# A Low Frequency Pilot Survey of Southern Hii Regions in the Vela
Constellation

Chenoa D.Tremblay,1 Tyler L. Bourke,2 James A. Green,1 John M. Dickey,3 O. Ivy
Wong,1,4 E-mail<EMAIL_ADDRESS>and Tim J. Galvin1,5

1CSIRO, Space and Astronomy, Australian Telescope National Faciliy, PO Box
1130, Bentley WA 6102, Australia
2SKA Observatory, Jodrell Bank, Macclesfield SK11 9FT, United Kingdom
3School of Maths and Physics, University of Tasmania, Hobart, TAS 7001,
Australia
4 International Centre for Radio Astronomy Research, UWA, 35 Stirling Hwy,
Crawley, WA 6009, Australia
5International Centre for Radio Astronomy Research, Curtin University, GPO Box
U1987, Perth WA 6845, Australia

(Accepted XXX. Received YYY; in original form ZZZ)

###### Abstract

Atomic ionised regions with strong continuum emission are often associated
with regions of high-mass star formation and low-frequency ($<$2 GHz)
observations of these regions are needed to help build star formation models.
The region toward the Vela Supernova Remnant is particularly interesting as it
is a complex structure of recent supernova explosions and molecular clouds
containing a number of Hii regions that are not well characterised. We
searched publicly available catalogues for Hii regions, both candidate and
identified, which also have low-frequency emission. In the area of $\sim$400
square degrees toward the Vela Supernova remnant, we found 10 such Hii
regions, some of which have multiple components in catalogues. In this work we
use data from the Australian Square Kilometre Array Pathfinder and previously
unpublished data from the Murchison Widefield Array and the Australian
Telescope Compact Array to analyse these sources. The high-mass star forming
region RCW 38, with observations specifically targeted on the source, is used
as a pilot study to demonstrate how low-frequency, wide-field continuum
observations can identify and study Hii regions in our Galaxy. For the 9 other
Hii regions, we discuss their properties; including information about which
clouds are interacting, their ages, whether they are dominated by infrared or
optical H$\alpha$ lines, distances, ionising photon flux, and upper limits on
the infrared luminosity. In future work, these 9 regions will be analysed in
more detail, similar to the result for RCW 38 presented here.

###### keywords:

stars: formation $-$ molecular data $-$ radio lines: ISM $-$ stars: surveys
$-$ ISM:Hii Regions $-$ ISM:molecules

††pubyear: 2021††pagerange: A Low Frequency Pilot Survey of Southern Hii
Regions in the Vela Constellation–A.3

##  1 INTRODUCTION

Hii regions are characterised as areas where interstellar atomic hydrogen is
ionised and where star formation has recently taken, or is, taking place. The
far-ultraviolet (FUV) radiation from high-mass stars (spectral type late B or
O stars, $>$10 M⊙) embedded in their natal environment create a mechanism for
ionisation through feedback (Samal et al., 2018). This ionisation is strongly
correlated with: H$\alpha$ emission (although sometimes only weak emission is
detected) readily observable by optical telescopes (Seon et al., 2011);
infrared emission of the warm ionising medium (Anderson et al., 2012);
recombination lines often observed by radio telescopes (Hoglund & Mezger,
1965; Wenger et al., 2021); and sometimes radio continuum emission (i.e. Bania
et al. 2010; Wenger et al. 2021).

Within the Hii region, we do not know if the low-mass stars or the high-mass
stars are the first to form (Kumar et al., 2020). This creates some challenges
on observing the sources and impacts which observational methods are most
likely to detect the nebulae. The densest, brightest regions called “compact
Hii regions" (or “ultra-compact") are often heavily obscured by dust and
cannot be viewed by visible light, making infrared and radio measurements
crucial. Masqué et al. (2021) studied ultra-compact Hii regions and found that
nebulas have a mix of thermal and non-thermal emission. They suggest that the
thermal sources have free-free emission from the ionised gas and the non-
thermal sources have a nature that appears to be different from standard
expanding or in-fall scenarios. So observations across the electromagnetic
spectrum, where either free-free emission or synchrotron radiation dominate,
of the same Hii clouds are required to help build complete star formation
models.

The Vela constellation contains a giant molecular cloud along the
Carina–Sagittarius Spiral Arm of the Galactic Plane. The highly energetic
environment caused by recent supernova explosions in the near and far-field
allow for some unique Hii regions. This makes it a particularly interesting
target for wide-field surveys.

Although a number of large southern-sky surveys have been completed over the
last 40 years, new surveys with current telescope technology has provided
significant insight. Hindson et al. (2016) completed a low-frequency survey
for Hii regions with the Murchison Widefield Array (MWA) at
$260^{\circ}<\ell<340^{\circ}$, $-3^{\circ}<b<2^{\circ}$. This region
approaches the IRAS radio shell of the Vela supernova remnant but does not
extend beyond or around the remnant. Wenger et al. (2021) recently completed a
blind census of Hii regions using the Australian Telescope Compact Array
(ATCA) at 4.8 GHz as part of the “Southern Hii Region Discovery Survey
(SHRDS)". In their survey they detected hydrogen radio recombination lines
(RRLs) towards 208 previously known nebulae and 438 Hii candidates along
$259^{\circ}<\ell<346^{\circ}$, $|b|<4^{\circ}$. Some of the candidate regions
had recombination line detections but no detectable radio continuum emission
at 4.8 GHz.

Often Galactic Hii regions are identified through the warm dust emission
observed by infrared telescopes or based on their morphology. Radio
observations $<$10 GHz focus on young or compact Hii regions (e.g. CORNISH;
Hoare et al. 2012). Many of the surveys of the Galactic Plane have focused
closer to the Galactic Centre and many of the low-frequency surveys suffer
from low resolution (i.e. Green et al. 1999; Hindson et al. 2016). With a
combination of the new wide-field and high-resolution capabilities of the
Australia Square Kilometre Array Pathfinder (ASKAP) and the Murchison
Widefield Array (MWA; with its Phase II upgrade) we can efficiently identify
Hii regions and provide information on their morphological and physical
characteristics, especially when combined with infrared and optical surveys.

In this paper we utilise new and archival data to extend the knowledge of Hii
regions at $259^{\circ}<\ell<275^{\circ}$, $-6^{\circ}<b<4^{\circ}$, some of
which is shown in Figure 1. The observations from ASKAP and the MWA represent
a single wide-field footprint from each telescope. We have broken the analysis
in the paper into two parts. In the first part we provide an in-depth analysis
of RCW 38 (Section 4). This source was chosen for extra attention as it is the
densest (in stellar membership) embedded region of high-mass star formation
within 2 kpc after the Orion Nebula Cluster (ONC), but is not as well studied
(Wolk et al., 2008; Lada & Lada, 2003). In the second part we compare
catalogues of known and candidate Hii regions from the WISE Catalogue of
Galactic Hii Regions V2.3 (Anderson et al.,
2014)111http://astro.phys.wvu.edu/wise/ and from SHRDS. These were overlaid on
the ASKAP and MWA images to search for sources that show low-frequency (114
MHz and/or 887.5 MHz) emission (Section 5). For each region that shows radio
continuum emission at these frequencies, we provide a short analysis.

##  2 Observations

New observations centred on the Vela Supernova Remnant were completed using
the MWA and matched with archival observations from ASKAP and WISE (at 22
$\mu$m) as shown in Figure 1. Both the ASKAP and MWA telescopes are located in
the Murchison Radio-astronomy Observatory (MRO), the site designated for the
low-frequency component of the future Square Kilometre Array (SKA). We also
used previously unpublished data at 1.6 GHz of RCW 38 from the Australian
Telescope Compact Array (ATCA).

Figure 1: An image showing a region covered by new data from Rapid ASKAP
Continuum Survey (left) at 887.5 MHz and MWA image at 114 MHz (Middle). The
striations around RCW 38 and other bright sources in the ASKAP image are due
to the shallow, 15 minute integrations and small calibration errors ($<$1 per
cent;McConnell et al. 2020) which are obvious around RCW 38 as is the source
is bright. The WISE 22$\mu$m (right) shows the infrared emission and
highlights some of the Hii regions discussed in this work as marked on the MWA
image.

