T. Shanks

Durham University, Durham, England, United Kingdom

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Publications (365)1152.68 Total impact

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    ABSTRACT: We apply a cross-correlation technique to infer the $S>3$mJy radio luminosity function (RLF) from the NRAO VLA sky survey (NVSS) to $z\sim3.5$. We measure $\Sigma$ the over density of radio sources around spectroscopically confirmed quasars. $\Sigma$ is related to the space density of radio sources at the distance of the quasars and the clustering strength between the two samples, hence knowledge of one constrains the other. Under simple assumptions we find $\Phi\propto (1+z)^{3.7\pm0.7}$ out to $z\sim2$. Above this redshift the evolution slows and we constrain the evolution exponent to $<1.01$ ($2\sigma$). This behaviour is almost identical to that found by previous authors for the bright end of the RLF potentially indicating that we are looking at the same population. This suggests that the NVSS is dominated by a single population; most likely radio sources associated with high-excitation cold-mode accretion. Inversely, by adopting a previously modelled RLF we can constrain the clustering of high-redshift radio sources and find a clustering strength consistent with $r_0=15.0\pm 2.5$ Mpc up to $z\sim3.5$. This is inconsistent with quasars at low redshift and some measurements of the clustering of bright FRII sources. This behaviour is more consistent with the clustering of lower luminosity radio galaxies in the local universe. Our results indicate that the high-excitation systems dominating our sample are hosted in the most massive galaxies at all redshifts sampled.
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    ABSTRACT: We present the discovery of two z > 6 quasars, selected as i band dropouts in the VST ATLAS survey. Our first quasar has redshift, z = 6.31 \pm 0.03, z band magnitude, zAB = 19.63 \pm 0.08 and rest frame 1450A absolute magnitude, M1450 = -27.8 \pm 0.2, making it one of the two most luminous quasars known at z > 6. The second quasar has z = 6.02 \pm 0.03, zAB = 19.54 \pm 0.08 and M1450 = -27.0 \pm 0.1. We also recover a z = 5.86 quasar discovered by Venemans et al. (2015, in prep.). To select our quasars we use a new 3D colour space, combining the ATLAS optical colours with mid-infra-red data from the WISE Space Telescope. We use iAB - zAB colour to exclude main sequence stars, galaxies and lower redshift quasars, W1 - W2 to exclude L dwarfs and zAB - W2 to exclude T dwarfs. A restrictive set of colour cuts returns only our three high redshift quasars and no contaminants, albeit with a sample completeness of \sim50%. We discuss how less restrictive cuts in our 3D colour space can be used to reject the majority of contaminants from samples of bright 5.7 < z < 6.4 quasars, replacing follow-up near-infra-red photometry, whilst retaining high completeness.
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    ABSTRACT: The VLT Survey Telescope (VST) ATLAS is an optical ugriz survey aiming to cover ~4700deg^2 of the Southern sky to similar depths as the Sloan Digital Sky Survey (SDSS). From reduced images and object catalogues provided by the Cambridge Astronomical Surveys Unit we first find that the median seeing ranges from 0.8 arcsec FWHM in i to 1.0 arcsec in u, significantly better than the 1.2-1.5 arcsec seeing for SDSS. The 5 sigma magnitude limit for stellar sources is r_AB=22.7 and in all bands these limits are at least as faint as SDSS. SDSS and ATLAS are more equivalent for galaxy photometry except in the z band where ATLAS has significantly higher throughput. We have improved the original ESO magnitude zeropoints by comparing m<16 star magnitudes with APASS in gri, also extrapolating into u and z, resulting in zeropoints accurate to ~+-0.02 mag. We finally compare star and galaxy number counts in a 250deg^2 area with SDSS and other count data and find good agreement. ATLAS data products can be retrieved from the ESO Science Archive, while support for survey science analyses is provided by the OmegaCAM Science Archive (OSA), operated by the Wide-Field Astronomy Unit in Edinburgh.
