arXiv subjects
Publications and source records attributed to John M. Dickey.
The ngVLA will create a Galaxy-wide, volume-limited sample of HII regions; solve some long standing problems in the physics of HII regions; and provide an extinction-free star formation tracer in nearby galaxies.
We identify a counterpart to a Galactic supershell in diffuse radio polarisation, and use this to determine the magnetic fields associated with this object. GSH 006$-$15$+$7 has perturbed the polarised emission at 2.3$\,$GHz, as observed in the S-band Polarisation All Sky Survey (S-PASS), acting as a Faraday screen. We model the Faraday rotation over the shell, and produce a map of Faraday depth over the area across it. Such models require information about the polarised emission behind the screen, which we obtain from the Wilkinson Microwave Anisotropy Probe (WMAP), scaled from 23$\,$GHz to 2.3$\,$GHz, to estimate the synchrotron background behind GSH 006$-$15$+$7. Using the modelled Faraday thickness we determine the magnitude and the plane-of-the-sky structure of the line-of-sight magnetic field in the shell. We find a peak line-of-sight field strength of $|B_\parallel|_\text{peak} = 2.0\substack{+0.01 \\ -0.7}\,μ$G. Our measurement probes weak magnetic fields in a low-density regime (number densities of $\sim0.4\,$cm$^{-3}$) of the ISM, thus providing crucial information about the magnetic fields in the partially-ionised phase.
We present 21-cm Spectral Line Observations of Neutral Gas with the VLA (21-SPONGE), a Karl G. Jansky Very Large Array (VLA) large project (~600 hours) for measuring the physical properties of Galactic neutral hydrogen (HI). 21-SPONGE is distinguished among previous Galactic HI studies as a result of: (1) exceptional optical depth sensitivity ($σ_τ < 10^{-3}$ per $0.42\rm\,km\,s^{-1}$ channels over 57 lines of sight); (2) matching 21 cm emission spectra with highest-possible angular resolution (~4') from the Arecibo Observatory; (3) detailed comparisons with numerical simulations for assessing observational biases. We autonomously decompose 21 cm spectra and derive the physical properties (i.e., spin temperature, $T_s$, column density) of the cold neutral medium (CNM; $T_s<250\rm\,K$), thermally unstable medium (UNM; $250< T_s < 1000\rm\,K$) and warm neutral medium (WNM; $T_s > 1000\rm\,K$) simultaneously. We detect 50% of the total HI mass in absorption, the majority of which is CNM (56 +/- 10%, corresponding to 28% of the total HI mass). Although CNM is detected ubiquitously, the CNM fraction along most lines of sight is <50%. We find that 20% of the total HI mass is thermally unstable (41 +/- 10% of HI detected in absorption), with no significant variation with Galactic environment. Finally, although the WNM comprises 52% of the total HI mass, we detect little evidence for WNM absorption with $1000<T_s<4000\rm\,K$. Following spectral modeling, we detect a stacked residual absorption feature corresponding to WNM with $T_s\sim10^4\rm\,K$. We conclude that excitation in excess of collisions likely produces significantly higher WNM $T_s$ than predicted by steady-state models.
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.
We analyze synthetic neutral hydrogen (HI) absorption and emission spectral lines from a high- resolution, three-dimensional hydrodynamical simulation to quantify how well observational methods recover the physical properties of interstellar gas. We present a new method for uniformly decomposing HI spectral lines and estimating the properties of associated gas using the Autonomous Gaussian Decomposition (AGD) algorithm. We find that Hi spectral lines recover physical structures in the simulation with excellent completeness at high Galactic latitude, and this completeness declines with decreasing latitude due to strong velocity-blending of spectral lines. The temperature and column density inferred from our decomposition and radiative transfer method agree with the simulated values within a factor of < 2 for the majority of gas structures. We next compare synthetic spectra with observations from the 21-SPONGE survey at the Karl G. Jansky Very Large Array using AGD. We find more components per line of sight in 21-SPONGE than in synthetic spectra, which reflects insufficient simulated gas scale heights and the limitations of local box simulations. In addition, we find a significant population of low-optical depth, broad absorption components in the synthetic data which are not seen in 21-SPONGE. This population is not obvious in integrated or per-channel diagnostics, and reflects the benefit of studying velocity-resolved components. The discrepant components correspond to the highest spin temperatures (1000 < Ts < 4000 K), which are not seen in 21-SPONGE despite sufficient observational sensitivity. We demonstrate that our analysis method is a powerful tool for diagnosing neutral ISM conditions, and future work is needed to improve observational statistics and implementation of simulated physics.