###  2.1 Murchison Widefield Array (MWA)

Using the Murchison Widefield Array (Tingay et al., 2013; Wayth et al., 2018;
Beardsley et al., 2019) we observed the region of the Vela and Puppis
Constellation for 30 hours between 2018 January 05 – 23, of which 17 hours are
used in this analysis. The remaining data suffered significant artefacts
(visible in the $u,v$ data as deformations of the beam, likely caused by a
malfunction in the electronics during the extended array deployment) or radio
frequency interference and were not used for analysis. These observations
represent early science data taken during the Phase II upgrade (Wayth et al.,
2018; Beardsley et al., 2019) and use 91 dipole tiles with baseline lengths
ranging from 22 m to 5.1 km at a central frequency of 114 MHz.

The MWA correlator offers 24 $\times$ 1.28 MHz coarse channels across a
bandwidth of 30.72 MHz. The data are calibrated and imaged as described in
Tremblay et al. (2020). For each night, 2-minute observations of the galaxy
Hydra A (245.8$\pm$31.9 Jy at 160 MHz; Kuehr et al. 1981) are used to perform
primary bandpass and phase calibration. The solutions are further refined by
using self calibration prior to being applied to each set of visibilities. A
robust weighting with a factor of “0.5” was used for the images presented
here, resulting in a synthesised beam of $129\arcsec\ \times\ 105\arcsec$ at
$-$29$\degr$ position angle.

The Earth’s atmosphere can create spatially variant refraction and propagation
delays that are more evident at lower frequencies. For each 5-minute
observation, the source positions are compared to the Molonglo Reference
Catalogue (MRC; Large et al. 1981) to correct for shifts in the source
positions. After correction, the error in a stacked continuum image was
–10$\pm$26 arc seconds in Right Ascension and 6$\pm$11 arc seconds in
Declination. Both of these values are smaller than the synthesised beam of
1.03 arc minute. The ratio of integrated intensity and peak intensity,
determined from point sources, is 1.05, suggesting that the ionospheric
distortions are well corrected for in these observations.

###  2.2 Australian Telescope Compact Array (ATCA)

The Australian Telescope Compact Array (ATCA) is a 6-element predominantly
East-West interferometer222it has a small northern spur available for some
compact configurations with 22 m antennas operating at cm and mm wavelengths.
To complement Parkes single-dish OH observations published in Bourke et al.
(2001), RCW 38 was observed at 1666 MHz with the ATCA in its 1.5D and 6.0C
configurations in 1996 May and June (preliminary results are presented in
Bourke et al. (2004)). On 1996 May 10 a 13.7 hour track was obtained with the
1.5D configuration array, while on 1996 June 15 a 11 hour track was obtained
with the 6.0C configuration. In order to obtain the highest spectral
resolution available with the correlator available at the time of the
observations, while still being able to observe both the 1665 and 1667 OH
transitions, a 4 MHz bandpass with 4096 channels centred on 1666.0 MHz with
one linear polarisation was used, resulting in a resolution of 976.56 Hz or
0.176 km s-1. With the correlator available at ATCA at the time these
observations were taken, correlating all polarisation products – XX*, YY*,
XY*, YX* (where * denotes the complex conjugate) – from the two orthogonal
linear polarisations, X and Y, would result in a decrease in the spectral
resolution by a factor of 4, resulting in a minimum spectral resolution of 0.8
km s-1 at the OH 1.6 GHz frequencies. However, the full-width-half-maximum
(FWHM) of the Gaussian component responsible for the observed OH Zeeman effect
in RCW 38 using the Parkes Radio telescope is only 2.4 km s-1 (Bourke et al.,
2001),. Thus, any OH Zeeman experiment on RCW 38 with ATCA at the time of the
observations using both OH lines would result in only 3 spectral channels
across the FWHM of this Gaussian component, which is insufficient to derive
and fit the Stokes V spectrum. Therefore, the observations were restricted to
only one polarisation with the full spectral resolution, in order to study the
kinematics of the OH .

The primary flux calibrator used was PKS 1934–638 with an assumed flux of
14.16 Jy and the complex gain calibrator was 0823–500. The visibility data was
processed in through standard methodologies using the MIRIAD (Sault et al.,
1995) package. Bandpass solutions were determined for PKS 1934–638 and copied
to PKS 0823–500. The time dependent antenna based gains were determined for
0823–500, bad data were flagged, and the flux scale was bootstrapped to
1934–638. The gain solutions were copied to RCW 38, the continuum was
constructed from the line-free channels in the visibility data set and the
line and continuum visibilities split. Very little data needed to be flagged
and very little interference was observed ($<<$10 %). The data for the two
configurations were calibrated separately and combined at the imaging stage.
Maps were made by Fourier transforming the visibilities, using the CLEAN
algorithm down to the rms (root mean squared) noise level, and restoring the
image using a Gaussian beam. A robust weighting with a factor of “0.5" was
used for the images presented here, resulting in a synthesised beam of 9.8
$\times$ 8.3 arc second at 16.8∘ position angle for the line data. The primary
beam size at these frequencies is $\approx$25 arc minutes.

###  2.3 Ancillary Data

In addition to the observations above, we used the public catalogues of 887.5
MHz Stokes I images from the recent Australian Square Kilometre Array survey
named Rapid ASKAP Continuum Survey (RACS;McConnell et al. 2020) in this work.
These images were downloaded from The CSIRO ASKAP Science Data Archive
(CASDA333https://research.csiro.au/casda/) and represent publicly available
data sets. We received additional ATCA radio astronomy data from Wolk et al.
(2008) at 4800 MHz for RCW 38.

Additionally, we used data from the Wide-field Infrared Survey Explorer
(WISE), which is a joint project of the University of California, Los Angeles,
and the Jet Propulsion Laboratory/California Institute of Technology. To
understand the dust emission we used the 90 $\mu$m survey from AKARI Far-
Infrared All-Sky Survey by Doi et al. (2015) and Herschel Space Observatory
Archive444http://archives.esac.esa.int/hsa/whsa/. For the H-alpha emission we
used The Southern H-Alpha Sky Survey Atlas (SHASSA; Gaustad et al. 2001),
Wisconsin H-Alpha Mapper (WHAM; Hill et al. 2014), and AAO/UKST SuperCOSMOS
H$\alpha$ survey (SHS;Parker et al. 2005). Images from these surveys were
obtained through $SkyView$555https://skyview.gsfc.nasa.gov/ or the individual
survey web servers.

We note that the sources were not covered in the MWA Galactic and Extra-
galactic all-sky survey catalogue (GLEAM; Hurley-Walker et al. 2017, 2019). We
do note that one source, RCW 39 was covered by Hindson et al. (2016) in their
survey of Hii regions using GLEAM Galactic Plane data.

##  3 Hii Region Identification

Atomic ionised regions with strong low-frequency radio continuum emission can
be a population of a mix of high-mass star forming regions and planetary
nebula (or other dying stars) (Spitzer, 1998; Hoare et al., 2012; Anderson et
al., 2012). Although, at times these two are grouped into the category of “Hii
regions”, as both are composed of gas ionised by photons with energies above
the Hydrogen ionisation energy of 13.6 eV, they represent different phases of
stellar life. The standard Hii region is ionised by hot spectral type O or
early B stars (or clusters of such stars) and associated with regions of
recent high-mass ($>$10 M⊙) star formation (e.g. Zinnecker & Yorke 2007).
Planetary nebula are the ejected outer envelopes of asymptotic giant branch
(AGB) stars photoionised by the hot remnant stellar core (e.g. Peña et al.
2021). There are different methods of distinguishing these two stages, such as
the detection of radio recombination lines. However, as Hajduk et al. (2021)
show, low frequency emission and recombination lines can also be detected in
planetary nebula. One way of differentiating sources within these two
evolutionary stages is through distinct 22 $\mu$m infrared emission, which
traces the stochastically heated small dust grains (Watson et al., 2008).
Calzetti et al. (2007) suggest that the mid-infrared emission traces the
dust–obscured star–formation while H$\alpha$ optical emission traces the
unobscured one. As some of the regions are candidates based on recombination
line detections, to verify if the sources studied here are high-mass star
forming regions, we compare with WISE 22 $\mu$m emission and the ASKAP
continuum emission or MWA continuum emission, as shown in Figures 2 and 3.

Observations of Hii regions below $\sim$200 MHz can appear optically thick and
display a positive spectral index (Hindson et al., 2016). The MWA observations
used in this paper were completed using the Phase II “long-baseline array”
with baselines from 22 m to 5.5 km. This means there is low surface brightness
sensitivity and a synthesised beam of $\sim$1 arc minute, allowing for the
regions to be less confused than the survey by Hindson et al. (2016), but with
a reduction in the reported flux in comparison to observations with a single-
dish telescope. The observations of the field using ASKAP have a $\sim$13 arc
second beam and, at a frequency of 887.5 MHz, can be in the optically thin
regime. Therefore, the continuum emission from RACS in the regions should be
more comparable to WISE, as the resolution of $\sim$12 arc second and a 6 mJy
sensitivity limit are well matched.