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    ABSTRACT: We present a survey of Ly$\alpha$ emitting galaxies in the fields of the VLT LBG Redshift Survey, incorporating the analysis of narrow band number counts, the rest frame UV luminosity function and the two-point correlation function of Ly$\alpha$ emitters at $z\approx3.1$. Our photometric sample consists of 750 LAE candidates, over an area of 1.07 deg$^2$, with estimated equivalent widths of $\gtrsim65$ \AA, from 5 fields based on deep Subaru Suprime-Cam imaging data. Added to this we have obtained spectroscopic follow-up observations, which successfully detected Ly$\alpha$ emission in 35 galaxies. Based on the spectroscopic results, we refined our photometric selection constraints, with the resulting sample having a success rate of $78\pm18\%$. We calculate the narrow band number counts for our photometric sample and find these to be consistent with previous studies of LAEs at this redshift. We find the $R$-band continuum luminosity function to be $\sim10\times$ lower than the equivalent luminosity function of LBGs at this redshift. The results are consistent with the LAE fraction of the LBG population being constant or marginally increasing to fainter magnitudes at $R<26$. Finally, we calculate the LAE auto-correlation function and find a low clustering amplitude compared to the $z\sim3$ LBG population. We calculate a clustering length of $2.87\pm0.70~h^{-1}{\rm Mpc}$, which corresponds to a clustering bias of $b=2.13\pm0.47$ and a median halo mass of $M_{\rm DM}=10^{11.0\pm0.6}~h^{-1}~{\rm M_\odot}$. Overall, we conclude that LAEs inhabit primarily low mass halos, but are a relatively small component of the galaxy population found in such halos.
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    ABSTRACT: The SAMI Galaxy Survey will observe 3400 galaxies with the Sydney-AAO Multi-object Integral-field spectrograph (SAMI) on the Anglo-Australian Telescope (AAT) in a 3-year survey which began in 2013. We present the throughput of the SAMI system, the science basis and specifications for the target selection, the survey observation plan and the combined properties of the selected galaxies. The survey includes four volume limited galaxy samples based on cuts in a proxy for stellar mass, along with low-stellar mass dwarf galaxies all selected from the Galaxy And Mass Assembly (GAMA) survey. The GAMA regions were selected because of the vast array of ancillary data available, including ultraviolet through to radio bands. These fields are on the celestial equator at 9, 12, and 14.5 hours, and cover a total of 144 square degrees (in GAMA-I). Higher density environments are also included with the addition of eight clusters. The clusters have spectroscopy from 2dFGRS and SDSS and photometry in regions covered by the Sloan Digital Sky Survey (SDSS) and/or VLT Survey Telescope/ATLAS. The aim is to cover a broad range in stellar mass and environment, and therefore the primary survey targets cover redshifts 0.004 < z < 0.095, magnitudes r$_{pet}$ < 19.4, stellar masses $10^{7} - 10^{12}$ M$_{sol}$, and environments from isolated field galaxies through groups to clusters of $10^{15}$ M$_{sol}$.
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    ABSTRACT: We report the serendipitous detection of a 0.2 L$^*$, Lyman-$\alpha$ emitting galaxy at redshift 2.5 at an impact parameter of 50 kpc from a brightbackground QSO sightline. A high-resolution spectrum of the QSO reveals a partial Lyman-limit absorption system ($N_\mathrm{HI}=10^{16.94\pm0.10}$ cm$^{-2}$) with many associated metal absorption lines at the same redshift as the foreground galaxy. Using photoionization models that carefully treat measurement errors and marginalise over uncertainties in the shape and normalisation of the ionizing radiation spectrum, we precisely measure the total hydrogen column density $N_\mathrm{H}=10^{19.4\pm0.3}$ cm$^{-2}$, and show that all the absorbing clouds are highly enriched, with $Z=0.1$-$0.6~Z_\odot$. These high metallicities and the system's large velocity width ($436$ km$\,$s$^{-1}$) suggest the gas is produced by an outflowing wind. Using an expanding shell model we estimate a mass outflow rate of $\sim5~M_\odot\,$yr$^{-1}$. Our photoionization model yields extremely small sizes ($<$100-500 pc) for the absorbing clouds, which we argue is typical of high column density absorbers in the circumgalactic medium (CGM). Given these small sizes and extreme kinematics, it is unclear how the clumps survive in the CGM without being destroyed by hydrodynamic instabilities. The small cloud sizes imply that even state-of-the-art cosmological simulations require more than a $1000$-fold improvement in resolution to resolve the hydrodynamics relevant for cool gas in the CGM.
