arXiv subjects

T. Shanks

Publications and source records attributed to T. Shanks.

At least 91 records · Page 5Linked to original sources

arXiv2005

Radio-quiet objects in the 2QZ survey

Co-addition of blank-field FIRST data at the location of over 8000 QSOs in the 2QZ survey has yielded statistical detections of radio quiet QSOs with median flux levels of 20-40microJy. We show that the total radio flux of radio-quiet QSOs in the 2QZ is consistent with a smooth extrapolation of the 2QZ radio-loud QSO number-flux distribution based on the slope of the relation flattening near the FIRST flux limit. However, we are unable to distinguish a smooth extrapolation of the luminosity function to faint levels from a bimodal luminosity function with a break close to or below the FIRST radio detection limit. We also demonstrate that the redshift dependence of the median radio-to-optical spectral index for these radio quiet QSOs is consistent with that obtained for individual radio-loud 2QZ QSOs detected by FIRST.

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arXiv2004

2MASS Constraints on the Local Large-Scale Structure: A Challenge to LCDM?

We investigate the large-scale structure of the local galaxy distribution using the recently completed 2 Micron All Sky Survey (2MASS). First, we determine the K-band number counts over the 4000 sq.deg. APM survey area where evidence for a large-scale `local hole' has previously been detected and compare them to a homogeneous prediction. Considering a LCDM form for the 2-point angular correlation function, the observed deficiency represents a 5 sigma fluctuation in the galaxy distribution. We check the model normalisation using faint K-band data compiled from the literature; the normalisation used in this paper is in excellent agreement, and the observed counts over the APM survey area would require the model to be lowered by 3.8 sigma. However, the issue is complicated by the b>20 and b<-20 2MASS counts which lie below the best-fit model normalisation. Second, since the K-band counts over the APM survey area continue to suggest the possible presence of excess clustering over the LCDM prediction, we next probe the power at large scales by comparing the 2MASS and LCDM mock galaxy angular power spectra. We find a 3 sigma excess in the 2MASS catalogue over the LCDM prediction at large scales (l<30). However, this excess is not enough to account for the low counts over the APM survey area. Finally, we apply a counts in cells analysis to the 2MASS data and mock catalogues; on the assumption that the 2MASS catalogue at |b|>20 is representative, we find excellent agreement between the biased LCDM mocks and the 2MASS catalogue to 30 deg. The crux of the interpretation of these results appears to be whether the 2MASS volume is yet big enough to constitute a fair sample of the Universe. (abridged)

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arXiv2004

The 2dF QSO Redshift Survey - XIV. Structure and evolution from the two-point correlation function

We present a clustering analysis of QSOs using over 20000 objects from the final catalogue of the 2dF QSO Redshift Survey (2QZ), measuring the z-space correlation function, xi(s). When averaged over the range 0.3<z<2.2 we find that xi(s) is flat on small scales, steepening on scales above ~25h-1Mpc. In a WMAP/2dF cosmology we find a best fit power law with s_0=5.48+0.42-0.48h-1Mpc and gamma=1.20+-0.10 on scales s=1-25h-1Mpc. A CDM model assuming WMAP/2dF cosmological parameters is a good description of the QSO xi(s) after accounting for non-linear clustering and z-space distortions, and a linear bias of b_qso(z=1.35)=2.02+-0.07. We subdivide the 2QZ into 10 redshift intervals from z=0.53 to 2.48 and find a significant increase in clustering amplitude at high redshift in the WMAP/2dF cosmology. We derive the bias of the QSOs which is a strong function of redshift with b_qso(z=0.53)=1.13+-0.18 and b_qso(z=2.48)=4.24+-0.53. We use these bias values to derive the mean dark matter halo (DMH) mass occupied by the QSOs. At all redshifts 2QZ QSOs inhabit approximately the same mass DMHs with M_DH=(3.0+-1.6)x10^12h-1M_sun, which is close to the characteristic mass in the Press-Schechter mass function, M*, at z=0. If the relation between black hole (BH) mass and M_DH or host velocity dispersion does not evolve, then we find that the accretion efficiency (L/L_edd) for L* QSOs is approximately constant with redshift. Thus the fading of the QSO population from z~2 to 0 appears to be due to less massive BHs being active at low redshift. We apply different methods to estimate, t_qso, the active lifetime of QSOs and constrain this to be in the range 4x10^6-6x10^8 years at z~2. (Abridged).

