Search

We present the first search for pulsed CH maser emission potentially stimulated by PSR J1644$-$4559, conducted using the ultra-wide-bandwidth low-frequency receiver on Murriyang, CSIRO's Parkes Radio Telescope. Observations targeted three CH $\Lambda$-doublet transitions at 3264, 3335, and 3349 MHz, with a variability timescale of 78 ms. We detected ten CH emission features at 3335 and 3349 MHz, and seven features at 3264 MHz, during both pulsar-ON and pulsar-OFF phases. The observed velocities align with the OH emission and absorption reported by a previous study, suggesting a close spatial association between CH and OH molecules. The derived column densities for CH clouds within the Parkes beam range from $0.05$ to $9.8 \times 10^{13}$ cm$^{-2}$, indicating that these clouds are likely in diffuse and translucent states. Upper limits for CH column densities within the pulsar beam ranged from $0.3$ to $9.8 \times 10^{13}$ cm$^{-2}$. Comparison of these column densities suggests that CH clouds may exhibit clumpiness and substructure. No significant stimulated emission feature was detected in the optical depth spectra. Additionally, as part of our search for pulsed stimulated emission, we investigated the potential CH absorption of the pulsar signal and found none, in agreement with astrophysical expectations. The upper limits for the potential maser amplification factors towards PSR J1644$-$4559 at 3264, 3335, and 3349 MHz are 1.014, 1.009, and 1.009, respectively. This study demonstrates the feasibility of detecting pulsed CH maser emission in the interstellar medium stimulated by pulsar photons., Comment: If you make use of the CH spectral line dataset in a publication, we request that you acknowledge the source of the information by referencing the DOI: https://doi.org/10.57760/sciencedb.Fastro.00022 and by quoting the web address of "NAOC - The Commensal Radio Astronomy FAST Survey (CRAFTS) Data Community (ScienceDB . Fastro)": https://www.scidb.cn/en/c/o00138

Context. The Galactic ASKAP collaboration (GASKAP) is undertaking an HI emission survey of the 21cm line to map the Magellanic system and the Galactic plane with the Australian Square Kilometre Array Pathfinder (ASKAP). One of the first areas observed in the Pilot Phase I of the survey was the Small Magellanic Cloud (SMC). Previous surveys of the SMC have uncovered new structures in the periphery of the SMC, along relatively low column density lines of sight. Aims. In this work we aimed to uncover the phase distribution of three distinct structures in the periphery of the SMC. This work will add to the constraints we have on the existence and survival of the cold neutral medium (CNM) in the SMC. Methods. We used ROHSA, a Gaussian decomposition algorithm, to model the emission across each cloud and classify the HI emission into their respective phases based on the linewidths of the fitted Gaussians. We created maps of velocity and column density of each phase of the HI across these three clouds. We measured the HI mass and CNM number density for each cloud. We also compared the HI results across the different phases with other gas tracers. Results. We find that in two clouds, the ends of each cloud are almost completely CNM dominated. Analysis of these two clouds indicates they are experiencing a compressive force from the direction of the SMC main body. In the third cloud we find a uniform CNM distribution along one wall of what is likely a supershell structure. Comparison with previous measurements of CO clumps in two of the clouds show the CO and HI are co-moving within a few km/s in regions of high HI column density, particularly when considering just the CNM., Comment: 16 pages, 11 figures, accepted for publication in Astronomy & Astrophysics

Recently, a class of long-period radio transients (LPTs) has been discovered, exhibiting emission on timescales thousands of times longer than radio pulsars. Several models had been proposed implicating either a strong magnetic field neutron star, isolated white dwarf pulsar, or a white dwarf binary system with a low-mass companion. While several models for LPTs also predict X-ray emission, no LPTs have been detected in X-rays despite extensive searches. Here we report the discovery of an extremely bright LPT (10-20 Jy in radio), ASKAP J1832-0911, which has coincident radio and X-ray emission, both with a 44.2-minute period. The X-ray and radio luminosities are correlated and vary by several orders of magnitude. These properties are unique amongst known Galactic objects and require a new explanation. We consider a $\gtrsim0.5$ Myr old magnetar with a $\gtrsim 10^{13}$ G crustal field, or an extremely magnetised white dwarf in a binary system with a dwarf companion, to be plausible explanations for ASKAP J1832-0911, although both explanations pose significant challenges to formation and emission theories. The X-ray detection also establishes a new class of hour-scale periodic X-ray transients of luminosity $\sim10^{33}$ erg/s associated with exceptionally bright coherent radio emission., Comment: 52 pages, 17 figures, 3 tables

The Magellanic Stream, a tidal tail of diffuse gas falling onto the Milky Way, formed by interactions between the Small and Large Magellanic Clouds, is primarily composed of neutral atomic hydrogen (HI). The deficiency of dust and the diffuse nature of the present gas make molecular formation rare and difficult, but if present, could lead to regions potentially suitable for star formation, thereby allowing us to probe conditions of star formation similar to those at high redshifts. We search for HCO$^+$, HCN, HNC, and C$_2$H using the highest sensitivity observations of molecular absorption data from the Atacama Large Millimeter Array to trace these regions, comparing with HI archival data to compare these environments in the Magellanic Stream to the HI column density threshold for molecular formation in the Milky Way. We also compare the line of sight locations with confirmed locations of stars, molecular hydrogen, and OI detections, though at higher sensitivities than the observations presented here. We find no detections to a 3$\sigma$ significance, despite four sightlines having column densities surpassing the threshold for molecular formation in the diffuse regions of the Milky Way. Here we present our calculations for the upper limits of the column densities of each of these molecular absorption lines, ranging from $3 \times 10^{10}$ to $1 \times 10^{13}$ cm$^{-2}$. The non-detection of HCO$^+$ suggests that at least one of the following is true: (i) $X_{HCO^+, \mathrm{MS}}$ is significantly lower than the Milky Way value; (ii) that the widespread diffuse molecular gas observed in the Milky Way's diffuse ISM does not have a direct analog in the MS; (iii) the HI-to-H$_2$ transition occurs in the MS at a higher surface density in the MS than in the LMC or SMC; or (iv) molecular gas exists in the MS, but only in small, dense clumps., Comment: 19 pages, 7 figures, 4 tables, Accepted for publication in PASA

We present the largest Galactic neutral hydrogen HI absorption survey to date, utilizing the Australian SKA Pathfinder Telescope at an unprecedented spatial resolution of 30''. This survey, GASKAP-HI, unbiasedly targets 2,714 continuum background sources over 250 square degrees in the direction of the Magellanic Clouds, a significant increase compared to a total of 373 sources observed by previous Galactic absorption surveys across the entire Milky Way. We aim to investigate the physical properties of cold (CNM) and warm (WNM) neutral atomic gas in the Milky Way foreground, characterized by two prominent filaments at high Galactic latitudes (between $-45^{\circ}$ and $-25^{\circ}$). We detected strong HI absorption along 462 lines of sight above the 3$\sigma$ threshold, achieving an absorption detection rate of 17%. GASKAP-HI's unprecedented angular resolution allows for simultaneous absorption and emission measurements to sample almost the same gas clouds along a line of sight. A joint Gaussian decomposition is then applied to absorption-emission spectra to provide direct estimates of HI optical depths, temperatures, and column densities for the CNM and WNM components. The thermal properties of CNM components are consistent with those previously observed along a wide range of Solar neighborhood environments, indicating that cold HI properties are widely prevalent throughout the local interstellar medium. Across our region of interest, CNM accounts for ~30% of the total HI gas, with the CNM fraction increasing with column density toward the two filaments. Our analysis reveals an anti-correlation between CNM temperature and its optical depth, which implies that CNM with lower optical depth leads to a higher temperature., Comment: Largest Galactic HI Absorption Survey To Date (GASKAP-HI): Cold Atomic Gas in the Magellanic Cloud foreground using Australian SKA Pathfinder. This paper has 19 pages, 17 figures. This paper has been accepted for publication in Monthly Notices of the Royal Astronomical Society Main Journal

