# ProfessorStephenWarren

Faculty of Natural SciencesDepartment of Physics

Professor of Astrophysics

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### Contact

+44 (0)20 7594 7554s.j.warren

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### Location

1115Blackett LaboratorySouth Kensington Campus

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## Publications

Publication Type
Year
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81 results found

O'Riordan CM, Warren SJ, Mortlock DJ, 2021, Galaxy mass profiles from strong lensing III: The two-dimensional broken power-law model, Monthly Notices of the Royal Astronomical Society, Vol: 501, Pages: 3687-3694, ISSN: 0035-8711

When modelling strong gravitational lenses, i.e., where there are multipleimages of the same source, the most widely used parameterisation for the massprofile in the lens galaxy is the singular power-law model $\rho(r)\proptor^{-\gamma}$. This model may be insufficiently flexible for very accurate work,for example measuring the Hubble constant based on time delays between multipleimages. Here we derive the lensing properties - deflection angle, shear, andmagnification - of a more adaptable model where the projected mass surfacedensity is parameterised as a continuous two-dimensional broken power-law(2DBPL). This elliptical 2DBPL model is characterised by power-law slopes$t_1$, $t_2$ either side of the break radius $\theta_\mathrm{B}$. The key tothe 2DBPL model is the derivation of the lensing properties of the truncatedpower law (TPL) model, where the surface density is a power law out to thetruncation radius $\theta_\mathrm{T}$ and zero beyond. This TPL model is alsouseful by itself. We create mock observations of lensing by a TPL profile wherethe images form outside the truncation radius, so there is no mass in theannulus covered by the images. We then show that the slope of the profileinterior to the images may be accurately recovered for lenses of moderateellipticity. This demonstrates that the widely-held notion that lensingmeasures the slope of the mass profile in the annulus of the images, and isinsensitive to the mass distribution at radii interior to the images, isincorrect.

Journal article

Warren SJ, Ahmed S, Laithwaite RC, 2021, The local vertical density distribution of ultracool dwarfs M7 to L2.5 and their luminosity function, The Open Journal of Astrophysics, Vol: 4, Pages: 1-13

We investigate the form of the local vertical density profile of the stars in the Galactic disk, close to the Galactic plane. We use a homogeneous sample of 34000 ultracool dwarfs M7 to L2.5 that all lie within 350 pc of the plane. We fit a profile of the form sechα, where α=2 is the theoretically preferred isothermal profile and α=0 is the exponential function. Larger values of α correspond to greater flattening of the profile towards the plane. We employ a likelihood analysis that accounts in a direct way for unresolved binaries in the sample, as well as for the spread in absolute magnitude MJ within each spectral sub-type (Malmquist bias). We measure α=0.29+0.12−0.13. The α=1 (sech) and flatter profiles are ruled out at high confidence for this sample, while α=0 (exponential) is included in the 95% credible interval. Any flattening relative to exponential is modest, and is confined to within 50 pc of the plane. The measured value of α is consistent with the results of the recent analysis by Xiang et al. Our value for α is also similar to that determined for nearby spiral galaxies by de Grijs et al., measured from photometry of galaxies viewed edge on. The measured profile allows an accurate determination of the local space density of ultracool dwarfs M7 to L2.5, and we use this to make a new determination of the luminosity function at the bottom of the main sequence. Our results for the luminosity function are a factor two to three lower than the recent measurement by Bardalez Gagliuffi et al., that uses stars in the local 25 pc radius bubble, but agree well with the older study by Cruz et al.