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arXiv2004

The Large Local Hole in the Galaxy Distribution: The 2MASS Galaxy Angular Power Spectrum

We present new evidence for a large deficiency in the local galaxy distribution situated in the 4000 sq.deg. APM survey area. We use models guided by the 2dF Galaxy Redshift Survey (2dFGRS) n(z) as a probe of the underlying LSS. We first check the usefulness of this technique by comparing the 2dFGRS n(z) model prediction with the K-band and B-band number counts extracted from the 2MASS and 2dFGRS parent catalogues over the 2dFGRS Northern and Southern declination strips, before turning to a comparison with the APM counts. We find that the APM counts in both the B and K-bands indicate a deficiency in the local galaxy distribution of approx. 30% to z=0.1 over the entire APM survey area. We examine the implied significance of such a large local hole, considering several possible forms for the real-space correlation function. We find that such a deficiency in the APM survey area indicates an excess of power at large scales over what is expected from the correlation function observed in the 2dFGRS correlation function or predicted from $Λ$CDM Hubble Volume mock catalogues. In order to check further the clustering at large scales in the 2MASS data, we have calculated the angular power spectrum for 2MASS galaxies. Although in the linear regime (l<30), LCDM models can give a good fit to the 2MASS angular power spectrum, over a wider range (l<100) the power spectrum from Hubble Volume mock catalogues suggests that scale-dependent bias may be needed for LCDM to fit. However, the modest increase in large-scale power observed in the 2MASS angular power spectrum is still not enough to explain the local hole.

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arXiv2004

The XMM-Newton/2dF survey IV. The X-ray spectral properties of the hard sources

We present an analysis of the X-ray spectral properties of 61 hard X-ray selected (2-8 keV) sources from the bright ($f_{2-8 keV}>10^{-14}$ \funits) {\it XMM-Newton}/2dF survey. This comprises of 9 {\it XMM-Newton} pointings in the North Galactic Pole region ($\sim 1.6 \rm deg^2$) and overlaps with the SDSS, 2QZ and 2dFGRS surveys. Our sources contribute about 50 per cent of the 2-10 keV X-ray background down to the flux limit of $10^{-14}$ cgs. The hardness ratio distribution of the sample suggests a deficit of heavily absorbed sources. A spectral fit to the co-added total source spectrum yields a steep photon index, $Γ=1.83^{+0.04}_{-0.05}$. All but 8 sources have optical counterparts down to the SDSS photometric limit of $r\approx22.5$. Spectroscopic identifications exist for 34 sources. The vast majority are associated with Broad-Line (BL) AGN (24 sources) while only 7 present narrow or no emission lines. Five sources are probably associated with Galactic stars. Finally, for another 17 probable AGN we present photometric redshifts. The combined spectrum of the 24 spectroscopically identified BL AGN is steep ($Γ=2.02^{+0.04}_{-0.05}$), while that of the 7 AGN, which do not present broad lines is flatter with $Γ=1.64^{+0.11}_{-0.11}$. The spectrum of the 8 optically unidentified sources is flat with $Γ\approx 1.1$. Spectral fits to the individual BL AGN reveal large absorption (rest-frame column density $\rm>10^{22} cm^{-2}$) in only two cases. The individual spectra of the NL AGN present significant evidence for even a moderate absorption ($\rm 3\times 10^{21} cm^{-2}$) in only one case.

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arXiv2004

The 2dF QSO Redshift Survey - XII. The spectroscopic catalogue and luminosity function

