arXiv subjects

T. Shanks

Publications and source records attributed to T. Shanks.

At least 109 records · Page 6Linked to original sources

arXiv2003

The 2dF QSO Redshift Survey - XI. The QSO Power Spectrum

We present a power spectrum analysis of the final 2dF QSO Redshift Survey catalogue containing 22652 QSOs. Utilising the huge volume probed by the QSOs, we can accurately measure power out to scales of ~500Mpc and derive new constraints, at z~1.4, on the matter and baryonic contents of the Universe. Importantly, these new cosmological constraints are derived at an intermediate epoch between the CMB observations at z~1000, and local (z~0) studies of large-scale structure; the average QSO redshift corresponds to a look-back time of approximately two-thirds of the age of the Universe. We find that the amplitude of clustering of the QSOs at z~1.4 is similar to that of present day galaxies. The power spectra of the QSOs at high and low redshift are compared and we find little evidence for any evolution in the amplitude. Assuming a lambda cosmology to derive the comoving distances, r(z), to the QSOs, the power spectrum derived can be well described by a model with shape parameter Gamma=0.13+-0.02. If an Einstein-de Sitter model r(z) is instead assumed, a slightly higher value of Gamma=0.16+-0.03 is obtained. A comparison with the Hubble Volume LCDM simulation shows very good agreement over the whole range of scales considered. A standard (Omega_m=1) CDM model, however, predicts a much higher value of Gamma than is observed, and it is difficult to reconcile such a model with these data. We fit CDM model power spectra (assuming scale-invariant initial fluctuations), convolved with the survey window function, and corrected for redshift space distortions, and find that models with baryon oscillations are slightly preferred, with the baryon fraction Omega_b/Omega_m=0.18+-0.10. The overall shape of the power spectrum provides a strong constraint on Omega_m*h (where h is the Hubble parameter), with Omega_m*h=0.19+-0.05.

astro-ph↗

arXiv2002

Cepheid, Tully-Fisher and SNIa Distances

We first discuss why the uncomfortable fine-tuning of the parameters of the Lambda-CDM cosmological model provides continuing, strong motivation to investigate Hubble's Constant. Then we review evidence from the HST Key Project that there is a significant scale error between raw Cepheid and Tully-Fisher distances. An analysis of mainly HST Distance Scale Key Project data shows a correlation between host galaxy metallicity and the rms scatter around the Cepheid P-L relation, which may support a recent suggestion that the P-L metallicity dependence is stronger than expected. If Cepheids do have a significant metallicity dependence then the Tully-Fisher scale error increases and the distances of the Virgo and Fornax clusters extend to more than 20Mpc, decreasing the value of Ho. Finally, if the Cepheids have a metallicity dependence then so do Type Ia Supernovae since the metallicity corrected Cepheid distances to eight galaxies with SNIa would then suggest that the SNIa peak luminosity is fainter in metal poor galaxies, with important implications for SNIa estimates of qo as well as Ho.

astro-ph↗

arXiv2002

The correlation of line strength with luminosity and redshift from composite QSO spectra

We have generated a series of composite QSO spectra using over 22000 individual low resolution (~8A) QSO spectra obtained from the 2dF (18.25<bj<20.85) and 6dF (16<bj<18.25) QSO Redshift Surveys. The large size of the catalogue has enabled us to construct composite spectra in narrow redshift (dz=0.25) and absolute magnitude (dMb=0.5) bins. The median number of QSOs in each composite is ~200, yielding typical S/N of ~100. For a given redshift interval, the composite spectra cover a factor of over 25 in luminosity. Using the composite spectra we have measured the equivalent widths (EWs) of the major broad and narrow emission lines, and the CaII K absorption feature due to the host galaxy of the AGN. Assuming a fixed host galaxy spectral energy distribution (SED), the correlation between CaII K EW and luminosity implies Lgal proportional to Lqso**{0.42+-0.05}. We find strong anti-correlations with luminosity for the EWs of [OII] and [NeV]. These provide hints to the general fading of the NLR in high luminosity sources which we attribute to the NLR dimensions becoming larger than the host galaxy. If average AGN host galaxies have SEDs similar to average galaxies, then the observed narrow [OII] emission could be solely due to the host galaxy at low luminosities (M_B~-20). We measure highly significant Baldwin effects for most broad emission lines (CIV, CIII], MgII, Hbeta, Hgamma) and show that they are predominantly due to correlations with luminosity, not redshift. We find that the Hbeta and Hgamma Balmer lines show an inverse Baldwin effect and are positively correlated with luminosity, unlike the broad UV lines. We postulate that this previously unknown effect is due to a luminosity dependent change in the the ratio of disk to non-disk continuum components (abridged).