Journal article

Barnett R, Warren SJ, Cross NJG, Mortlock DJ, Fan X, Wang F, Hewett PCet al., 2020, A complete search for redshift z>6.5 quasars in the VIKING survey, Monthly Notices of the Royal Astronomical Society, Vol: 501, Pages: 1663-1676, ISSN: 0035-8711

We present the results of a new, deeper, and complete search forhigh-redshift $6.5<z<9.3$ quasars over 977deg$^2$ of the VISTA Kilo-DegreeInfrared Galaxy (VIKING) survey. This exploits a new list-driven datasetproviding photometry in all bands ZYJHKs, for all sources detected by VIKING in$J$. We use the Bayesian model comparison (BMC) selection method of Mortlock etal., producing a ranked list of just 21 candidates. The sources ranked 1, 2, 3and 5 are the four known $z>6.5$ quasars in this field. Additional observationsof the other 17 candidates, primarily DESI Legacy Survey photometry and ESOFORS2 spectroscopy, confirm that none is a quasar. This is the first completesample from the VIKING survey, and we provide the computed selection function.We include a detailed comparison of the BMC method against two other selectionmethods: colour cuts and minimum-$\chi^2$ SED fitting. We find that: i) BMCproduces eight times fewer false positives than colour cuts, while alsoreaching 0.3 mag. deeper, ii) the minimum-$\chi^2$ SED fitting method isextremely efficient but reaches 0.7 mag. less deep than the BMC method, andselects only one of the four known quasars. We show that BMC candidates,rejected because their photometric SEDs have high $\chi^2$ values, includebright examples of galaxies with very strong [OIII]$\lambda\lambda$4959,5007emission in the $Y$ band, identified in fainter surveys by Matsuoka et al. Thisis a potential contaminant population in Euclid searches for faint $z>7$quasars, not previously accounted for, and that requires bettercharacterisation.

Journal article

Laithwaite RC, Warren SJ, 2020, The absolute magnitudes $M_J$, the binary fraction, and the binary mass ratios of M7 to M9.5 dwarfs, Monthly Notices of the Royal Astronomical Society, Vol: 499, Pages: 2587-2597, ISSN: 0035-8711

We use the large homogeneous sample of late M dwarfs, M7 to M9.5, of Ahmed &Warren (2019) matched to $\textit{Gaia}$ DR2, to measure the relation betweenabsolute magnitude and spectral type, and to infer the multiplicity fraction ofthe population, and the distribution of mass ratios in the binary systems.Binaries are identified photometrically as overluminous sources. In order todefine a sample that is unbiased with respect to multiplicity we use distancelimits that are a function of $G-J$ colour to define a volume-complete sampleof 2706 systems. The $G-J$ colours are very precise, with random errors allless than 0.02. We measure absolute magnitudes $M_J$ that are on average 0.5mag. brighter than previous determinations. We find evidence that thediscrepancies arise from differences in spectral types in different samples.The measured binary fraction is $16.5\pm0.8\%$, of which $98\%$ are unresolved:both values are consistent with results of previous studies. The distributionof excess flux in the binaries, compared to the singles, is used to infer themass ratio distribution $f(q)\propto q^\gamma$, where $q=M_s/M_p$. We infer avery steep distribution over this spectral range, with $\gamma>10$ ($99\%$probability). This says that unresolved ultracool M dwarf binaries residealmost exclusively in equal mass systems, and implies that the spectral typesof the unresolved binaries match to with 0.5 spectral subtypes. The intrinsicscatter in absolute magnitude $M_J$ for ultracool M dwarfs at fixed $G-J$colour is measured to be 0.21 mag.

Journal article

ORiordan CM, Warren SJ, Mortlock DJ, 2020, Galaxy mass profiles from strong lensing II: The elliptical power-law model, Monthly Notices of the Royal Astronomical Society, Vol: 496, Pages: 3424-3435, ISSN: 0035-8711