We present the final catalogue of the 2dF QSO Redshift Survey (2QZ), based on Anglo-Australian Telescope 2dF spectroscopic observations of 44576 colour-selected (u b_J r) objects with 18.25<b_J<20.85 selected from APM scans of UK Schmidt Telescope (UKST) photographic plates. The 2QZ comprises 23338 QSOs, 12292 galactic stars (including 2071 white dwarfs) and 4558 compact narrow-emission-line galaxies. We obtained a reliable spectroscopic identification for 86 per cent of objects observed with 2dF. We also report on the 6dF QSO Redshift Survey (6QZ), based on UKST 6dF observations of 1564 brighter 16<b_J<18.25 sources selected from the same photographic input catalogue. In total, we identified 322 QSOs spectroscopically in the 6QZ. The completed 2QZ is, by more than a factor 50, the largest homogeneous QSO catalogue ever constructed at these faint limits (b_J<20.85) and high QSO surface densities (35 QSOs deg^-2). As such it represents an important resource in the study of the Universe at moderate-to-high redshifts. As an example of the results possible with the 2QZ, we also present our most recent analysis of the optical QSO luminosity function and its cosmological evolution with redshift. For a flat, Omega_m=0.3 and Omega_lam=0.7, Universe, we find that a double power law with luminosity evolution that is exponential in look-back time, t, of the form L*(z) exp(6.15t), equivalent to an e-folding time of 2Gyr, provides an acceptable fit to the redshift dependence of the QSO luminosity function over the range 0.4 < z < 2.1 and M_bJ<-22.5. Evolution described by a quadratic in redshift is also an acceptable fit, with L*(z)~10^(1.39z-0.29z^2).

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arXiv2004

Problems with the Current Cosmological Paradigm

We note that the standard LCDM cosmological model continues to face fundamental problems. First, the model continues to depend wholly on two pieces of undiscovered physics, namely dark energy and cold dark matter. Then, the implied dark energy density is so small that it is unstable to quantum correction and its size is fine-tuned to the almost impossible level of one part in ~10^102; it is also difficult to explain the coincidence between the dark energy, dark matter and baryon densities at the present day. Moreover, any model with a positive cosmological constant also creates fundamental difficulties for superstring theories of quantum gravity. We also review the significant number of astrophysical observations which are now in contradiction with the LCDM model. On the grounds that the SNIa Hubble diagram is prone to evolutionary corrections and also that the CMB power spectrum may be contaminated by the effects of foreground ionised gas, we argue that the existence of such systematics could still allow more satisfactory, alternative, models to appear. We suggest that if H_0<50kms^-1Mpc^-1 then a simpler, inflationary model with Omega_baryon=1 might still be allowed with no need for dark energy or cold dark matter.

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arXiv2004

Evidence for an Extended SZ Effect in WMAP Data

We have cross-correlated the WMAP data with several surveys of extragalactic sources and find evidence for temperature decrements associated with galaxy clusters and groups detected in the APM Galaxy Survey survey and the ACO catalogue. We interpret this as evidence for the thermal Sunyaev-Zeldovich (SZ) effect from the clusters. Most interestingly, the signal may extend to ~1 deg around both groups and clusters and we suggest that this may be due to hot `supercluster' gas. We have further cross-correlated the WMAP data with clusters identified in the 2MASS galaxy catalogue and also find evidence for temperature decrements there. From the APM group data we estimate the mean Compton parameter as y(z<0.2)=7x10^(-7). We have further estimated the gas mass associated with the galaxy group and cluster haloes. Assuming temperatures of 5 keV for ACO clusters and 1 keV for APM groups and clusters, we derive average gas masses. Using the space density of APM groups we then estimate Omega_gas. For an SZ extent of theta_max=20', kT=1 keV and h=0.7, this value of Omega_gas=0.04 is consistent with the standard value of Omega_baryon=0.044 but if the indications we have found for a more extended SZ effect out to theta=60' are confirmed, then higher values of Omega_gas will be implied. Finally, the contribution to the WMAP temperature power spectrum from the extended SZ effect around the z<0.2 APM+ACO groups and clusters is 1-2 orders of magnitude lower than the l=220 first acoustic peak. But if a similar SZ effect arises from more distant clusters then this contribution could increase by a factor >10 and then could seriously affect the WMAP cosmological fits.