Radio astronomy file formats are now required to store wide frequency bandwidths and multiple simultaneous receiver beams and must be able to account for versatile observing modes and numerous calibration strategies. The need to capture and archive high-time and high frequency-resolution data, along with the comprehensive metadata that fully describe the data, implies that a new data format and new processing software are required. This requirement is suited to a well-defined, hierarchically-structured and flexible file format. In this paper we present the Spectral-Domain Hierarchical Data Format (`SDHDF') -- a new file format for radio astronomy data, in particular for single dish or beam-formed data streams. Since 2018, SDHDF has been the primary format for data products from the spectral-line and continuum observing modes at Murriyang, the CSIRO Parkes 64-m radio telescope, and we demonstrate that this data format can also be used to store observations of pulsars and fast radio bursts., Comment: Supplementary material (SDHDF definition): https://ars.els-cdn.com/content/image/1-s2.0-S2213133724000192-mmc1.pdf

In this paper, we investigate the conditions for the HI-to-H$_{2}$ transition in the solar neighborhood by analyzing HI emission and absorption measurements toward 58 Galactic lines of sight (LOSs) along with $^{12}$CO(1$-$0) (CO) and dust data. Based on the accurate column densities of the cold and warm neutral medium (CNM and WNM), we first perform a decomposition of gas into atomic and molecular phases and show that the observed LOSs are mostly HI-dominated. In addition, we find that the CO-dark H$_{2}$, not the optically thick HI, is a major ingredient of the dark gas in the solar neighborhood. To examine the conditions for the formation of CO-bright molecular gas, we analyze the kinematic association between HI and CO and find that the CNM is kinematically more closely associated with CO than the WNM. When CNM components within CO line widths are isolated, we find the following characteristics: spin temperature $<$ 200 K, peak optical depth $>$ 0.1, CNM fraction of $\sim$0.6, and $V$-band dust extinction $>$ 0.5 mag. These results suggest that CO-bright molecular gas preferentially forms in environments with high column densities where the CNM becomes colder and more abundant. Finally, we confront the observed CNM properties with the steady-state H$_{2}$ formation model of Sternberg et al. and infer that the CNM must be clumpy with a small volume filling factor. Another possibility would be that missing processes in the model, such as cosmic-rays and gas dynamics, play an important role in the HI-to-H$_{2}$ transition., Comment: 25 pages, 12 figures, 6 tables, Accepted for publication in ApJ

We present observations of the four 2 Pi 3/2 J = 3/2 ground-rotational state transitions of the hydroxyl molecule (OH) along 107 lines of sight both in and out of the Galactic plane: 92 sets of observations from the Arecibo telescope and 15 sets of observations from the Australia Telescope Compact Array (ATCA). Our Arecibo observations included off-source pointings, allowing us to measure excitation temperature (Tex) and optical depth, while our ATCA observations give optical depth only. We perform Gaussian decomposition using the Automated Molecular Excitation Bayesian line-fitting Algorithm 'AMOEBA' (Petzler, Dawson, and Wardle 2021) fitting all four transitions simultaneously with shared centroid velocity and width. We identify 109 features across 38 sightlines (including 58 detections along 27 sightlines with excitation temperature measurements). While the main lines at 1665 and 1667 MHz tend to have similar excitation temperatures (median Tex(main) difference = 0.6 K, 84% show Tex(main) difference < 2 K), large differences in the 1612 and 1720 MHz satellite line excitation temperatures show that the gas is generally not in LTE. For a selection of sightlines we compare our OH features to associated (on-sky and in velocity) HI cold gas components (CNM) identified by Nguyen et al. (2019) and find no strong correlations. We speculate that this may indicate an effective decoupling of the molecular gas from the CNM once it accumulates., Comment: Accepted for publication to PASA. 41 pages, 27 figures

Pulsar timing array collaborations have recently reported evidence for a noise process with a common spectrum among the millisecond pulsars in the arrays. The spectral properties of this common-noise process are consistent with expectations for an isotropic gravitational-wave background (GWB) from inspiralling supermassive black-hole binaries. However, recent simulation analyses based on Parkes Pulsar Timing Array data indicate that such a detection may arise spuriously. In this paper, we use simulated pulsar timing array datasets to further test the robustness of the inference methods for spectral and spatial correlations from a GWB. Expanding on our previous results, we find strong support (Bayes factors exceeding $10^5$) for the presence of a common-spectrum noise process in datasets where no common process is present, under a wide range of timing noise prescriptions per pulsar. We show that these results are highly sensitive to the choice of Bayesian priors on timing noise parameters, with priors that more closely match the injected distributions of timing noise parameters resulting in diminished support for a common-spectrum noise process. These results emphasize shortcomings in current methods for inferring the presence of a common-spectrum process, and imply that the detection of a common process is not a reliable precursor to detection of the GWB. Future searches for the nanohertz GWB should remain focussed on detecting spatial correlations, and make use of more tailored specifications for a common-spectrum noise process., Comment: 12 pages, 12 figures. Accepted for publication in MNRAS

We present the full data release for the Southern Parkes Large-Area Survey in Hydroxyl (SPLASH), a sensitive, unbiased single-dish survey of the Southern Galactic Plane in all four ground-state transitions of the OH radical at 1612, 1665, 1667 and 1720 MHz. The survey covers the inner Galactic Plane, Central Molecular Zone and Galactic Centre over the range $|b|<$ 2$^{\circ}$, 332$^{\circ}$ $< l <$ 10$^{\circ}$, with a small extension between 2$^{\circ}$ $< b <$ 6$^{\circ}$, 358$^{\circ}$ $< l <$ 4$^{\circ}$. SPLASH is the most sensitive large-scale survey of OH to-date, reaching a characteristic root-mean-square sensitivity of $\sim15$ mK for an effective velocity resolution of $\sim0.9$ km/s. The spectral line datacubes are optimised for the analysis of extended, quasi-thermal OH, but also contain numerous maser sources, which have been confirmed interferometrically and published elsewhere. We also present radio continuum images at 1612, 1666 and 1720 MHz. Based on initial comparisons with $^{12}$CO(J=1-0), we find that OH rarely extends outside CO cloud boundaries in our data, but suggest that large variations in CO-to-OH brightness temperature ratios may reflect differences in the total gas column density traced by each. Column density estimation in the complex, continuum-bright Inner Galaxy is a challenge, and we demonstrate how failure to appropriately model sub-beam structure and the line-of-sight source distribution can lead to order-of-magnitude errors. Anomalous excitation of the 1612 and 1720 MHz satellite lines is ubiquitous in the inner Galaxy, but is disabled by line overlap in and around the Central Molecular Zone., Comment: Main paper: 22 pages, 15 figures. Online-only material (appended to ArXiv PDF): 11 pages, 3 figure sets. Accepted to MNRAS. Survey data is available from: https://docs.datacentral.org.au/splash/