For this survey, the region was searched in
$SIMBAD$666http://simbad.u-strasbg.fr/simbad/, SHRDS catalogue, and the WISE
Hii catalogue for known or candidate Hii regions. This cross match found 10
regions which are shown in Figure 2 and Figure 3 and summarised in Table 1.
All sources have well matched radio continuum and 22 $\mu$m infrared emission.
This, in combination with the detection of radio recombination lines and some
masers, confirms the identification of the sources being Hii regions
containing high-mass stars. Within the literature, a number of these have been
characterised as ultra-compact Hii regions through their methanol maser
emission (e.g. Walsh et al. 1997). In Section 4 and Section 5 we provide
analysis of the age and condition of these regions.

Figure 2: The black contours from the ASKAP radio continuum emission are
overlaid onto an 22 $\mu$m infrared image of the region from the Wide-field
Infrared Survey Explorer (WISE) survey. The contours show the positions of
highest intensity are well matched between the radio and infrared emission.
The contours are set at levels of 0.02–0.20 Jy in increments of 0.03 Jy and
the bottom right-hand corner shows the size of the ASKAP synthesised beam

Figure 3: The cyan contours from the MWA radio continuum emission are overlaid onto an 22 $\mu$m infrared image of the region from the Wide-field Infrared Survey Explorer (WISE) survey. The contours show the positions of highest intensity are well matched between the radio and infrared emission. The contours are set at levels of 0.05, 0.1, 0.15, 0.2, 0.25, 0.3 Jy and the bottom right-hand corner shows the size of the MWA synthesised beam Table 1: Information on the sources observed in this survey and have shown continuum emission at either 114 MHz or 887.5 MHz or both.  Cluster Name | Coordinates | SIMBAD Class. | Obs. Telescope | Maser Detection* | RRL Detection** | Molecular Obs.***
---|---|---|---|---|---|---
| (J2000) |  |  |  |  |
RCW38 | 08 59 05.50 –47 30 39.4 |  Hii Region | MWA & ASKAP | No Obs. | Yes |  CH, OH, 18CO,
|  |  |  |  |  |  12CO, CS
IRAS08563-4711 | 08 58 04.2 –47 22 57 |  Hii Region | MWA & ASKAP | No Detection | Yes |
RCW39 | 09 03 21.4 –48 25 48 |  Hii Region | MWA & ASKAP | No Detection | Yes |
RCW40 | 09 02 21.3 –48 41 55 |  Hii Region | MWA & ASKAP | No Detection | Yes |
RCW42 | 09 24 30.1 –51 59 07 |  Hii Region | MWA & ASKAP | No Obs. | Yes |
GAL268.4–00.9 | 09 01 54.3 –47 43 59 |  Hii Region | MWA & ASKAP | No Detection | Yes |
IRAS09015-4843 | 09 03 13.5 –48 55 21 |  Hii Region | MWA & ASKAP |  H2O,CH3OH(II) | Yes |  12CO
GAL263.62–00.53 | 08 45 34.9 –43 51 07 |  Hii Region | MWA | No Detection | Yes |
RCW41 | 09 16 43.2 –47 56 26 | Composite Object | MWA & ASKAP |  H2O,CH3OH | No |  12CO
GAL281.01–01.53 | 09 59 08.5 –56 52 44 |  Hii Region | MWA | No Detection | Yes |  13CO
* Information from http://maserdb.net/. ** From Southern Hii Region Discovery Survey (Wenger et al., 2021) and RCW 38 Study by Torii et al. (2021). ***As listed in SIMBAD or WISE Catalogue.

##  4 RCW 38

###  4.1 Background

The compact Hii region RCW 38 is a relatively nearby region of massive star
formation ($\sim$1.63 kpc; Zucker et al. 2020). It stands out as one of the
brightest regions in the southern sky at radio wavelengths (Wilson et al.,
1970), and is one of the few regions within 2 kpc, along with the Orion Nebula
Cluster (ONC), to contain over 2000 young stars identified at infrared
wavelengths and in X-rays, although it is more embedded and younger ($\sim$1
Myr) than the ONC (Wolk et al., 2006; Wolk et al., 2008; Winston et al.,
2011). These studies have identified around 30 OB star candidates, some of
which have been confirmed (DeRose et al., 2009; Povich et al., 2017). At
infrared wavelengths the region is dominated by two sources, the mid-infrared
bright RCW 38 IRS1 and the near-infrared bright RCW 38 IRS2 (Figure 4; Furniss
et al. 1975; Frogel & Persson 1974; Smith et al. 1999). IRS1 is the peak of an
extended emission region on the western side of a ring-like structure of size
0.1 pc, and is an active site of on-going star formation (DeRose et al., 2009;
Winston et al., 2011; Torii et al., 2021). Within this ring, near its centre
but slightly offset, is IRS2, a binary O5.5 stellar system that is the
dominant source of energy for the region, whose winds are responsible for
carving out the ring (likely a partial bubble seen in projection) within the
natal molecular cloud and clearing out the dust within it (Smith et al., 1999;
DeRose et al., 2009), forming the compact Hii region. The ring-like structure,
first revealed at mid-infrared wavelengths, has subsequently been observed at
wavelengths from the near-infrared through the far-infrared, millimetre, and
the radio (e.g., Bourke et al. 2004; Vigil 2004; Wolk et al. 2006; Wolk et al.
2008; Winston et al. 2011; Fukui et al. 2016; Torii et al. 2021; Izumi et al.
2021).

Figure 4: Near-infrared overview of the central 2$\aas@@fstack{\prime\prime}$5
($\sim$1.2 pc) of RCW 38 main ring centred on RA 08:59:05, Dec –47:30:39. The
filters used are Z (blue), H (green) and K (red). The Figure is inspired by
Fig. 3 of Wolk et al. (2008) and Fig 1a. of Fukui et al. (2016). The top
right-hand corner shows the ASKAP radio continuum image with a white box
showing the region which the VLT image covers. The VLT image shows the
separation of IRS1 (shock/ionisation front) and IRS2 (O5.5 binary) that is not
easily discerned at the radio wavelengths presented here, due to the lack of
emission from IRS2. Image Credit: ESO

The infrared ring is surrounded by dense gas ( 103-4 cm-3, up to 106 cm-3) on
large scales, as observed in e.g. 13CO 1–0 and CS 2–1, with significant clumps
to the north north-east and south south-west (Bourke et al., 2004; Kaneda et
al., 2013; Fukui et al., 2016). More compact dense gas traced by CS 2–1 and
C18O 2–1 is evident to the west and bordering IRS1, indicating that the
emission from IRS1 is due to irradiation and ionization of the outer edge of
this dense gas (Wang et al., 2016; Torii et al., 2021). Observations at mm
wavelengths trace both free-free and dust emission; in particular the
continuum emission at 1.3 mm at IRS1 is a mix of both types of emission with a
significant free-free component (tens of percent), as indicated by
extrapolation of radio fluxes, and the presence of compact mm recombination
lines (M. Vigil private communication; Wang et al. 2016; Torii et al. 2021).

###  4.2 Radio observations

In Hii regions, the structure can be traced by emission of optical H$\alpha$
(Balmer lines), infrared emission, and radio continuum. The H$\alpha$ emission
traces the heated ionised gas in the diffuse layers of gas and dust while in
the compact, dense regions the dust absorbs the radiation and re-emits it in
the infrared (Spitzer, 1998). The continuum emission traces the free-free
emission and synchrotron radiation associated with the electron density of the
region (Thompson et al., 1986).

The area around RCW 38 at 114 MHz from the MWA and 887 MHz from ASKAP is shown
in Figure 5. There is broad agreement between the 114 and 887 MHz images,
keeping in mind the 114 MHz is of lower resolution and does not show the
details afforded by the higher resolution of the 887 MHz image. As shown in
Figure 6, there is also agreement for the IRS1 and IRS2 region between ASKAP
at 887 MHz and ATCA at 1666 and 4800 MHz (the latter from Wolk et al. (2008),
their Figure 5). When comparing the two lower frequencies of 114 MHz and 887
MHz, two interesting features emerge. One is that the 114 MHz emission peaks
to the South South-West of RCW 38, whereas the 887 MHz emission peaks at RCW
38, in the same ring-like structure centred on IRS2 shown in Figure 6.