We present a systematic analysis of the constraints σγ on the mass profile slope γ obtainable when fitting a singular power-law ellipsoid model to a typical strong lensing observation of an extended source. These results extend our previous analysis of circular systems, Paper I. We draw our results from 676 mock observations covering a range of image configurations, each created with a fixed signal to noise ratio S = 100 in the images. We analyse the results using a combination of theory and a simplified model which identifies the contribution to the constraints of the individual fluxes and positions in each of the lensed images. The main results are: 1. For any lens ellipticity, the constraints σγ for two image systems are well described by the results of Paper I, transformed to elliptical coordinates; 2. We derive an analytical expression for σγ for systems with the source aligned with the axis of the lens; 3. For both two-image systems and aligned systems, σγ is limited by the flux uncertainties; 4. The constraints for off-axis four-image systems are a factor of two to eight better, depending on source size, than for two-image systems, and improve with increasing lens ellipticity. We show that the constraints on γ in these systems derive from the complementary positional information of the images alone, without using flux. The complementarity improves as the offset of the source from the axis increases, such that the best constraints σγ < 0.01, for S = 100, occur when the source approaches the caustic.

Journal article

Dobbie PS, Warren SJ, 2020, A bayesian approach to the vertical structure of the disk of the Milky Way, The Open Journal of Astrophysics, Vol: 3, ISSN: 2565-6120

This work investigates the vertical profile of the stars in the disk of the Milky Way. The models investigated are of the form sech2/n(nz/(2H)) where, setting α = 2/n, the three functions of the sequence α= 0, 1, 2 correspond to exponential, sech, and sech2 functions respectively. We consider symmetric models and asymmetric models, above and below the plane. The study uses the large sample of K and M stars of Ferguson et al. (2017) and applies the methods of Bayesian model comparison to discriminate between the 6 models. Two inconsistencies in Ferguson et al. (2017), concerning the vertical height cut and the model continuity across the plane, are noted and addressed. We find that (1) in the Milky Way the symmetric disc models are decisively ruled out, with northern thin disc scale heights about 25% larger than southern, (2) there is moderate evidence for the exponential and sech models over the sech2 model, though a sample extending further into the Galactic mid-plane is needed to strengthen this result, (3) the photometric distances used by Ferguson et al. underestimate the GAIA distances by a factor of roughly 1.16, and (4) the increase of scale height with Galactic latitude observed by Ferguson et al. is due to incorrect cuts to the data.

Journal article

Barnett R, Warren SJ, Mortlock DJ, Cuby J-G, Conselice C, Hewett PC, Willott CJet al., 2019, Euclid preparation. V. Predicted yield of redshift 7<z<9 quasars from the wide survey, Astronomy & Astrophysics, Vol: 631, ISSN: 0004-6361

We provide predictions of the yield of 7 < z < 9 quasars from the Euclid wide survey, updating the calculation presented in theEuclid Red Book in several ways. We account for revisions to the Euclid near-infrared filter wavelengths; we adopt steeper ratesof decline of the quasar luminosity function (QLF; Φ) with redshift, Φ ∝ 10k(z−6), k = −0.72, and a further steeper rate of decline,k = −0.92; we use better models of the contaminating populations (MLT dwarfs and compact early-type galaxies); and we make useof an improved Bayesian selection method, compared to the colour cuts used for the Red Book calculation, allowing the identificationof fainter quasars, down to JAB ∼ 23. Quasars at z > 8 may be selected from Euclid OY JH photometry alone, but selection overthe redshift interval 7 < z < 8 is greatly improved by the addition of z-band data from, e.g., Pan-STARRS and LSST. We calculatepredicted quasar yields for the assumed values of the rate of decline of the QLF beyond z = 6. If the decline of the QLF acceleratesbeyond z = 6, with k = −0.92, Euclid should nevertheless find over 100 quasars with 7.0 < z < 7.5, and ∼ 25 quasars beyond thecurrent record of z = 7.5, including ∼ 8 beyond z = 8.0. The first Euclid quasars at z > 7.5 should be found in the DR1 data release,expected in 2024. It will be possible to determine the bright-end slope of the QLF, 7 < z < 8, M1450 < −25, using 8 m class telescopesto confirm candidates, but follow-up with JWST or E-ELT will be required to measure the faint-end slope. Contamination of thecandidate lists is predicted to be modest even at JAB ∼ 23. The precision with which k can be determined over 7 < z < 8 depends onthe value of k, but assuming k = −0.72 it can be measured to a 1σ uncertainty of 0.07.