We demonstrate how radio pulsars can be used as random number generators. Specifically, we focus on publicly verifiable randomness (PVR), in which the same sequence of trusted and verifiable random numbers is obtained by multiple parties. PVR is a critical building block for many processes and algorithms (including cryptography, scientific trials, electoral audits and international treaties). However, current approaches (based on number theory) may soon become vulnerable to quantum computers, motivating a growing demand for PVR based on natural physical phenomena. In this context, we explore pulsars as a potential physical PVR source. We first show that bit sequences extracted from the measured flux densities of a bright millisecond pulsar can pass standardised tests for randomness. We then quantify three illustrative methods of bit-extraction from pulsar flux density sequences, using simultaneous observations of a second pulsar carried out with the Parkes telescope in Australia and the Five-hundred-metre Aperture Spherical radio Telescope (FAST) in China, supported by numerical simulations. We demonstrate that the same bit sequence can indeed be obtained at both observatories, but the ubiquitous presence of radiometer noise needs to be accounted for when determining the expected bit error rate between two independent sequences. We discuss our results in the context of an imaginary use-case in which two mutually distrusting parties wish to obtain the same random bit sequence, exploring potential methods to mitigate against a malicious participant., Comment: 12 pages, 4 figures, accepted to Astronomy & Computing

We present the most sensitive and detailed view of the neutral hydrogen (HI) emission associated with the Small Magellanic Cloud (SMC), through the combination of data from the Australian Square Kilometre Array Pathfinder (ASKAP) and Parkes (Murriyang), as part of the Galactic Australian Square Kilometre Array Pathfinder (GASKAP) pilot survey. These GASKAP-HI pilot observations, for the first time, reveal HI in the SMC on similar physical scales as other important tracers of the interstellar medium, such as molecular gas and dust. The resultant image cube possesses an rms noise level of 1.1 K (1.6 mJy/beam) per 0.98 km s$^{-1}$ spectral channel with an angular resolution of 30$''$ ($\sim$10 pc). We discuss the calibration scheme and the custom imaging pipeline that utilizes a joint deconvolution approach, efficiently distributed across a computing cluster, to accurately recover the emission extending across the entire $\sim$25 deg$^2$ field-of-view. We provide an overview of the data products and characterize several aspects including the noise properties as a function of angular resolution and the represented spatial scales by deriving the global transfer function over the full spectral range. A preliminary spatial power spectrum analysis on individual spectral channels reveals that the power-law nature of the density distribution extends down to scales of 10 pc. We highlight the scientific potential of these data by comparing the properties of an outflowing high velocity cloud with previous ASKAP+Parkes HI test observations., Comment: Accepted for publication in PASA, 34 pages, 18 figures, 5 tables

Using the Australian Square Kilometre Array Pathfinder to measure 21-cm absorption spectra toward continuum background sources, we study the cool phase of the neutral atomic gas in the far outer disk, and in the inner Galaxy near the end of the Galactic bar at longitude 340 degrees. In the inner Galaxy the cool atomic gas has a smaller scale height than in the solar neighborhood, similar to the molecular gas and the superthin stellar population in the bar. In the outer Galaxy the cool atomic gas is mixed with the warm, neutral medium, with the cool fraction staying roughly constant with Galactic radius. The mean spin temperature, i.e. the ratio of the emission brightness temperature to the absorption, is roughly constant for velocities corresponding to Galactic radius greater than about twice the solar circle radius. The ratio has a value of about 300 K, but this does not correspond to a physical temperature in the gas. If the gas causing the absorption has kinetic temperature of about 100 K, as in the solar neighborhood, then the value 300 K indicates that the fraction of the gas mass in this phase is one-third of the total HI mass., Comment: 29 pages, 12 figures, submitted to Astrophysical Journal

We investigate the kinematic properties of Galactic HII regions using radio recombination line (RRL) emission detected by the Australia Telescope Compact Array (ATCA) at 4-10 GHz and the Jansky Very Large Array (VLA) at 8-10 GHz. Our HII region sample consists of 425 independent observations of 374 nebulae that are relatively well isolated from other, potentially confusing sources and have a single RRL component with a high signal-to-noise ratio. We perform Gaussian fits to the RRL emission in position-position-velocity data cubes and discover velocity gradients in 178 (42%) of the nebulae with magnitudes between 5 and 200 m/s/arcsec. About 15% of the sources also have a RRL width spatial distribution that peaks toward the center of the nebula. The velocity gradient position angles appear to be random on the sky with no favored orientation with respect to the Galactic Plane. We craft HII region simulations that include bipolar outflows or solid body rotational motions to explain the observed velocity gradients. The simulations favor solid body rotation since, unlike the bipolar outflow kinematic models, they are able to produce both the large, > 40 m/s/arcsec, velocity gradients and also the RRL width structure that we observe in some sources. The bipolar outflow model, however, cannot be ruled out as a possible explanation for the observed velocity gradients for many sources in our sample. We nevertheless suggest that most HII region complexes are rotating and may have inherited angular momentum from their parent molecular clouds., Comment: Accepted to ApJ

We investigate the Hi envelope of the young, massive GMCs in the star-forming regions N48 and N49, which are located within the high column density Hi ridge between two kpc-scale supergiant shells, LMC 4 and LMC 5. New long-baseline Hi 21 cm line observations with the Australia Telescope Compact Array (ATCA) were combined with archival shorter baseline data and single dish data from the Parkes telescope, for a final synthesized beam size of 24.75" by 20.48", which corresponds to a spatial resolution of ~ 6 pc in the LMC. It is newly revealed that the Hi gas is highly filamentary, and that the molecular clumps are distributed along filamentary Hi features. In total 39 filamentary features are identified and their typical width is ~ 21 (8-49) [pc]. We propose a scenario in which the GMCs were formed via gravitational instabilities in atomic gas which was initially accumulated by the two shells and then further compressed by their collision. This suggests that GMC formation involves the filamentary nature of the atomic medium., Comment: 22 pages, 4 tables, 16 figures

Optical depth variations in the Galactic neutral interstellar medium (ISM) with spatial scales from hundreds to thousands of astronomical units have been observed through HI absorption against pulsars and continuum sources, while extremely small structures with spatial scales of tens of astronomical units remain largely unexplored. The nature and formation of such tiny-scale atomic structures (TSAS) need to be better understood. We report a tentative detection of TSAS with a signal-to-noise ratio of 3.2 toward PSR B1557$-$50 in the second epoch of two Parkes sessions just 0.36 yr apart, which are the closest-spaced spectral observations toward this pulsar. One absorption component showing marginal variations has been identified. Based on the pulsar's proper motion of 14 mas $\rm yr^{-1}$ and the component's kinematic distance of 3.3 kpc, the corresponding characteristic spatial scale is 17 au, which is among the smallest sizes of known TSAS. Assuming a similar line-of-sight (LOS) depth, the tentative TSAS cloud detected here is overdense and overpressured relative to the cold neutral medium (CNM), and can radiatively cool fast enough to be in thermal equilibrium with the ambient environment. We find that turbulence is not sufficient to confine the overpressured TSAS. We explore the LOS elongation that would be required for the tentative TSAS to be at the canonical CNM pressure, and find that it is $\sim5000$ -- much larger than filaments observed in the ISM. We see some evidence of line width and temperature variations in the CNM components observed at the two epochs, as predicted by models of TSAS-like cloud formation colliding warm neutral medium flows., Comment: 11 pages, 2 figures, 2 tables, accepted for publication in the ApJL

