arXiv subjects

W. K. Gear

Publications and source records attributed to W. K. Gear.

69 records · Page 4Linked to original sources

arXiv2005

First ground-based 200-um observing with THUMPER on JCMT - sky characterisation and planet maps

We present observations that were carried out with the Two HUndred Micron PhotometER (THUMPER) mounted on the James Clerk Maxwell Telescope (JCMT) in Hawaii, at a wavelength of 200 um (frequency 1.5 THz). The observations utilise a small atmospheric window that opens up at this wavelength under very dry conditions at high-altitude observing sites. The atmosphere was calibrated using the sky-dipping method and a relation was established between the optical depth, tau, at 1.5 THz and that at 225 GHz: tau_1.5THz = (95 +/- 10)*tau_225GHz. Mars and Jupiter were mapped from the ground at this wavelength for the first time, and the system characteristics measured. A noise equivalent flux density (NEFD) of ~65 +/- 10 Jy (1 sigma 1 second) was measured for the THUMPER-JCMT combination, consistent with predictions based upon our laboratory measurements. The main-beam resolution of 14 arcsec was confirmed and an extended error-beam detected at roughly two-thirds of the magnitude of the main beam. Measurements of the Sun allow us to estimate that the fraction of the power in the main beam is ~15%, consistent with predictions based on modelling the dish surface accuracy. It is therefore shown that the sky over Mauna Kea is suitable for astronomy at this wavelength under the best conditions. However, higher or drier sites should have a larger number of useable nights per year.

astro-ph↗

arXiv2005

XMM-Newton Surveys of the Canada-France Redshift Survey Fields - III: The Environments of X-ray Selected AGN at 0.4<z<0.6

The environmental properties of a sample of 31 hard X-ray selected AGN are investigated, from scales of 500 kpc down to 30 kpc, and are compared to a control sample of inactive galaxies. The AGN all lie in the redshift range 0.4 =0 clusters. It is suggested that minor mergers with low mass companions is a likely candidate for the mechanism by which these modest luminosity AGN are fuelled.

astro-ph↗

arXiv2004

Dust in spiral galaxies: global properties

We present and analyse high-quality SCUBA 850- and 450-um images of 14 local spiral galaxies, including the detection of dust well out into the extended disk in many cases. We use these data in conjunction with published far-IR flux densities from IRAS and ISO, and millimetre-wave measurements from ground-based facilities to deduce the global properties of the dust in these galaxies; in particular temperature and mass. We find that simple two-temperature greybody models of fixed dust emissivity index beta=2 and with typical temperatures of 25 < T(warm) < 40 K and 10 < T(cold) < 20 K provide good fits to the spectral energy distributions. The dust mass in the cold component correlates with the mass in atomic hydrogen and the mass in the warm component correlates with the mass in molecular hydrogen. These results thus fit the simple picture in which the cold dust is heated predominantly by the interstellar radiation field while the hot dust is heated predominantly by OB stars in more active regions, although we argue that there is some mixing. The mean gas-to-dust ratio is 120+/-60, very similar to that found within our own galaxy and roughly a factor of 10 lower than that derived from IRAS data alone. The gas-to-dust ratios in the warm, molecular component are on average higher than those in the cold, atomic component. We compare our modelling results with similar results for more luminous spiral galaxies selected at far-IR wavelengths and find that whilst the total dust mass distributions of the two samples are indistinguishable, they have significantly different dust temperature distributions in both the warm and cold components. We suggest that this difference...(abridged)

astro-ph↗

arXiv2004

CLOVER - A new instrument for measuring the B-mode polarization of the CMB

We describe the design and expected performance of Clover, a new instrument designed to measure the B-mode polarization of the cosmic microwave background. The proposed instrument will comprise three independent telescopes operating at 90, 150 and 220 GHz and is planned to be sited at Dome C, Antarctica. Each telescope will feed a focal plane array of 128 background-limited detectors and will measure polarized signals over angular multipoles 20 < l < 1000. The unique design of the telescope and careful control of systematics should enable the B-mode signature of gravitational waves to be measured to a lensing-confusion-limited tensor-to-scalar ratio r~0.005.

