Characteristics of the supernova remnant G351.7+0.8 and a distance argument against its association with PSR J1721−3532

New images of the supernova remnant (SNR) G351.7+0.8 are presented based on 21-cm H  i -line emission and continuum emission data from the Southern Galactic Plane Survey. SNR G351.7+0.8 has a flux density of 8.4 ± 0.7 Jy at 1420 MHz. Its spectral index is 0.52 ± 0.25 ( S = v ^−α) between 1420 and 843 MHz, typical of adiabatically expanding shell-like remnants. H  i observations show structures possibly associated with the SNR in the radial velocity range of −10 to −18 km s⁻¹, and suggest a distance of 13.2 kpc and a radius of 30.7 pc. The estimated Sedov age for G351.7+0.8 is less than  6.8×10⁴ yr  . A young radio pulsar PSR J1721−3532 lies close to SNR G351.7+0.8 on the sky. The new distance and age of G351.7+0.8 and recent proper motion measurements of the pulsar strongly argue against an association between SNR G351.7+0.8 and PSR J1721−3532. There is an unidentified, faint X-ray point source 1RXS J172055.3−353937 which is close to G351.7+0.8. This may be a neutron star potentially associated with G351.7+0.8.     

G332.5−5.6, a new Galactic supernova remnant

We present radio observations of the source G332.5−5.6, a candidate supernova remnant (SNR). Observations have been performed with the Australia Telescope Compact Array (ATCA) at two frequencies, 1.4 and 2.4 GHz. Our results confirm that G332.5−5.6 is an SNR, with a spectral index  α=−0.7 ± 0.2  for the whole source and an average fractional polarization of ∼35 per cent at 2.4 GHz. The central component is coincident with extended X-ray emission, and the distance to the SNR is estimated to be ∼3.4 kpc. Based on its radio and X-ray morphology, this SNR should be classified as a composite, and we suggest that it belongs to a trident-shaped subclass like G291.0−0.1.     

G309.2–00.6 and jets in supernova remnants

We present Australia Telescope Compact Array observations of the supernova remnant (SNR) G309.2–00.6. In a 1.3-GHz continuum image the remnant appears as a near-circular shell, but with two brightened and distorted arcs of emission on opposite sides. H  i absorption against the SNR yields a distance in the range 5.4 to 14.1 kpc, corresponding to an age (1−20) × 10³ yr. On the basis of the morphology of the SNR we argue that it is a younger analogue of the W 50/SS 433 system, and that its unusual appearance is a result of opposed jets or outflows from a central source. A jet-like feature and breaks in the shell can both be seen along the axis of proposed outflow, providing further support for this interpretation; the central source itself is not detected. The SNR may be interacting with the adjacent H  ii region RCW 80 through an extension of the proposed outflow beyond its shell. This would put the SNR at the lower limit of its distance range and would imply an age 4000 yr. We consider other SNRs similar to G309.2–00.6, and propose remnants whose shells are affected by jets as one of several classes of SNR from which the presence of a central source can be inferred.     

Australia Telescope Compact Array H i observations of the NGC 6845 galaxy group

We present the results of Australia Telescope Compact Array (ATCA) H  i line and 20-cm radio continuum observations of the galaxy quartet NGC 6845. The H  i emission extends over all four galaxies but can only be associated clearly with the two spiral galaxies, NGC 6845A and B, which show signs of strong tidal interaction. We derive a total H  i mass of at least  1.8 × 10¹⁰ M_⊙  , most of which is associated with NGC 6845A, the largest galaxy of the group. We investigate the tidal interaction between NGC 6845A and B by studying the kinematics of distinct H  i components and their relation to the known H  ii regions. No H  i emission is detected from the two lenticular galaxies, NGC 6845C and D. A previously uncatalogued dwarf galaxy, ATCA  J2001−4659  , was detected 4.4 arcmin NE from NGC 6845B and has an H  i mass of  ∼5 × 10⁸ M_⊙  . No H  i bridge is visible between the group and its newly detected companion. Extended 20-cm radio continuum emission is detected in NGC 6845A and B as well as in the tidal bridge between the two galaxies. We derive star formation rates of  15–40 M_⊙ yr⁻¹  .     