    Monthly Notices of the Royal Astronomical Society 06/2014; 446(1). DOI:10.1093/mnras/stu2088 · 5.23 Impact Factor
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    ABSTRACT: We announce the discovery of a new Galactic companion found in data from the ESO VST ATLAS survey, and followed up with deep imaging on the 4m William Herschel Telescope. The satellite is located in the constellation of Crater (the Cup) at a distance of 170 kpc. Its half-light radius is $r_h=30$ pc and its luminosity is $M_V=-5.5$. The bulk of its stellar population is old and metal-poor. We would probably have classified the newly discovered satellite as an extended globular cluster were it not for the presence of a handful of Blue Loop stars and a sparsely populated red clump. The existence of this core helium burning population implies that star-formation occurred in Crater perhaps as recently as 400 Myr ago. No globular cluster has ever accomplished the feat of prolonging its star-formation by several Gyrs. Therefore, if our hypothesis that the blue bright stars in Crater are Blue Loop giants is correct, the new satellite should be classified as a dwarf galaxy with unusual properties. Note that only ten degrees to the North of Crater, two ultra-faint satellites Leo IV and V orbit the Galaxy at approximately the same distance. This hints that all three galaxies may once have been closely associated before falling togther into the Milky Way halo.
    Monthly Notices of the Royal Astronomical Society 03/2014; 441(3). DOI:10.1093/mnras/stu626 · 5.23 Impact Factor
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    ABSTRACT: The current standard cosmological model, LCDM, provides an excellent fit to the WMAP and Planck CMB data. However, the model has well known problems. For example, the cosmological constant is fine tuned to 1 part in 10^100 and the cold dark matter (CDM) particle is not yet detected in the laboratory. Here we seek an alternative model to LCDM which makes minimal assumptions about new physics. This is based on previous work by Shanks who investigated a model which assumed neither exotic particles nor a cosmological constant but instead postulated a low Hubble constant (H_0) to help allow a baryon density which was compatible with an inflationary model with zero spatial curvature. However, the recent Planck results make it more difficult to reconcile such a model with the cosmic microwave background (CMB) temperature fluctuations. Here we relax the previous assumptions to assess the effects of assuming standard model neutrinos of moderate mass (~5eV) but with no CDM and no cosmological constant. If we assume a low H_0~45kms^-1Mpc^-1 then we now find a reasonable fit to the Planck CMB power spectrum. This model is related to the 11eV sterile neutrino model of Angus (2009) except there a cosmological constant was assumed with H_0~70kms^-1Mpc^-1. The problem for both these models is that the amplitude of fluctuations is low (sigma_8~0.2) making it difficult to form galaxies by the present day. Angus advocated a modified gravity model to increase the growth rate of perturbations. Another approach is to invoke seeds for galaxy formation that might have existed just after decoupling. One possibility is cosmic string wakes but we believe seeds from a primordial magnetic field are more in the spirit of our `what you see is what you get' model.
    Monthly Notices of the Royal Astronomical Society 02/2014; 445(3). DOI:10.1093/mnras/stu1956 · 5.23 Impact Factor
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    ABSTRACT: The prime evidence underpinning the standard Λ cold dark matter cosmological model is the cosmic microwave background (CMB) power spectrum as observed by Wilkinson Microwave Anisotropy Probe (WMAP) and other microwave experiments. But Sawangwit and Shanks have recently shown that the WMAP CMB power spectrum is highly sensitive to the beam profile of the WMAP telescope. Here, we use the source catalogue from the Planck early data release to test further the WMAP beam profiles. We confirm that stacked beam profiles at Q, V and particularly at W, appear wider than expected when compared to the Jupiter beam, normalized either directly to the radio source profiles or using Planck fluxes. The same result is also found based on WMAP-CMB-free source catalogues and NRAO VLA Sky Survey (NVSS) sources. The accuracy of our beam profile measurements is supported by analysis of CMB sky simulations. However, the beam profiles from WMAP7 at the W band are narrower than previously found in WMAP5 data and the rejection of the WMAP beam is now only at the ≈3σ level. We also find that the WMAP source fluxes demonstrate possible non-linearity with Planck fluxes. But including ground-based and Planck data for the bright Weiland et al. sources may suggest that the discrepancy is a linear offset rather than a non-linearity. Additionally, we find that the stacked Sunyaev-Zel'dovich (SZ) decrements of ≈151 galaxy clusters observed by Planck are in agreement with the WMAP data. We find that there is no evidence for a WMAP SZ deficit as has previously been reported. In the particular case of Coma, we find evidence for the presence of an {O}(0.1 mK) downwards CMB fluctuation. We conclude that beam profile systematics can have significant effects on both the amplitude and position of the acoustic peaks, with potentially important implications for cosmology parameter fitting.