Figure 5: Image of the continuum emission around RCW 38 observed by the MWA at
114 MHz (left) and ASKAP at 887.5 MHz (right). The coordinates for each
itemised position listed in the WISE catalogue is marked with an “*”. The
bottom right-hand corner shows the size and shape of the synthesised beam for
each radio interferometer.

Figure 6: Comparison of the emission morphological structure observed by ASKAP
at 887.5 MHz (left) by ATCA at 1666MHz (middle) and at 4800 MHz (right). The
synthesised beam for each image is shown on the bottom right of each image.
The bottom row of images are the contour images for the same observations with
contour levels set at 0.1–1.3 Jy with an increment of 0.2 Jy. The bottom
right-hand corner shows the size and shape of the synthesised beam for each
radio interferometer.

Second is that the emission at 114 MHz above the RCW 38 main lobe shows a
clear asymmetric ring-like structure of its own, limb brightened in the south
edge, which is barely visible at 887 MHz (even with an intensity stretch).
This asymmetric ring shows good correspondence with IRAC band 2 emission at
4.8 $\mu$m, band 4 emission at 8.0 $\mu$m and the WISE 3.4 and 4.6 $\mu$m
(likely from shocks and PAHs, respectively), and seems to be associated with a
candidate O-star TYC 8156-728-1 (Figure 7; see also Figure 8; Winston et al.
2011). The 4.6$\mu$m emission lies inside the South side of the ring, and
corresponds to the region of bright 114 MHz emission on the South side of the
ring. Not much is known about this feature or the O-star candidate, further
investigations are needed with high-resolution radio telescopes at a range of
frequencies.

Figure 7: Bubble surrounding the O-star candidate TYC 8156-720-1. The right-
hand image is the same field-of-view from the MWA 114 MHz data on the left in
Figure 5 centred at RA 8:59:26, Dec –47:26:12. On the left is a Spitzer IRAC
3.6, 4.5, and 8 $\mu$m three-coloured (blue, green, and red respectively)
image of the region to the north of RCW 38 (visible at lower centre). The oval
shows the location of the proposed bubble due to TYC 8156, whose position is
indicated on the Spitzer image. Figure 8: Image showing the radio emission at
114 MHz (right; similar to Figure 5 centred at RA 8:59:26, Dec -47:26:12,
J2000) of RCW 38 overlaid onto the high-resolution H$\alpha$ optical emission
from SuperCOSMOS H-alpha Survey (left) and the Herschel 165 $\mu$m emission
(middle). The contours span from 0 to 25 Jy at six equal increments.

The tail toward the East of RCW 38, not clearly detected in either the 1666
MHz or 4800 MHz images, has faint emission in WISE 4.6 $\mu$m, 887 MHz
continuum and 165 $\mu$m emission and is strongest in the 114 MHz and
H$\alpha$. Urquhart et al. (2007) observed 13CO 1–0 emission in this region
and Broos et al. (2013) discovered a young high-mass star candidate (2MASS
J08595792-4732147) in the region using x-ray and infrared data. It may be
possible that this region is in the later stages of development where the
thick ionising dust cloud is being swept away by the stellar winds. It is also
possible that a single O star will have a less dense Hii region as they form
at a faster rate than B stars; thus not having the time to build up and ionise
a thick cloud. However, as little is known about this region, future surveys
and additional information will allow us to better interpret the data for this
region.

The high resolution ($\sim$10”) radio maps of RCW 38 at 1666 MHz (this work;
see also Bourke et al. (2004)) and 4800 MHz (Wolk et al., 2008) obtained with
the ATCA resolve the central region and show the main RCW 38 ring-like
structure as seen at mid-infrared wavelengths, marking the interface of the
Hii region and the surrounding dense gas. These data are shown in Figure 6,
along with the 887.5 MHz image from ASKAP, in which the ring is also evident.
The relationship of the 165 $\mu$m emission in Figure 8 and the WISE 22 $\mu$m
emission in Figure 2, suggests that the continuum emission in the larger
region called RCW 38 (the southern feature, eastern tail and the main lobe) is
a mix of thermal and synchrotron radiation (Tabatabaei et al., 2007; Calzetti
et al., 2007; Bendo et al., 2008). Masqué et al. (2021) discuss that thermal
emission is often dominant in the densest regions and the emission of non-
thermal synchrotron radiation is dependant the presence of magnetic fields and
shocks.

The integrated main line OH 1666 MHz absorption is shown in Figure 9. No OH
masers are present. While the absorption “peaks” (i.e., troughs) fall on top
of the radio continuum ring, there is extended absorption to the south, where
the 114 MHz continuum peaks (and where the radio optical depth is modest). A
comparison of the Parkes data published in Bourke et al. (2001) and ATCA
spectral line data indicates that the component giving rise to the Parkes
Zeeman detection is probably a compact, isolated absorption feature located on
the IRS1 ridge. The interface between the radio continuum ring and the IRS1
dense gas is a photodissociation region, where the stellar winds and ultra-
violet radiation from IRS2 are impacting the cloud, perhaps compressing it and
causing triggered star-formation. Additional observations to study this region
in all four of the OH transitions, including Zeeman splitting will be
undertaken shortly with ATCA, to better understand the role the magnetic field
is playing in the star formation within IRS1. A comparison of the integrated
OH absorption spectrum with the integrated 13CO 1-0 spectrum (Bourke, 1999)
shows their line shapes are very similar. As these two transitions are
expected to be co-located, tracing similar material, this suggests that the
13CO can be used to infer the physical conditions of the OH gas. This will be
explored in a future study.

Figure 9: Image of integrated 1667 MHz OH absorption (contours) over the 1666
MHz (18 cm) continuum (greyscale) observed with ATCA. The OH contours on this
figure are from -0.65 to -7.15 in steps of 0.65 Jy km s-1/beam. The greyscale
ranges from -0.2 to 1.2 Jy/beam. Velocity of integration is $-$4.5 to $+$7.5
km s-1. The beam is shown at lower left, and the vertical bar indicates 0.5 pc
at the distance of RCW 38.

###  4.3 Radio Spectral Index

Although the $u,v$ coverage of the 1666 MHz data is more sparse than that of
the 887.5 MHz image777ATCA has six antennas, and for our observations, the
baselines ranged between 105 m and 6 km and ASKAP has 36 antennas, with
baselines between 22 m and 6 km., particularly at short $u,v$ distances, such
that some of the more extended emission is missing from the 1666 MHz image, we
have constructed a two-point spectral index map between 887.5 and 1666 MHz. We
have not added in the MWA data at 114 MHz as the resolution of $\sim$1 arcmin
does not allow for the same detail within the region as the ASKAP and ATCA
data. Both the ATCA & ASKAP images have been convolved to the same resolution
of 15 arc sec. At these frequencies we expect any Hii region to be primarily
in the optically thick regime, or at least in the turn-over from optically
thin at higher frequencies (spectral index $-$0.1) to thick at lower
frequencies (spectral index 2.0). The spectral index should therefore be
mostly positive throughout the region but not greater than 2, assuming the
region is well resolved at both wavelengths.

The spectral index map shown in Figure 10 supports these assumptions, with the
median optical depth value of 0.2$\pm$0.07 representing most of the map,
peaking in the ring around IRS2 with a value of 1.6$\pm$0.3. The errors here
are generated from a root mean square error map of the two input maps.
Although we note this may overestimate the errors (Dickey et al., 2021), it
demonstrates that the different segments of the region can be correctly
differentiated. This map illustrates the potential value in constructing
similar maps of candidate Hii regions in a larger survey with matching $u,v$
coverage using frequencies at the low, mid, and high end of the ASKAP band
(upcoming RACS survey full data release), to study and confirm the nature of
Hii regions. Additionally with these upcoming wide-field, high-resolution
surveys, there will be significant improvements in our understanding of the
optical depths and relationships to the spectral index and electron densities
(Dickey et al., 2021).

Figure 10: Radio spectral index map of RCW 38 using ATCA 1666 MHz and ASKAP
887.5 MHz data.