An Australia Telescope Compact Array 20-cm radio continuum study of the Large Magellanic Cloud

We present a mosaic image of the 1.4-GHz radio continuum emission from the Large Magellanic Cloud (LMC) observed with the Australia Telescope Compact Array (ATCA) and the Parkes Telescope. The mosaic covers     with an angular resolution of 40 arcsec, corresponding to a spatial scale of ∼10 pc in the LMC. The final image is suitable for studying emission on all scales between 40 arcsec and the surveyed area. In this paper, we discuss (i) the characteristics of the LMC's diffuse and compact radio continuum emission, (ii) the fraction of the emission produced by thermal processes and the implied star formation rate in the LMC and (iii) variations in the radio spectral index across the LMC. Two non-standard reduction techniques that we used to process the ATCA visibility data may be of interest for future wide-field radio continuum surveys. The data are open to the astronomical community and should be a rich resource for studies of individual objects such as supernova remnants, H  ii regions and planetary nebulae as well as extended features such as the diffuse emission from synchrotron radiation.     

Re-identification of G35.6−0.4 as a supernova remnant

G35.6−0.4 is an extended radio source in the Galactic plane which has previously been identified as either a supernova remnant or an H  ii region. Observations from the Very Large Array Galactic Plane Survey at 1.4 GHz with a resolution of 1 arcmin allow the extent of G35.6−0.4 to be defined for the first time. Comparison with other radio survey observations show that this source has a non-thermal spectral index, with   S ∝ν^{−0.47±0.07}  . G35.6−0.4 does not have obvious associated infrared emission, so it is identified as a Galactic supernova remnant, not an H  ii region. It is  ≈15 × 11 arcmin²  in extent, showing partial limb brightening.     

Radio continuum and H I observations of the supernova remnant G296.8–00.3

We present Australia Telescope Compact Array (ATCA) observations of the supernova remnant (SNR) G296.8–00.3. A 1.3-GHz continuum image shows the remnant to have a complex multi-shelled appearance, with an unusual rectangular strip running through its centre. H I absorption yields a kinematic distance to the remnant of 9.6 ± 0.6 kpc, from which we estimate an age in the range (2–10) × 10³ yr. The ATCA's continuum mode allows a measurement of the Faraday rotation across the band, from which we derive a mean rotation measure towards the SNR of 430 rad m⁻². We consider possible explanations for the morphology of G296.8–00.3, and conclude that either it has a biannular structure, as might be produced through interaction with an asymmetric progenitor wind, or its appearance is caused by the effects of the surrounding interstellar medium.   We argue that the adjacent pulsar J1157–6224 is at a similar distance to the SNR, but that a physical association is highly unlikely. The pulsar is the only detectable source in the field in circular polarization, suggesting a method for finding pulsars during aperture synthesis.     

Galactic H i on the 50-au scale in the direction of three extragalactic sources observed with MERLIN

We present MERLIN observations of Galactic 21-cm H  i absorption at an angular resolution of  ∼0.1–0.2  arcsec and a velocity resolution of 0.5 km s⁻¹, in the direction of three moderately low latitude  (−8° < b < −12°)  extragalactic radio sources, 3C 111, 3C 123 and 3C 161, all of which are heavily reddened. H  i absorption is observed against resolved background emission sources up to ∼2 arcsec in extent and we distinguish details of the opacity distribution within 1–1.5 arcsec regions towards 3C 123 and 3C 161. This study is the second MERLIN investigation of small-scale structure in interstellar H  i (earlier work probed Galactic H  i in the directions of the compact sources 3C 138 and 3C 147). The 0.1-arcsec scale is intermediate between H  i absorption studies made with other fixed element interferometers with resolution of 1–10 arcsec and very long baseline interferometry studies with resolutions of 10–20 mas. At a scale of 1 arcsec (about 500 au), prominent changes in Galactic H  i opacity in excess of 1–1.5 are determined in the direction of 3C 161 with a signal-to-noise ratio of at least 10σ. Possible fluctuations in the H  i opacity at the level of about 1 are detected at the  2.5–3σ  level in the direction of 3C 123.     