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arXiv2003

The XMM-Newton/2dF survey - II. The nature of X-ray faint optically bright X-ray sources

In this paper we investigate the properties of low X-ray-to-optical flux ratio sources detected in a wide area (2.5deg^2) shallow (f(0.5-8keV)~10e-14cgs) XMM-Newton survey. We find a total of 26 sources (5% of the total X-ray selected population) with log f_X/f_{opt}<-0.9 to the above flux limit. Optical spectroscopy is available for 20 of these low X-ray-to-optical flux ratio objects. Most of them are found to be associated with Galactic stars (total of 8) and broad line AGNs (total of 8).We also find two sources with optical spectra showing absorption and/or narrow emission lines and X-ray/optical properties suggesting AGN activity. Another two sources are found to be associated with low redshift galaxies with narrow emission line optical spectra, X-ray luminosities L_X(0.5-8keV)~10e41cgs and logf_X/f_opt ~ -2 suggesting `normal' star-forming galaxies. Despite the small number statistics the sky density of `normal' X-ray selected star-forming galaxies at the flux limit of the present sample is low consistent with previous ROSAT HRI deep surveys. Also, the number density estimated here is in good agreement with both the logN-logS of `normal' galaxies in the Chandra Deep Field North (extrapolated to bright fluxes) and model predictions based on the X-ray luminosity function of local star-forming galaxies.

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arXiv2003

The 2dF QSO Redshift Survey - XIII. A Measurement of Lambda from the QSO Power Spectrum

We report on measurements of the cosmological constant, Lambda, and the redshift space distortion parameter beta=Omega_m^0.6/b, based on an analysis of the QSO power spectrum parallel and perpendicular to the observer's line of sight, from the final catalogue of the 2dF QSO Redshift Survey. We derive a joint Lambda - beta constraint from the geometric and redshift-space distortions in the power spectrum. By combining this result with a second constraint based on mass clustering evolution, we break this degeneracy and obtain strong constraints on both parameters. Assuming a flat cosmology and a Lambda cosmology r(z) function to convert from redshift into comoving distance, we find best fit values of Omega_Lambda=0.71^{+0.09}_{-0.17} and beta(z~1.4)=0.45^{+0.09}_{-0.11}. Assuming instead an EdS cosmology r(z) we find that the best fit model obtained, with Omega_Lambda=0.64^{+0.11}_{-0.16} and beta(z~1.4)=0.40^{+0.09}_{-0.09}, is consistent with the Lambda r(z) results, and inconsistent with a Lambda=0 flat cosmology at over 95 per cent confidence.

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arXiv2003

Possible arcminute-separation gravitational lensed QSOs in the 2dF QSO survey

We report the possible discovery of multiple gravitationally-lensed images of QSOs with angular separations on arcminute scales. The QSOs were selected from the completed 2dF QSO survey as having redshifts and optical colours which are statistically consistent. In this paper we present higher-quality optical spectra of the candidates and discuss the likelihood of their genuinely being systems lensed by massive clusters of galaxies. From a comparison of the spectra it appears that up to six pairs of QSOs may be lensed multiple images, although the true number may be less than that and further observations should be undertaken to amass more evidence and to detect the lensing clusters. Two of the candidates may be associated with low redshift clusters of galaxies.

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arXiv2003

The XMM-Newton/2dF survey I: X-ray properties of normal galaxies

This paper explores the X-ray properties of `normal' galaxies using a shallow XMM-Newton survey covering an area of ~1.5deg2. The X-ray survey overlaps with the 2dF Galaxy Redshift Survey. Compared with previous studies this has the advantage of high quality spectra and spectral classifications to bj=19.4. Moreover, sources with optical spectra revealing powerful AGNs can easily be discarded from the normal galaxy sample used here. In particular, we present stacking analysis results for 200 galaxies from the 2dFGRS at =0.1. We detect a strong signal for the whole sample (~6sigma) in the soft 0.5-2keV band corresponding to a flux of ~7*10^-16cgs and a luminosity of ~2*10^40cgs. A statistically significant signal is also detected for both the early and late galaxy sub-samples with X-ray luminosities of ~3*10^40 and ~5*10^39cgs respectively. In contrast, no signal is detected in the hard 2-8keV band for any of the above samples. The mean L_X/L_B ratio of the spiral galaxy sample is consistent with both local (<100Mpc) and distant (z~1) samples suggesting little or no evolution of the X-ray emission mechanisms relative to the optical. The 0.5-2keV XRB contribution of the spiral galaxy sub-sample at z~0.1 is estimated to be 0.4% in broad agreement with the XRB fractions estimated in previous studies.Assuming that star-forming galaxies evolve with redshift as (1+z)^{k} the present data combined with previous studies suggest k<3. The k values are constrained by the relatively low fraction of the soft X-ray background that remains unresolved by deep surveys (6-26%). The mean X-ray emissivity of spiral galaxies at z~0.1 is also estimated and is found to be consistent within the uncertainties with that of local HII galaxy samples.

