T. Shanks

Durham University, Durham, England, United Kingdom

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Publications (374)1172.37 Total impact

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    T. Shanks
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    ABSTRACT: I first review the status of Digital Sky Surveys. The focus will be on extragalactic surveys with an area of more than 100 sq.deg. The Sloan Digital Sky Survey is the archetype of such imaging surveys and it is its great success that has prompted great activity in this field. The latest surveys explore wider, fainter and higher resolution and also a longer wavelength range than SDSS. Many of these surveys overlap particularly in the S Hemisphere where we now have Pan-STARRS, DES and the ESO VST surveys, and our aim here is to compare their properties. Since there is no dedicated article on the VST ATLAS in this symposium, we shall especially review the properties of this particular survey. This easily fits onto our other main focus which is to compare overlapping Southern Surveys and see how they best fit with the available NIR imaging data. We conclude that the Southern Hemisphere will soon overtake the North in terms of multiwavelength imaging. However we note that the South has more limited opportunities for spectroscopic follow-up and this weakness will persist during the LSST era. Some new perspectives are offered on this and other aspects of survey astronomy.
    Preview · Article · Jul 2015
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    ABSTRACT: We present a panoramic study of the Fornax dwarf spheroidal galaxy, using data obtained as part of the VLT Survey Telescope (VST) ATLAS Survey. The data presented here – a subset of the full survey – uniformly cover a region of 25 deg2 centred on the galaxy, in g, r and i bands. This large area coverage reveals two key differences to previous studies of Fornax. First, data extending beyond the nominal tidal radius of the dwarf highlight the presence of a second distinct red giant branch population. This bluer red giant branch appears to be co-eval with the horizontal branch population. Secondly, a shell structure located approximately 1 $_{.}^{\circ}$4 from the centre of Fornax is shown to be a mis-identified background overdensity of galaxies. This last result casts further doubt on the hypothesis that Fornax underwent a gas-rich merger in its relatively recent past.
    Preview · Article · Jul 2015 · Monthly Notices of the Royal Astronomical Society
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    S. Fine · T. Shanks · R. Johnston · M. J. Jarvis · T. Mauch
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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 ∼ 3.5. We measure Σ the over density of radio sources around spectroscopically confirmed quasars. Σ 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 Φ ∝ (1 + z)3.7 ± 0.7 out to z ∼ 2. Above this redshift the evolution slows and we constrain the evolution exponent to <1.01 (2σ). 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 r0 = 15.0 ± 2.5 Mpc up to z ∼ 3.5. This is inconsistent with quasars at low redshift and some measurements of the clustering of bright FR II 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.
    Full-text · Article · May 2015 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: We present the selection algorithm and anticipated results for the Time Domain Spectroscopic Survey (TDSS). TDSS is an SDSS-IV eBOSS subproject that will provide initial identification spectra of approximately 220,000 luminosity-variable objects (variable stars and AGN) across 7,500 square degrees selected from a combination of SDSS and multi-epoch Pan-STARRS1 photometry. TDSS will be the largest spectroscopic survey to explicitly target variable objects, avoiding pre-selection on the basis of colors or detailed modeling of specific variability characteristics. Kernel Density Estimate (KDE) analysis of our target population performed on SDSS Stripe 82 data suggests our target sample will be 95% pure (meaning 95% of objects we select have genuine luminosity variability of a few magnitudes or more). Our final spectroscopic sample will contain roughly 135,000 quasars and 85,000 stellar variables, approximately 4,000 of which will be RR Lyrae stars which may be used as outer Milky Way probes. The variability-selected quasar population has a smoother redshift distribution than a color-selected sample, and variability measurements similar to those we develop here may be used to make more uniform quasar samples in large surveys. The stellar variable targets are distributed fairly uniformly across color space, indicating that TDSS will obtain spectra for a wide variety of stellar variables including pulsating variables, stars with significant chromospheric activity, cataclysmic variables and eclipsing binaries. TDSS will serve as a pathfinder mission to identify and characterize the multitude of variable objects that will be detected photometrically in even larger variability surveys such as LSST.
    Full-text · Article · May 2015 · The Astrophysical Journal
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    ABSTRACT: We present the discovery of two z > 6 quasars, selected as i-band dropouts in the Very Large Telescope Survey Telescope ATLAS survey. Our first quasar has redshift, z = 6.31 ± 0.03, z-band magnitude, zAB = 19.63 ± 0.08 and rest frame 1450 Å absolute magnitude, M1450 = −27.8 ± 0.2, making it the joint second most luminous quasar known at z > 6. The second quasar has z = 6.02 ± 0.03, zAB = 19.54 ± 0.08 and M1450 = −27.0 ± 0.1. We also recover a z = 5.86 quasar discovered by Venemans et al., in preparation. To select our quasars, we use a new 3D colour space, combining the atlas optical colours with mid-infrared data from the Wide-field Infrared Survey Explorer. 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 ∼50 per cent. We discuss how our 3D colour space can be used to reject the majority of contaminants from samples of bright 5.7 < z < 6.3 quasars, replacing follow-up near-infrared photometry, whilst retaining high completeness.