GMRT observations of four suspected supernova remnants near the Galactic Centre

We have observed two fields – Field I     ,     and Field II     ,     – with the Giant Metrewave Radio Telescope (GMRT) at 330 MHz. In the first field, we have studied the candidate supernova remnant (SNR) G3.1−0.6 and, based on its observed morphology, spectral index and polarization, confirmed it to be an SNR. We find this supernova to have a double ring appearance with a strip of emission on its western side passing through its centre.
We have discovered two extended curved objects in the second field, which appears to be part of a large shell-like structure. It is possibly the remains of an old supernova in the region. Three suspected SNRs, G356.3−0.3, G356.6+0.1 and G357.1−0.2, detected in the MOST 843-MHz survey of the GC region appear to be located on this shell-like structure. While both G356.3−0.3 and G356.6+0.1 seem to be parts of this shell, G357.1−0.2, which has a steeper spectrum above 1 GHz, could be a background SNR seen through the region. Our H  i absorption observation towards the candidate SNR G357.1−0.2 indicates that it is at a distance of more than 6 kpc from us.     

The temperature of the warm neutral medium in the Milky Way

We report high-spectral-resolution Australia Telescope Compact Array (ATCA) H  i 21-cm observations resulting in the detection of the warm neutral medium (WNM) of the Galaxy in absorption against two extragalactic radio sources, PKS 1814−637 and PKS 0407−658. The two lines of sight were selected on the basis of the simplicity of their absorption profiles and the strength of the background sources; the high velocity resolution of the spectra then enabled us to estimate the kinetic temperatures of the absorbing gas by fitting multiple Gaussians to the absorption profiles. Four separate WNM components were detected towards the two sources, with peak optical depths  τ_max= (1.0 ± 0.08) × 10⁻², (1.4 ± 0.2) × 10⁻³, (2.2 ± 0.5) × 10⁻³  and  (3.4 ± 0.5) × 10⁻³  and kinetic temperatures   T _k= 3127 ± 300, 3694 ± 1595, 3500 ± 1354  and  2165 ± 608 K  , respectively. All four components were thus found to have temperatures in the thermally unstable range  500 < T _k < 5000 K  ; this suggests that thermal equilibrium has not been reached throughout the WNM.     

Multifrequency study of the Large Magellanic Cloud supernova remnant (SNR) B0513−692 and new SNR candidate J051327−6911

We present a new multiwavelength study of supernova remnant (SNR) B0513−692 in the Large Magellanic Cloud (LMC). The remnant also has a strong, superposed, essentially unresolved, but unrelated radio source at its north-western edge, J051324−691049. This is identified as a likely compact H  ii region based on related optical imaging and spectroscopy. We use the Australia Telescope Compact Array (ATCA) at 4790 and 8640 MHz  (λ≃ 6 cm and λ≃ 3.5 cm)  to determine the large-scale morphology, spectral index and polarization characteristics of B0513−692 for the first time. We detect a strongly polarized region (49 per cent) in the remnant's southern edge  (λ≃ 6 cm)  . Interestingly, we also detect a small (∼40 arcsec) moderately bright, but distinct optical, circular shell in our Hα imagery which is adjacent to the compact H  ii region and just within the borders of the north-eastern edge of B0513−692. We suggest that this is a separate new SNR candidate based on its apparently distinct character in terms of optical morphology in three imaged emission lines and indicative SNR optical spectroscopy (including enhanced optical [S  ii ] emission relative to Hα).     

A deep search for pulsar wind nebulae using pulsar gating

Using the Australia Telescope Compact Array (ATCA) we have imaged the fields around five promising pulsar candidates to search for radio pulsar wind nebulae (PWNe). We have used the ATCA in its pulsar-gating mode; this enables an image to be formed containing only off-pulse visibilities, thereby dramatically improving the sensitivity to any underlying PWN. Data from the Molonglo Observatory Synthesis Telescope were also used to provide sensitivity on larger spatial scales. This survey found a faint new PWN around PSR B0906−49; here we report on non-detections of PWNe towards PSRs B1046−58, B1055−52, B1610−50 and J1105−6107. Our radio observations of the field around PSR B1055−52 argue against previous claims of an extended X-ray and radio PWN associated with the pulsar. If these pulsars power unseen, compact radio PWNe, upper limits on the radio flux indicate that a fraction of less than 10⁻⁶ of their spin-down energy is used to power this emission. Alternatively, PSRs B1046−58 and B1610−50 may have relativistic winds similar to other young pulsars and the unseen PWN may be resolved and fainter than our surface brightness sensitivity threshold. We can then determine upper limits on the local interstellar medium (ISM) density of 2.2×10⁻³ and 1×10⁻² cm⁻³, respectively. Furthermore, we derive the spatial velocities of these pulsars to be ∼450 km s⁻¹ and thus rule out the association of PSR B1610−50 with supernova remnant (SNR) G332.4+00.1 (Kes 32). Strong limits on the ratio of unpulsed to pulsed emission are also determined for three pulsars.     