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arXiv2003

Emission line widths and QSO black hole mass estimates from the 2dF QSO Redshift Survey

We have used composite spectra generated from more than 22000 QSOs observed in the course of the 2dF and 6dF QSO Redshift Surveys to investigate the relationship between the velocity width of emission lines and QSO luminosity. We find that the velocity width of the broad emission lines Hbeta, Hgamma, MgII, CIII] and CIV are correlated with the continuum luminosity, with a significance of more than 99 per cent. Of the major narrow emission lines ([OIII] 5007, [OII] 3727, NeIII 3870 and NeV 3426) only [OIII] exhibits a significant correlation between line width and luminosity. Assuming that the gas is moving in Keplerian orbits and that the radius of the broad line region is related to the QSO continuum luminosity, we use the velocity widths of the broad lines to derive average black hole masses for the QSOs contributing to the composite spectra. The resultant QSO mass-luminosity relationship is consistent with M ~ L^0.97+-0.16. We find that the correlation between line width and redshift, if present, must be weak, and only CIV shows significant evidence of evolution. This enables us to constrain the redshift evolution of the black hole mass-luminosity ratio to be ~(1+z)^beta with beta ~< 1, much less than the ~(1+z)^3 evolution seen in QSO luminosity evolution. Assuming that the motion of the broad line region gas is Keplerian and that its radius depends on the QSO luminosity, our models indicate that the observed weak redshift dependence is too small for the observed QSO luminosity function to be due to the evolution of a single long-lived population of sources.

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arXiv2003

Ghosts of the Milky Way: a search for topology in new quasar catalogues

We revisit the possibility that we inhabit a compact multi-connected flat, or nearly-flat, Universe. Analysis of COBE data has shown that, for such a case, the size of the fundamental domain must be a substantial fraction of the horizon size. Nevertheless, there could be several copies of the Universe within the horizon. If the Milky Way was once a quasar we might detect its `ghost' images. Using new large quasar catalogues we repeat the search by Fagundes & Wichoski for antipodal quasar pairs. By applying linear theory to account for the peculiar velocity of the local group, we are able to narrow the search radius to 134 arcsec. We find seven candidate antipodal quasar pairs within this search radius. However, a similar number would be expected by chance. We argue that, even with larger quasar catalogues, and more accurate values of the cosmological parameters, it is unlikely to be possible to identify putative ghost pairs unambiguously, because of the uncertainty of the correction for peculiar motion of the Milky Way.

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arXiv2003

XMM-Newton observations of an absorbed z=0.67 QSO: no dusty torus?

We present XMM-Newton observations of AXJ0341.4-4453, a mildly reddened A_V<7 QSO at a redshift of z=0.672. The XMM-Newton spectrum shows a large obscuring column N_H~10^23 cm^{-2} corresponding to A_V~70, in agreement with previous results based on the lower limit of the ASCA hardness ratio. The X-ray spectrum is represented by a 'scattering' model with Gamma~2.0 with the scattered power-law normalization being a few per cent of the hard component. No FeK line is detected with a 90 per cent upper limit on its equivalent width of ~360eV. The large discrepancy between the column density observed in X-rays and that inferred from the Balmer decrement can be explained by dust sublimation near the nucleus. Then, the X-ray and the optical obscuration come from two different regions: the X-ray close to the accretion disk while the optical at much larger >0.25pc scales.