The Southern HII Region Discovery Survey (SHRDS) is a 900 hour Australia Telescope Compact Array 4-10 GHz radio continuum and radio recombination line (RRL) survey of Galactic HII regions and infrared-identified HII region candidates in the southern sky. For this data release, we reprocess all previously published SHRDS data and include an additional ~450 hours of observations. The search for new HII regions is now complete over the range 259 deg < Galactic longitude < 346 deg, |Galactic latitude| < 4 deg for HII region candidates with predicted 6 GHz continuum peak brightnesses >30 mJy/beam. We detect radio continuum emission toward 730 targets altogether including previously known nebulae and HII region candidates. By averaging ~18 RRL transitions, we detect RRL emission toward 206 previously known HII regions and 436 HII region candidates. Including the northern sky surveys, over the last decade the HII Region Discovery Surveys have more than doubled the number of known Galactic HII regions. The census of HII regions in the WISE Catalog of Galactic HII Regions is now complete for nebulae with 9 GHz continuum flux densities > 250 mJy. We compare the RRL properties of the newly discovered SHRDS nebulae with those of all previously known HII regions. The median RRL full-width at half-maximum line width of the entire WISE Catalog HII region population is 23.9 km/s and is consistent between Galactic quadrants. The observed Galactic longitude-velocity asymmetry in the population of HII regions probably reflects underlying spiral structure in the Milky Way., Comment: 37 pages, 10 figures, 11 tables (full tables in machine readable format available in the online journal), 1 appendix. Survey website: https://www.cadc-ccda.hia-iha.nrc-cnrc.gc.ca/en/community/shrds/

From targeted observations of ground-state OH masers towards 702 Multibeam (MMB) survey 6.7-GHz methanol masers, between Galactic longitudes 186$^{\circ}$ through the Galactic centre to 20$^{\circ}$, made as part of the `MAGMO' project, we present the physical and polarisation properties of the 1720-MHz OH maser transition, including the identification of Zeeman pairs. We present 10 new and 23 previously catalogued 1720-MHz OH maser sources detected towards star formation regions. In addition, we also detected 16 1720-MHz OH masers associated with supernova remnants and two sites of diffuse OH emission. Towards the 33 star formation masers, we identify 44 Zeeman pairs, implying magnetic field strengths ranging from $-$11.4 to $+$13.2 mG, and a median magnetic field strength of $|B_{LOS}|$ $\sim$ 6 mG. With limited statistics, we present the in-situ magnetic field orientation of the masers and the Galactic magnetic field distribution revealed by the 1720-MHz transition. We also examine the association statistics of 1720-MHz OH SFR masers with other ground-state OH masers, excited-state OH masers, class I and class II methanol masers and water masers, and compare maser positions with mid-infrared images of the parent star forming regions. Of the 33 1720-MHz star formation masers, ten are offset from their central exciting sources, and appear to be associated with outflow activity.

Carbon-bearing molecules, particularly CO, have been widely used as tracers of molecular gas in the interstellar medium (ISM). In this work, we aim to study the properties of molecules in diffuse, cold environments, where CO tends to be under-abundant and/or sub-thermally excited. We performed one of the most sensitive (down to $\mathrm{\tau_{rms}^{CO} \sim 0.002}$ and $\mathrm{\tau_{rms}^{HCO^+} \sim 0.0008}$) sub-millimeter molecular absorption line observations towards 13 continuum sources with the ALMA. CO absorption was detected in diffuse ISM down to $\mathrm{A_v< 0.32\,mag}$ and \hcop was down to $\mathrm{A_v < 0.2\,mag}$, where atomic gas and dark molecular gas (DMG) starts to dominate. Multiple transitions measured in absorption toward 3C454.3 allow for a direct determination of excitation temperatures $\mathrm{T_{ex}}$ of 4.1\,K and 2.7\,K, for CO and for \hcop, respectively, which are close to the cosmic microwave background (CMB) and provide explanation for their being undercounted in emission surveys. A stronger linear correlation was found between $\mathrm{N_{HCO^+}}$ and $\mathrm{N_{H_2}}$ (Pearson correlation coefficient P $\sim$ 0.93) than that of $\mathrm{N_{CO}}$ and $\mathrm{N_{H_2}}$ (P $\sim$ 0.33), suggesting \hcop\ being a better tracer of H$_2$ than CO in diffuse gas. The derived CO-to-\h2 conversion factor (the CO X-factor) of (14 $\pm$ 3) $\times$ 10$^{20}$ cm$^{-2}$ (K \kms)$^{-1}$ is approximately 6 times larger than the average value found in the Milky Way.

We present high spatial resolution observations of ground-state OH masers achieved with the Australia Telescope Compact Array (ATCA). These observations targeted 253 pointing centres containing OH maser candidates at all four ground-state OH transitions identified in the Southern Parkes Large-Area Survey in Hydroxyl (SPLASH) across 96 square degrees of the Southern Galactic plane (332\degree$

We describe an ultra-wide-bandwidth, low-frequency receiver ("UWL") recently installed on the Parkes radio telescope. The receiver system provides continuous frequency coverage from 704 to 4032 MHz. For much of the band (~60%) the system temperature is approximately 22K and the receiver system remains in a linear regime even in the presence of strong mobile phone transmissions. We discuss the scientific and technical aspects of the new receiver including its astronomical objectives, as well as the feed, receiver, digitiser and signal-processor design. We describe the pipeline routines that form the archive-ready data products and how those data files can be accessed from the archives. The system performance is quantified including the system noise and linearity, beam shape, antenna efficiency, polarisation calibration and timing stability., Comment: submitted to PASA

We report Arecibo 21 cm absorption-emission observations to characterise the physical properties of neutral hydrogen (HI) in the proximity of five giant molecular clouds (GMCs): Taurus, California, Rosette, Mon OB1, and NGC 2264. Strong HI absorption was detected toward all 79 background continuum sources in the ~60x20 square degree region. Gaussian decompositions were performed to estimate temperatures, optical depths and column densities of the cold and warm neutral medium (CNM, WNM). The properties of individual CNM components are similar to those previously observed along random Galactic sightlines and in the vicinity of GMCs, suggesting a universality of cold HI properties. The CNM spin temperature (Ts) histogram peaks at ~50K. The turbulent Mach numbers of CNM vary widely, with a typical value of ~4, indicating that their motions are supersonic. About 60% of the total HI gas is WNM, and nearly 40% of the WNM lies in thermally unstable regime 500-5000K. The observed CNM fraction is higher around GMCs than in diffuse regions, and increases with increasing column density (NHI) to a maximum of ~75%. On average, the optically thin approximation (N*(HI)) underestimates the total N(HI) by ~21%, but we find large regional differences in the relationship between N(HI) and the required correction factor, f=N(HI)/N*(HI). We examine two different methods (linear fit of f vs log10(N*(HI)) and uniform Ts) to correct for opacity effects using emission data from the GALFA-HI survey. We prefer the uniform Ts method, since the linear relationship does not produce convincing fits for all subregions., Comment: This article (22 pages, 23 figures) has been accepted for publication in The Astrophysical Journal (ApJ)