    Preview · Article · Feb 2015 · Monthly Notices of the Royal Astronomical Society Letters
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    ABSTRACT: The VLT Survey Telescope ATLAS survey is an optical ugriz survey aiming to cover ≈4700 deg2 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 (full width at half-maximum) in i to 1.0 arcsec in u, significantly better than the 1.2–1.5 arcsec seeing for SDSS. The 5σ mag limit for stellar sources is rAB = 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 zero-points by comparing m < 16 star magnitudes with the AAVSO Photometric All-Sky Survey in gri, also extrapolating into u and z, resulting in zero-points accurate to ≈ ± 0.02 mag. We finally compare star and galaxy number counts in a 250 deg2 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, operated by the Wide-Field Astronomy Unit in Edinburgh.
    Preview · Article · Feb 2015 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: We present a survey of z ∼ 3 Lyα emitters (LAEs) within the fields of the VLT Lyman break galaxies (LBG) redshift survey. The data encompass five independent survey fields co-spatial with spectroscopic LBG data and covering a larger total area than previously analysed for LAE number counts and clustering. This affords an improved analysis over previous work by minimizing the effects of cosmic variance and allowing the cross-clustering analysis of LAEs and LBGs. Our photometric sample consists of ≈600 LAE candidates, over an area of 1.07 deg2, with equivalent widths of ≳65 Å and a flux limit of ≈2 × 10−17 erg cm−2 s−1. From spectroscopic follow-up, we measured a success rate of 78 ± 18 per cent. We find the R-band continuum luminosity function to be ∼10 times lower than the luminosity function of LBGs at this redshift, consistent with previous studies. Exploiting the large area of the survey, we estimate the LAE auto-correlation function and find a clustering length of r0 = 2.86 ± 0.33 h−1 Mpc, low compared to the z ∼ 3 LBG population, but somewhat higher than previous LAE measurements. This corresponds to a median halo mass of MDM = 1011.0±0.3 h−1 M⊙. We present an analysis of clustering length versus continuum magnitude and find that the measurements for LAEs and LBGs are consistent at faint magnitudes. Our combined data set of LAEs and LBGs allows us to measure, for the first time, the LBG–LAE cross-correlation, finding a clustering length of r0 = 3.29 ± 0.57 h−1 Mpc and a LAE halo mass of 1011.1±0.4 h−1 M⊙. Overall, we conclude that LAEs inhabit primarily low-mass haloes, but form a relatively small proportion of the galaxy population found in such haloes.
    Preview · Article · Jan 2015 · Monthly Notices of the Royal Astronomical Society
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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 in a 3-yr 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 h, and cover a total of 144 deg2 (in GAMA-I). Higher density environments are also included with the addition of eight clusters. The clusters have spectroscopy from 2-degree Field Galaxy Redshift Survey (2dFGRS) and Sloan Digital Sky Survey (SDSS) and photometry in regions covered by the 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 rpet < 19.4, stellar masses 107–1012 M⊙, and environments from isolated field galaxies through groups to clusters of ∼1015 M⊙.
    Full-text · Article · Jul 2014 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: We report the serendipitous detection of a 0.2 L*, Lyα emitting galaxy at redshift 2.5 at an impact parameter of 50 kpc from a bright background QSO sightline. A high-resolution spectrum of the QSO reveals a partial Lyman-limit absorption system (${\it N}_\mathrm{H\,\small {i}}=10^{16.94\pm 0.10}$ 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 marginalize over uncertainties in the shape and normalization of the ionizing radiation spectrum, we derive the total hydrogen column density ${\it N}_\mathrm{H}=10^{19.4\pm 0.3}\,\rm cm^{-2}$, and show that all the absorbing clouds are metal enriched, with Z = 0.1–0.6 Z⊙. These 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 ∼5 M⊙ 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 mass resolution to resolve the hydrodynamics relevant for cool gas in the CGM.
    Full-text · Article · Jun 2014 · Monthly Notices of the Royal Astronomical Society
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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 4-m 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 rh = 30 pc and its luminosity is MV = −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 the 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 Gyr. 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 10°to the north of Crater, two ultrafaint galaxies Leo IV and Leo V orbit the Galaxy at approximately the same distance. This hints that all three satellites may once have been closely associated before falling together into the Milky Way halo.