Journal article

O'Riordan CM, Warren SJ, Mortlock DJ, 2019, Galaxy mass profiles from strong lensing I: the circular power-law model, Monthly Notices of the Royal Astronomical Society, ISSN: 0035-8711

In this series of papers we develop a formalism for constraining mass profiles in strong gravitational lenses with extended images, using fluxes in addition to positional information. We start in this paper with a circular power-law profile and show that the slope γ is uniquely determined by only two observables: the flux ratio f1/f2 and the image position ratio θ1/θ2 of the two images. We derive an analytic expression relating these two observables to the slope, a result which does not depend on the Einstein angle or the structure or brightness of the source. We then find an expression for the uncertainty on the slope σγ that depends only on the position ratio θ1/θ2 and the total S/N in the images. For example, in a system with position ratio θ1/θ2=0.5, S/N =100 and γ=2 we find that γ is constrained to a precision of ±0.03. We then test these results against a series of mock observations. We invert the images and fit an 11 parameter model, including ellipticity and position angle for both lens and source and measure the uncertainty on γ. We find agreement with the theoretical estimate for all mock observations. In future papers we will examine the radial range of the galaxy over which the constraint on the slope applies, and extend the analysis to elliptical lenses.

Journal article

Ahmed S, Warren SJ, 2019, A homogeneous sample of 34 000 M7-M9.5 dwarfs brighter than J=17.5 with accurate spectral types, Astronomy and Astrophysics, Vol: 623, ISSN: 0004-6361

The space density of late M dwarfs, subtypes M7–M9.5, is not well determined. We applied the photo-type method to iz photometry from the Sloan Digital Sky Survey and YJHK photometry from the UKIRT Infrared Deep Sky Survey, over an effective area of 3070 deg2, to produce a new, bright J(Vega) <  17.5, homogeneous sample of 33 665 M7–M9.5 dwarfs. The typical S/N of each source summed over the six bands is > 100. Classifications are provided to the nearest half spectral subtype. Through a comparison with the classifications in the BOSS Ultracool Dwarfs (BUD) spectroscopic sample, the typing is shown to be accurately calibrated to the BUD classifications and the precision is better than 0.5 subtypes rms; i.e. the photo-type classifications are as precise as good spectroscopic classifications. Sources with large χ2 >  20 include several catalogued late-type subdwarfs. The new sample of late M dwarfs is highly complete, but there is a bias in the classification of rare peculiar blue or red objects. For example, L subdwarfs are misclassified towards earlier types by approximately two spectral subtypes. We estimate that this bias affects only ∼1% of the sources. Therefore the sample is well suited to measure the luminosity function and investigate the softening towards the Galactic plane of the exponential variation of density with height.

Journal article

Barnett R, Warren SJ, Becker GD, Mortlock DJ, Hewett PC, McMahon RG, Simpson C, Venemans BPet al., 2017, Observations of the Lyman series forest towards the redshift 7.1 quasar ULAS J1120+0641, Astronomy and Astrophysics: a European journal, Vol: 601, ISSN: 0004-6361

