arXiv subjects

T. Shanks

Publications and source records attributed to T. Shanks.

At least 127 records · Page 7Linked to original sources

arXiv2000

Starburst activity in a ROSAT Narrow Emission-Line Galaxy

We present multiwaveband photometric and optical spectropolarimetric observations of the R=15.9 narrow emission line galaxy R117_A which lies on the edge of the error circle of the ROSAT X-ray source R117 (from McHardy et al 1998). The overall spectral energy distribution of the galaxy is well modelled by a combination of a normal spiral galaxy and a moderate-strength burst of star formation. The far infra-red and radio emission is extended along the major axis of the galaxy, indicating an extended starburst. On positional grounds, the galaxy is a good candidate for the identification of R117 and the observed X-ray flux is very close to what would be expected from a starburst of the observed far infra-red and radio fluxes. Although an obscured high redshift QSO cannot be entirely ruled out as contributing some fraction of the X-ray flux, we find no candidates to K=20.8 within the X-ray errorbox and so conclude that R117_A is responsible for a large fraction, if not all, of the X-ray emission from R117. Searches for indicators of an obscured AGN in R117_A have so far proven negative; deep spectropolarimetric observations show no signs of broad lines to a limit of one per cent and, for the observed far infra-red and radio emission, we would expect a ten times greater X-ray flux if the overall emission were powered by an AGN. We therefore conclude that the X-ray emission from R117 is dominated by starburst emission from the galaxy R117_A.

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arXiv2000

Galaxy number counts - V. Ultra-deep counts: The Herschel and Hubble Deep Fields

We present u,b,r & i galaxy number counts and colours both from the North and South Hubble Space Telescope Deep Fields and from the William Herschel Deep Field. The latter comprises a 7'x7' area of sky reaching b~28.5 at its deepest. We show that simple Bruzual & Charlot evolutionary models which assume exponentially increasing star-formation rates with look-back time and q_0=0.05 continue to give excellent fits to galaxy counts and colours in the deep imaging data. With q_0=0.5, an extra population of `disappearing dwarf' galaxies is required to fit the optical counts. When we compare the observed and predicted galaxy counts for UV dropouts in the range 2<z<3.5 we find excellent agreement, indicating that the space density of galaxies may not have changed much since z=0 and identifying the Lyman break galaxies with the bright end of the evolved spiral luminosity function. Making the same comparison for B dropout galaxies in the range 3.5<z<4.5 we find that the space density of intrinsically bright galaxies remains the same but the space density of faint galaxies drops by a factor of ~5 compared to the models, consistent with the idea that L* galaxies were already in place at z~4 but that dwarf galaxies may have formed later at 3<z<4.

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arXiv2000

The Durham/UKST Galaxy Redshift Survey - VII. Redshift-space distortions in the power spectrum

We investigate the effect of redshift-space distortions in the power spectrum parallel and perpendicular to the observer's line of sight, P(k_par,k_perp), using the optically selected Durham/UKST Galaxy Redshift Survey. On small, non-linear scales anisotropy in the power-spectrum is dominated by the galaxy velocity dispersion; the `Finger of God' effect. On larger, linear scales coherent peculiar velocities due to the infall of galaxies into overdense regions are the main cause of anisotropy. According to gravitational instability theory these distortions depend only on the density and bias parameters via beta. Geometrical distortions also occur if the wrong cosmology is assumed, although these would be relatively small given the low redshift of the survey. To quantify these effects, we assume the real-space power spectrum of the APM Galaxy Survey, and fit a simple model for the redshift-space and geometrical distortions. Assuming a flat Omega = 1 universe, we find values for the one-dimensional pairwise velocity dispersion of sigma_p = 410 +- 170 km/s, and beta = 0.38 +- 0.17. An open Omega = 0.3, and a flat Omega = 0.3, Lambda = 0.7 universe yield sigma_p = 420 km/s, beta = 0.40, and sigma_p = 440 km/s, beta = 0.45 respectively, with comparable errors. These results are consistent with estimates using the two-point galaxy correlation function, xi(sigma,pi), and favour either a low-density universe with Omega ~ 0.3 if galaxies trace the underlying mass distribution, or a bias factor of b ~ 2.5 if Omega = 1.