##  5 Other Prominent Sources in the Field

Within the Vela region, the Gum Catalogue
888http://galaxymap.org/cat/list/gum/1 and RCW Catalogue (Rodgers et al.,
1960) provide the identification of several Hii regions observable in the
southern hemisphere. Other candidate and known Hii regions are also collected
in the WISE Hii Catalogue (Anderson et al., 2019), in Wenger et al. (2021),
and in SIMBAD. In the foreground, at $\approx$300-450 pc away is the Gum
Nebula, the Vela Supernova Remnant, and prominent high-mass stars. In the
background are a series of Hii regions associated with a giant molecular cloud
along the Galactic Plane, at a distance of 1.7 to 1.8 kpc away and some
sources are thought to be further in the background. These are summarised in
Table 2.

Overall, the field includes a complex structure of foreground and background
sources that are difficult to disentangle. Below are details of new, wide-
field observations of Hii regions observed with ASKAP and the MWA which
provide greater detail around the clusters containing Hii regions. Any Hii
regions that were analysed and discussed in Hindson et al. (2016), except for
RCW39, are not included in this survey as it would be a repetition of
information. RCW39 is included here as the ASKAP image provides greater detail
of the source structure than previously observed in other surveys.

Table 2: Characterisation of Hii regions in the field covered in this paper.
The distances listed in column 3 are from various sources and are sometimes
assumed in literature, as specified as “M" for measured and “A" for assumed.
The values in columns 4–7 with an “*" denote measurements determined within
this work. The total luminosity (Log Lum.) and Lyman Continuum Photon Flux
(Nc) are calculated based on the equations and assumption listed in Appendix
A. The luminosity values calculated from IRAS fluxes are an upper limit as the
beam for IRAS was large. Ionised gas velocity measurements are derived from
hydrogen radio recombination lines, except where noted. All information from
previous work is referenced in each section for the individual sources and
summarised here.

Source | WISE Name | Distance | Stellar Pop. | Age | Log (Nc) | Log (Lum.) | Velocity
---|---|---|---|---|---|---|---
|  | (kpc) |  | (Myr) |  (photons s-1) |  (L⊙) |  (km s-1)
RCW38 (Main Ring) | G267.935–01.075 |  1.63$\pm$0.17 | 40 OB Stars | <1 | 49.4 | 6.09 |  1.00$\pm$0.1a
IRAS08563-4711 | G267.730–01.09 | 2.1 |  At least 1 B1∗∗ |  | 45.63* | 3.66 |  6.8$\pm$0.3a
RCW39 | G269.068–01.114 | 2.6 |  At least 1 O9.5∗∗ |  | 45.9* |  4.6∗ |  13.5$\pm$0.1a
RCW40 | G269.174–01.436 | 1.7 | O8 | Indeter. | 48.8 | 3.5* |  7.5$\pm$2.1a
RCW42 | G274.004–01.152 | 6.4 | >12 OStars | Late Stage | 50.30 | Confused |  39.5$\pm$0.1a
GAL268.4–0.9 | G268.419–0.843 |  1.7$-$1.8 | At least 1 B0 Star | 0.8 |  47.0∗ | 5.0* |  5.4$\pm$0.1a
IRAS09015-4843 | G269.454–01.468 |  7.80$\pm$0.40 |  At least 1O9.5∗∗ |  | 47.7* | 2.78 |  67.0$\pm$0.3a
GAL263.62–00.53 | G263.615–0.534 | 0.7 | B1 | 1–2 | ND | 3.0* |  11.0-17.5b
RCW41 | G270.255+00.85 |  1.3 $\pm$ 0.2 | O9V star | 2.5–5 | 45.5* | 3.7* |  2.5-5.5b
GAL281.01–01.53 | G281.047–01.541 | 1.9 |  |  | ND | ND |  –5.6$\pm$0.3a

*Determined as part of this work. All other values are determined in other publications as referenced in the text.
**Based on Table 1 in Panagia (1973)
ND=Not Determined
RRL Velocity Measurements: a Wenger et al. (2021) Table 10 (H88-H112)
bMethanol maser peak velocity measurements associated with the complex from
Green et al. (2012), which can be used as a proxy for the systemic velocity to
within $\sim$3 km/s (Green & McClure-Griffiths, 2011).

###  5.1 IRAS08563-4711

The candidate Hii region IRAS08562-4711 is one arc minute in RA from the main
lobe of RCW 38 at a position of RA 08:58:04.2, Dec -47:22:57 (J2000). The
$Gaia$ distance catalogue lists the photometric distance for stellar
candidates in the region as ranging from 1.3 to 2.5 kpc and the 18CO
observations from Torii et al. (2021) suggests there may be some interaction
with RCW 38.

The source is unresolved with both ASKAP and the MWA, as shown in Figure 5 and
2. Navarete et al. (2015) observed the source with MSX for H2 (source 217 in
their paper) and classified it as being two diffuse or filamentary high-mass
star clouds containing a cluster of embedded stars. They determined a
kinematic distance of 2.1 kpc and luminosity of 103.66 L⊙. Valdettaro et al.
(2007) searched the region for water masers, a powerful tracer of embedded
stars, but did not detect any with the Tidbinbilla 70 m dish. Using the flux
density from the ASKAP observations and the distance of 2.1 kpc, we determined
the the stellar Hii region to contains at least one B1 star from using Panagia
(1973) and the ionising photon flux listed in Table 2.

As shown in Figure 8, the region has both strong H$\alpha$ and 165 $\mu$m
emission. This suggests it is a compact region of hot, dense ionising gas
consistent with an Hii region.

###  5.2 RCW 39

The GAL269.13–1.14 Hii region within the cluster RCW 39, at the position of RA
09:03:23, Dec –48:26:34 (J2000), is comprised of two ring-like clouds as shown
in Figure 11. This source was included in a census of Hii regions by Caswell &
Haynes (1987) with the Parkes Radio Telescope at 5 GHz. They determined a
continuum flux density of 6.8 Jy for the source and velocities of 10 and 15 km
s-1 for the H2CO and Hydrogen recombination lines respectively. Wenger et al.
(2021) measured the velocity for the Eastern lobe as 14.3 km s-1 and 12 km s-1
for the south-western region. Bronfman et al. (1996) measured CS(2-1)
transition with the Swedish-ESO Submillimetre Telescope with a velocity of
10.3 km s-1. Hou & Han (2014) report the region to be at the distance of 2.6
kpc in their catalogue via the method of kinematic distance using a Galaxy
rotation curve with the current IAU standard, but within the WISE Catalogue it
is listed at a distance of 3.09$\pm$0.62 kpc.

In our observations, the ASKAP data separates the region into two components
where previous studies have shown one blurred region similar to what is
observed with the MWA (Figure 11). This source was also observed by Hindson et
al. (2016) with the MWA from 70–200 MHz finding a spectral index at these low
frequencies of 2.2$\pm$0.4. From the observations so far reported in this
region, it is unknown if these two clouds are interacting.

Using the flux density from the ASKAP observations and the distance of 2.6
kpc, we determined the the stellar Hii region located in the north-east
contains at least one O9.5 star from using Panagia (1973). By using the
equations for the total luminosity from IRAS fluxes and the continuum photon
flux (Nc) in Appendix A, we characterise this lesser known region. We put an
upper limit for the total luminosity as 104.6 L⊙. These values are collected
in Table 2.

Figure 11: RCW 39 (GAL269 -1.14) HII region imaged in continuum with MWA at
114 MHz (left) and ASKAP at 887.5 MHz (right). The position of the “*” marks
are from the WISE Catalog of sources. The bottom right-hand corner shows the
size and shape of the synthesised beam for each radio interferometer.

###  5.3 RCW40

RCW40 (within the Gum 25 cluster) is a bright southern Hii region
characterised as containing a cluster of hot, young, massive stars at the
position RA 09:02:21.3, Dec –48:41:55 (J2000). Copetti (2000) measured the
distance to be 1.7 kpc with integrated photometry which is updated from the
distance of 1.4 kpc reported by Shaver et al. (1983). Reylé & Robin (2002)
found the cluster to have a large dispersive colour in infrared, suggesting a
strongly varying extinction exists in front of or within the cluster. This
means that they could not reliably determine the age of the cluster.

Reylé & Robin (2002) obtained an average visual extinction (AV) of 9 mag
toward the cluster using the Deep Near Infrared Survey of the Southern Sky
(DENIS), and so it contains an embedded young cluster. This is a more
sensitive measurement over the 2MASS catalogue values (Cutri et al., 2003).
Kennicutt et al. (2000) studied Hii regions within the Milky Way, Small
Magellanic Clouds, and Large Magellanic Clouds. Using optical data they
calculated the stellar temperature (T∗) of 38,400 K and the ionising
luminosity of the stars of 1048.80 photon s-1. Hawley (2011) reported a
comparison of Oii, Oiii, and Nii abundances for the region. In a survey of Hii
regions by Goy (1980), they classified the region as being dominated by an O8
spectral type high-mass star.

