arXiv subjects

M. J. Kesteven

Publications and source records attributed to M. J. Kesteven.

32 records · Page 2Linked to original sources

arXiv2005

Imaging of the Radio Remnant of SN 1987A at 12 mm Wavelength

Observations of the radio remnant of Supernova 1987A using the Australia Telescope Compact Array in the 12-mm band on 2003 July 31 (day 6002.7 after the explosion) give the first fully resolved radio image of the supernova remnant. The diffraction-limited image has a resolution of about 0.45 arcsec, a factor of two better than that of the 3-cm images previously obtained. There is excellent agreement between the 12-mm image and a contemporaneous super-resolved 3-cm image. Super-resolution of the 12-mm image gives a further factor of two improvement in resolution, to 0.25 arcsec, albeit with limited dynamic range. While the spatial distributions of the radio and X-ray emission are broadly similar, there are significant differences in detail with no correspondence in the regions of brightest emission. The 12-mm image is well modelled by a thick equatorial ring inclined at 43 deg to the line of sight. This, together with the common east-west asymmetry and the relatively steady increase in the radio flux density, suggests that the reverse shock is the main site for generation of the radio emission.

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arXiv2005

The HIPASS Catalogue: III - Optical Counterparts & Isolated Dark Galaxies

We present the largest catalogue to date of optical counterparts for HI radio-selected galaxies, Hopcat. Of the 4315 HI radio-detected sources from the HI Parkes All Sky Survey (Hipass) catalogue, we find optical counterparts for 3618 (84%) galaxies. Of these, 1798 (42%) have confirmed optical velocities and 848 (20%) are single matches without confirmed velocities. Some galaxy matches are members of galaxy groups. From these multiple galaxy matches, 714 (16%) have confirmed optical velocities and a further 258 (6%) galaxies are without confirmed velocities. For 481 (11%), multiple galaxies are present but no single optical counterpart can be chosen and 216 (5%) have no obvious optical galaxy present. Most of these 'blank fields' are in crowded fields along the Galactic plane or have high extinctions. Isolated 'Dark galaxy' candidates are investigated using an extinction cut of ABj < 1 mag and the blank fields category. Of the 3692 galaxies with an ABj extinction < 1 mag, only 13 are also blank fields. Of these, 12 are eliminated either with follow-up Parkes observations or are in crowded fields. The remaining one has a low surface brightness optical counterpart. Hence, no isolated optically dark galaxies have been found within the limits of the Hipass survey.

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arXiv2004

First results from the ATCA 18 GHz pilot survey

As a pilot study for the first all-sky radio survey at short wavelengths, we have observed 1216 sq. deg. of the southern sky at 18GHz (16mm) using a novel wide-band (3.4GHz bandwidth) analogue correlator on one baseline of the Australia Telescope Compact Array (ATCA). We scanned a region of sky between declination -71 and -59 degrees with an rms noise level of 15mJy. Follow-up radio imaging of candidate sources above a 4 sigma detection limit of 60mJy resulted in 221 confirmed detections, for which we have measured accurate positions and flux densities. For extragalactic sources, the survey is roughly 70% complete at a flux density of 126mJy and 95% complete above 300mJy. Almost half the detected sources lie within a few degrees of the Galactic plane, but there are 123 sources with |b|>5 degrees which can be assumed to be extragalactic. The differential source counts for extragalactic sources in the range 0.1Jy < S_{18GHz} < 3Jy are well fitted by a relation of the form n(S)=57(S/{Jy})^{-2.2\pm0.2} [Jy^-1 sr^-1], in good agreement with the 15GHz counts published by Taylor et al. (2001) and Waldram et al. (2003). Over 70% of the extragalactic sources have a flat radio spectrum (α_{0.843}^{18}> -0.5, S_ν\proptoν^α), and 29% have inverted radio spectra (α_{0.843}^{18}> 0). The optical identification rate is high: 51% of the extragalactic sources are identified with stellar objects (candidate QSOs), 22% with galaxies and only 27% with faint optical objects or blank fields.

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arXiv2003

Polarization observations in a low synchrotron emission field at 1.4 GHz

We present the first observation of the diffuse polarized synchrotron radiation of a patch ($\sim 3^\circ \times 3^\circ$) in the BOOMERanG field, one of the areas with the lowest CMB foreground emission. The work has been carried out with the Australia Telescope Compact Array at 1.4 GHz with 3.4 arcmin resolution and sensitivity of $\sim 0.18$ mJy beam$^{-1}$. The mean polarized signal has been found to be $P_{rms} = \sqrt{(Q_{rms}^2 + U_{rms}^2)} = 11.6 \pm 0.6$ mK, nearly one order of magnitude below than in the Galactic Plane. Extrapolations to frequencies of interest for cosmological investigations suggest that polarized synchrotron foreground noise should allow the detection of the CMB Polarization $E$--mode already at 32 GHz and make us confident that, at 90 GHz, it is accessible with no relevant foreground contamination. Last but not least, even the $B$--mode detection for $T/S > 0.01$ is not ruled out in such a low emission patch.

