A Large-Scale, Low-Frequency Murchison Widefield Array Survey of Galactic H ii Regions between 260 < l < 340

Abstract

We have compiled a catalogue of H ii regions detected with the Murchison Widefield Array between 72 and 231 MHz. The multiple frequency bands provided by the Murchison Widefield Array allow us identify the characteristic spectrum generated by the thermal Bremsstrahlung process in H ii regions. We detect 306 H ii regions between 260° &lt; l &lt; 340° and report on the positions, sizes, peak, integrated flux density, and spectral indices of these H ii regions. By identifying the point at which H ii regions transition from the optically thin to thick regime, we derive the physical properties including the electron density, ionised gas mass, and ionising photon flux, towards 61 H ii regions. This catalogue of H ii regions represents the most extensive and uniform low frequency survey of H ii regions in the Galaxy to date.

A spatially resolved radio spectral index study of the dwarf irregular galaxy NGC 1569

“…In the conversion, we assume an electron temperature of 10 4 K (Deeg et al 1997). It should be noted that this value can vary significantly, as shown in studies of a large sample of galactic HII regions (Hindson et al 2016). Furthermore, Nicholls et al (2014) find that an electron temperature of 1.4 × 10 4 K is more appropriate in metal poor dwarf galaxies.…”

“…A thermal component, originating from ionized plasma surrounding massive stars (HII regions), and a non-thermal component, originating from CRe that are accelerated in SNR shock fronts (see Condon 1992 for a review). As shown by Hindson et al (2016), HII regions can be assumed to be optically thin at frequencies above 1 GHz. Therefore, an expression for the radio continuum spectrum can be written as:…”

“…There are several additions that could be made to the fitted model. For example, if additional lower frequency observations are included, we could include the effects of free-free or synchrotron self absorption (Tingay & de Kool 2003;Callingham et al 2015;Hindson et al 2016;Kapińska et al 2017). Towards higher frequencies, a break in the non-thermal spectrum due to spectral ageing and inverse Compton scattering (Lisenfeld et al 2004;Harwood et al 2013;Heesen et al 2015) could also be included.…”

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

“…When the distance to the H i i region is known, the difference between its surface brightness and a nearby (source-free) region gives a direct measurement of the integrated cosmic-ray electron emissivity between the H i i region and the edge of the Galactic plane. Phase I of the MWA was successfully used by Hindson et al (2016) to detect and measure 306 H i i regions. Su et al (2017Su et al ( , 2018) went on to use the distinct spectral signature of these regions to perform cosmic ray tomography of the Galactic Plane.…”

Workshop Summary “The Power of Faraday Tomography”

“…Tessa Vernstrom gave an overview of the Murchison Widefield Array (MWA), and a wide variety of MWA science results including limits on synchrotron radiation from the cosmic web [48] or absorption studies of Galactic HII regions [49]. She presented the GaLactic and Extragalactic All-Sky MWA Survey (GLEAM, [50] catalog, visualization, science results, and follow-up surveys.…”