Wenger et al. (2021) measured recombination lines at the two locations marked
with an “*” on Figure 12. The southern lobe has a velocity of 3.8 km s-1 and
the northern lobe has a velocity of 2.7 km s-1. Previous to this work the
focus of Kennicutt et al. (2000) and other surveys is on the larger ring
structure of the source.

The continuum images from the MWA and ASKAP show a consistent structure for
this source. The ASKAP image may suggest there is a separation between the two
“lobes” of the Hii region as seen in the optical images from the Digital Sky
Survey (DSS). Using the equations in Appendix A, we put an upper limit on the
of the total infrared luminosity of 103.5 L⊙.

Figure 12: RCW40 (Gum 25 Cluster) imaged in continuum with MWA at 114 MHz
(left) and ASKAP at 887.5 MHz (right). The “*” on each image marks the
positions of RRL detections. The bottom right-hand corner shows the size and
shape of the synthesised beam for each radio interferometer.

###  5.4 RCW42 (Gum 26 Cluster)

RCW42 (Gum 26 Cluster) is a stellar cluster at RA 09:24:30.1, Dec –51:59:07
(J2000). This Galactic Hii region is part of a larger structure shell called
GSH 277$+$0.36 at a distance of 6.5 kpc (McClure-Griffiths et al., 2003).
Moisés et al. (2011) studied RCW42 as part of a survey of Galactic Hii regions
in the near and far-infrared. They found the region to be a densely-pact
cluster of stars surrounded by a reddish nebula of about 600 pc in diameter
and with emission dominated at 8.0 $\mu$m.

Conti & Crowther (2004) used the distance of 6.4 kpc from Russeil (2003) to
calculate the Lyman continuum luminosity of (Nc)=1050.30. From this Moisés et
al. (2011) determined at least a dozen early O-type stars are associated with
the region. Although the region is complicated, Moisés et al. (2011) suggested
that the kinematic distance and stellar sequence are correctly interpreted
based on the near-infrared J, H and K three coloured image and Spitzer images
999Courtesy of NASA/JPL-Caltech. http://www.spitzer.caltech.edu/. They also
classify the region as a “naked photosphere" and suggest it is quite late in
the evolutionary phase.

Figure 13: RCW 42 Hii region, part of the Gum 26 Cluster, imaged in continuum
with MWA at 114 MHz (left) and ASKAP at 887.5 MHz (right). There is a strong
correlation with the MWA features with optical H$\alpha$. The bottom right-
hand corner shows the size and shape of the synthesised beam for each radio
interferometer.

The MWA continuum image mimics the structure detected in H$\alpha$ by the
SHASSA survey (Gaustad et al., 2001) and WISE 22 $\mu$m suggesting a warmer
gas is present. This is consistent with the idea the region is in the late
stages of star formation and thus containing hotter dust emission. The ASKAP
image shows greater structure detail. The different points entries (marked
with “*”) from the WISE catalogue are on each image in Figure 13. The RRL
detection by Anderson et al. (2012) at a velocity of 39 km s-1, is in the
central location of the low-frequency emission region. The source was included
in the Class II methanol maser survey by Walsh et al. (1997), but no methanol
was detected.

###  5.5 GAL 268.4-00.9 Hii Region

The Hii region GAL 228.4–00.9 (also known as IRAS 09002-4732) is located at RA
09:01:54.3, Dec –47:43:59 (J2000). The parallax measurements from ESA’s Gaia
satellite suggests a distance of 1.7–1.8 kpc for the cluster (Getman et al.,
2019) which places it along the edge of the Vela Molecular cloud, inline with
RCW 38. Getman et al. (2019) completed a survey of the little-known region
using x-ray and infrared data. They found the cluster to be only about 0.8 Myr
old but twice as dense in stellar objects as the ONC. They also classified it
as a compact Hii region within a filament of the Vela Molecular Cloud.

Lenzen (1991) studied the region and with an assumed distance of 1.8 kpc
calculated the Bolometric Luminosity of 105.0 L$\odot$ and determined the
region has at least one spectral type B0 star. We use the Equation 1 and 3
from Kurtz et al. (1994) and found a Lyman continuum photons per second ((Nc)
to be 1047.0 photon s-1 when making the same assumption as Lenzen (1991) of a
distance of 1.8 kpc, a Te of 104 K and a dust to gas ratio of 100. We use the
flux density at 887.5 MHz from the ASKAP data of 54.5 mJy.

Figure 14 shows that the MWA image at 114 MHz only contains a very faint
emission toward the Hii region. There is strong emission toward the Hii region
with ASKAP at 887.5 MHz as an unresolved source. The 90 $\mu$m dust emission
map in Figure 14 shows a matched structure with ASKAP toward the Hii region
but little H$\alpha$ emission. This suggests the region is dominated by cold
dense gas, which is consistent with the analysis done by Getman et al. (2019)
that the Hii region is young. The “*” on the images marks the location of the
Hii region as listed in the WISE Catalogue. The RRL detection by Anderson et
al. (2012) is at a velocity of 5 km s-1.

Figure 14: Gal 268.4–00.9 Hii imaged in continuum with the MWA at 114 MHz (top
left) and ASKAP at 887.5 MHz(top right). The bottom right image shows the dust
emission at 90 $\mu$m surveyed by AKARI. The bottom left image is optical
H$\alpha$ emission from the Southern H-Alpha Sky Survey Atlas (SHASSA;
Finkbeiner 2003). The “*” is the position of the Hii region sepcified in the
WISE catalogue. The bottom right-hand corner shows the size and shape of the
synthesised beam for each radio interferometer.

###  5.6 IRAS09015-4843

The compact Hii region known as IRAS09015-4843 is located at RA 09:03:13.5,
Dec –48:55:21 (J2000). The distance is listed in the WISE Hii catalogue as
7.80$\pm$0.40 kpc (Anderson et al., 2015). Walsh et al. (1997) observed Class
II methanol masers (CH3OH) toward the region and Breen & Ellingsen (2011)
found water masers, both signs of active star formation. Anderson et al.
(2012) observed hydrogen RRLs at a velocity of 71 km s-1. Green et al. (2012)
measured the velocity of 6.7 GHz methanol masers for the region between 53.5
to 56.6 km s-1. Observations of 12CO (Wouterloot & Brand, 1989) were also made
toward the ionisation centre labelled by an “*” on Figure 15. Both the ASKAP
and MWA images shows a non-resolved point source.

Hill et al. (2009) completed an analysis of Galactic Hii regions through the
infrared spectral index. They found the cold gas temperature between 2.7–35 K,
the clump mass to be at least 3.1$\times$102 M⊙ and a luminosity of at least
6.1$\times$102 L⊙. Using the continuum emission from RACS at 887 MHz, we
determine the continuum photon flux $N_{c}=10^{47.7}$ photons s-1.

Figure 15: IRAS09015-4843 Hii imaged in continuum with the MWA (left) at 114
MHz and ASKAP (right) at 887.5 MHz. The “*” symbol marks the position of the
ionisation source as listed in the WISE catalog. The bottom right-hand corner
shows the size and shape of the synthesised beam for each radio
interferometer.

###  5.7 GAL263.62-00.53

The Hii region GAL263.62-00.53 is in the embedded cluster IRAS08438-4340
(IRS16) (Massi et al., 2010) and one of six clusters noted within Vela
Molecular Ridge Cloud D (Murphy & May, 1991). The position of the region is at
RA 08:45:34.9, Dec, –43:51:07 (J2000). Massi et al. (2006) studied the region
and by using the initial mass function found the clusters in Vela D to be of
similar age which was constrained to 1–5 MYr and IRS16 had a total cloud mass
of 99 M⊙. Massi et al. (2010) further studied the region in detail using near
and far infrared analysis. Using two different methodologies they constrained
the age of the cluster to 1–2 MYr and found the Hii region was fuelled by a
Early B star.

Caswell & Haynes (1987) constrained the size of the radio emission to one arc
second at a distance of 700 pc. Massi et al. (2010) suggested that this means
that the Hii region would be greater than the Strömgren sphere only for an
ionising star later than B1, which was consistent with previous results by
Massi et al. (2003) which suggested a spectral type B0-B1 star fuelled the Hii
region.