astro-ph↗

arXiv2002

Clustering in the 2dF QSO Redshift Survey

We present clustering results from the 2dF QSO Redshift Survey (2QZ) which currently contains over 20,000 QSOs at z<3. The two-point correlation function of QSOs averaged over the entire survey ( ~1.5) is found to be similar to that of local galaxies. When sub-dividing the sample as a function of redshift, we find that for an Einstein-de Sitter universe QSO clustering is constant (in comoving coordinates) over the entire redshift range probed by the 2QZ, while in a universe with Omega_0=0.3 and Lambda_0=0.7 there is a marginal increase in clustering with redshift. Sub-dividing the 2QZ on the basis of apparent magnitude we find only a slight difference between the clustering of QSOs of different apparent brightness, with the brightest QSOs having marginally stronger clustering. We have made a first measurement of the redshift space distortion of QSO clustering, with the goal of determining the value of cosmological parameters (in partcular Lambda_0) from geometric distortions. The current data do not allow us to discriminate between models, however, in combination with constraints from the evolution of mass clustering we find Omega_0=1-Lambda_0=0.23 +0.44-0.13 and beta(z~1.4)=0.39 +0.18-0.17. The full 2QZ data set will provide further cosmological constraints.

astro-ph↗

arXiv2002

The 2dF QSO Redshift Survey - IX. A measurement of the luminosity dependence of QSO clustering

In this Paper we present a clustering analysis of QSOs as a function of luminosity over the redshift range z=0.3-2.9. We use a sample of 10566 QSOs taken from the preliminary data release catalogue of the 2dF QSO Redshift Survey (2QZ). We analyse QSO clustering as a function of apparent magnitude. The strong luminosity evolution of QSOs means that this is approximately equivalent to analysing the data as a function of absolute magnitude relative to M* over the redshift range that the 2QZ probes. Over the relatively narrow range in apparent magnitude of the 2QZ we find no significant (>2sigma) variation in the strength of clustering, however, there is marginal evidence for QSOs with brighter apparent magnitudes having a stronger clustering amplitude. QSOs with 18.25<bj<19.80 show a correlation scale length s_0=5.50+-0.79h-1Mpc in an Einstein-de Sitter (EdS) universe and s_0=8.37+-1.17h-1Mpc in a universe with omega_0=0.3 and lambda_0=0.7 (Lambda), while the best fit values for the full magnitude interval (18.25<bj<20.85) over the same spatial scales are s_0=4.29+-0.30h-1Mpc (EdS) and s_0=6.35+-0.45h-1Mpc (Lambda). We can therefore determine that the bias of the brightest sub-sample is a factor 1.22+-0.15 (EdS) or 1.24+-0.15 (Lambda) larger than that of the full data set. An increase in clustering with luminosity, if confirmed, would be in qualitative agreement with models in which the luminosity of a QSO is correlated to the mass of the dark halo in which it resides, implying that the mass of the host plays at least some part in determining a QSO's formation and evolution. These models predict that the clustering in brighter QSO data sets, such as Sloan Digital Sky Survey QSO sample or the bright extension of the 2QZ should show a higher clustering amplitude than the 2QZ.

astro-ph↗

arXiv2002

The 2dF BL Lac Survey

We have optically identified a sample of 56 featureless continuum objects without significant proper motion from the 2dF QSO Redshift Survey (2QZ). The steep number--magnitude relation of the sample, $n(\bj) \propto 10^{0.7\bj}$, is similar to that derived for QSOs in the 2QZ and inconsistent with any population of Galactic objects. Follow up high resolution, high signal-to-noise, spectroscopy of five randomly selected objects confirms the featureless nature of these sources. Assuming the objects in the sample to be largely featureless AGN, and using the QSO evolution model derived for the 2QZ, we predict the median redshift of the sample to be $z=1.1$. This model also reproduces the observed number-magnitude relation of the sample using a renormalisation of the QSO luminosity function, $Φ^* = Φ^*_{\rm \sc qso}/66 \simeq 1.65 \times 10^{-8} $mag$^{-1}$Mpc$^{-3}$. Only $\sim$20 per cent of the objects have a radio flux density of $S_{1.4}>3 $mJy, and further VLA observations at 8.4 GHz place a $5σ$ limit of $S_{8.4} < 0.2$mJy on the bulk of the sample. We postulate that these objects could form a population of radio-weak AGN with weak or absent emission lines, whose optical spectra are indistinguishable from those of BL Lac objects.

astro-ph↗

arXiv2001

The 2dF QSO Redshift Survey - IV. The QSO Power Spectrum from the 10k Catalogue

(ABRIDGED) We present a power spectrum analysis of the 10K catalogue from the 2dF QSO Redshift Survey. We compare the redshift-space power spectra of QSOs to those measured for galaxies and Abell clusters at low redshift and find that they show similar shapes in their overlap range, 50-150h^{-1}Mpc, with P_QSO(k)\propto k^{-1.4}. The amplitude of the QSO power spectrum at z~1.4 is almost comparable to that of galaxies at the present day if Omega_m=0.3 and Omega_Lambda=0.7 (the Lambda cosmology), and a factor of ~3 lower if Omega_m=1 (the EdS cosmology) is assumed. The amplitude of the QSO power spectrum is a factor of ~10 lower than that measured for Abell clusters at the present day. At larger scales, the QSO power spectra continue to rise robustly to ~400 h^{-1}Mpc, implying more power at large scales than in the APM galaxy power spectrum measured by Baugh & Efstathiou. We split the QSO sample into two redshift bins and find little evolution in the amplitude of the power spectrum. The QSO power spectrum may show a spike feature at ~90h^{-1}Mpc assuming the Lambda cosmology or ~65 h^{-1}Mpc assuming an EdS cosmology. Although the spike appears to reproduce in both the North and South strips and in two independent redshift ranges, its statistical significance is still marginal and more data is needed to test further its reality. We compare the QSO power spectra to CDM models to obtain a constraint on the shape parameter, Gamma. For two choices of cosmology (Omega_m=1, Omega_Lambda=0 and Omega_m=0.3, Omega_Lambda=0.7), we find the best fit model has Gamma~0.1 +-0.1.

