Radio emission from the Galaxy and from extragalactic sources is both a complicating foreground for EoR observations and an interesting scientific target that forms the second key science theme for the MWA.
In the MWA frequency range, there are a number of science cases for studying astronomical phenomena in molecular and atomic spectral lines: high-mass star formation, detection of complex organics to look for signs of life in stars and planets, reviving investigation of the disportionate ratio of organic and inorganic molecules, and determining the physical properties of the interstellar and intergalactic medium, including its cold diffuse component. A number of completed surveys to look at the Galactic Centre, Orion constellation, and towards distant quasars were completed during Phase I and Phase II of the MWA to look for simple molecules, carbon recombination lines, and high-redshifted (z > 5) neutral hydrogen, with some producing tentative detections. With the compact configuration of the MWA and the large collecting area, large surveys of these sources enable orders of magnitude larger studies of these phenomena then previous surveys.
Only with low-frequency radio telescopes like the MWA can we start to probe the physical conditions of the intergalactic medium around the end of the EoR through recombination lines and neutral hydrogen toward distant radio active galactic nuclei. In the frequency range of 80—270MHz, these spectral features are observed in absorption toward bright continuum objects. This also enables the study of the cold gas feeding massive galaxy formation in the early Universe.
An exciting data product of the MWA is a cube of diffuse Galactic polarized emission and its corresponding Faraday rotation across the band, enabling studies of magnetic fields and turbulence in the local interstellar medium (ISM) with unprecedented detail. The broad continuous bandwidth allows the technique of “rotation measure synthesis” to be applied, in which a spectral cube of polarization position angle is Fourier transformed to yield the polarized signal as a function of Faraday depth. This three-dimensional dissection of magnetized gas, which will reveal the shells, filaments, and sheets, provides information on how the magnetic fields are distributed throughout the local ISM.
The MWA is particularly sensitive to the study of compact polarized sources, such as pulsars, FRII radio AGN, and m-dwarf stellar flares, both in linear and circular polarization.
Star formation theories predict the existence of over 1000 Galactic supernova remnants (SNRs) in the Milky Way, with only about 30% currently detected. The detection of SNRs in the Galaxy allows for a better handle on the production and energy density of Galactic cosmic rays, a better understanding of nucleosynthesis in the ISM, turbulence, the ISM energy budget, and triggered star formation. In addition, there exists the possibility of finding SNRs with unique properties. Since SNRs are spatially extended (~0.2-2 deg) and their emission is non-thermal, the optimal survey strategy is with low-frequency interferometers, as shown with already published results with the MWA.
At low frequencies, SNRs spectra also show spectral curvature – this information can be used to probe shock acceleration and the foreground free-free absorption. Resolved spectral index maps at low frequencies provide information on the foreground absorbing clouds and the ejecta associated with SNRs. Additionally, it has allowed for a better classification of these sources where in some surveys they were mis-identified as HII regions (earlier stages of star formation).
HII regions become optically thick at low frequencies (<150MHz) and appear as “absorption holes” because the smooth synchrotron background is resolved out. This allows the diffuse synchrotron emission to be separated into foreground and background components. Since distances to HII regions are known, a 3D map of the Galactic cosmic ray distribution can be obtained. When this is combined with data on the diffuse gamma-ray background, it results in a large-scale 3D map of Galactic magnetic fields. The first phase of this work done by the MWA found that there is an increase in emissivity toward the Galactic Centre and a decrease with galactocentric radius, consistent with other results in the literature. Using the MWA paired with large-area HII region surveys for distances, allows for a tantalizing three-dimensional view of our Galaxy at scales not previously available.
Many of the sources within the large field-of-view are radio AGN and a few star forming galaxies, as cataloged through the GLEAM survey. The wide-area coverage and large spectral bandwidth allow for fast coverage of low-frequency emitting galaxies to study populations of sources and their properties, increasing our understanding of galaxy evolution. By combining the large bandwidth of low-frequency data provided by the MWA and the vast number of higher-frequency surveys on telescopes such as ASKAP and uGMRT, classifications of galaxies can be made on scales previously not available.
The MWA is sensitive to some of the most powerful distant AGN, allowing for a better study of the synchrotron emission, jet morphology, and physics. Additionally, the population of sources which have spectral energy distributions that peak at these lower frequencies have been thought to be young AGN where the radio emission has only recently ignited. If so, these sources which are primarily identified through this low-frequency information, allow for the study of jet creation and formation.
From the study of new AGN to the old relics of AGN, MWA has excellent capability to detect the old, dead or dying AGN that emit low-surface brightness, diffuse radiation. Work with the MWA and other low-frequency telescopes has uncovered a previously unknown prevalent population of radio sources, with the significant unknowns of the expected size of this population. With the compact core, and large field-of-view, the MWA is uniquely situated to study these large radio sources.
The MWA’s large field-of-view allows for resolved ISM studies of physical processes occurring in of star-forming galaxies, in particular the details of those in the local group. Through the use of broadband spectral coverage the MWA enables the study of galaxy evolution by studying the physical processes of the galaxy’s interstellar medium and its interaction with other nearby galaxies which drive its evolution over time. With the increased resolution capabilities and low frequencies, the MWA is used to quantify the cosmic ray energy spectrum and the measurement of spatial variations that arise from shock re-acceleration, spectral aging, and absorption effects.
The MWA’s unique design allows for the study of diffuse, low surface brightness emission in galaxy clusters and the large-scale faint emission from the synchrotron cosmic web. Large intergalactic shocks produce faint synchrotron emission, which can act as a tracer of large-scale structure, cosmic filaments and primordial magnetic fields. With this information, observational results are compared with models to understand the large scale structures that make up the Universe. The steep spectral index of galaxy cluster emission (< -1.3) makes low frequencies optimal for their study, and the MWA’s unique large field of view means the entire cluster can be captured in a single observation, making this a powerful telescope to study these objects.