astro-ph↗

arXiv2004

Scientific optimization of a ground-based CMB polarization experiment

We investigate the science goals achievable with the upcoming generation of ground-based Cosmic Microwave Background polarization experiments and calculate the optimal sky coverage for such an experiment including the effects of foregrounds. We find that with current technology an E-mode measurement will be sample-limited, while a B-mode measurement will be detector-noise-limited. We conclude that a 300 sq deg survey is an optimal compromise for a two-year experiment to measure both E and B-modes, and that ground-based polarization experiments can make an important contribution to B-mode surveys. Focusing on one particular experiment, QUaD, a proposed bolometric polarimeter operating from the South Pole, we find that a ground-based experiment can make a high significance measurement of the acoustic peaks in the E-mode spectrum, and will be able to detect the gravitational lensing signal in the B-mode spectrum. Such an experiment could also directly detect the gravitational wave component of the B-mode spectrum if the amplitude of the signal is close to current upper limits. We also investigate how a ground-based experiment can improve constraints on the cosmological parameters. We estimate that by combining two years of QUaD data with the four-year WMAP data, an optimized ground-based polarization experiment can improve constraints on cosmological parameters by a factor of two. If the foreground contamination can be reduced, the measurement of the tensor-to-scalar ratio can be improved by up to a factor of six over that obtainable from WMAP alone.

astro-ph↗

arXiv2004

XMM-Newton Surveys of the Canada-France Redshift Survey Fields - II: The X-ray Catalogues, the Properties of the Host Galaxies, and the Redshift Distribution

We present the X-ray source catalogues for the XMM surveys of the 3-h and 14-h Canada-France Redshift Survey fields (0.5-10 keV flux range ~2E-15 - 1E-13 erg cm^-2 s^-1). We use a subset of the XMM sources, which have Chandra positions, to determine the best method of obtaining optical identifications of sources with only XMM positions. We find optical identifications for 79% of the XMM sources for which there are deep optical images. The sources without optical identifications are likely to be optically fainter and have higher redshifts than the sources with identifications. We have estimated `photometric redshifts' for the identified sources, calibrating our method using ~200 galaxies in the fields with spectroscopic redshifts. We find that the redshift distribution has a strong peak at z~0.7. The host galaxies of AGN identified in this work cover a wide range of optical properties with every galaxy type being represented, and no obvious preference for one type over another. Redder types tend to be more luminous than blue types, particularly at lower redshifts. The host galaxies also span a wide range of optical luminosity, in contrast to the narrow range found for the starburst galaxies detected in micro Jy radio surveys. We find a strong correlation between optical and X-ray luminosity similar to the Magorrian relation, although selection effects cannot be ruled out.

astro-ph↗

arXiv2002

The Canada-UK Deep Submillimeter Survey V: The Submillimeter Properties of Lyman-Break Galaxies

We have used 850$μ$m maps obtained as part of the Canada-UK Deep Submillimeter Survey (CUDSS) to investigate the sub-mm properties of Lyman-break galaxies (LBGs). We used three samples of Lyman-break galaxies: two from the Canada-France Deep Fields (CFDF) survey covering CUDSS-14 and CUDSS-3, and one from Steidel and collaborators also covering CUDSS-14. We measure a mean flux from both CFDF LBG samples at a level of $\sim2σ$ of 0.414 $\pm$ 0.263 mJy for CUDSS-03 and 0.382 $\pm$ 0.206 mJy for CUDSS-14, but the Steidel et al. sample is consistent with zero flux. From this we place upper limits on the Lyman-break contribution to the $850μm$ background of $\sim$20%. We have also measured the cross-clustering between the LBGs and SCUBA sources. From this measurement we infer a large clustering amplitude of $r_o$ = 11.5 $\pm$ 3.0 $\pm$ 3.0 $h^{-1}$Mpc for the Steidel et al. sample (where the first error is statistical and the second systematic), $r_o$ = 4.5 $\pm$ 7.0 $\pm$ 5.0 $h^{-1}$Mpc for CFDF-14 and $r_o$ = 7.5 $\pm$ 7.0 $\pm$ 5.0 $h^{-1}$Mpc for CFDF-3. The Steidel et al sample, for which we have most only significant detection of clustering is also the largest of the three samples and has spectroscopically confirmed redshifts.

astro-ph↗

arXiv2002

The Relative Performance of Filled and Feedhorn-Coupled Focal-plane Architectures