We present a 30 h integration Very Large Telescope X-shooter spectrum of the Lyman series forest towards the z = 7.084 quasar ULAS J1120+0641. The only detected transmission at S/N > 5 is confined to seven narrow spikes in the Lyα forest, over the redshift range 5.858 <z < 6.122, just longward of the wavelength of the onset of the Lyβ forest. There is also a possible detection of one further unresolved spike in the Lyβ forest at z = 6.854, with S/N = 4.5. We also present revised Hubble Space Telescope F814W photometry of the source. The summed flux from the transmission spikes is in agreement with the F814W photometry, so all the transmission in the Lyman series forest may have been detected. There is a Gunn-Peterson (GP) trough in the Lyα forest from z = 6.122 all the way to the quasar near zone at z = 7.04. The trough, of comoving length 240 h-1 Mpc, is over twice as long as the next longest known GP trough. We combine the spectroscopic and photometric results to constrain the evolution of the Lyα effective optical depth (τGPeff) with redshift, extending a similar analysis by Simpson et al. We find τGPeff ∝ (1 + z)ξ where ξ = 11.2+ 0.4-0.6, for z > 5.5. The data nevertheless provide only a weak limit on the volume-weighted intergalactic medium (IGM) hydrogen neutral fraction at z ~ 6.5, xH i > 10-4, similar to limits at redshift z ~ 6 from less distant quasars. The new observations cannot extend measurements of the neutral fraction of the IGM to higher values because absorption in the Lyα forest is already saturated near z ~ 6. For higher neutral fractions, other methods such as measuring the red damping wing of the IGM will be required.

Journal article

Skrzypek N, Warren SJ, Faherty JK, 2016, Photometric brown-dwarf classification. II. A homogeneous sample of 1361 L and T dwarfs brighter than J = 17.5 with accurate spectral types, Astronomy & Astrophysics, Vol: 589, ISSN: 0004-6361

We present a homogeneous sample of 1361 L and T dwarfs brighter than J = 17.5 (of which 998 are new), from an effective areaof 3070 deg2, classified by the photo-type method to an accuracy of one spectral sub-type using izY JHKW1W2 photometry fromSDSS+UKIDSS+WISE. Other than a small bias in the early L types, the sample is shown to be effectively complete to the magnitudelimit, for all spectral types L0 to T8. The nature of the bias is an incompleteness estimated at 3% because peculiar blue L dwarfs oftype L4 and earlier are classified late M. There is a corresponding overcompleteness because peculiar red (likely young) late M dwarfsare classified early L. Contamination of the sample is confirmed to be small: so far spectroscopy has been obtained for 19 sources inthe catalogue and all are confirmed to be ultracool dwarfs. We provide coordinates and izY JHKW1W2 photometry of all sources. Weidentify an apparent discontinuity, ∆m ∼ 0.4 mag., in the Y − K colour between spectral types L7 and L8. We present near-infraredspectra of nine sources identified by photo-type as peculiar, including a new low-gravity source ULAS J005505.68+013436.0, withspectroscopic classification L2γ. We provide revised izY JHKW1W2 template colours for late M dwarfs, types M7 to M9.

Journal article

Skrzypek N, Warren SJ, Faherty JK, Mortlock DJ, Burgasser AJ, Hewett PCet al., 2015, Photometric brown-dwarf classification I. A method to identify and accurately classify large samples of brown dwarfs without spectroscopy, Astronomy & Astrophysics, Vol: 574, ISSN: 1432-0746

Aims. We present a method, named photo-type, to identify and accurately classify L and T dwarfs onto the standard spectral classification system using photometry alone. This enables the creation of large and deep homogeneous samples of these objects efficiently, without the need for spectroscopy.Methods. We created a catalogue of point sources with photometry in 8 bands, ranging from 0.75 to 4.6 μm, selected from an area of 3344 deg2, by combining SDSS, UKIDSS LAS, and WISE data. Sources with 13.0 <J< 17.5, and Y − J> 0.8, were then classified by comparison against template colours of quasars, stars, and brown dwarfs. The L and T templates, spectral types L0 to T8, were created by identifying previously known sources with spectroscopic classifications, and fitting polynomial relations between colour and spectral type.Results. Of the 192 known L and T dwarfs with reliable photometry in the surveyed area and magnitude range, 189 are recovered by our selection and classification method. We have quantified the accuracy of the classification method both externally, with spectroscopy, and internally, by creating synthetic catalogues and accounting for the uncertainties. We find that, brighter than J = 17.5, photo-type classifications are accurate to one spectral sub-type, and are therefore competitive with spectroscopic classifications. The resultant catalogue of 1157 L and T dwarfs will be presented in a companion paper.