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arXiv2003

The 1000 Brightest HIPASS Galaxies: HI Mass Function and Omega_HI

We present a new accurate measurement of the HI mass function of galaxies from the HIPASS Bright Galaxy Catalog, a sample of 1000 galaxies with the highest HI peak flux densities in the southern hemisphere (Koribalski et al. 2003). This sample spans nearly four orders of magnitude in HI mass (from log M_HI/M_sun=6.8 to 10.6, H0=75) and is the largest sample of HI selected galaxies to date. We develop a bivariate maximum likelihood technique to measure the space density of galaxies, and show that this is a robust method, insensitive to the effects of large scale structure. The resulting HI mass function can be fitted satisfactorily with a Schechter function with faint-end slope alpha=-1.30. This slope is found to be dependent on morphological type, with later type galaxies giving steeper slopes. We extensively test various effects that potentially bias the determination of the HI mass function, including peculiar motions of galaxies, large scale structure, selection bias, and inclination effects, and quantify these biases. The large sample of galaxies enables an accurate measurement of the cosmological mass density of neutral gas: Omega_HI=(3.8 +/- 0.6) x 10^{-4}. Low surface brightness galaxies contribute only 15% to this value, consistent with previous findings.

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arXiv2002

A New Look at the Large-Scale HI Structure of the LMC

We present a Parkes multibeam \HI survey of the Large Magellanic Cloud (LMC). This survey, which is sensitive to spatial structure in the range 200 pc to 10 kpc, complements the Australia Telescope Compact survey, which is sensitive to structure in the range 15 pc to 500 pc. With an rms column density sensitivity of 8 x 10^16/cm^2 for narrow lines and 4 x 10^17/cm^2 for typical linewidths of 40 km/s, emission is found to be extensive well beyond the main body of the LMC. Arm-like features extend from the LMC to join the Magellanic Bridge and the Leading Arm, a forward counterpart to the Magellanic Stream. These features, whilst not as dramatic as those in the SMC, appear to have a common origin in the Galactic tidal field, in agreement with recent 2MASS and DENIS results for the stellar population. The diffuse gas which surrounds the LMC, particularly at pa's 90 to 330 deg, appears to be loosely associated with tidal features, but loosening by the ram pressure of tenuous Galactic halo gas against the outer parts of the LMC cannot be discounted. High-velocity clouds, which lie between the Galaxy and the LMC in velocity and which appear in the UV spectra of some LMC stars, are found to be associated with the LMC if their heliocentric velocity exceeds about +100 km/s. They are possibly the product of energetic outflows from the LMC disk. The HI mass of the LMC is found to be (4.8+/-0.2) x 10^8 Msun (for an assumed distance of 50 kpc), substantially more than previous recent measurements.

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arXiv2000

Removing radio interference from contaminated astronomical spectra using an independent reference signal and closure relations

The growing level of radio frequency interference (RFI) is a recognized problem for research in radio astronomy. This paper describes an intuitive but powerful RFI cancellation technique that is suitable for radio spectroscopy where time-averages are recorded. An RFI ``reference signal,'' is constructed from the cross power spectrum of the signals from the two polarizations of a reference horn pointed at the source of the RFI signal. The RFI signal paths obey simple phase and amplitude closure relations, which allows computation of the RFI contamination in the astronomical data and the corrections to be applied to the astronomical spectra. Since the method is immune to the effects of multipath scattering in both the astronomy and reference signal channels, ``clean copies'' of the RFI signal are not required. The method could be generalized (1) to interferometer arrays, (2) to correct for scattered solar radiation that causes spectral ``standing waves'' in single-dish spectroscopy, and (3) to pulsar survey and timing applications where a digital correlator plays an important role in broadband pulse dedispersion. Future large radio telescopes, such as the proposed LOFAR and SKA arrays, will require a high degree of RFI suppression and could implement the technique proposed here with the benefit of faster electronics, greater digital precision and higher data rates.

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arXiv2000

HI Bright Galaxies in the Southern Zone of Avoidance

A blind survey for HI bright galaxies in the southern Zone of Avoidance, (212 deg < l < 36 deg; |b| < 5 deg), has been made with the 21 cm multibeam receiver on the Parkes 64 m radiotelescope. The survey, sensitive to normal spiral galaxies to a distance of about 40 Mpc and more nearby dwarfs, detected 110 galaxies. Of these, 67 have no counterparts in the NASA/IPAC Extragalactic Database. In general, the uncataloged galaxies lie behind thicker obscuration than do the cataloged objects. All of the newly-discovered galaxies have HI flux integrals more than an order of magnitude lower than the Circinus galaxy. The survey recovers the Puppis cluster and foreground group (Kraan-Korteweg & Huchtmeier 1992), and the Local Void remains empty. The HI mass function derived for the sample is satisfactorily fit by a Schechter function with parameters alpha = 1.51 +- 0.12, Phi* = 0.006 +- 0.003, and log M* = 9.7 +- 0.10.