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arXiv2000

Galaxy clustering in the Herschel deep field

(abridged) We present a study of the angular correlation function as measured in the William Herschel Deep Field. This field covers 50 arcmin$^2$ and with it we are able to investigate the scaling of the angular correlation function to $B\sim28$, $R, I\sim 26$,$K\sim20$ and $H\sim22.5$. We compare our measurements to results obtained from the smaller Hubble Deep Field. To interpret our results, we use a model which correctly predicts colours, number counts and redshift distributions for the faint galaxy population. We find that at fixed separation the amplitude of $ω(θ)$ measured in $BRI$ bandpasses is lower than the predictions of a model containing with no luminosity evolution and stable clustering growth in proper co-ordinates. However, in the near-infrared bandpasses, our measurements are consistent with the predictions of an essentially non-evolving $K-$ selected galaxy redshift distribution. In the range $B\sim27-28$ we find that our correlation amplitudes are independent of magnitude, which is consistent with the observed flattening of the number count slope and correspondingly slower increase of the cosmological volume element expected at these magnitudes. Our measurements in all bandpasses are consistent with a rapid growth of galaxy clustering ($0 < ε< 2 $ in the normal parametrisation) on the sub-Mpc scales which our survey probes. We demonstrate that this rapid growth of clustering is consistent with the predictions of biased models of galaxy formation, which indicate that a rapid rate of clustering growth is expected for the intrinsically faint galaxies which dominate our survey.

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arXiv2000

The 2dF QSO Redshift Survey - I. The Optical QSO Luminosity Function

We present a determination of the optical QSO luminosity function and its cosmological evolution with redshift for a sample of over 6000 QSOs identified primarily from the first observations of the 2dF QSO Redshift Survey (2QZ). For QSOs with -26 < M_B < -23 and 0.35 < z < 2.3, we find that pure luminosity evolution (PLE) models provide an acceptable fit to the observed redshift dependence of the luminosity function. The luminosity function is best fit by a two-power-law function of the form. Exponential luminosity evolution models, both as a function of look-back time, and as a general second-order polynomial with redshift, were found to provide acceptable fits to the dataset comprising the 2QZ and the Large Bright Quasar Survey. Exponential evolution with look-back time is prefered for q_0=0.05, while the polynomial evolution model is prefered for q_0=0.5. The shape and evolution of the LF at low redshifts (z<0.5) and/or high luminosities, not currently well sampled by the 2dF QSO survey, may show departures from pure luminosity evolution, but the results presented here show that over a significant range of redshift, PLE is a good description of QSO evolution.

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arXiv2000

X-ray variability in a deep, flux limited sample of QSOs

We present an analysis of X-ray variability in a flux limited sample of QSOs. Selected from our deep ROSAT survey, these QSOs span a wide range in redshift ($0.1 0.5$) do not show the anti-correlation with luminosity seen in local AGN. We speculate on the implications of these results for physical models of AGN and their evolution. Finally, we find evidence for X-ray variability in an object classified as a narrow emission-line galaxy, suggesting the presence of an AGN.

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arXiv2000

2dF QSO Redshift Survey

With approximately 6000 QSO redshifts,the 2dF QSO redshift survey is already the biggest complete QSO survey. The aim for the survey is to have 25000 QSO redshifts, providing an order of magnitude increase in QSO clustering statistics. We first describe the observational parameters of the 2dF QSO survey. We then describe several highlights of the survey so far, including new estimates of the QSO luminosity function and its evolution. We also review the current status of QSO clustering analyses from the 2dF data. Finally, we discuss how the complete QSO survey will be able to constrain the value of Omega_o by measuring the evolution of QSO clustering, place limits on the cosmological constant via a direct geometrical test and determine the form of the fluctuation power-spectrum out to the approximately 1000 Mpc scales only previously probed by COBE.

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arXiv2000

Distances to Cepheid Open Clusters Via Optical and K-Band Imaging

We investigate the reddening and Main Sequence fitted distances to eleven young, Galactic open clusters that contain Cepheids. Each cluster contains or is associated with at least one Cepheid variable star. Reddening to the clusters is estimated using the U-B:B-V colours of the OB stars and the distance modulus to the cluster is estimated via B-V:V and V-K:V colour-magnitude diagrams. By main-sequence fitting we proceed to calibrate the Cepheid P-L relation and find M_V=-2.81xlogP-1.33 +/-0.32 and M_K=-3.44xlogP-2.20 +/-0.29 and a distance modulus to the LMC of 18.55+/-0.32 in the V-band and 18.47+/-0.29 in the K-band giving an overall distance modulus to the LMC of 18.51+/-0.3. In the case of two important clusters we find that the U-B:B-V diagram in these clusters is not well fitted by the standard Main Sequence line. In one case, NGC7790, we find that the F stars show a UV excess which if caused by metallicity would imply Fe/H ~ -1.5; this is anomalously low compared to what is expected for young open clusters. In a second case, NGC6664, the U-B:B-V diagram shows too red U-B colours for the F stars which in this case would imply a higher than solar metallicity. If these effects are due to metallicity then it would imply that the Cepheid PL(V) and PL(K) zeropoints depend on metallicity according to delta(M)/delta(Fe/H) ~0.66 in the sense that lower metallicity Cepheids are intrinsically fainter. Medium-high resolution spectroscopy for the main-sequence F stars in these two clusters is needed to determine if metallicity really is the cause or whether some other explanation applies.