Journal article

Barnett R, Warren SJ, Banerji M, McMahon RG, Hewett PC, Mortlock DJ, Simpson C, Venemans BP, Ota K, Shibuya Tet al., 2015, The spectral energy distribution of the redshift 7.1 quasar ULAS J1120+0641, Astronomy & Astrophysics, Vol: 575, ISSN: 1432-0746

Journal article

Simpson C, Mortlock D, Warren S, Cantalupo S, Hewett P, McLure R, McMahon R, Venemans Bet al., 2014, No excess of bright galaxies around the redshift 7.1 quasar ULAS J1120+0641, MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Vol: 442, Pages: 3454-3461, ISSN: 0035-8711

Journal article

Page MJ, Simpson C, Mortlock DJ, Warren SJ, Hewett PC, Venemans BP, McMahon RGet al., 2014, X-rays from the redshift 7.1 quasar ULAS J1120+0641, MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Vol: 440, Pages: L91-L95, ISSN: 0035-8711

Journal article

Venemans BP, McMahon RG, Walter F, Decarli R, Cox P, Neri R, Hewett P, Mortlock DJ, Simpson C, Warren SJet al., 2012, Detection of atomic carbon [C II] 158 mu m and dust emission from A z=7.1 quasar host galaxy, Letters of the Astrophysical Journal, Vol: 751, ISSN: 2041-8205

Using the IRAM Plateau de Bure Interferometer, we report the detection of the 158 μm [C II] emission line and underlying dust continuum in the host galaxy of the quasar ULAS J112001.48+064124.3 (hereafter J1120+0641) at z = 7.0842 ± 0.0004. This is the highest redshift detection of the [C II] line to date, and allows us to put the first constraints on the physical properties of the host galaxy of J1120+0641. The [C II] line luminosity is 1.2 ± 0.2 × 109 L ☉, which is a factor ~4 lower than observed in a luminous quasar at z = 6.42 (SDSS J1148+5251). The underlying far-infrared (FIR) continuum has a flux density of 0.61 ± 0.16 mJy, similar to the average flux density of z ~ 6 quasars that were not individually detected in the rest-frame FIR. Assuming that the FIR luminosity of L FIR = 5.8 × 1011-1.8 × 1012 L ☉ is mainly powered by star formation, we derive a star formation rate in the range 160-440 M ☉ yr–1 and a total dust mass in the host galaxy of 6.7 × 107-5.7 × 108 M ☉ (both numbers have significant uncertainties given the unknown nature of dust at these redshifts). The [C II] line width of σ V = 100 ± 15 km s–1 is among the smallest observed when compared to the molecular line widths detected in z ~ 6 quasars. Both the [C II] and dust continuum emission are spatially unresolved at the current angular resolution of 2.0 × 1.7 arcsec2 (corresponding to 10 × 9 kpc2 at the redshift of J1120+0641).

Journal article

Mortlock DJ, Patel M, Warren SJ, Hewett PC, Venemans BP, McMahon RG, Simpson Cet al., 2012, Probabilistic selection of high-redshift quasars, Monthly Notices of the Royal Astronomical Society, Vol: 419, Pages: 390-410, ISSN: 0035-8711