    Full-text · Article · Mar 2014 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: The standard cosmological model, Λ cold dark matter (ΛCDM), provides an excellent fit to cosmic microwave background (CMB) data. However, the model has well-known problems. For example, the cosmological constant, Λ, is fine-tuned to 1 part in 10100 and the CDM particle is not yet detected in the laboratory. Shanks previously investigated a model which assumed neither exotic particles nor a cosmological constant but instead postulated a low Hubble constant (H0) to allow a baryon density compatible with inflation and zero spatial curvature. However, recent Planck results make it more difficult to reconcile such a model with CMB power spectra. Here, we relax the previous assumptions to assess the effects of assuming three active neutrinos of mass ≈5 eV. If we assume a low H0 ≈ 45 km s−1 Mpc−1 then, compared to the previous purely baryonic model, we find a significantly improved fit to the first three peaks of the Planck power spectrum. Nevertheless, the goodness of fit is still significantly worse than for ΛCDM and would require appeal to unknown systematic effects for the fit ever to be considered acceptable. A further serious problem is that the amplitude of fluctuations is low (σ8 ≈ 0.2), making it difficult to form galaxies by the present day. This might then require seeds, perhaps from a primordial magnetic field, to be invoked for galaxy formation. These and other problems demonstrate the difficulties faced by models other than ΛCDM in fitting ever more precise cosmological data.
    Preview · Article · Feb 2014 · Monthly Notices of the Royal Astronomical Society
  • J. R. Whitbourn · T. Shanks · U. Sawangwit
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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.
    No preview · Article · Jan 2014 · Monthly Notices of the Royal Astronomical Society
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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.
    No preview · Article · Nov 2013
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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 Λcold dark matter (ΛCDM) model to calculate the linear bias of the quasar sample, b = 3.74 ± 0.12. The measured value of bias is consistent with the findings of White et al., where they analyse almost the same quasar sample, although only in the range s < 40 h−1 Mpc. At large scales we observe an excess or plateau in the clustering correlation function. 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 < fNLlocal < 190 at the 95 per cent confidence level. We correct the sample from systematics according to the methods of Ross et al. and Ho et al., with the fNLlocal measurements after the application of the two methods being consistent with each other. Finally, we use cross-correlations across redshift slices to test the corrected sample for any remaining unknown sources of systematics, but the results give no indication of any such further errors. We consider as our final results on non-Gaussianity, 46 < fNLlocal < 158 at 95 per cent confidence, after correcting the sample with the weights method of Ross et al. These results are consistent with previous tight constraints on non-Gaussianity from other Large-Scale Structures surveys, but are in tension with the latest results from the cosmic microwave background.
    Preview · Article · Oct 2013 · Monthly Notices of the Royal Astronomical Society
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    J. R. Whitbourn · T. Shanks
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    ABSTRACT: The redshifts of ≈250 000 galaxies are used to study the local hole and its associated peculiar velocities. The sample, compiled from the 6dF Galaxy Redshift Survey and Sloan Digital Sky Survey, provides wide sky coverage to a depth of ≈300 h−1 Mpc. 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 underdensities ranging from ≈4–40 per cent in these regions to depths of ≈150 h−1 Mpc with the deepest underdensity being over the southern Galactic cap. Using the Galaxy and Mass Assembly survey, we then establish the normalization of galaxy counts at fainter magnitudes and thus confirm that the underdensity over all three fields at K < 12.5 is ≈15 ± 3 per cent. Finally, we further use redshift catalogues to map sky-averaged peculiar velocities over the same areas using the average redshift–magnitude, $\overline{z}(m)$, technique of Soneira. After accounting for the direct effect of the large-scale structure on $\overline{z}(m),$ we can then search for peculiar velocities. Taking all three regions into consideration, the data reject at the ≈4σ level the idea that we have recovered the cosmic microwave background rest frame in the volume probed. We therefore conclude that there is some consistent evidence from both counts and Hubble diagrams for a ‘local hole’ with an ≈150 h−1 Mpc underdensity that deeper counts and redshifts in the northern Galactic cap suggest may extend to ≈300 h−1 Mpc.
    Preview · Article · Jul 2013 · Monthly Notices of the Royal Astronomical Society
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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.
    No preview · Article · Jul 2013 · Monthly Notices of the Royal Astronomical Society
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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.
    Full-text · Article · May 2013
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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)
    Full-text · Article · Apr 2013 · Monthly Notices of the Royal Astronomical Society
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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.
    Full-text · Article · Jan 2013 · Remote Sensing
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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.
    Full-text · Article · Sep 2012 · Monthly Notices of the Royal Astronomical Society

Publication Stats

9k Citations
1,172.37 Total Impact Points

Institutions

  • 1970-2015
    • Durham University
      • Department of Physics
      Durham, England, United Kingdom
  • 2002
    • National Optical Astronomy Observatory
      Tucson, Arizona, United States
  • 1983-1998
    • University of Cambridge
      • Institute of Astronomy
      Cambridge, England, United Kingdom
  • 1996
    • Johns Hopkins University
      Baltimore, Maryland, United States
  • 1987
    • Cardiff University
      Cardiff, Wales, United Kingdom