H i 21-cm absorption at z∼ 3.39 towards PKS 0201+113

We report the Giant Metrewave Radio Telescope detection of H  i 21-cm absorption from the z ∼ 3.39 damped Lyman α absorber (DLA) towards PKS 0201+113, the highest redshift at which 21-cm absorption has been detected in a DLA. The absorption is spread over ∼115 km s⁻¹ and has two components, at   z = 3.387 144(17)  and   z = 3.386 141  (45). The stronger component has a redshift and velocity width in agreement with the tentative detection of Briggs, Brinks & Wolfe, but a significantly lower optical depth. The core size and DLA covering factor are estimated to be ≲100 pc and f ∼ 0.69, respectively, from a Very Long Baseline Array 328-MHz image. If one makes the conventional assumption that the H  i column densities towards the optical and radio cores are the same, this optical depth corresponds to a spin temperature of T _s∼[(955 ± 160) × ( f /0.69)] K. However, this assumption may not be correct, given that no metal-line absorption is seen at the redshift of the stronger 21-cm component, indicating that this component does not arise along the line of sight to the optical quasi-stellar object (QSO), and that there is structure in the 21-cm absorbing gas on scales smaller than the size of the radio core. We model the 21-cm absorbing gas as having a two-phase structure with cold dense gas randomly distributed within a diffuse envelope of warm gas. For such a model, our radio data indicate that, even if the optical QSO lies along a line of sight with a fortuitously high (∼50 per cent) cold gas fraction, the average cold gas fraction is low, ≲17 per cent, when averaged over the spatial extent of the radio core. Finally, the large mismatch between peak 21-cm and optical redshifts and the complexity of both profiles makes it unlikely that the z ∼ 3.39 DLA will be useful in tests of fundamental constant evolution.     

Magnetic field evolution of accreting neutron stars – IV. Effect of the curvature of space–time

We report near-infrared molecular hydrogen and Brackett γ observations towards the massive star formation site G323.74−0.26. The region contains an H  ii region, ∼30 arcsec across, and two Class II methanol maser sites, which are separate from the H  ii region. We show that the spectral type of the star powering the H  ii region is B0. We also show that at least one of the maser sites is powered by an infrared source that appears to be at least as luminous as the star responsible for the H  ii region. However, neither of the two stars associated with the methanol maser sites shows any signs of radio continuum emission above 0.2 mJy. For at least one of these maser sites, this shows a real deficiency in the radio continuum flux, which we suggest is an indication that the star is in an early stage of development, before its H  ii region becomes visible, or it is a multiple intermediate mass star system. A shocked molecular hydrogen outflow is seen extending from one of the maser sites towards the west and possibly in a fan shape, suggesting that the stars associated with the maser sites are indeed at a very early stage of evolution.     

Extragalactic sources towards the central region of the Galaxy

We have observed a sample of 64 small-diameter sources towards the central  −6° < l < 6°, −2° < b < 2°  of the Galaxy with the aim of studying the Faraday rotation measure near the Galactic Centre region. All the sources were observed at 6- and 3.6-cm wavelengths using the ATCA and the VLA. 59 of these sources are inferred to be extragalactic. The observations presented here constitute the first systematic study of the radio polarization properties of the background sources towards this direction and increase the number of known extragalactic radio sources in this part of the sky by almost an order of magnitude. Based on the morphology, spectral indices and lack of polarized emission, we identify four Galactic H  ii regions in the sample.     

Spin temperatures and covering factors for H i 21-cm absorption in damped Lyman α systems

We investigate the practice of assigning high spin temperatures to damped Lyman α absorption systems (DLAs) not detected in H  i 21-cm absorption. In particular, Kanekar & Chengalur have attributed the mix of 21-cm detections and non-detections in low-redshift  ( z _abs≤ 2.04) DLAs  to a mix of spin temperatures, while the non-detections at high redshift were attributed to high spin temperatures. Below   z _abs= 0.9  , where some of the DLA host galaxy morphologies are known, we find that 21-cm absorption is normally detected towards large radio sources when the absorber is known to be associated with a large intermediate (spiral) galaxy. Furthermore, at these redshifts, only one of the six 21-cm non-detections has an optical identification and these DLAs tend to lie along the sight-lines to the largest background radio continuum sources. For these and many of the high-redshift DLAs occulting large radio continua, we therefore expect covering factors of less than the assumed/estimated value of unity. This would have the effect of introducing a range of spin temperatures considerably narrower than the current range of  Δ T _s≳ 9000 K  , while still supporting the hypothesis that the high-redshift DLA sample comprises a larger proportion of compact galaxies than the low-redshift sample.     