    Monthly Notices of the Royal Astronomical Society 01/2014; 437(1):622-640. DOI:10.1093/mnras/stt1912 · 5.23 Impact Factor
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    ABSTRACT: The VST ATLAS is a ugriz-imaging survey targeting ~ 4500 square degrees of the southern sky. It reaches similar magnitude limits to the Sloan Digital Sky Survey in the north, i.e., r ~ 22.5, but ATLAS has better median seeing of 1 arcsecond full width half maximum. ATLAS is a companion survey to the VISTA Hemisphere Survey, which supplies YJHK imaging over much of its area. In addition, the Wide-field Infrared Survey Explorer (WISE) satellite supplies a further four mid-infrared bands. Together these surveys complement each other and provide excellent multi-wavelength data for both Galactic and extragalactic science projects.
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    ABSTRACT: We analyse the clustering of 22,361 quasars between redshift 2.2 < z < 2.9 observed with the Sloan Digital Sky Survey (SDSS)-III Baryon Oscillation Spectroscopic Survey (BOSS), which are included in the ninth data release (DR9). We fit the clustering results with a LCDM model to calculate the linear bias of the quasar sample, b=3.74\pm 0.12. The measured value of bias is consistent with the findings of White et al. (2012), where they analyse almost the same quasar sample, although only in the range, s < 40 Mpc/h. Fitting the standard cosmological model at small and intermediate scales (3-120 Mpc/h) of the quasar clustering produces a rejection at the 2.2 sigma significance level. At larger scales we observe an excess or plateau in the clustering correlation function. Including this large-scale feature in the fit produces a rejection of LCDM at the 2.7 sigma level. By fitting a model that incorporates a scale dependent additional term in the bias introduced by primordial non-Gaussianity of the local type, we calculate the amplitude of the deviation from the Gaussian initial conditions as 70< f_NL <190 at the 95% confidence level. We apply stricter cuts on Galactic extinction, where after keeping regions with A_r < 0.14 mag we measure, 81< f_NL < 170 at 95% CL. In this case the goodness-of-fit of LCDM shows a rejection of the model at a significance level of 1.7 sigma. Investigating systematics further, we make corrections according to the methods of Ross et al. (2011); Ho et al. (2012) and Ross et al. (2011, 2012b), with the f_NL measurements after the application of the two methods being consistent with each other. We consider as our final results on non-Gaussianity those originating after the correction of the sample with the weights method of Ross et al. (2011, 2012b), giving 46< f_NL <158 at 95% confidence. The rejection value of LCDM in this case is at the 2.3 sigma level.
    Monthly Notices of the Royal Astronomical Society 10/2013; 441(1). DOI:10.1093/mnras/stu590 · 5.23 Impact Factor
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    J. R. Whitbourn, T. Shanks
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    ABSTRACT: The redshifts of ~250000 galaxies are used to study the Local Hole and its associated peculiar velocities. The sample, compiled from 6dFGS and SDSS provides wide sky coverage to a depth of ~300h-1Mpc. We have therefore examined K and r limited galaxy redshift distributions and number counts to map the local density field. Comparing observed galaxy n(z) distributions to homogeneous models in three large regions of the high latitude sky, we find evidence for under-densities ranging from ~4-40% in these regions to depths of ~150h-1Mpc with the deepest under-density being over the Southern Galactic cap. Using the Galaxy and Mass Assembly (GAMA) survey we then establish the normalisation of galaxy counts at fainter magnitudes and thus confirm that the underdensity over all three fields at K<12.5 is ~15+-3%. Finally, we further use redshift catalogues to map peculiar velocities over the same areas using the average redshift - magnitude, zbar(m), technique of Soneira (1979). After accounting for the direct effect of large-scale structure on zbar(m) we can then search for peculiar velocities. Taking all three regions into consideration the data reject at the ~4sigma level the idea that we have recovered the CMB rest frame in the volume probed. There is therefore some consistent evidence from counts and Hubble diagram for a local ~150h-1Mpc underdensity that deeper counts and redshifts in the Northern Galactic cap suggest may extend to ~300h-1Mpc.