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arXiv2000

A deep ROSAT survey XV. the average QSO spectrum and its evolution

Using a sample of 165 X-ray selected QSOs from seven deep ROSAT fields we investigate the X-ray spectral properties of an ``average''radio-quiet broad line QSO as a function of redshift. The QSO stacked spectra, in the observers 0.1-2 keV band, in five redshift bins over the range 0.1 1.0) the single power-law model gives an acceptable fit to the data over the full energy band, at lower redshifts the spectra need a second component at low energies, a `soft excess'. Inclusion of a simple model for the soft excess, i.e. a black-body component (kT~100 eV), results in a significant improvement to the model fit, and yields power-law slopes of 1.8-1.9, for all redshift bins. This power-law is not inconsistent, within the error bars, with those of nearby AGN in the 2-10 keV band, suggesting that the same intrinsic power-law slope may continue from 10 keV down to below 0.5 keV. We caution that there is a possibility that the spectral upturn observed may not represent a real physical component but could be due to co-adding spectra with a large dispersion in spectral indices. Regardless of the origin of the soft excess, the average QSO spectrum has important consequences for the origin of the X-ray background: the average spectra of the typical, faint, high redshift QSO are significantly steeper than the spectrum of the X-ray background extending the spectral paradox into the soft 0.1-2 keV X-ray band.

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arXiv1999

The Durham/UKST Galaxy Redshift Survey - VI. Power spectrum analysis of clustering

We present the power spectrum analysis of clustering in the Durham/UKST Galaxy Redshift Survey, which covers 1450 square degrees and consists of 2501 galaxy redshifts sampled at a rate of 1 in 3 to b_J = 17. The fluctuations that we measure can be expressed as sigma_{8} = 1.01 +/- 0.17 . We find remarkably good agreement between the power spectrum measured for the Durham/UKST Survey and those from other optical studies, in spite of the very different sampling rates, magnitude limits and survey geometries employed, on scales up to lambda = (2 pi)/k = 80/h Mpc. On scales larger than this we find good agreement with the power measured from the Stromlo-APM Survey (Tadros & Efstathiou), but find more power than estimated from the Las Campanas Redshift Survey (Lin et al). We apply a simple model for redshift space distortions to the real space APM Galaxy Survey power spectrum and find a shape and amplitude that is in very good agreement with the power spectrum of the Durham/UKST Survey. This implies beta=Omega^{0.6}/b=0.60 +/- 0.35, where Omega is the density parameter and b is the bias between fluctuations in the galaxy and mass distributions, and also suggests a one dimensional velocity dispersion of sigma = 320 +/-140 km/s. We find that for any choice of normalisation, the standard CDM model has a shape that cannot be reconciled with the Durham/UKST Survey power spectrum, unless either unacceptably high values of the one dimensional velocity dispersion are adopted or the assumption that bias is constant is invalid on scales greater than 20/h Mpc. Over the range of wavenumbers for which we have a robust measurement of the power spectrum, we find the best agreement is obtained for a critical density CDM model in which the shape of the power spectrum is modified. (Abridged)

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arXiv1999

Implications of an Obscured AGN Model for the X-ray Background at Sub-mm and Far Infra-Red Wavelengths

Models invoking large populations of obscured AGN are known to provide good fits to the spectrum of the X-ray background and the observed soft and hard X-ray number counts. An important consequence of these models is that significant quantities of dust are required in order to provide the obscuring medium, which will be heated by the nuclear radiation from the AGN, and will radiate in the thermal infra-red. We therefore model the properties of the dust, and predict the contribution of obscured AGN to the intensity of the far infra-red background and the sub-mm source counts and redshift distribution, in order to ascertain whether our models are constrained further by the data available at these wavelengths. Our conservative models predict a contribution of between 5 and 15 per cent of the far infra-red background intensity, whereas if we use more extreme values for several parameters, this value may reach 33 per cent. This suggests that AGN may only form a significant minority of sub-mm sources, in agreement with the results of spectroscopic follow-up. Our models thus show that there is no inconsistency between obscured AGN models for the X-ray background and either sub-mm source counts or the intensity of the far infra-red background. We further propose that obscured AGN may explain the sub-mm emission associated with Extremely Red Objects (EROs). Finally, we make predictions for the redshift distribution of AGN sources detected in faint sub-mm surveys, which should allow future tests of this obscured AGN model.