A set of diffuse interstellar clouds in the inner Galaxy within a few hundred pc of the Galactic plane has been observed at an angular resolution of ~1 arcmin combining data from the NRAO Green Bank Telescope and the Very Large Array. At the distance of the clouds the linear resolution ranges from ~1.9 pc to ~2.8 pc. These clouds have been selected to be somewhat out of the Galactic plane and are thus not confused with unrelated emission, but in other respects they are a Galactic population. They are located near the tangent points in the inner Galaxy, and thus at a quantifiable distance: $2.3 \leq R \leq 6.0$ kpc from the Galactic Center, and $-1000 \leq z \leq +610$ pc from the Galactic plane. These are the first images of the diffuse neutral HI clouds that may constitute a considerable fraction of the ISM. Peak HI column densities range from $N_{HI} = 0.8-2.9 \times 10^{20}$ cm$^{-2}$. Cloud diameters vary between about 10 and 100 pc, and their HI mass spans the range from less than a hundred to a few thousands Msun. The clouds show no morphological consistency of any kind except that their shapes are highly irregular. One cloud may lie within the hot wind from the nucleus of the Galaxy, and some clouds show evidence of two distinct thermal phases as would be expected from equilibrium models of the interstellar medium.
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 $Δv=4.5\pm 1.0\rm\,km\,s^{-1}$ and optical depth of $τ(\rm HCO^+)=0.10\pm 0.02$. 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.
We present methods and results from "21-cm Spectral Line Observations of Neutral Gas with the EVLA" (21-SPONGE), a large survey for Galactic neutral hydrogen (HI) absorption with the Karl G. Jansky Very Large Array (VLA). With the upgraded capabilities of the VLA, we reach median root-mean-square (RMS) noise in optical depth of $σ_τ=9\times 10^{-4}$ per $0.42\rm\,km\,s^{-1}$ channel for the 31 sources presented here. Upon completion, 21-SPONGE will be the largest HI absorption survey with this high sensitivity. We discuss the observations and data reduction strategies, as well as line fitting techniques. We prove that the VLA bandpass is stable enough to detect broad, shallow lines associated with warm HI, and show that bandpass observations can be combined in time to reduce spectral noise. In combination with matching HI emission profiles from the Arecibo Observatory ($\sim3.5'$ angular resolution), we estimate excitation (or spin) temperatures ($\rm T_s$) and column densities for Gaussian components fitted to sightlines along which we detect HI absorption (30/31). We measure temperatures up to $\rm T_s\sim1500\rm\,K$ for individual lines, showing that we can probe the thermally unstable interstellar medium (ISM) directly. However, we detect fewer of these thermally unstable components than expected from previous observational studies. We probe a wide range in column density between $\sim10^{16}$ and $>10^{21}\rm\,cm^{-2}$ for individual HI clouds. In addition, we reproduce the trend between cold gas fraction and average $\rm T_s$ found by synthetic observations of a hydrodynamic ISM simulation by Kim et al. (2014). Finally, we investigate methods for estimating HI $\rm T_s$ and discuss their biases.
As we strive to understand how galaxies evolve it is crucial that we resolve physical processes and test emerging theories in nearby systems that we can observe in great detail. Our own Galaxy, the Milky Way, and the nearby Magellanic Clouds provide unique windows into the evolution of galaxies, each with its own metallicity and star formation rate. These laboratories allow us to study with more detail than anywhere else in the Universe how galaxies acquire fresh gas to fuel their continuing star formation, how they exchange gas with the surrounding intergalactic medium, and turn warm, diffuse gas into molecular clouds and ultimately stars. The $λ$21-cm line of atomic hydrogen (HI) is an excellent tracer of these physical processes. With the SKA we will finally have the combination of surface brightness sensitivity, point source sensitivity and angular resolution to transform our understanding of the evolution of gas in the Milky Way, all the way from the halo down to the formation of individual molecular clouds.
We present an analysis of the HI and CO gas in conjunction with the Planck/IRAS submillimeter/far-infrared dust properties toward the most outstanding high latitude clouds MBM 53, 54, 55 and HLCG 92-35 at b = -30 deg to -45 deg. The CO emission, dust opacity at 353 GHz (tau353), and dust temperature (Td) show generally good spatial correspondence. On the other hand, the correspondence between the HI emission and the dust properties is less clear than in CO. The integrated HI intensity WHI and tau353 show a large scatter with a correlation coefficient of ~0.6 for a Td range from 16 K to 22 K. We find, however, that WHI and tau353 show better correlation for smaller ranges of Td every 0.5 K, generally with a correlation coefficient of 0.7-0.9. We set up a hypothesis that the HI gas associated with the highest Td >= 21.5 K is optically thin, whereas the HI emission is generally optically thick for Td lower than 21.5 K. We have determined a relationship for the optically thin HI gas between atomic hydrogen column density and tau353, NHI (cm-2) = (1.5 x 10^26) x tau353, under the assumption that the dust properties are uniform and we have applied this to estimate NHI from tau353 for the whole cloud. NHI was then used to solve for Ts and tauHI over the region. The result shows that the HI is dominated by optically thick gas having a low spin temperature of 20-40 K and a density of 40-160 cm-3. The HI envelope has a total mass of ~1.2 x 10^4 Msol, an order of magnitude larger than that of the CO clouds. The HI envelope properties derived by this method do not rule out a mixture of HI and H2 in the dark gas, but we present indirect evidence that most of the gas mass is in the atomic state.
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.