The census of Galactic HII regions is vastly incomplete in the Southern sky. We use the Australia Telescope Compact Array (ATCA) to observe 4-10 GHz radio continuum and hydrogen radio recombination line (RRL) emission from candidate HII regions in the Galactic zone 259 deg < l < 344 deg, |b| < 4 deg. In this first data release, we target the brightest HII region candidates and observe 282 fields in the direction of at least one previously-known or candidate HII region. We detect radio continuum emission and RRL emission in 275 (97.5%) and 258 (91.5%) of these fields, respectively. We catalog the ~7 GHz radio continuum peak flux densities and positions of 80 previously-known and 298 candidate HII regions. After averaging ~18 RRL transitions, we detect 77 RRL velocity components towards 76 previously-known HII regions and 267 RRL velocity components towards 256 HII region candidates. The discovery of RRL emission from these nebulae increases the number of known Galactic HII regions in the surveyed zone by 82%, to 568 nebulae. In the fourth quadrant we discover 50 RRLs with positive velocities, placing those sources outside the Solar circle. Including the pilot survey, the SHRDS has now discovered 295 Galactic HII regions. In the next data release we expect to add ~200 fainter and more distant nebulae., Comment: 113 pages, 15 figures, 11 tables (at the end); accepted for publication in ApJS

We present high spatial resolution observations of ground-state OH masers, achieved using the Australia Telescope Compact Array (ATCA). These observations were conducted towards 171 pointing centres, where OH maser candidates were identified previously in the Southern Parkes Large-Area Survey in Hydroxyl (SPLASH) towards the Galactic Center region, between Galactic longitudes of $355^{\circ}$ and $5^{\circ}$ and Galactic latitudes of $-2^{\circ}$ and $+2^{\circ}$. We detect maser emission towards 162 target fields and suggest that 6 out of 9 non-detections are due to intrinsic variability. Due to the superior spatial resolution of the follow-up ATCA observations, we have identified 356 OH maser sites in the 162 of the target fields with maser detections. Almost half (161 of 356) of these maser sites have been detected for the first time in these observations. After comparing the positions of these 356 maser sites to the literature, we find that 269 (76\%) sites are associated with evolved stars (two of which are planetary nebulae), 31 (9\%) are associated with star formation, four are associated with supernova remnants and we were unable to determine the origin of the remaining 52 (15\%) sites. Unlike the pilot region (\citealt{Qie2016a}), the infrared colors of evolved star sites with symmetric maser profiles in the 1612 MHz transition do not show obvious differences compared with those of evolved star sites with asymmetric maser profiles., Comment: 24 pages, 12 figures, accepted by ApJS

Observations of interstellar dust are often used as a proxy for total gas column density $N_\mathrm{H}$. By comparing $\textit{Planck}$ thermal dust data (Release 1.2) and new dust reddening maps from Pan-STARRS 1 and 2MASS (Green et al. 2018), with accurate (opacity-corrected) HI column densities and newly-published OH data from the Arecibo Millennium survey and 21-SPONGE, we confirm linear correlations between dust optical depth $\tau_{353}$, reddening $E(B{-}V)$ and the total proton column density $N_\mathrm{H}$ in the range (1$-$30)$\times$10$^{20}$cm$^{-2}$, along sightlines with no molecular gas detections in emission. We derive an $N_\mathrm{H}$/$E(B{-}V)$ ratio of (9.4$\pm$1.6)$\times$10$^{21}$cm$^{-2}$mag$^{-1}$ for purely atomic sightlines at $|b|$$>$5$^{\circ}$, which is 60$\%$ higher than the canonical value of Bohlin et al. (1978). We report a $\sim$40$\%$ increase in opacity $\sigma_{353}$=$\tau_{353}$/$N_\mathrm{H}$, when moving from the low column density ($N_\mathrm{H}$$<$5$\times$10$^{20}$cm$^{-2}$) to moderate column density ($N_\mathrm{H}$$>$5$\times$10$^{20}$cm$^{-2}$) regime, and suggest that this rise is due to the evolution of dust grains in the atomic ISM. Failure to account for HI opacity can cause an additional apparent rise in $\sigma_{353}$, of the order of a further $\sim$20$\%$. We estimate molecular hydrogen column densities $N_{\mathrm{H}_{2}}$ from our derived linear relations, and hence derive the OH/H$_2$ abundance ratio of $X_\mathrm{OH}$$\sim$1$\times$10$^{-7}$ for all molecular sightlines. Our results show no evidence of systematic trends in OH abundance with $N_{\mathrm{H}_{2}}$ in the range $N_{\mathrm{H}_{2}}$$\sim$(0.1$-$10)$\times$10$^{21}$cm$^{-2}$. This suggests that OH may be used as a reliable proxy for H$_2$ in this range, which includes sightlines with both CO-dark and CO-bright gas., Comment: The revised manuscript is accepted for publication in The Astrophysical Journal

HI self absorption (HISA) clouds are clumps of cold neutral hydrogen (HI) visible in front of warm background gas, which makes them ideal places to study the properties of the cold atomic component of the interstellar medium (ISM). The Riegel-Crutcher (R-C) cloud is the most striking HISA feature in the Galaxy. It is one of the closest HISA clouds to us and is located in the direction of the Galactic Centre, which provides a bright background. High-resolution interferometric measurements have revealed the filamentary structure of this cloud, however it is difficult to accurately determine the temperature and the density of the gas without optical depth measurements. In this paper we present new HI absorption observations with the Australia Telescope Compact Array (ATCA) against 46 continuum sources behind the Riegel-Crutcher cloud to directly measure the optical depth of the cloud. We decompose the complex HI absorption spectra into Gaussian components using an automated machine learning algorithm. We find 300 Gaussian components, from which 67 are associated with the R-C cloud (0 < v_{LSR} < 10 kms, FWHM < 10 kms). Combining the new HI absorption data with HI emission data from previous surveys we calculate the spin temperature and find it to be between 20 and 80 K. Our measurements uncover a temperature gradient across the cloud with spin temperatures decreasing towards positive Galactic latitudes. We also find three new OH absorption lines associated with the cloud, which support the presence of molecular gas., Comment: Accepted to MNRAS, 27 pages, 26 figures, 7 tables

Hydroxyl (OH) is expected to be abundant in diffuse interstellar molecular gas as it forms along with $H_2$ under similar conditions and within a similar extinction range. We have analyzed absorption measurements of OH at 1665 MHz and 1667 MHz toward 44 extragalactic continuum sources, together with the J=1-0 transitions of $^{12}$CO, $^{13}$CO , and C$^{18}$O, and the J=2-1 of $^{12}$CO. The excitation temperature of OH were found to follow a modified log-normal distribution, $ f(T{\rm_{ex}}) \propto \frac{1}{ \sqrt{2\pi}\sigma } \rm{exp}\left[-\frac{[ln(\textit{T}_{ex})-ln(3.4\ K)]^2}{2\sigma^2}\right] $, the peak of which is close to the temperature of the Galactic emission background (CMB+synchron). In fact, 90% of the OH has excitation temperature within 2 K of the Galactic background at the same location, providing a plausible explanation for the apparent difficulty to map this abundant molecule in emission. The opacities of OH were found to be small and peak around 0.01. For gas at intermediate extinctions (A$\rm_V$ $\sim$ 0.05-2 mag), the detection rate of OH with detection limit $N(\mathrm{OH})\simeq 10^{12}$ cm$^{-2}$ is approximately independent of $A\rm_V$. We conclude that OH is abundant in the diffuse molecular gas and OH absorption is a good tracer of `dark molecular gas (DMG)'. The measured fraction of DMG depends on assumed detection threshold of the CO data set. The next generation of highly sensitive low frequency radio telescopes, FAST and SKA, will make feasible the systematic inventory of diffuse molecular gas, through decomposing in velocity the molecular (e.g. OH and CH) absorption profiles toward background continuum sources with numbers exceeding what is currently available by orders of magnitude., Comment: 24 pages, 23 figures. Accepted for publication in ApJS