    Monthly Notices of the Royal Astronomical Society 07/2013; 437(3). DOI:10.1093/mnras/stt2024 · 5.23 Impact Factor
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    ABSTRACT: Over the past 20 years reverberation mapping has proved one of the most successful techniques for studying the local (<1 pc) environment of supermassive black holes that drive active galactic nuclei. Key successes of reverberation mapping have been direct black hole mass estimates, the radius-luminosity relation for the Hβ line and the calibration of single-epoch mass estimators commonly employed up to z ˜ 7. However, observing constraints mean that few studies have been successful at z > 0.1, or for the more-luminous quasars that make up the majority of current spectroscopic samples, or for rest-frame ultraviolet emission lines available in optical spectra of z > 0.5 objects. Previously, we described a technique for stacking cross-correlations to obtain reverberation mapping results at high z. Here, we present the first results from a campaign designed for this purpose. We construct stacked cross-correlation functions for the C IV and Mg II lines and find a clear peak in both. We find that the peak in the Mg II correlation is at longer lags than C IV consistent with previous results at low redshift. For the C IV sample, we are able to bin by luminosity and find evidence for increasing lags for more-luminous objects. This C IV radius-luminosity relation is consistent with previous studies but with a fraction of the observational cost.
    Monthly Notices of the Royal Astronomical Society 07/2013; 434(1):L16-L20. DOI:10.1093/mnrasl/slt069 · 5.23 Impact Factor
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    ABSTRACT: Over the past 20years reverberation mapping has proved one of the most successful techniques for studying the local (<1pc) environment of super-massive black holes that drive active galactic nuclei. Key successes of reverberation mapping have been direct black hole mass estimates, the radius-luminosity relation for the Hbeta line and the calibration of single-epoch mass estimators commonly employed up to z~7. However, observing constraints mean that few studies have been successful at z>0.1, or for the more-luminous quasars that make up the majority of current spectroscopic samples, or for the rest-frame ultra-violet emission lines available in optical spectra of z>0.5 objects. Previously we described a technique for stacking cross correlations to obtain reverberation mapping results at high z. Here we present the first results from a campaign designed for this purpose. We construct stacked cross-correlation functions for the CIV and MgII lines and find a clear peak in both. We find the peak in the MgII correlation is at longer lags than CIV consistent with previous results at low redshift. For the CIV sample we are able to bin by luminosity and find evidence for increasing lags for more-luminous objects. This CIV radius-luminosity relation is consistent with previous studies but with a fraction of the observational cost.