Neutral hydrogen gas in interacting galaxies: the NGC 1511 galaxy group

We present H  i line and 20-cm radio continuum observations of the NGC 1511 galaxy group obtained with the Australia Telescope Compact Array. The data reveal an extended, rather disturbed H  i distribution for the peculiar starburst galaxy NGC 1511 and a narrow bridge to its small companion galaxy, NGC 1511B, which has been severely distorted by the interaction/collision between the two galaxies. No stellar counterpart to the gaseous bridge has been detected. In addition, we find that the peculiar optical ridge to the east of NGC 1511 is probably the stellar remnant of a galaxy completely disrupted by interactions with NGC 1511. The slightly more distant neighbour, NGC 1511A, shows a regular H  i velocity field and no obvious signs of interactions.
Radio continuum emission from NGC 1511 reveals three prominent sources on top of a more diffuse, extended distribution. We derive an overall star formation rate of  7 M_⊙ yr⁻¹  . The most enhanced star formation is found in the south-eastern part of the disc, coincident with several bright H  ii regions, and closest to the peculiar optical ridge. No continuum emission was detected in the companions, but NGC 1511B appears to show an H  ii region at its faint western edge, closest to NGC 1511. The group displays a prime example of interaction-induced star formation activity.     

12-μm fine-structure emission line and continuum images of G333.6–0.2

We present high spatial resolution (∼0.8 arcsec) diffraction-limited 12.8-μm Ne  ii fine-structure emission line and 12.5-μm continuum images of the bright southern compact H  ii region G333.6–0.2, taken with the mid-infrared imaging polarimeter NIMPOL. The two images show remarkably similar, compact, yet asymmetric, flux distributions. The [Ne  ii ] image shows a complex structure near the ionizing source(s) which we interpret in terms of the ionization structure of the H  ii region. It is found that G333.6–0.2 is more likely to be excited by a cluster of O and B stars than by a single star.     

G353.9−2.0, a new Galactic supernova remnant identified from the NRAO Very Large Array Sky Survey

G353.9−2.0 is a likely Galactic supernova remnant (SNR) identified from a visual inspection of the National Radio Astronomy Observations (NRAO) Very Large Array (VLA) Sky Survey (NVSS) observations in the Galactic plane. It shows a shell structure, about 13 arcmin in diameter, with a central extended source. VLA observations of G353.9−2.0 at 1.4 GHz, with a resolution of ≈40 arcsec, and a significantly better sensitivity than the NVSS observations, are presented here, together with observations at 327 MHz and observations of the central source at 8.4 GHz. These new observations and existing results from the literature are discussed. G353.9−2.0 is confirmed as a SNR, and the central source as a likely extragalactic double source.     

An H i interstellar bubble surrounding WR 85 and RCW 118

We analyse the distribution of the interstellar matter in the environs of the Wolf-Rayet star LSS 3982 (= WR 85, WN6+OB?) linked to the optical ring nebula RCW 118. Our study is based on neutral hydrogen 21-cm line data belonging to the Southern Galactic Plane Survey (SGPS).
The analysis of the H  i data allowed the identification of a neutral hydrogen interstellar bubble related to WR 85 and the 25-arcmin-diameter ring nebula RCW 118. The H  i bubble was detected at a systemic velocity of −21.5 km s⁻¹, corresponding to a kinematical distance of 2.8 ± 1.1 kpc, compatible with the stellar distance. The neutral structure is about 25 arcmin in radius or 21 ± 8 pc, and is expanding at 9 ± 2 km s⁻¹. The associated ionized and neutral masses amount to  3000 M_⊙  . The carbon monoxide (CO) emission distribution depicts a region lacking CO coincident in position and velocity with the H  i structure. The 9.3-arcmin-diameter inner optical nebula appears to be related to the approaching part of the neutral atomic shell. The H  i void and shell are the neutral gas counterparts of the optical bubble and have very probably originated in the action of the strong stellar wind of the central star during the O-type and WR phases on the surrounding interstellar medium. The H  i bubble appears to be in the momentum conserving stage.