Massi et al. (2007) studied the millimetre continuum emission of the region
and found 3 dense cores around the GAL263.62-00.53 region. They suggest that
the expanding ionisation cloud is spurring further star formation in the area.
As shown in Figure 16, GAL263.62-00.53 is unresolved in the MWA image. The “*”
on the image marks the location of the RRL detection by Wenger et al. (2021),
which had a velocity of -0.2 km s-1. Green et al. (2012) measured the 6.7 GHz
methanol maser velocity to range from 11.0 to 17.5 km s-1 for the region. This
source was between two footprints from the RACS survey, so a high resolution
image was not obtained. This also meant we could not obtain a good measure of
the photon flux for the region. Using the IRAS fluxes and the equations in
Appendix A, we determined the upper limit of the luminosity of 103.0 L⊙.

Figure 16: GAL263.62-00.53 Hii imaged in continuum with the MWA at 114 MHz.
The “*” symbol marks the position of the ionisation source as listed in the
WISE catalog. The bottom right-hand corner shows the size and shape of the
synthesised beam for each radio interferometer.

###  5.8 RCW41

Within the infrared cloud IRAS09149-4743, the Hii region RCW41 (also known as
TYC 8170-333-1, and BRAN 246) is situated at RA 09:16:58, Dec –47:57:47
(J2000), embedded in the Vela Molecular Ridge. This cluster is shown to be
part of Cloud A (Murphy & May, 1991) with two distinct regions; Object 2 to
the south fuelled by a B0V star at a distance of 1.2 $\pm$ 0.12 kpc and Object
1 fuelled by a O9V star at 1.27 $\pm$ 0.13 kpc (Roman-Lopes et al., 2009).
These two regions are marked by “*” on Figure 17. Santos et al. (2012) studied
the polarimetric properties of Object 1 and the sources within the cluster
with deep infrared observations. They determined the cluster age of 2.5–5 MYr
using a set of pre-main-sequence isochrones. In the analysis completed by
Santos et al. (2012), they found several sources exhibiting infrared excess
consistent with pre-main-sequence stellar objects and suggests there is a O9V
star in Object 1, where the main cluster of stellar objects are found. They
also suggest the extinction gradient in the south–north direction may suggest
the O9V star, possibly the most massive in the cluster, has already swept out
dust and gas.

Object 1 shows a strong filament structure when viewed in H$\alpha$ and
matches with the magnetic field directions reported by Santos et al. (2012).
This region (G270.2614 +00.8358) also includes strong masers prominently
detected, which all represent sign-post for high-mass star formation activity.
This has included water masers (Breen & Ellingsen, 2011) at 22 GHz, Class I
and II methanol masers at 6.7, 36 and 44 GHz (i.e Green et al. 2012; Voronkov
et al. 2014; Walsh et al. 1997) but no detection of hydrogen recombination
lines were noted by Wenger et al. (2021), also suggesting the high-mass star
is in the late stages of star formation. The LSR velocity for the 6.7 GHz of
3.9km/s could be within 2.8km/s of the systemic (with reference there for
Green & McClure-Griffiths 2011) and the kinematic distance was not determined
on the basis of Hi self absorption, making it more ambiguous.

Neichel et al. (2015) studied both Objects in the region of RCW41 in detail
using the Gemini-GeMS/GSAOI instrument for wide-field optic infrared. They
find an age gradient along the cluster within and found 1/3 of the young
stellar objects (YSOs) are in a range between 3 to 5 Myr, while 2/3 of the YSO
are $<$3/,Myr old. They also measured the total cluster mass of 78$\pm$18 M⊙
and that the initial mass function matches that of the Trapezium (around the
Orion Nebula).

Emission of H$\alpha$ and 5.8 $\mu$m infrared suggests Object 1 is dominated
by warm gas (Neichel et al., 2015). The MWA and ASKAP data shows a
concentrated region of emission toward the North that matches the WISE
emission in Figure 2 and matches the source structure commented by Santos et
al. (2012). The high-resolution RACS image shows detailed structure in the
emission which matches well with the source positions within the WISE
Catalogue, marked with “*” on Figure 17. The MWA image is confused with
emission from an (likely) unrelated star to the north of the region.

Figure 17: RCW41 Hii imaged in continuum with the MWA (left) at 114 MHz and
ASKAP (right) at 887.5 MHz. The “*” symbol marks unique source positions
listed in the WISE HII catalogue. The red diamond marks the location of the
maser detections. The bottom right-hand corner shows the size and shape of the
synthesised beam for each radio interferometer.

###  5.9 GAL281.01–01.53

This Hii region, located at RA 09:59:08.5, Dec –56:52:44 (J2000) was first
observed in the survey by Caswell & Haynes (1987) and estimated to be at a
distance of 1.9 kpc. The H2CO velocity was measured at –7 km s-1 and the
hydrogen RRLs at 4.8 GHz at –5 km s-1 and they noted that there were no
optical counterparts detected. Kuchar & Clark (1997) observed 760 Hii regions
with Parkes 64m and National Radio Astronomy Observatory Green Bank 100 m
telescopes and compared their flux densities at 4.85 GHz with the those from
Caswell & Haynes (1987). At these frequencies the flux density was measured to
be 8.2 Jy.

In Figure 18 the “*” marks the position of the RRL detection by Wenger et al.
(2021) and 13CO by Urquhart et al. (2007), which had velocities of –6.8 and
–8.7 km s${{}^{-}1}$ respectively. Although the source was included in the
region of methanol maser surveys by Walsh et al. (1997) and van der Walt et
al. (1995), no emission was detected. The MWA continuum image is only slightly
resolved with the emission slightly to the left of the RRL source position.
The source was on the boarder of two RACS footprints and a high resolution
image was not obtained. In the IRAS survey, emission was not detected at all
frequencies so an upper limit on the luminosity was not determined.

Figure 18: GAL281.01–1.53 Hii imaged in continuum with the MWA at 114 MHz The
“*” symbol marks unique source positions listed in the WISE HII catalogue. The
bottom right-hand corner shows the size and shape of the synthesised beam for
each radio interferometer.

##  6 Discussion

Yamaguchi et al. (1999) observed the Gum Nebula in J=1–0 12CO and found 82
molecular clouds and a number cometary globules thought to be nebula.
Observations of CO trace the molecular gas associated with structures within
giant molecular clouds and the study of the kinematics or velocity structures
allows for an assessment of the population density of nebulae, which may
contain Hii regions. They determined that the velocity of these clouds
differed from that of Galactic rotation and had a high velocity dispersion;
suggesting the clouds are expanding under the dynamical motion caused by high-
mass stars and supernova explosions, making it an interesting region to study
with wide-field surveys. It may also suggest that by combining these surveys,
a significant number of new Hii regions may be uncovered.

Radio astronomy surveys, in particular of those less than 1 GHz, provide
important detail on the non-thermal emission toward high-mass star forming
regions. Often, surveys done at high resolution are limited in field-of-view
due to the large quantities of observational time and data processing required
for wide-field analysis. However, with Square Kilometre Array (SKA)
pathfinders, such as ASKAP and the MWA, we are learning the scope of science
that can be accomplished with these new, high-resolution, wide-field
telescopes.

Due to a number of long-running radio astronomy programs and telescopes in the
northern hemisphere, the region above Declination –20 is reasonably well
characterised for known sources to help study star formation models. This
survey is a pilot study to help build a database of southern hemisphere
sources that could broaden our knowledge of star formation and important links
in evolutionary phases which could be further explored as the next generation
southern telescopes come online, like the Square Kilometre Array.

As shown in Figure 19, we are entering into a new phase in radio astronomy
with an abundance of all-sky, high-resolution, low-frequency surveys that will
allow us to study star formation regions to the further reaches of our Galaxy.
The plot from Figure 5 in Hindson et al. (2016), shows the expected SEDs for
compact to ultra compact Hii regions for sources out to 20 kpc based on
equations 1–4 of Mezger & Henderson (1967a). In their paper, they included the
5$\sigma$ limits for the GLEAM survey (Hurley-Walker et al., 2017). We have
added the target sensitivity limits for upcoming surveys expected to be
completed around 2021–2023. This includes the Evolutionary Map of the Universe
(EMU;Norris et al. 2011, 2021), the GLEAM extended survey
(GLEAM-X101010https://www.mwatelescope.org/gleam-x), and the Galactic ASKAP
Survey (GASKAP;Dickey et al. 2013). The fully realised RACS survey (including
mid and high-band ASKAP data;McConnell et al. 2020) will also help in this
analysis. The vertical bands represent the expected frequency range for these
surveys and the horizontal bands represent the expected sensitivity (line
width for the sensitivity are arbitrary).