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    ABSTRACT: We use observations and simulations to study the relationship between star-forming galaxies and the IGM at z~3. We use redshift data taken from the VLT LBG Redshift Survey (VLRS) and Keck surveys in fields centred on bright background QSOs, whilst the simulated data is taken from GIMIC. In the simulations, we find that the dominant peculiar velocities are in the form of large-scale coherent motions of gas and galaxies. Gravitational infall of galaxies towards each other is also seen, consistent with linear theory. Peculiar velocity pairs with separations smaller than 1Mpc have a smaller dispersion and better explain the simulated z-space correlations. Lyman-alpha auto- and cross-correlations in the simulations appear to show smaller infall than implied by the expected beta_Lyman-alpha ~ 1.3. Galaxy-wide outflows implemented in the simulations may contribute to this effect. When velocity errors are taken into account, the LBG correlation function prefers the high clustering amplitude shown by higher mass galaxies in the simulation. The simulation and observations also show similar neutral gas densities around galaxies. Finally, little direct evidence is seen in either simulations or observations for high transmission near galaxies due to feedback in agreement with the results of Rakic et al 2012. However, these authors claim evidence for random peculiar velocities between gas and galaxies at small scales and strong infall at larger scales which are much less evident in our GIMIC simulations or indeed in the observed Keck+VLRS data.(abridged)
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    ABSTRACT: S.B.); colin.dunlop@durham.ac.uk (C.D.); d.a.nandi@durham.ac.uk (D.N.); r.m.sharples@durham.ac.uk (R.S.); r.g.talbot@durham.ac.uk (G.T.); pluke1974@btinternet.com (P.L.) Abstract: We present the results of a project to develop a proof of concept for a novel hyperspectral imager based on the use of advanced micro-optics technology. The technology gives considerably more spatial elements than a classic pushbroom which translates into far more light being integrated per unit of time. This permits us to observe at higher spatial and/or spectral resolution, darker targets and under lower illumination, as in the early morning. Observations of faint glow at night should also be possible but need further studies. A full instrument for laboratory demonstration and field tests has now been built and tested. It has about 10,000 spatial elements and spectra 150 pixel long. It is made of a set of cylindrical fore-optics followed by a new innovative optical system called a microslice Integral Field Unit (IFU) which is itself followed by a standard spectrograph. The fore-optics plus microslice IFU split the field into a large number of small slit-like images that are dispersed in the spectrograph. Our goal is to build instruments with at least hundreds of thousands of spatial elements.
    Remote Sensing 01/2013; 5(3):1204-1219. DOI:10.3390/rs5031204 · 2.62 Impact Factor
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    ABSTRACT: [Abridged] Sub-mm observations of the William Herschel Deep Field using LABOCA revealed possible counterparts for 2 X-ray absorbed QSOs. The aim here is to exploit EVLA imaging at 8.4 GHz to establish the QSOs as radio/sub-mm sources. The challenge in reducing the EVLA data was the presence of a strong 4C source in the field. A new calibration algorithm was applied to the data to subtract it. The resulting thermal noise limited radio map covers the 16'x16' Extended WHDF. It contains 41 sources above a 4-sigma limit, 17 of which have primary beam corrected flux. The radio observations show that the absorbed AGN with LABOCA detections are coincident with radio sources, confirming the tendency for X-ray absorbed AGN to be sub-mm bright. These sources show strong ultraviolet excess (UVX) suggesting the nuclear sightline is gas- but not dust-absorbed. Of the 3 remaining LABOCA sources within the ~5' half-power beam width, 1 is identified with a faint nuclear X-ray/radio source in a nearby galaxy, 1 with a faint radio source and 1 is unidentified in any other band. More generally, differential radio source counts are in good agreement with previous observations, showing at S<50 micro-Jy a significant excess over a pure AGN model. In the full area, of 10 sources fainter than this limit, 6 have optical counterparts of which 3 are UVX (i.e. likely QSOs) including the 2 absorbed quasar LABOCA sources. The other faint radio counterparts are not UVX but are only slightly less blue and likely to be star-forming/merging galaxies, predominantly at lower luminosities and redshifts. The 4 faint, optically unidentified radio sources may be either dust obscured QSOs or galaxies. These high-z obscured AGN and lower-z star-forming populations are thus the main candidates to explain the observed excess in faint source counts and hence the excess radio background found previously by the ARCADE2 experiment.
    Monthly Notices of the Royal Astronomical Society 09/2012; 428(2). DOI:10.1093/mnras/sts046 · 5.23 Impact Factor
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    ABSTRACT: We developed the technology of microslice integral field units some years ago as the next step in SAURON type microlens IFU design with typically 5 times more spatial elements (spaxels) for the same spectrograph and spectral length aiming at 1,000,000 spaxels IFUs. A full instrument for laboratory demonstration composed of the fore-optics, the IFU, the spectrograph and the detector has now been built and tested. It has about 10,000 spatial elements and spectra 150 pixel long. Our IFU has 5 cylindrical microlens arrays along the optical axis as opposed to one hexagonal array in the previous design. Instead of imaging pupils on the spectrograph input focal plane, our IFU images short slitlets 17 pixel long that keep the spatial information along the spatial direction then giving 17 spaxels per slitlet instead of one in pupil imaging. This removes most of the lost space between spectra leaving place for more and keeps the spatial information over the element size while pupil images lose it. The fore-optics re-images the field on the input of the IFU. They are made of cylindrical optics to get the desired different magnifications in both directions. All the optics and detector fit in a cylinder 35 mm in diameter and 280 mm long. With a different set of fore-optics on a 4-m telescope, a field of 43" x 6.7" with spatial elements of 0.14" x 0.22" could be observed so 12 of these mini-spectrographs would cover a field surface area of about 1 arcmin2 and 120,000 spaxels.