The relative orientation between filamentary structures in molecular clouds and the ambient magnetic field provides insight into filament formation and stability. To calculate the relative orientation, a measurement of filament orientation is first required. We propose a new method to calculate the orientation of the one pixel wide filament skeleton that is output by filament identification algorithms such as \textsc{filfinder}. We derive the local filament orientation from the direction of the intensity gradient in the skeleton image using the Sobel filter and a few simple post-processing steps. We call this the `Sobel-gradient method'. The resulting filament orientation map can be compared quantitatively on a local scale with the magnetic field orientation map to then find the relative orientation of the filament with respect to the magnetic field at each point along the filament. It can also be used in constructing radial profiles for filament width fitting. The proposed method facilitates automation in analysis of filament skeletons, which is imperative in this era of `big data'., Comment: 12 pages, 7 figures. Accepted for publication in ApJS, August 2017

We present a study of the three-dimensional structure of the molecular clouds in the Galactic Centre (GC) using CO emission and OH absorption lines. Two CO isotopologue lines, $^{12}$CO ($J$=1$\rightarrow$0) and $^{13}$CO ($J$=1$\rightarrow$0), and four OH ground-state transitions, surveyed by the Southern Parkes Large-Area Survey in Hydroxyl (SPLASH), contribute to this study. We develop a novel method to calculate the OH column density, excitation temperature, and optical depth precisely using all four OH lines, and we employ it to derive a three-dimensional model for the distribution of molecular clouds in the GC for six slices in Galactic latitude. The angular resolution of the data is 15.5 arcmin, which at the distance of the GC (8.34 kpc) is equivalent to 38 pc. We find that the total mass of OH in the GC is in the range 2400-5100 Solar mass . The face-on view at a Galactic latitude of b = 0{\deg} displays a bar-like structure with an inclination angle of 67.5 $\pm$ 2.1{\deg} with respect to the line of sight. No ring-like structure in the GC is evident in our data, likely due to the low spatial resolution of the CO and OH maps., Comment: Accepted for publication in MNRAS, 15 pages, 12 figures

The Southern HII Region Discovery Survey (SHRDS) is a survey of the third and fourth quadrants of the Galactic plane that will detect radio recombination line and continuum emission at cm-wavelengths from several hundred HII region candidates using the Australia Telescope Compact Array. The targets for this survey come from the WISE Catalog of Galactic HII Regions, and were identified based on mid-infrared and radio continuum emission. In this pilot project, two different configurations of the Compact Array Broad Band receiver and spectrometer system were used for short test observations. The pilot surveys detected radio recombination line emission from 36 of 53 HII region candidates, as well as seven known \hii regions that were included for calibration. These 36 recombination line detections confirm that the candidates are true HII regions, and allow us to estimate their distances., Comment: 19 pages, 5 figures, Astronomical Journal in press

Different combinations of input parameters to filament identification algorithms, such as Disperse and FilFinder, produce numerous different output skeletons. The skeletons are a one pixel wide representation of the filamentary structure in the original input image. However, these output skeletons may not necessarily be a good representation of that structure. Furthermore, a given skeleton may not be as good a representation as another. Previously there has been no mathematical `goodness-of-fit' measure to compare output skeletons to the input image. Thus far this has been assessed visually, introducing visual bias. We propose the application of the mean structural similarity index (MSSIM) as a mathematical goodness-of-fit measure. We describe the use of the MSSIM to find the output skeletons most mathematically similar to the original input image (the optimum, or `best', skeletons) for a given algorithm, and independently of the algorithm. This measure makes possible systematic parameter studies, aimed at finding the subset of input parameter values returning optimum skeletons. It can also be applied to the output of non-skeleton based filament identification algorithms, such as the Hessian matrix method. The MSSIM removes the need to visually examine thousands of output skeletons, and eliminates the visual bias, subjectivity, and limited reproducibility inherent in that process, representing a major improvement on existing techniques. Importantly, it also allows further automation in the post-processing of output skeletons, which is crucial in this era of `big data'., Comment: 8 pages, 3 figures, Accepted for publication in ApJL, April 2017

We have obtained OH spectra of four transitions in the $^2\Pi_{3/2}$ ground state, at 1612, 1665, 1667, and 1720 MHz, toward 51 sightlines that were observed in the Herschel project Galactic Observations of Terahertz C+. The observations cover the longitude range of (32$^\circ$, 64$^\circ$) and (189$^\circ$, 207$^\circ$) in the northern Galactic plane. All of the diffuse OH emissions conform to the so-called 'Sum Rule' of the four brightness temperatures, indicating optically thin emission condition for OH from diffuse clouds in the Galactic plane. The column densities of the HI `halos' N(HI) surrounding molecular clouds increase monotonically with OH column density, N(OH), until saturating when N(HI)=1.0 x 10$^{21}$ cm$^{-2}$ and N (OH) $\geq 4.5\times 10^{15}$ cm$^{-2}$, indicating the presence of molecular gas that cannot be traced by HI. Such a linear correlation, albeit weak, is suggestive of HI halos' contribution to the UV shielding required for molecular formation. About 18% of OH clouds have no associated CO emission (CO-dark) at a sensitivity of 0.07 K but are associated with C$^+$ emission. A weak correlation exists between C$^+$ intensity and OH column density for CO-dark molecular clouds. These results imply that OH seems to be a better tracer of molecular gas than CO in diffuse molecular regions., Comment: 13 pages, 12 figures, accepted for publication in ApJ

We report on high spatial resolution observations, using the Australia Telescope Compact Array (ATCA), of ground-state OH masers. These observations were carried out toward 196 pointing centres previously identified in the Southern Parkes Large-Area Survey in Hydroxyl (SPLASH) pilot region, between Galactic longitudes of $334^{\circ}$ and $344^{\circ}$ and Galactic latitudes of $-2^{\circ}$ and $+2^{\circ}$. Supplementing our data with data from the MAGMO (Mapping the Galactic Magnetic field through OH masers) survey, we find maser emission towards 175 of the 196 target fields. We conclude that about half of the 21 non-detections were due to intrinsic variability. Due to the superior sensitivity of the follow-up ATCA observations, and the ability to resolve nearby sources into separate sites, we have identified 215 OH maser sites towards the 175 fields with detections. Among these 215 OH maser sites, 111 are new detections. After comparing the positions of these 215 maser sites to the literature, we identify 122 (57 per cent) sites associated with evolved stars (one of which is a planetary nebula), 64 (30 per cent) with star formation, two sites with supernova remnants and 27 (13 per cent) of unknown origin. The infrared colors of evolved star sites with symmetric maser profiles tend to be redder than those of evolved star sites with asymmetric maser profiles, which may indicate that symmetric sources are generally at an earlier evolutionary stage., Comment: 29 pages, 17 figures, accepted by ApJS, 162 on-line figures

We present the first detection of HCO$^+$ absorption in the Magellanic System. Using the Australia Telescope Compact Array (ATCA), we observed 9 extragalactic radio continuum sources behind the Magellanic System and detected HCO$^+$ absorption towards one source located behind the leading edge of the Magellanic Bridge. The detection is located at LSR velocity of $v=214.0 \pm 0.4\rm\,km\,s^{-1}$, with a full width at half maximum of $\Delta v=4.5\pm 1.0\rm\,km\,s^{-1}$ and optical depth of $\tau(\rm HCO^+)=0.10\pm 0.02$. Although there is abundant neutral hydrogen (HI) surrounding the sightline in position-velocity space, at the exact location of the absorber the HI column density is low, $<10^{20}\rm\,cm^{-2}$, and there is little evidence for dust or CO emission from Planck observations. While the origin and survival of molecules in such a diffuse environment remains unclear, dynamical events such as HI flows and cloud collisions in this interacting system likely play an important role., Comment: Accepted for publication in ApJ. 6 pages, 2 figures, 2 tables