Figure 19: Figure 5 from Hindson et al. 2016 showing model spectral energy
distributions (SEDs) for different types of Hii regions at a distance of 20
kpc, with the expected detection ranges of upcoming surveys; The Galactic and
Extra-galacitc All-sky MWA eXtended Survey (GLEAM-X; blue bars), Evolutionary
Map of the Universe (EMU; Yellow bars) and Galactic ASKAP Survey (GASKAP;
Orange bars). The green, blue, and red curves show model SEDs for
hypercompact, ultracompact, and compact HII regions. The black (solid and
dashed) curves show model SEDs for classical HII regions of different sizes.
As shown, the next generation ASKAP surveys will be sensitive to the
hypercompact regions (green line) out to 20 kpc.

##  7 Conclusion

In this paper we provide a new multi-wavelength analysis and low-frequency
information on RCW 38 and a selection of Hii regions observable in the
southern hemisphere. Additionally, we use the wide-field capabilities to
simultaneously study the sources with ASKAP and MWA. With new upcoming surveys
like Evolutionary Map of the Universe (EMU;Norris et al. 2011), the completion
of the RACS survey along the ASKAP frequency band, and the upcoming extension
to the Galactic and Extra-Galactic All-sky MWA Survey (GLEAM;Hurley-Walker et
al. 2017), many more surveys of this style will be possible, along with
sensitivity to a broader range of Hii regions.

Many of these sources are interesting as they represent young Hii regions that
are densely packed with high-mass stars. Orion has long been a region of
intense interest for testing models due to its close proximity and large
number of high-mass stars. However, to truly test the models, a larger
population of sources need to be identified. This work provides a glimpse of
what can be achieved with these larger surveys but also what is possible with
the future Square Kilometre Array, in regards to unprecedented sensitivity and
survey speeds.

## Acknowledgements

This publication makes use of data products from the Wide-field Infrared
Survey Explorer, which is a joint project of the University of California, Los
Angeles, and the Jet Propulsion Laboratory/California Institute of Technology,
funded by the National Aeronautics and Space Administration. This scientific
work makes use of the Murchison Radio-astronomy Observatory, operated by
CSIRO. We acknowledge the Wajarri Yamatji people as the traditional owners of
the Observatory site. Support for the operation of the MWA is provided by the
Australian Government (NCRIS), under a contract to Curtin University
administered by Astronomy Australia Limited. The Australian SKA Pathfinder is
part of the Australia Telescope National Facility which is managed by CSIRO.
Operation of ASKAP is funded by the Australian Government with support from
the National Collaborative Research Infrastructure Strategy. Establishment of
ASKAP, the Murchison Radio-astronomy Observatory and the Pawsey Supercomputing
Centre are initiatives of the Australian Government, with support from the
Government of Western Australia and the Science and Industry Endowment Fund.
We acknowledge the Pawsey Supercomputing Centre which is supported by the
Western Australian and Australian Governments. The Australia Telescope Compact
Array is part of the Australia Telescope National Facility which is funded by
the Australian Government for operation as a National Facility managed by
CSIRO. We acknowledge the Gomeroi people as the traditional owners of the
Observatory site.

For this work, we used the following software:

  * •

aoflagger and cotter – Offringa et al. (2015)

  * •

wsclean – Offringa et al. (2014); Offringa & Smirnov (2017)

  * •

Aegean – Hancock et al. (2018)

  * •

topcat – Taylor (2005)

  * •

NumPy v1.11.3 (Dubois et al., 1996), AstroPy v2.0.6 (Astropy Collaboration et
al., 2013), SciPy v0.17.0 (Oliphant, 2007), Matplotlib v1.5.3 (Hunter, 2007)

  * •

CARTA – Comrie et al. (2020)

##  8 Data Availability

A description and citation to relevant survey papers, as available, are listed
in Section 2 of this paper.

The MWA & ATCA data underlying this article will be shared on reasonable
request to the corresponding author. The raw visibilities for the MWA are
available to registered users via the Murchison Widefield Array All-Sky
Virtual Observatory at https://asvo.mwatelescope.org/.

The WISE, AKARI, and SHASSA data underlying this article are available in
$SkyView$ at https://skyview.gsfc.nasa.gov/current/cgi/titlepage.pl, and can
be accessed with the relevant survey and wavelength name.

The WHAM survey data underlying this article are available in the University
of Wisconsin archive at http://www.astro.wisc.edu/wham-site/wham-sky-
survey/wham-ss/ via their web interface.

The Herschel Space Observatory data used in this article are available on the
NASA/IPAC Infrared Science Archive at https://irsa.ipac.caltech.edu/ and is
available on this web service.

The ASKAP RACS Survey data used in this article are available through the
CSIRO ASKAP Science Data Archive at https://research.csiro.au/casda/ and is
available to all registered users on the archive.

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##  Appendix A Calculations

###  A.1 Luminosity from IRAS Fluxes

To estimate the upper limit on the luminosity we use the IRAS fluxes with
equation 3 from Walsh et al. (1997) which is modified from Casoli et al.
(1986). The IRAS measurements can be considered as a strong upper limit on the
bolometric luminosity as the large IRAS beam encapsulates all emission and
there is no loss of flux due to extended emission.

$F_{tot}=(f_{12}\delta\nu_{25}+f_{25}\delta\nu_{12}+f_{60}\delta\nu_{60}+f_{100}\delta\nu_{100})/0.61$
(1)

where $f_{x}$ is the flux measured by Infrared astronomical satellite (IRAS)
catalogs and atlases (IRAS, 1988) and $\delta\nu_{x}$ is the bandwidth for
each band. The value of 0.61 was derived by Walsh et al. (1997) based on the
analysis by Chini et al. (1986) for compact Hii regions. Tremblay et al.
(2015) found this upper limit to be comparable values determined by LABOCA and
other infrared instruments. This equation produces different values than the
Equation 1 from Helou et al. (1985) used for estimating the total luminosity
for extra-galactic sources.

An upper limit on the luminosity is then derived by:

$L_{\odot}=F_{tot}*4\pi r^{2}$ (2)

where r is the distance to the source in kpc.

###  A.2 Lyman Photon Flux

To determine the Lyman Photon Flux values we are using the equations by Mezger
& Henderson (1967b) but with the modifications and assumptions from equations
1 and 3 in Kurtz et al. (1994).

$N_{\mathrm{c}}=8.04\times 10^{46}T_{\mathrm{e}}^{-0.85}U^{3}$ (3)

In this equation $T_{\mathrm{e}}$ is the electron temperature in Kelvins,
which a value of 104 from Haynes et al. (1978). The value $U$ is derived as:

$U=4.553[\frac{1}{\alpha(\nu,T_{\mathrm{e}})}(\frac{\nu}{GHz})^{0.1}(\frac{T_{\mathrm{e}}}{K})^{0.35}(\frac{S_{\nu}}{Jy})(\frac{D}{kpc})^{2}]^{\frac{1}{3}}$
(4)

For these values in Table 1, we used the flux density and frequencies from the
ASKAP data, as the surface brightness sensitivity is better than the MWA with
the short baselines removed.

###  A.3 Data Availability

A description and citation to relevant survey papers, as available, are listed
in Section 2 of this paper.

The MWA & ATCA data underlying this article will be shared on reasonable
request to the corresponding author. The raw visibilities for the MWA are
available to registered users via the Murchison Widefield Array All-Sky
Virtual Observatory at https://asvo.mwatelescope.org/.

The WISE, AKARI, and SHASSA data underlying this article are available in
$SkyView$ at https://skyview.gsfc.nasa.gov/current/cgi/titlepage.pl, and can
be accessed with the relevant survey and wavelength name.

The WHAM survey data underlying this article are available in the University
of Wisconsin archive at http://www.astro.wisc.edu/wham-site/wham-sky-
survey/wham-ss/ via their web interface.

The Herschel Space Observatory data used in this article are available on the
NASA/IPAC Infrared Science Archive at https://irsa.ipac.caltech.edu/ and is
available on this web service.

The ASKAP RACS Survey data used in this article are available through the
CSIRO ASKAP Science Data Archive at https://research.csiro.au/casda/ and is
available to all registered users on the archive.