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arXiv1999

A Spectacular Post-Starburst Quasar

We report the discovery of a spectacular ``post-starburst quasar'' UN J1025-0040 (B=19; z=0.634). The optical spectrum is a chimera, displaying the broad Mg II emission line and strong blue continuum characteristic of quasars, but is dominated in the red by a large Balmer jump and prominent high-order Balmer absorption lines indicative of a substantial young stellar population at similar redshift. Stellar synthesis population models show that the stellar component is consistent with a 400 Myr old instantaneous starburst with a mass of less than or about 10^{11} solar masses. A deep, K_s-band image taken in 0.5 arcsec seeing shows a point source surrounded by asymmetric extended fuzz. Approximately 70% of the light is unresolved, the majority of which is expected to be emitted by the starburst. While starbursts and galaxy interactions have been previously associated with quasars, no quasar ever before has been seen with such an extremely luminous young stellar population.

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arXiv1999

Statistical lensing of faint QSOs by galaxy clusters

We investigate the anti-correlation between faint high redshift QSOs and low redshift galaxy groups found by Boyle, Fong & Shanks (1988), on the assumption that it is caused by gravitational lensing of a flat QSO number count, rather than any other systematic effect (e.g dust). An isothermal sphere lens model, requires a velocity dispersion of 1286+72-91 km/s. Addition of a constant density plane reduces this to 1143+109-153 km/s, while the plane density is 0.081+-0.032h g/cm**2. Both these values are considerably larger than the 400-600km/s expected for poor clusters and groups and imply that the mass associated with such groups is ~4 times larger than inferred from virial analyses. If due to lensing, this measurement clearly tends to favour high values of Omega_0. We demonstrate how an estimate of Omega_0 may be obtained, finding Omega_0=1.3(n/3x10^-4h^3Mpc^-3)(r/1Mpc)(sigma/1286km/s)^2. Systematic uncertainties in r, the extent of the anti-correlation, and n, the group space density, currently dominate this determination, but this will be a route to estimating Omega_0 in improved galaxy-QSO datasets where systematics can be better controlled. We compared our result to that of Williams & Irwin (1998) who find a positive correlation between bright LBQS QSOs and APM galaxies. Adapting the analysis of Williams & Irwin to our use of galaxy groups, we find agreement between the amplitude of the positive cross-correlation found for bright QSOs and the negative cross-correlation found for faint QSOs. This analysis leads to a common estimate of Omega_0*sigma_8~3-4. This, however, is significantly higher than indicated from several other analyses. Further tests of the accuracy of galaxy-QSO cross-correlations and their implications for Omega_0 and sigma_8 will soon be available from the new 2dF QSO catalogue.

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arXiv1999

Faint UBVRI CCD sequences for wide-field surveys -- II. UBVR sequences at declination=-30 deg

We present results from a continuing campaign to secure deep multi-colour CCD sequences for photometric calibration in UK Schmidt fields with galactic latitudes |b|>50deg. In this paper we present UBVR photometry in 12 fields and BR photometry in a further 14 fields observed within UK Schmidt survey fields centred at declination=-30deg. Photometric errors are at the 0.05 level at 20.2, 21, 20.5 and 20 mag for UBVR sequences respectively. Our observations were carried out with the 0.9m Telescope at the Cerro-Tololo Inter-American Observatory. These data are not intended for use as highly accurate individual photometric standards, but rather for use as sequences, using a large number of stars to calibrate wide-area data such as photographic plates. The data are available electronically at http://icstar5.ph.ic.ac.uk/~scroom/phot/photom.html

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arXiv1999

Galaxy number counts- IV. surveying the Herschel deep field in the near-infrared

(abridged) We present results from two new near-infrared imaging surveys. One covers 47.2 arcmin^2 to K(3σ)<20 whilst a second, deeper survey covers a sub-area of 1.8 arcmin^2 to K(3σ)<22.75. Over the entire area we have extremely deep UBRI photometry. Our K- counts are consistent with the predictions of non-evolving models with 0 < q0 <0.5. The K-selected (B-K) galaxy colour distributions move sharply bluewards fainter than K~20 and at at brighter magnitudes (K<20) our observed colour distributions indicate a deficiency of red, early-type galaxies at z~1 in comparison with passively evolving models. This implies either a pure luminosity evolution (PLE) model with a low level of continuing star-formation following an an initial burst, or dynamical merging. At fainter magnitudes, the continuing bluewards trend observed in (B-K) can be explained purely in terms of passively evolving PLE models. Our observed numbers of (I-K)>4 galaxies at K<20 exhibit the same deficiency, suggesting that at least part of the larger deficit observed in (B-K) at K<20 may be due to star-formation rather than dynamical merging. Finally, as we and others have noted, the number-redshift distribution at 18 1. Dynamical merging is one possible solution to reduce the numbers of these galaxies but a dwarf-dominated IMF for early-type galaxies could offer an alternative explanation; we show that this model reproduces both the optical-infrared colour distributions and the K- band galaxy counts.