We present distributions of two molecular clouds having velocities of 2 km s$^{-1}$ and 14 km s$^{-1}$ toward RCW 38, the youngest super star cluster in the Milky Way, in the $^{12}$CO ($J=$1--0 and 3--2) and $^{13}$CO ($J=$1--0) transitions. The two clouds are likely physically associated with the cluster as verified by the high intensity ratio of the $J$=3--2 emission to the $J$=1--0 emission, the bridging feature connecting the two clouds in velocity and their morphological correspondence with the infrared dust emission. The total mass of the clouds and the cluster is too small to gravitationally bind the velocity difference. We frame a hypothesis that the two clouds are colliding with each other by chance to trigger formation of the $\sim$20 candidate O stars which are localized within $\sim$0.3 pc of the cluster center in the 2 km s$^{-1}$ cloud. We suggest that the collision is currently continuing toward part of the 2 km s$^{-1}$ cloud where the bridging feature is localized. This is the third super star cluster alongside of Westerlund2 and NGC3603 where cloud-cloud collision triggered the cluster formation. RCW38 is the most remarkable and youngest cluster, holding a possible sign of on-going O star formation, and is the most promising site where we may be able to witness the moment of O-star formation.

We have conducted the first parallax and proper motion measurements of 6.7 GHz methanol maser emission using the Australian Long Baseline Array (LBA). The parallax of G339.884$-$1.259 measured from five epochs of observations is 0.48$\pm $0.08 mas, corresponding to a distance of $2.1^{+0.4}_{-0.3}$ kpc, placing it in the Scutum spiral arm. This is consistent (within the combined uncertainty) with the kinematic distance estimate for this source at 2.5$\pm $0.5 kpc using the latest Solar and Galactic rotation parameters. We find from the Lyman continuum photon flux that the embedded core of the young star is of spectral type B1, demonstrating that luminous 6.7 GHz methanol masers can be associated with high-mass stars towards the lower end of the mass range.

Dense, star-forming gas is believed to form at the stagnation points of large-scale ISM flows, but observational examples of this process in action are rare. We here present a giant molecular cloud (GMC) sandwiched between two colliding Milky Way supershells, which we argue shows strong evidence of having formed from material accumulated at the collision zone. Combining 12CO, 13CO and C18O(J=1-0) data with new high-resolution, 3D hydrodynamical simulations of colliding supershells, we discuss the origin and nature of the GMC (G288.5+1.5), favoring a scenario in which the cloud was partially seeded by pre-existing denser material, but assembled into its current form by the action of the shells. This assembly includes the production of some new molecular gas. The GMC is well interpreted as non-self-gravitating, despite its high mass (MH2 ~ 1.7 x 10^5 Msol), and is likely pressure confined by the colliding flows, implying that self-gravity was not a necessary ingredient for its formation. Much of the molecular gas is relatively diffuse, and the cloud as a whole shows little evidence of star formation activity, supporting a scenario in which it is young and recently formed. Drip-like formations along its lower edge may be explained by fluid dynamical instabilities in the cooled gas., Comment: 13 pages, 9 figures, accepted for publication in ApJ

We present a complete catalog of H I emission and absorption spectrum pairs, toward H II regions, detectable within the boundaries of the Southern Galactic Plane Survey (SGPS I), a total of 252 regions. The catalog is presented in graphical, numerical and summary formats. We demonstrate an application of this new dataset through an investigation of the locus of the Near 3kpc Arm., Comment: Accepted for publication by ApJS Feb 6, 2014. Data files and Figure Set (252 images) to appear in the on-line version of the journal

We have made new CO observations of two molecular clouds, which we call "jet" and "arc" clouds, toward the stellar cluster Westerlund 2 and the TeV gamma-ray source HESS J1023-575. The jet cloud shows a linear structure from the position of Westerlund 2 on the east. In addition, we have found a new counter jet cloud on the west. The arc cloud shows a crescent shape in the west of HESS J1023-575. A sign of star formation is found at the edge of the jet cloud and gives a constraint on the age of the jet cloud to be ~Myrs. An analysis with the multi CO transitions gives temperature as high as 20 K in a few places of the jet cloud, suggesting that some additional heating may be operating locally. The new TeV gamma-ray images by H.E.S.S. correspond to the jet and arc clouds spatially better than the giant molecular clouds associated with Westerlund 2. We suggest that the jet and arc clouds are not physically linked with Westerlund 2 but are located at a greater distance around 7.5 kpc. A microquasar with long-term activity may be able to offer a possible engine to form the jet and arc clouds and to produce the TeV gamma-rays, although none of the known microquasars have a Myr age or steady TeV gamma-rays. Alternatively, an anisotropic supernova explosion which occurred ~Myr ago may be able to form the jet and arc clouds, whereas the TeV gamma-ray emission requires a microquasar formed after the explosion.

SPLASH (the Southern Parkes Large-Area Survey in Hydroxyl) is a sensitive, unbiased and fully-sampled survey of the Southern Galactic Plane and Galactic Centre in all four ground-state transitions of the hydroxyl (OH) radical. The survey provides a deep census of 1612-, 1665-, 1667- and 1720-MHz OH absorption and emission from the Galactic ISM, and is also an unbiased search for maser sources in these transitions. We present here first results from the SPLASH pilot region, which covers Galactic longitudes 334 to 344 degrees and latitudes of -2 to +2 degrees. Diffuse OH is widely detected in all four transitions, with optical depths that are always small (averaged over the Parkes beam), and with departures from LTE common even in the 1665- and 1667-MHz main lines. To a 3$\sigma$ sensitivity of 30 mK, we find no evidence of OH envelopes extending beyond the CO-bright regions of molecular cloud complexes, and conclude that the similarity of the OH excitation temperature and the level of the continuum background is at least partly responsible for this. We detect masers and maser candidates in all four transitions, approximately 50 per cent of which are new detections. This implies that SPLASH will produce a substantial increase in the known population of ground-state OH masers in the Southern Galactic Plane., Comment: 20 pages, 9 figures, accepted for publication in MNRAS

We make a comprehensive study of HI absorption toward HII regions located within Galactic longitudes less than 10 degrees. Structures in the extreme inner Galaxy are traced using the longitude-velocity space distribution of this absorption. We find significant HI absorption associated with the Near and Far 3kpc Arms, the Connecting Arm, Banias Clump 1 and the H I Tilted Disk. We also constrain the line of sight distances to HII regions, by using HI absorption spectra together with the HII region velocities measured by radio recombination lines., Comment: Complete figure set available in online version of journal. Accepted by ApJ August 8, 2013

The accumulation, compression and cooling of the ambient interstellar medium (ISM) in large-scale flows powered by OB cluster feedback can drive the production of dense molecular clouds. We review the current state of the field, with a strong focus on the explicit modelling and observation of the neutral interstellar medium. Magneto-hydrodynamic simulations of colliding ISM flows provide a strong theoretical framework in which to view feedback-driven cloud formation, as do models of the gravitational fragmentation of expanding shells. Rapid theoretical developments are accompanied by growing body of observational work that provides good evidence for the formation of molecular gas via stellar feedback - both in the Milky Way and the Large Magellanic Cloud. The importance of stellar feedback compared to other major astrophysical drivers of dense gas formation remains to be investigated further, and will be an important target for future work., Comment: 20 pages, 6 figures. Accepted for publication in PASA