High-redshift quasars (HZQs) with redshifts of z ≳ 6 are so rare that any photometrically selected sample of sources with HZQ-like colours is likely to be dominated by Galactic stars and brown dwarfs scattered from the stellar locus. It is impractical to re-observe all such candidates, so an alternative approach was developed in which Bayesian model comparison techniques are used to calculate the probability that a candidate is a HZQ, Pq, by combining models of the quasar and star populations with the photometric measurements of the object. This method was motivated specifically by the large number of HZQ candidates identified by cross-matching the UKIRT (United Kingdom Infrared Telescope) Infrared Deep Sky Survey (UKIDSS) Large Area Survey (LAS) to the Sloan Digital Sky Survey (SDSS): in the forumla covered by the LAS in the UKIDSS Eighth Data Release (DR8) there are ∼9 × 103 real astronomical point sources with the measured colours of the target quasars, of which only ∼10 are expected to be HZQs. Applying Bayesian model comparison to the sample reveals that most sources with HZQ-like colours have Pq≲ 0.1 and can be confidently rejected without the need for any further observations. In the case of the UKIDSS DR8 LAS, there were just 107 candidates with Pq≥ 0.1; these objects were prioritized for re-observation by ranking according to Pq (and their likely redshift, which was also inferred from the photometric data). Most candidates were rejected after one or two (moderate-depth) photometric measurements by recalculating Pq using the new data. That left 12 confirmed HZQs, six of which were previously identified in the SDSS and six of which were new UKIDSS discoveries. The high efficiency of this Bayesian selection method suggests that it could usefully be extended to other HZQ surveys (e.g. searches by the Panoramic Survey Telescope And Rapid Response System, Pan-STARRS, or the Visible and Infrared Survey

Journal article

Bolton JS, Haehnelt MG, Warren SJ, Hewett PC, Mortlock DJ, Venemans BPet al., 2011, How neutral is the intergalactic medium surrounding the redshift z = 7.085 quasar ULAS J1120+0641?, Monthly Notices of the Royal Astronomical Society, Vol: 416, Pages: 70-74, ISSN: 0035-8711

The quasar ULAS J1120+0641 at redshift z = 7.085 has a highly ionized near zone which is smaller than those around quasars of similar luminosity at z ≃ 6. The spectrum also exhibits evidence for a damping wing extending redward of the systemic Lyα redshift. We use radiative transfer simulations in a cosmological context to investigate the implications for the ionization state of the inhomogeneous intergalactic medium (IGM) surrounding this quasar. Our simulations show that the transmission profile is consistent with an IGM in the vicinity of the quasar with a volume averaged H I fraction of ? and that ULAS J1120+0641 has been bright for 10^6-10^7 yr. The observed spectrum is also consistent with smaller IGM neutral fractions, ?, if a damped Lyα system in an otherwise highly ionized IGM lies within 5 proper Mpc of the quasar. This is, however, predicted to occur in only ˜5 per cent of our simulated sightlines for a bright phase of 10^6-10^7 yr. Unless ULAS J1120+0641 grows during a previous optically obscured phase, the low age inferred for the quasar adds to the theoretical challenge of forming a 2 × 10^9 Msun black hole at this high redshift.

Journal article

Mortlock DJ, Warren SJ, Venemans BP, Patel M, Hewett PC, McMahon RG, Simpson C, Theuns T, Gonzales-Solares EA, Adamson A, Dye S, Hambly NC, Hirst P, Irwin MJ, Kuiper E, Lawrence A, Rottgering HJAet al., 2011, A luminous quasar at a redshift of z=7.085, NATURE, Vol: 474, Pages: 616-619, ISSN: 0028-0836

Journal article

Driver SP, Hill DT, Kelvin LS, Robotham ASG, Liske J, Norberg P, Baldry IK, Bamford SP, Hopkins AM, Loveday J, Peacock JA, Andrae E, Bland-Hawthorn J, Brough S, Brown MJI, Cameron E, Ching JHY, Colless M, Conselice CJ, Croom SM, Cross NJG, De Propris R, Dye S, Drinkwater MJ, Ellis S, Graham AW, Grootes MW, Gunawardhana M, Jones DH, van Kampen E, Maraston C, Nichol RC, Parkinson HR, Phillipps S, Pimbblet K, Popescu CC, Prescott M, Roseboom IG, Sadler EM, Sansom AE, Sharp RG, Smith DJB, Taylor E, Thomas D, Tuffs RJ, Wijesinghe D, Dunne L, Frenk CS, Jarvis MJ, Madore BF, Meyer MJ, Seibert M, Staveley-Smith L, Sutherland WJ, Warren SJet al., 2011, Galaxy and Mass Assembly (GAMA): survey diagnostics and core data release, MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Vol: 413, Pages: 971-995, ISSN: 0035-8711