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arXiv1999

A Test of Tully-Fisher Distance Estimates Using Cepheids and Type Ia Supernovae

We update and extend the results of Shanks (1997, MNRAS, 290, L77) by making a direct test of Tully-Fisher distance estimates to thirteen spiral galaxies with HST Cepheid distances and to ten spiral galaxies with Type Ia supernova (SNIa) distances. The results show that the Tully-Fisher distance moduli are too short with respect to the Cepheid distances by 0.46+-0.11mag and too short with respect to the SNIa distances by 0.49+-0.18mag. Combining the HST Cepheid and the best SNIa data suggests that, overall, previous Tully-Fisher distances at v~1000 kms-1 were too short by 0.43+-0.09mag, a result which is significant at the 4.6 sigma level. These data therefore indicate that previous Tully-Fisher distances should be revised upwards by 22+-5% implying, for example, a Virgo distance of 19.0+-1.8Mpc. The value of Ho from Tully-Fisher estimates is correspondingly revised downwards from Ho=84+-10kms-1Mpc-1 to Ho=69+-8kms-1Mpc-1. There is evidence that the Tully-Fisher relation at large distances is affected by Malmquist bias. In this case, we argue that Ho<50kms-1Mpc-1 cannot be ruled out by Tully-Fisher considerations.

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arXiv1998

ASCA observations of deep ROSAT fields IV.IR and hard X-ray observations of an obscured high-redshift QSO

We use UKIRT and ASCA observations to determine the nature of a high redshift (z=2.35) narrow-line AGN, previously discovered by Almaini et al. (1995). The UKIRT observations show a broad Halpha line while no Hbeta line is detected. This together with the red colour (B-K=5.4) suggest that our object is a moderately obscured QSO (Av>3), in optical wavelengths. The ASCA data suggest a hard spectrum, probably due to a large obscuring column, with a photon index of 1.93 (+0.62, -0.46), NH~10^23. The combined ASCA and ROSAT data again suggest a heavily obscured spectrum (NH~10^23 or Av~100). In this picture, the ROSAT soft X-ray emission may arise from electron scattering, in a similar fashion to local Seyfert 1.9. Then, there is a large discrepancy between the moderate reddening witnessed by the IR and the large X-ray absorbing column. This could be possibly explained on the basis of e.g. high gas metallicities or assuming that the X-ray absorbing column is inside the dust sublimation radius. An alternative explanation can be obtained when we allow for variability between the ROSAT and ASCA observations. Then the best fit spectrum is still flat, 1.35 (+0.16, -0.14), but with low intrinsic absorption in better agreement with the IR data, while the ROSAT normalization is a factor of two below the ASCA normalization. This object may be one of the bright examples of a type-II QSO population at high redshift, previously undetected in optical surveys. The hard X-ray spectrum of this object suggests that such a population could make a substantial contribution to the X-ray background.

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arXiv1998

Radio-quiet QSO environments - I. The correlation of QSOs and bj<23 galaxies

In this paper we present results of an analysis of radio-quiet QSO environments. The aim is to determine the relation between QSOs and galaxies as a function of redshift. We cross-correlate a sample of ~150 QSOs from optically and X-ray selected catalogues with faint, bj<23, galaxies. These data allow us to probe the galaxy clustering environment of QSOs out to z~1.0-1.5. Far from giving a positive correlation, at z<1.5 the QSO-galaxy cross-correlation function is marginally negative, with w(theta<120'')=-0.027+-0.020. Colour information suggests that the anti-correlation is most significant between the QSOs and the red galaxy population. We have constructed models to predict the QSO-galaxy cross-correlation, using the known form of the galaxy N(z) at bj<23, and assuming a variety of clustering evolution rates. Cases in which QSOs exist in rich cluster environments are comfortably ruled out at more than 5sigma and the results are more consistent with a `normal' galaxy environment for radio-quiet QSOs. If the small anti-correlation is interpreted as an effect of gravitational lensing, this conclusion is not altered. In this case, the data are only ~1sigma below the low clustering amplitude models, while the high amplitude models are still comfortably rejected. We therefore conclude that these QSOs may not be much more highly biased than optically selected galaxies.

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