We have used the Australia Telescope National Facility Mopra 22-m antenna to search for 37.7-GHz (7(-2) - 8(-1}E) methanol masers towards a sample of thirty six class II methanol masers. The target sources are the most luminous class II methanol masers not previously searched for this transition, with isotropic peak 12.2-GHz maser luminosity greater than 250 Jy/kpc^2 and isotropic peak 6.7-GHz maser luminosity greater than 800 Jy/kpc^2. Seven new 37.7-GHz methanol masers were detected as a result of the search. The detection rate for 37.7-GHz methanol masers towards a complete sample of all such class II methanol maser sites south of declination -20 deg is at least 30 percent. The relatively high detection rate for this rare methanol transition is in line with previous predictions that the 37.7-GHz transition is associated with a late stage of the class II methanol maser phase of high-mass star formation. We find that there is a modest correlation between the ratio of the 6.7- and 37.7-GHz maser peak intensity and the 6.7- and 12.2-GHz maser peak intensity (correlation coefficient 0.63 in a log-log plot). We detected one new 38.3-GHz (6(2) - 5(3)A-) methanol maser towards G335.789+0.174. This is only the fourth source for which maser emission has been detected in this transition and it is the only one for which emission is not also observed in the 38.5-GHz 6(2) - 5(3)A+ transition., Comment: Accepted for publication in MNRAS, 18 pages, 6 figures

We investigate the influence of large-scale stellar feedback on the formation of molecular clouds in the Large Magellanic Cloud (LMC). Examining the relationship between HI and 12CO(J=1-0) in supergiant shells (SGSs), we find that the molecular fraction in the total volume occupied by SGSs is not enhanced with respect to the rest of the LMC disk. However, the majority of objects (~70% by mass) are more molecular than their local surroundings, implying that the presence of a supergiant shell does on average have a positive effect on the molecular gas fraction. Averaged over the full SGS sample, our results suggest that ~12-25% of the molecular mass in supergiant shell systems was formed as a direct result of the stellar feedback that created the shells. This corresponds to ~4-11% of the total molecular mass of the galaxy. These figures are an approximate lower limit to the total contribution of stellar feedback to molecular cloud formation in the LMC, and constitute one of the first quantitative measurements of feedback-triggered molecular cloud formation in a galactic system., Comment: 14 pages, 6 figures. Accepted for publication in ApJ

The role of large-scale stellar feedback in the formation of molecular clouds has been investigated observationally by examining the relationship between HI and 12CO(J=1-0) in supershells. Detailed parsec-resolution case studies of two Milky Way supershells demonstrate an enhanced level of molecularisation over both objects, and hence provide the first quantitative observational evidence of increased molecular cloud production in volumes of space affected by supershell activity. Recent results on supergiant shells in the LMC suggest that while they do indeed help to organise the ISM into over-dense structures, their global contribution to molecular cloud formation is of the order of only ~10%., Comment: Proceedings of IAUS 292 - Molecular Gas, Dust, and Star Formation in Galaxies, eds. T. Wong & J. Ott. 4 pages, 3 figures

We present an in-depth case study of three molecular clouds associated with the walls of the Galactic supershells GSH 287+04-17 and GSH 277+00+36. These clouds have been identified in previous work as examples in which molecular gas is either being formed or destroyed due to the influence of the shells. 12CO(J=1-0), 13CO(J=1-0) and C18O(J=1-0) mapping observations with the Mopra telescope provide detailed information on the distribution and properties of the molecular gas, enabling an improved discussion of its relationship to the wider environment in which it resides. We find that massive star formation is occurring in molecular gas likely formed in-situ in the shell wall, at a Galactic altitude of ~200 pc. This second-generation star formation activity is dominating its local environment; driving the expansion of a small HII region which is blistering out of the atomic shell wall. We also find new morphological evidence of disruption in two smaller entrained molecular clouds thought to pre-date the shells. We suggest that at the present post-interaction epoch, the lifetime of this surviving molecular material is no longer strongly determined by the shells themselves., Comment: 26 pages, 10 figures, accepted for publication in the Astrophysical Journal

A large-scale study of the molecular clouds toward the Trifid nebula, M20, has been made in the J=2-1 and J=1-0 transitions of 12CO and 13CO. M20 is ionized predominantly by an O7.5 star HD164492. The study has revealed that there are two molecular components at separate velocities peaked toward the center of M20 and that their temperatures - 30-50 K as derived by an LVG analysis - are significantly higher than the 10 K of their surroundings. We identify that the two clouds as the parent clouds of the first generation stars in M20. The mass of each cloud is estimated to be ~10^3 Msun and their separation velocity is ~8 km/s over ~1-2 pc. We find the total mass of stars and molecular gas in M20 is less than ~3.2 \times 103 Msun, which is too small by an order of magnitude to gravitationally bind the system. We argue that the formation of the first generation stars, including the main ionizing O7.5 s tar, was triggered by the collision between the two clouds in a short time scale of ~1 Myrs, a second example alongside Westerlund 2, where a super star cluster may have been formed due to cloud-cloud collision triggering., Comment: 32 pages, 8 figures, 2 tables, accepted to ApJ

We present parsec-scale resolution observations of the atomic and molecular ISM in two Galactic supershells, GSH 287+04-17 and GSH 277+00+36. HI synthesis images from the Australia Telescope Compact Array are combined with 12CO(J=1-0) data from the NANTEN telescope to reveal substantial quantities of molecular gas closely associated with both shells. These data allow us to confirm an enhanced level of molecularization over the volumes of both objects, providing the first direct observational evidence of increased molecular cloud production due to the influence of supershells. We find that the atomic shell walls are dominated by cold gas with estimated temperatures and densities of T ~ 100 K and n0 ~ 10 cm-3. Locally, the shells show rich substructure in both tracers, with molecular gas seen elongated along the inner edges of the atomic walls, embedded within HI filaments and clouds, or taking the form of small CO clouds at the tips of tapering atomic `fingers'. We discuss these structures in the context of different formation scenarios, suggesting that molecular gas embedded within shell walls is well explained by in-situ formation from the swept up medium, whereas CO seen at the ends of fingers of HI may trace remnants of molecular clouds that pre-date the shells. A preliminary assessment of star formation activity within the shells confirms ongoing star formation in the molecular gas of both GSH 287+04-17 and GSH 277+00+36., Comment: 42 pages, 14 figures, accepted for publication in the Astrophysical Journal

We present new parsec-scale resolution data from a multi-phase study of the ISM in the walls of Galactic supershells. HI synthesis images and CO survey data reveal a wealth of substructure, including dense-tipped fingers and extended molecular clouds embedded in shell walls. We briefly consider formation scenarios for these features, and suggest that both the interaction of an expanding shell with pre-existing dense clouds, as well as in-situ formation of CNM and molecular gas, are likely to be relevant. Future work will also examine the role of instabilities in structure formation and breakup, and will investigate the presence of high-altitude gas associated with supershells and chimneys., Comment: 4 pages, to appear in conference proceedings "The Role of Disk-Halo Interaction in Galaxy Evolution, Outflow vs. Infall?" ed. M. A. de Avillez, European Astronomical Society Publications, 2009 in press