Journal article

Patel M, Warren SJ, Mortlock DJ, Fynbo JPUet al., 2010, The reanalysis of spectra of GRB 080913 to estimate the neutral fraction of the IGM at a redshift of 6.7, ASTRONOMY & ASTROPHYSICS, Vol: 512, ISSN: 0004-6361

Journal article

Leggett SK, Burningham B, Saumon D, Marley MS, Warren SJ, Smart RL, Jones HRA, Lucas PW, Pinfield DJ, Tamura Met al., 2010, MID-INFRARED PHOTOMETRY OF COLD BROWN DWARFS: DIVERSITY IN AGE, MASS, AND METALLICITY, ASTROPHYSICAL JOURNAL, Vol: 710, Pages: 1627-1640, ISSN: 0004-637X

Journal article

Smart RL, Jones HRA, Lattanzi MG, Leggett SK, Warren SJ, Adamson AJ, Burningham B, Casali M, Evans DW, Irwin MJ, Pinfield Det al., 2010, The distance to the cool T9 brown dwarf ULAS J003402.77-005206.7, ASTRONOMY & ASTROPHYSICS, Vol: 511, ISSN: 0004-6361

Journal article

Mortlock DJ, Patel M, Warren SJ, Venemans BP, McMahon RG, Hewett PC, Simpson C, Sharp RG, Burningham B, Dye S, Ellis S, Gonzales-Solares EA, Huelamo Net al., 2009, Discovery of a redshift 6.13 quasar in the UKIRT infrared deep sky survey, Astronomy and Astrophysics: a European journal, Vol: 505, Pages: 97-104, ISSN: 0004-6361

Optical and near-infrared (NIR) spectra are presented for ULAS J131911.29+095051.4 (hereafter ULAS J1319+0950), a new redshift z = 6.127 ± 0.004 quasar discovered in the Third Data Release (DR3) of the UKIRT Infrared Deep Sky Survey (UKIDSS). The source has = 19.10 ± 0.03, corresponding to = -27.12, which is comparable to the absolute magnitudes of the z 6 quasars discovered in the Sloan Digital Sky Survey (SDSS). ULAS J1319+0950 was, in fact, registered by SDSS as a faint source with = 20.13 ± 0.12, just below the signal-to-noise ratio limit of the SDSS high-redshift quasar survey. The faint z-band magnitude is a consequence of the weak Ly α/N V emission line, which has a rest-frame equivalent width of ~20Å and provides only a small boost to the z-band flux. Nevertheless, there is no evidence of a significant new population of high-redshift quasars with weak emission lines from this UKIDSS-based search. The Ly α optical depth to ULAS J1319+0950 is consistent with that measured towards similarly distant SDSS quasars, implying that results from optical- and NIR-selected quasars may be combined in studies of cosmological reionization. Also presented is a new NIR-spectrum of the previously discovered UKIDSS quasar ULAS J020332.38+001229.2, which reveals the object to be a broad absorption line quasar. The new spectrum shows that the emission line initially identified as Ly α is actually N V, leading to a revised redshift of z = 5.72, rather than z = 5.86 as previously estimated.

Journal article

Leggett SK, Cushing MC, Saumon D, Marley MS, Roellig TL, Warren SJ, Burningham B, Jones HRA, Kirkpatrick JD, Lodieu N, Lucas PW, Mainzer AK, Martin EL, McCaughrean MJ, Pinfield DJ, Sloan GC, Smart RL, Tamura M, Van Cleve Jet al., 2009, THE PHYSICAL PROPERTIES OF FOUR similar to 600 K T DWARFS, ASTROPHYSICAL JOURNAL, Vol: 695, Pages: 1517-1526, ISSN: 0004-637X

Journal article

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