecular abundances due to efficient freeze-out, and, in the case of mid-sized organics, a mismatch between expected excitation temperatures and accessible line upper energy levels. At ngVLA wavelengths, the dust is optically thin, enabling observations into the planet forming disk midplane. ngVLA also has the requisite sensitivity. Using TW Hya as a case study, we show that ngVLA will be able to map out the distributions of diagnostic organics, such as CH3CN, in nearby protoplanetary disks.
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