    Proceedings of SPIE - The International Society for Optical Engineering 09/2012; DOI:10.1117/12.927267 · 0.20 Impact Factor
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    ABSTRACT: A study for a spectrograph delivering at least 10000 slits for galaxies and 20000 for stars over a 2.5 deg2 field have been completed as an answer to the call for proposal for future VISTA MOS instrumentation. In a single night, 65000 galaxy redshifts can be measured to z~0.7 and beyond for measuring the Baryon Acoustic Oscillation (BAO) scale and many other science goals. The design features ten cloned spectrographs which give a smaller total weight and length than a unique spectrograph to make it placable in the space envelope of the Cassegrain focus. The clones use a transparent design including a grism in which all optics are about the size or smaller than the clone rectangular subfield so that they can be tightly packed with little gaps between subfields. Only low cost glasses are used; the variations in chromatic aberrations between bands are compensated by changing a box containing the grism and two adjacent lenses. Two bands cover the 550nm to 900nm wavelength range at resolution of 1100 for blue end and 3000 for red end while another cover the Calcium triplet at 5000. An optional box does imaging but we studied different innovative methods for acquisition without imaging. A new 2.3° corrector was designed that places the pupil before and relatively near the focal plane which permits to give more space at the back of the spectrographs by placing them in a hedgehog configuration. An offaxis field lens in each spectrograph permits to control the pupil position.
    Proceedings of SPIE - The International Society for Optical Engineering 09/2012; DOI:10.1117/12.927260 · 0.20 Impact Factor
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    ABSTRACT: Reverberation mapping offers one of the best techniques for studying the inner regions of QSOs. It is based on cross-correlating continuum and emission-line light curves. New time-resolved optical surveys will produce well sampled light curves for many thousands of QSOs. We explore the potential of stacking samples to produce composite cross-correlations for groups of objects that have well sampled continuum light curves, but only a few (~2) emission-line measurements. This technique exploits current and future wide-field optical monitoring surveys (e.g. Pan-STARRS, LSST) and the multiplexing capability of multi-object spectrographs (e.g. 2dF, Hectospec) to significantly reduce the observational expense of reverberation mapping, in particular at high redshift (0.5 to 2.5). We demonstrate the technique using simulated QSO light curves and explore the biases involved when stacking cross-correlations in some simplified situations. We show that stacked cross correlations have smaller amplitude peaks compared to well sampled correlation functions as the mean flux of the emission light curve is poorly constrained. However, the position of the peak remains intact. We find there can be `kinks' in stacked correlation functions due to different measurements contributing to different parts of the correlation function. Using the Pan-STARRS Medium-Deep Survey (MDS) as a template we show that cross-correlation lags should be measurable in a sample size of 500 QSOs that have weekly photometric monitoring and two spectroscopic observations. Finally we apply the technique to a small sample (42) of QSOs that have light curves from the MDS. We find no indication of a peak in the stacked cross-correlation. A larger spectroscopic sample is required to produce robust reverberation lags.
    Monthly Notices of the Royal Astronomical Society 05/2012; 427(4). DOI:10.1111/j.1365-2966.2012.21248.x · 5.23 Impact Factor

Publication Stats

7k Citations
1,152.68 Total Impact Points

Institutions

  • 1970–2015
    • Durham University
      • Department of Physics
      Durham, England, United Kingdom
  • 2002
    • National Optical Astronomy Observatory
      Tucson, Arizona, United States
  • 1983–1995
    • University of Cambridge
      • Institute of Astronomy
      Cambridge, England, United Kingdom
  • 1991
    • University of Leicester
      • Department of Physics and Astronomy
      Leiscester, England, United Kingdom
  • 1987
    • Cardiff University
      Cardiff, Wales, United Kingdom