Modern far infrared and submillimeter instruments require large format arrays. We consider the relative performance of filled array (bare pixel) and feedhorn-coupled architectures for bolometer focal planes. Based on typical array parameters, we quantify the relative observing speeds and comment on the merits of the different architectures. Filled arrays can provide higher mapping speed (by a factor of up to 3.5) and simpler observing modes at the expense of reduced sensitivity for pointed observations, increased detector numbers, and greater vulnerability to stray light and electromagnetic interference. Taking advantage of the filled array architecture requires strongly background limited detectors. At millimeter wavelengths, filled arrays must be surrounded by a sufficiently cold enclosure to minimize the background power from the instrument itself.

astro-ph↗

arXiv2002

The Canada-UK Deep Submillimeter Survey VI: The 3-Hour Field

We present the complete submillimeter data for the Canada-UK Deep Submillimeter Survey (CUDSS) 3-hour field. The obeservations were taken with the Submillimeter Common-User Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope on Mauna Kea. The 3-hour field is one of two main fields in our survey and covers 60 square arcminutes to a 3-sigma depth of 3 mJy. In this field we have detected 27 sources above 3-sigma and 15 above 3.5-sigma. We assume the source counts follow the form $N(S) {\propto} S^{-α}$ and measure $α$ = 3.3$^{+1.4}_{-1.0}$. This is in good agreement with previous studies and further supports our claim (Eales et al., 2000) that SCUBA sources brighter than 3 mJy produce ~20% of the 850$μ$m background energy. Using preliminary ISO 15 $μ$m maps and VLA 1.4 GHz data we have identified counterparts for six objects and have marginal detections at 450$μ$m for two additional sources. With this information we estimate a median redshift for the sample of 2.0$\pm$0.5, with $\sim$10% lying at $z<$ 1. We have measured the angular clustering of S850 > 3 mJy sources using the source catalogues from the CUDSS two main fields, the 3-hour and 14-hour fields, and find a marginal detection of clustering, primarily from the 14-hour field, of $ω(θ)=4.4\pm2.9 θ^{-0.8}$. This is consistent with clustering at least as strong as that seen for the Lyman-break galaxy population and the Extremely Red Objects. Since SCUBA sources are selected over a broader range in redshifts than these two populations the strength of the true spatial clustering is expected to be correspondingly stronger.

astro-ph↗

arXiv2000

High spatial resolution observations of CUDSS14A: a SCUBA-selected Ultraluminous galaxy at high redshift

We present a high-resolution millimetre interferometric image of the brightest SCUBA-selected galaxy from the Canada-UK deep SCUBA survey (CUDSS). We make a very clear detection at 1.3 mm, but fail to resolve any structure in the source. The interferometric position is within 1.5 arcsec of the SCUBA 850 $μ$m centroid, and also within 1.5 arcsec of a 44 $μ$Jy radio source and a very faint, extremely red galaxy which we had previously identified as the submillimetre source. We also present new optical and infrared imaging, and infrared spectroscopy of this source. We model the overall spectral energy distribution and conclude that it lies within the redshift range 2$< z < 4.5$. The submm/FIR luminosity of CUDSS14A is very weakly dependent on redshift within the constrained range, and is roughly 4$\times10^{12}$ L$_\odot$ (for H$_0$=75 and an assumed Arp220-like spectrum), which implies a star-formation rate $\sim$1000 M$_\odot$ yr$^{-1}$. We derive an approximate gas mass of $\sim 10^{10}$ M$_\odot$ which would imply the current star-forming activity cannot be sustained for longer than about 10 million years. With the present data however we are unable to rule out a significant AGN contribution to the total luminosity.

astro-ph↗

arXiv1999

Variations in the broadband spectra of BL Lac objects: millimetre observations of an X-ray-selected sample

Observations at millimetre wavelengths are presented for a representative sample of 22 X-ray-selected BL Lac objects (XBLs). This sample comprises 19 High-energy cutoff BL Lacs objects (HBLs), 1 Low-energy cutoff BL Lac object (LBL) and 2 `intermediate' sources. Data for LBLs, which are mostly radio selected BL Lac objects (RBLs) are taken from the literature. It is shown that the radio--millimetre spectral indices ($α_{5-230)$) of HBLs ($-0.29\pm0.15$) are slightly steeper than those of the LBLs ($-0.19\pm0.14$). A correlation exists between $α_{5-230}$ and 230 GHz luminosity. While this correlation could be an artefact of comparing two populations of BL Lac objects with intrinsically different radio properties it is also consistent with the predictions of existing unified schemes which relate BL Lac objects to Fanaroff- Riley class I radio galaxies. The HBLs have significantly flatter submillimetre- X-ray spectral indices ($α_{230-X}$) than the LBLs ($-0.68\pm0.07$ cf. $-1.08\pm0.06$)although the two intermediate sources also have intermediate values of $α_{230-X}\sim-0.9$. It is argued that this difference cannot be explained entirely by the viewing angle hypothesis and requires a difference in physical source parameters. The $α_{230-X}$ values for the HBLs are close to the canonical value found for large samples of radio sources and thus suggest that synchrotron radiation is the mechanism that produces the X-ray emission. As suggested by Padovani & Giommi, the inverse-Compton mechanism is likely to dominate in the LBLs requiring the synchrotron spectra of these sources to steepen or cutoff at lower frequencies than those of the HBLs.