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arXiv2000

An Australia telescope survey for CMB anisotropies

We have surveyed six distinct `empty fields' using the Australia Telescope Compact Array in an ultra-compact configuration with the aim of imaging, with a high brightness sensitivity, any arcmin-scale brightness-temperature anisotropies in the background radio sky. The six well-separated regions were observed at a frequency of 8.7 GHz and the survey regions were limited by the ATCA primary beams which have a full width at half maximum of 6 arcmin at this frequency; all fields were observed with a resolution of 2 arcmin and an rms thermal noise of 24 microJy/beam. After subtracting foreground confusion detected in higher resolution images of the fields, residual fluctuations in Stokes I images are consistent with the expectations from thermal noise and weaker (unidentified) foreground sources; the Stokes Q and U images are consistent with expectations from thermal noise. Within the sensitivity of our observations, we have no reason to believe that there are any Sunyaev-Zeldovich holes in the microwave sky surveyed. Assuming Gaussian-form CMB anisotropy with a `flat' spectrum, we derive 95 per cent confidence upper limits of Q_flat < 10--11 microK in polarized intensity and Q_flat < 25 microK in total intensity. The ATCA filter function peaks at l=4700 and has half maximum values at l=3350 and 6050.

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arXiv1999

Supernova 1987A: A young supernova remnant in an aspherical progenitor wind

The interaction between the ejecta from Supernova 1987A and surrounding material is producing steadily brightening radio and X-ray emission. The new-born supernova remnant has been significantly decelerated by this interaction, while its morphology reflects the axisymmetric nature of the progenitor wind.

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arXiv1998

New HI-detected Galaxies in the Zone of Avoidance

We present the first results of a blind HI survey for galaxies in the southern Zone of Avoidance with a multibeam receiver on the Parkes telescope. This survey is eventually expected to catalog several thousand galaxies within Galactic latitude |b|<5 degrees, mostly unrecognised before due to Galactic extinction and confusion. We present here results of the first three detections to have been imaged with the Australia Telescope Compact Array (ATCA). The galaxies all lie near Galactic longitude 325 degrees and were selected because of their large angular sizes, up to 1.3 degrees. Linear sizes range from 53 to 108 kpc. The first galaxy is a massive 5.7x10^11 solar mass disk galaxy with a faint optical counterpart, SGC 1511.1--5249. The second is probably an interacting group of galaxies straddling the Galactic equator. No optical identification is possible. The third object appears to be an interacting pair of low column density galaxies, possibly belonging to an extended Circinus or Centaurus A galaxy group. No optical counterpart has been seen despite the predicted extinction (A(B) = 2.7 - 4.4 mag) not being excessive. We discuss the implications of the results, in particular the low HI column densities (~10^19 atoms/sq.cm) found for two of the three galaxies.

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arXiv1998

Methanol Masers as Tracers of Circumstellar Disks

We show that in many methanol maser sources the masers are located in lines, with a velocity gradient along them which suggests that the masers are situated in edge-on circumstellar, or protoplanetary, disks. We present VLBI observations of the methanol maser source G309.92+0.48, in the 12.2 GHz transition, which confirm previous observations that the masers in this source lie along a line. We show that such sources are not only linear in space but, in many cases, also have a linear velocity gradient. We then model these and other data in both the 6.7 GHz and the 12.2 GHz transition from a number of star formation regions, and show that the observed spatial and velocity distribution of methanol masers, and the derived Keplerian masses, are consistent with a circumstellar disk rotating around an OB star. We consider this and other hypotheses, and conclude that about half of these methanol masers are probably located in edge-on circumstellar disks around young stars. This is of particular significance for studies of circumstellar disks because of the detailed velocity information available from the masers.

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arXiv1996

The asymmetric radio remnant of SN 1987A

We present seven years of radio observations of SN 1987A made with the Australia Telescope Compact Array. At 1.4, 2.4, 4.8 and 8.6 GHz, the flux density of the radio remnant has increased monotonically since emission was redetected 1200 days after the explosion. On day 3200, the remnant was expanding at 2800 +/- 400 km/s, which we interpret as indicating significant deceleration of the fastest moving ejecta. Since day 1787 the spectral index has remained constant at alpha = -0.95 +/- 0.04. These observations are all consistent with the shock having encountered a denser, shocked, component of the progenitor's stellar wind. At the current rate of expansion, the shock is expected to encounter the inner optical ring in the year 2006 +/- 3. Using super-resolution, we have also obtained 9 GHz images of the remnant (resolution approx 0".5) at four epochs. The emission is distributed around the rim of a near-circular shell, but has become increasingly asymmetric with time. There are two "hotspots" to the east and west, aligned along the major axis of the optical ring. This morphology is most likely indicative of an axisymmetric circumstellar medium into which the shock is expanding, consistent with present understanding of the progenitor star and its environment.

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arXiv1994

Compact Radio Cores in Seyfert Galaxies

We have observed a sample of 157 Seyfert galaxies with a 275 km baseline radio interferometer to search for compact, high brightness temperature radio emission from the active nucleus. We obtain the surprising result that compact radio cores are much more common in Seyfert 2 than in Seyfert 1 galaxies, which at first seems to be inconsistent with orientation unification schemes. We propose a model, involving optical depth effects in the narrow-line region, which can reconcile our result with the standard unified scheme. (Accepted for publication in ApJ 1994 Sep 10)

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