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arXiv2003

The 2dF QSO Redshift Survey - X. Lensing of Background QSOs by Galaxy Groups

We cross-correlate QSOs from the 2dF Survey with galaxy groups. The galaxy samples are limited to B < 20.5. We use an objective algorithm to detect galaxy groups. A 3sigma anti-correlation is observed between QSOs and galaxy groups. This paucity of faint QSOs around groups is neither a selection effect nor due to restrictions on the placement of 2dF fibres. By observing the colours of QSOs on the scales of the anti-correlation, we limit dust in galaxy groups, finding a maximum reddening of E(b_j-r) < 0.012 at the 95% level. The small amount of dust thus inferred is insufficient to cause the anti-correlation, supporting the suggestion by Croom & Shanks that the signal is due to gravitational lensing. The possibility remains that tailored dust models, such as grey dust, heavy patches of dust or a combination of dust and lensing, could explain the anti-correlation. Assuming the signal is caused by lensing rather than dust, we measure the average velocity dispersion of a Singular Isothermal Sphere that would cause the anti-correlation as around 1150 km/s. Simulations reject 600 km/s at the 5% significance level. We also model foreground lenses as NFW haloes and measure the typical mass within 1.5 Mpc/h of the halo centre as M_{1.5} = (1.2 +/- 0.9) x 10^{15} solarmasses/h. Regardless of whether we utilise a SIS or NFW dark matter profile, our model favours more mass in groups than accounted for in a universe with density parameter Omega_m = 0.3. Detailed simulations and galaxy group redshifts will significantly reduce the current systematic uncertainties in these $Ω_m$ estimates. Reducing the remaining uncertainty will require larger QSO and galaxy group surveys (abridged).

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arXiv2003

The Local Hole in the Galaxy Distribution: Evidence from 2MASS

Using the bright galaxy counts from the 2 Micron All Sky Survey (2MASS) second incremental release, two techniques for probing large-scale structure at distances of ~150 h-1 Mpc are investigated. First, we study the counts from two sets of six 5 degree by ~80 degree strips in the two galactic caps. In the six southern strips a deficit of ~30% was found relative to a predicted homogeneous distribution at K~11. These strips were also in good agreement with a model incorporating the radial density function of the southern 2dF Galaxy Redshift Survey (2dFGRS), which shows a deep underdensity between ~90 and 180 h-1 mpc. These results indicate a very large `local hole' in the Southern Galactic Cap (SGC) to >150 h-1 Mpc with a linear size across the sky of ~200 h-1 Mpc, but with a significantly lower mean underdensity of ~30% than that suggested by the bright APM SGC counts. The counts in the northern set of strips are low overall but indicate a more varied pattern. When all the available 2MASS data with |b|>30 were aggregated, they indicated underdensities of ~18% and ~30% at K~11 for the northern and southern areas respectively. Our second method uses the ratio of the counts with 11.38 300 h-1 Mpc in the galaxy power spectrum.

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arXiv2003

The Local Hole in the Galaxy Distribution: New Optical Evidence

We present a new CCD survey of galaxies within the N and S strips of the 2dFGRS areas. We use the new CCD data to check the photographic photometry scales of the 2dFGRS, APMBGC, APM-Stromlo Redshift Survey, Durham-UKST (DUKST) survey, Millenium Galaxy Catalogue (MGC) and Sloan Digital Sky Survey (SDSS). We find evidence for scale and zero-point errors in the 2dFGRS northern field, DUKST and APM data of 0.10, 0.24 and 0.31 mag. respectively; we find excellent agreement with the MGC and SDSS photometry. We find conclusive evidence that the S counts with B<17 mag are down by ~30% relative to both the N counts and to the models of Metcalfe et al. We further compare the n(z) distributions from the B<17 mag. DUKST and B<19.5 2dFGRS redshift surveys using the corrected photometry. While the N n(z) from 2dFGRS appears relatively homogeneous over its whole range, the S n(z) shows a 30% deficiency out to z=0.1; at higher redshifts it agrees much better with the N n(z) and the homogeneous model n(z). The DUKST n(z) shows that the S `hole' extends over a 20 by 75 degree squared area. The troughs with z<0.1 in the DUKST n(z) appear deeper than for the fainter 2dFGRS data. This may be evidence that the local galaxy distribution is biased on >50 h-1 Mpc scales which is unexpected in a Lambda-CDM cosmology. Finally, since the Southern local void persists over the full area of the APM and APMBGC with a ~25% deficiency in the counts below B~17, this means that its extent is ~300 h-1 Mpc by 300 h-1 Mpc on the sky as well as ~300 h-1 Mpc in the redshift direction. Such a 25% deficiency extending over ~10^7 h-3 Mpc^3 may imply that the galaxy correlation function's power-law behaviour extends to \~150 h-1 Mpc with no break and show more excess large-scale power than detected in the 2dFGRS correlation function or expected in the Lambda-CDM cosmology.

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