astro-ph↗

arXiv1998

Multiwaveband Observations of Quasars with Flat Radio Spectra and Strong Millimeter Emission

We present multiwaveband observations of a well selected sample of 28 quasars and two radio galaxies with flat radio spectra and strong millimeter wave emission (referred to here as FSRQ's). The observations include multifrequency VLBI measurements, X-ray observations with ROSAT and submillimeter observations with the JCMT. Particularly interesting among many findings is a correlation between the X-ray to millimeter spectral index and fraction of flux density contained in the VLBI core. This tendency toward higher X-ray fluxes from sources with stronger jet emission implies that the knots in the jet are the prominent source of X-rays.

astro-ph↗

arXiv1998

A dust ring around Epsilon Eridani: analogue to the young Solar System

Dust emission around the nearby star epsilon Eridani has been imaged using a new submillimetre camera (SCUBA at the JCMT). At 850 microns wavelength a ring of dust is seen, peaking at 60 AU from the star and with much lower emission inside 30 AU. The mass of the ring is at least 0.01 Earth masses in dust, while an upper limit of 0.4 Earth masses in molecular gas is imposed by CO observations. The total mass is comparable to the estimated amount of material, 0.04-0.3 Earth masses, in comets orbiting the Solar System. The most probable origin of the the ring structure is that it is a young analogue to the Kuiper Belt in our Solar System, and that the central region has been partially cleared by the formation of grains into planetesimals. Dust clearing around epsilon Eri is seen within the radius of Neptune's orbit, and the peak emission at 35-75 AU lies within the estimated Kuiper Belt zone of 30-100 AU radius. epsilon Eri is a main-sequence star of type K2V (0.8 Solar masses) with an estimated age of 0.5-1.0 Gyr, so this interpretation is consistent with the early history of the Solar System where heavy bombardment occurred up to approximately 0.6 Gyr. An unexpected discovery is substructure within the ring, and these asymmetries could be due to perturbations by planets.

astro-ph↗

arXiv1997

A Millimetre/Submillimetre Polarisation Survey of Compact Flat-Spectrum Radio Sources

We present the results of the first survey and monitoring study of the linear polarisation properties of compact, flat-spectrum radio sources at millimetre/submillimetre wavelengths and discuss the implications of the inferred magnetic field structure for the emission models involving shock waves in relativistic jets on sub-parsec scales. We find significant polarisation in most sources but, in general, the magnetic field on sub-parsec scales is less well ordered than on parsec scales. We observe no difference in polarisation properties between the BL Lac objects and compact flat-spectrum quasars at these wavelengths. Although we find the behaviour of some sources, particularly the most highly polarised, to be very consistent with the predictions of transverse shock-in-jet models, the detailed behaviour of most objects is not. Conical shock structures can more readily explain the observed diverse behaviour of the sample, although some degree of bending of the jet may still be necessary in some cases.

astro-ph↗

arXiv1994

Modelling the millimetre-infrared flaring behaviour of the quasar 1253-055 (3C 279)

The infrared through millimetre light curve of 3C 279 is investigated for the period of 1986 until mid-1994, during which time several flares were observed. A quiescent spectrum (identified with emission from an underlying jet) is presented. Both the near-IR and 375-150 GHz regimes are shown to be well described by power laws, with no evidence for any thermal contribution in the IR. Successful isolation of the flaring component by subtraction of a base level is found to be difficult. Dividing each individual flare into two regimes corresponding to before and after maximum flux, we find strong linear correlations between log 90 GHz flux and 22-90 GHz spectral slope. Furthermore, the gradient of the linear correlation steepens as the flare decays after maximum. This trend is observed for several successive flares, and can be successfully explained in terms of evolution of the flare according to the the shocked-jet model of Marscher & Gear (1985).

astro-ph↗