Graeme F Wong

A survey of the cold molecular gas in gravitationally lensed star-forming galaxies at z > 2

Using the Australia Telescope Compact Array, we conducted a survey of CO J = 1 − 0 and J = 2 − 1 line emission towards strongly lensed high-redshift dusty star-forming galaxies (DSFGs) previously discovered with the South Pole Telescope (SPT). Our sample comprises 17 sources that had CO-based spectroscopic redshifts obtained with the Atacama Large Millimeter/submillimeter Array and the Atacama Pathfinder Experiment. We detect all sources with known redshifts in either CO J = 1 − 0 or J = 2 − 1. 12 sources are detected in the 7-mm continuum. The derived CO luminosities imply gas masses in the range (0.5–11) × 10^(10) M⊙ and gas depletion time-scales t_(dep) < 200 Myr, using a CO to gas mass conversion factor αCO = 0.8 M⊙ (K km s^(−1) pc^2)^(−1). Combining the CO luminosities and dust masses, along with a fixed gas-to-dust ratio, we derive α_(CO) factors in the range 0.4–1.8 M⊙ (K km s^(−1) pc^2)^(−1), similar to what is found in other starbursting systems. We find small scatter in αCO values within the sample, even though inherent variations in the spatial distribution of dust and gas in individual cases could bias the dust-based α_(CO) estimates. We find that lensing magnification factors based on the CO linewidth to luminosity relation (μCO) are highly unreliable, but particularly when μ < 5. Finally, comparison of the gas and dynamical masses suggest that the average molecular gas fraction stays relatively constant at z = 2–5 in the SPT DSFG sample.

Sub-kiloparsec Imaging of Cool Molecular Gas in Two Strongly Lensed Dusty, Star-forming Galaxies

We present spatially-resolved imaging obtained with the Australia Telescope Compact Array (ATCA) of three CO lines in two high-redshift gravitationally lensed dusty star-forming galaxies, discovered by the South Pole Telescope. Strong lensing allows us to probe the structure and dynamics of the molecular gas in these two objects, at z=2.78 and z=5.66, with effective source-plane resolution of less than 1kpc. We model the lensed emission from multiple CO transitions and the dust continuum in a consistent manner, finding that the cold molecular gas as traced by low-J CO always has a larger half-light radius than the 870um dust continuum emission. This size difference leads to up to 50% differences in the magnification factor for the cold gas compared to dust. In the z=2.78 galaxy, these CO observations confirm that the background source is undergoing a major merger, while the velocity field of the other source is more complex. We use the ATCA CO observations and comparable resolution Atacama Large Millimeter/submillimeter Array dust continuum imaging of the same objects to constrain the CO-H_2 conversion factor with three different procedures, finding good agreement between the methods and values consistent with those found for rapidly star-forming systems. We discuss these galaxies in the context of the star formation - gas mass surface density relation, noting that the change in emitting area with observed CO transition must be accounted for when comparing high-redshift galaxies to their lower redshift counterparts.

THE SPECTRAL VARIABILITY OF THE GHZ-PEAKED SPECTRUM RADIO SOURCE PKS 1718-649 AND A COMPARISON OF ABSORPTION MODELS

Using the new wideband capabilities of the Australia Telescope Compact Array (ATCA), we obtain spectra for PKS 1718-649, a well-known gigahertz-peaked spectrum radio source. The observations, between approximately 1 and 10 GHz over three epochs spanning approximately 21 months, reveal variability both above the spectral peak at ~3 GHz and below the peak. The combination of the low and high frequency variability cannot be easily explained using a single absorption mechanism, such as free-free absorption or synchrotron self-absorption. We find that the PKS 1718-649 spectrum and its variability are best explained by variations in the free-free optical depth on our line-of-sight to the radio source at low frequencies (below the spectral peak) and the adiabatic expansion of the radio source itself at high frequencies (above the spectral peak). The optical depth variations are found to be plausible when X-ray continuum absorption variability seen in samples of Active Galactic Nuclei is considered. We find that the cause of the peaked spectrum in PKS 1718-649 is most likely due to free-free absorption. In agreement with previous studies, we find that the spectrum at each epoch of observation is best fit by a free-free absorption model characterised by a power-law distribution of free-free absorbing clouds. This agreement is extended to frequencies below the 1 GHz lower limit of the ATCA by considering new observations with Parkes at 725 MHz and 199 MHz observations with the newly operational Murchison Widefield Array. These lower frequency observations argue against families of absorption models (both free-free and synchrotron self-absorption) that are based on simple homogenous structures.

The Carina Nebula and Gum 31 molecular complex – I. Molecular gas distribution, column densities, and dust temperatures

We report high resolution observations of the

New 6 and 3-cm radio-continuum maps of the Small Magellanic Cloud : Part II - point source catalogue

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New 20-cm radio-continuum study of the Small Magellanic Cloud. Part II : Point sources

CO

Multi-frequency Observations of a Superbubble in the LMC: The Case of LHA 120-N 70

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Active galactic nuclei behind the SMC selected from radio and X-ray surveys

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RCW 36 in the Vela Molecular Ridge: Evidence for high-mass star-cluster formation triggered by cloud–cloud collision

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X-RAY SPECTROSCOPY OF POTENTIAL SMALL MAGELLANIC CLOUD TYPE Ia SUPERNOVA REMNANTS AND THEIR ENVIRONMENTS

CO