The Physics of the Cold Neutral Medium: Low-frequency Radio Recombination Lines with the Square Kilometre Array

Abstract

The Square Kilometre Array (SKA) will transform our understanding of the role of the cold, atomic gas in galaxy evolution. The interstellar medium (ISM) is the repository of stellar ejecta and the birthsite of new stars and, hence, a key factor in the evolution of galaxies over cosmic time. Cold, diffuse, atomic clouds are a key component of the ISM, but so far this phase has been diffcult to study, because its main tracer, the HI 21 cm line, does not constrain the basic physical information of the gas (e.g., temperature, density) well. The SKA opens up the opportunity to study this component of the ISM through a complementary tracer in the form of low-frequency (<350 MHz) carbon radio recombination lines (CRRL). These CRRLs provide a sensitive probe of the physical conditions in cold, diffuse clouds. The superb sensitivity, large field of view, frequency resolution and coverage of the SKA allows for efficient surveys of the sky, that will revolutionize the field of low-frequency recombination line studies. By observing these lines with the SKA we will be able determine the thermal balance, chemical enrichment, and ionization rate of the cold, atomic medium from degree-scales down to scales corresponding to individual clouds and filaments in our Galaxy, the Magellanic Clouds and beyond. Furthermore, being sensitive only to the cold, atomic gas, observations of low-frequency CRRLs with the SKA will aid in disentangling the warm and cold constituents of the HI 21 cm emission.Advancing Astrophysics with the Square Kilometre Array

Science with the Murchison Widefield Array: Phase I results and Phase II opportunities

“…There is a range of science cases for studying astronomical phenomena in molecular and atomic spectral lines at low frequencies: high-mass star formation (Codella et al 2015), detection of complex organics to look for signs of life in stars and planets (Danilovich et al 2016), reviving investigation of the disproportionate ratio of organic and inorganic molecules (Cosmovici et al 1979), and determining the physical properties of the ISM (Salgado et al 2017b), including its cold diffused component (Oonk et al 2015; Peters et al 2011). The study of molecules around high-mass stars is complicated partially due to strong emission of prominent molecules which would not be present at lower radio frequencies.…”

HI Science with the Square Kilometre Array

“…The following chapters concentrate on the far-reaching impact of SKA1 on our understanding of the formation and evolution of galaxies (Blyth et al 2015;Power et al 2015;Meyer et al 2015) including the build-up of angular momentum (Obreschkow et al 2015), the accretion of gas into the outskirts of galaxies (Popping et al 2015), the detailed understanding of the ISM of galaxies (de Blok et al 2015), and the impact of AGN on the large-scale gas distribution in galaxies (Morganti et al 2015). The impact of SKA1 on our understanding of the local Galactic ISM and halo is also explored, including detailed mapping via absorption-line studies of the density and temperature of the cold neutral medium (McClure-Griffiths et al 2015;Oonk et al 2015). This impressive set of science cases and chapters has been facilitated by the H I and Galaxy Evolution working group of the SKA Science Working Group.…”

SKA studies of nearby galaxies: star-formation, accretion processes and molecular gas across all environments

“…We are beginning to see that the properties and mass balance atomic/molecular/ionised change along individual outflows as well as among outflows. In this area HI absorption studies are absolutely fundamental, however observations of OH maser emission in dense outflows in ULIRGs also have a critical role and radio recombination lines (RRLs) also potential can play an important part in nearby sources (see section 4 and Morganti et al, 2015;Oonk et al, 2015, for HI absorption and RRL opportunities with the SKA). Very recently it has also become clear that AGN driven outflows may also have extremely interesting molecular properties (linked to their evolution) that can be studied in exquisite detail with the SKA.…”