Context. Frequent, simultaneous observations across the electromagnetic spectrum are essential to the study of a range of astrophysical phenomena including Active Galactic Nuclei. A key tool of such studies is the ability to observe an object when it flares i.e. exhibits a rapid and significant increase in its flux density. Aims. We describe the specific observational procedures and the calibration techniques that have been developed and tested to create a single baseline radio interferometer that can rapidly observe a flaring object. This is the only facility that is dedicated to rapid high resolution radio observations of an object south of -30 degrees declination. An immediate application is to provide rapid contemporaneous radio coverage of AGN flaring at γ-ray frequencies detected by the Fermi Gamma-ray Space Telescope. Methods. A single baseline interferometer was formed with radio telescopes in Hobart, Tasmania and Ceduna, South Australia. A software correlator was set up at the University of Tasmania to correlate these data. Results. Measurements of the flux densities of flaring objects can be made using our observing strategy within half an hour of a triggering event. These observations can be calibrated with amplitude errors better than 15%. Lower limits to the brightness temperatures of the sources can also be calculated using CHI.
The interstellar medium of the Milky Way is multi-phase, magnetized and turbulent. Turbulence in the interstellar medium produces a global cascade of random gas motions, spanning scales ranging from 100 parsecs to 1000 kilometres. Fundamental parameters of interstellar turbulence such as the sonic Mach number (the speed of sound) have been difficult to determine because observations have lacked the sensitivity and resolution to directly image the small-scale structure associated with turbulent motion. Observations of linear polarization and Faraday rotation in radio emission from the Milky Way have identified unusual polarized structures that often have no counterparts in the total radiation intensity or at other wavelengths, and whose physical significance has been unclear. Here we report that the gradient of the Stokes vector (Q,U), where Q and U are parameters describing the polarization state of radiation, provides an image of magnetized turbulence in diffuse ionized gas, manifested as a complex filamentary web of discontinuities in gas density and magnetic field. Through comparison with simulations, we demonstrate that turbulence in the warm ionized medium has a relatively low sonic Mach number, M_s <~ 2. The development of statistical tools for the analysis of polarization gradients will allow accurate determinations of the Mach number, Reynolds number and magnetic field strength in interstellar turbulence over a wide range of conditions.
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.
We report on the discovery of consecutive bright radio pulses from the Vela pulsar, a new phenomenon that may lead to a greater understanding of the pulsar emission mechanism. This results from a total of 345 hr worth of observations of the Vela pulsar using the University of Tasmania's 26 m radio telescope to study the frequency and statistics of abnormally bright pulses and sub-pulses. The bright pulses show a tendency to appear consecutively. The observations found two groups of six consecutive bright pulses and many groups of two to five bright pulses in a row. The strong radio emission process that produces the six bright pulses lasts between 0.4 and 0.6 s. The numbers of bright pulses in sequence far exceed what would be expected if individual bright pulses were independent random events. Consecutive bright pulses must be generated by an emission process that is long lived relative to the rotation period of the neutron star.
One of the Survey Science Projects that the Australian Square Kilometre Array Pathfinder (ASKAP) telescope will do in its first few years of operation is a study of the 21-cm line of HI and the 18-cm lines of OH in the Galactic Plane and the Magellanic Clouds and Stream. The wide-field ASKAP can survey a large area with very high sensitivity much faster than a conventional telescope because of its focal plane array of receiver elements. The brightness sensitivity for the widespread spectral line emission of the interstellar medium depends on the beam size and the survey speed. In the GASKAP survey, maps with different resolutions will be synthesized simultaneously; these will be matched to different scientific applications such as diffuse HI and OH emission, OH masers, and HI absorption toward background continuum sources. A great many scientific questions will be answered by the GASKAP survey results; a central topic is the exchange of matter and energy between the Milky Way disk and halo. The survey will show how neutral gas at high altitude (z) above the disk, like the Magellanic Stream, makes its way down through the halo, what changes it experiences along the way, and how much is left behind.
In order to establish the position of the center of mass of the Earth in the International Celestial Reference Frame, observations of the Global Positioning Satellite (GPS) constellation using the IVS network are important. With a good frame-tie between the coordinates of the IVS telescopes and nearby GPS receivers, plus a common local oscillator reference signal, it should be possible to observe and record simultaneously signals from the astrometric calibration sources and the GPS satellites. The standard IVS solution would give the atmospheric delay and clock offsets to use in analysis of the GPS data. Correlation of the GPS signals would then give accurate orbital parameters of the satellites {\bf in the ICRF reference frame}, i.e. relative to the positions of the astrometric sources. This is particularly needed to determine motion of the center of mass of the earth along the rotation axis.
Using H I absorption spectra taken from the recent surveys of 21-cm line and continuum emission in the Galactic plane, the distribution of cool atomic clouds in the outer disk of the Milky Way is revealed. The warp of the midplane is clearly seen in absorption, as it is in emission, and the cool, neutral medium also shows flaring or increase in scale height with radius similar to that of the warm atomic hydrogen. The mixture of phases, as measured by the fraction of H I in the cool clouds relative to the total atomic hydrogen, stays nearly constant from the solar circle out to about 25 kpc radius. Assuming cool phase temperature ~50 K this indicates a mixing ratio of 15% to 20% cool H I, with the rest warm.