astro-ph↗

arXiv2001

A first look at cataclysmic variable stars from the 2dF QSO survey

The 2dF QSO survey is a spectroscopic survey of 48,000 point-sources selected by colour with magnitudes in the range 18.35 < B < 20.95. Amongst QSOs, white dwarfs, narrow-line galaxies and other objects are some cataclysmic variables (CVs). This survey should be sensitive to intrinsically faint CVs. In the standard picture of CV evolution, these form the majority of the CV population. We present the spectra of 6 CVs from this survey. Four have the spectra of dwarf novae and two are magnetic CVs. We present evidence that suggests that the dwarf novae have period P < 2 h and are indeed intrinsically less luminous than average. However, it is not clear yet whether these systems are present in the large numbers predicted.

astro-ph↗

arXiv2001

The 2dF QSO Redshift Survey - VI. Measuring Lambda and Beta from Redshift-space Distortions in the Power Spectrum

When the 2dF QSO Redshift Survey (2QZ) is complete, a powerful geometric test for the cosmological constant will be available. By comparing the clustering along and across the line of sight and modelling the effects of peculiar velocities and bulk motions in redshift space, geometric distortions, which occur if the wrong cosmology is assumed, can be detected. In this paper we investigate the effect of geometric and redshift-space distortions in the power spectrum parallel and perpendicular to the observer's line of sight. Ballinger et al. developed a model to estimate the cosmological constant, $Λ$, and the important parameter $β\approx Ω_m^{0.6}/b$ from these distortions. We apply this model to a detailed simulation of the final 25k 2QZ, produced using the Virgo Consortium's huge {\it Hubble Volume} N-body $Λ$-CDM light cone simulation. We confirm the conclusions of Ballinger et al.; the shape of the redshift-space and geometric distortions are very similar. When all the uncertainties are taken into account we find that only a joint $Λ- β$ constraint is possible. By combining this result with a second constraint based on mass clustering evolution, however, we can make significant progress. We predict that this method should allow us to constrain $β$ to approximately $\pm0.1$, and $Ω_{m}$ to $\pm0.25$ using the final catalogue. We apply the method to the 2QZ catalogue of 10000 QSOs and find that this incomplete catalogue marginally favours a $Λ$ cosmology.

astro-ph↗

arXiv2001

The 2dF QSO Redshift Survey - VIII. Absorption systems in the 10k catalogue

We examine the highest S/N spectra from the 2QZ 10k release and identify over 100 new low-ionisation heavy element absorbers; DLA candidates suitable for higher resolution follow-up observations. These absorption systems map the spatial distribution of high-z metals in exactly the same volumes that the foreground 2QZ QSOs themselves sample and hence the 2QZ gives us the unique opportunity to directly compare the two tracers of large scale structure. We examine the cross-correlation of the two populations to see how they are relatively clustered, and, by considering the colour of the QSOs, detect a small amount of dust in these metal systems.

astro-ph↗

arXiv2001

The 2dF QSO Redshift Survey - VII. Constraining Cosmology from Redshift Space Distortions via xi(sigma,pi)

We describe a method from which cosmology may be constrained from the 2QZ Survey. By comparing clustering properties parallel and perpendicular to the line of sight and by modeling the effects of redshift space distortions, we are able to study geometric distortions in the clustering pattern which occur if a wrong cosmology is assumed when translating redshifts into comoving distances. Using mock 2QZ catalogues, drawn from the Hubble Volume simulation, we find, that there is a degeneracy between the geometric and the redshift-space distortions that makes it difficult to obtain an unambiguous estimate of Omega_{m}(0) from the geometric tests alone. However, we demonstrate a new method to determine the cosmology which works by combining the above geometric test with a test based on the evolution of the QSO clustering amplitude. We find that we are able to break the degeneracy and that independent constraints to +-20% (1 sigma) accuracy on Omega_{m}(0) and +-10% (1 sigma) accuracy on beta_{QSO}(z) should be possible in the full 2QZ survey. Finally we apply the method to the 10k catalogue of 2QZ QSOs. The smaller number of QSOs and the current status of the Survey mean that a strong result on cosmology is not possible but we do constrain beta_{QSO}(z) to 0.35+-0.2. By combining this constraint with the further constraint available from the amplitude of QSO clustering, we find tentative evidence favouring a model with non-zero Omega_{Lambda}(0), although an Omega_{m}(0)=1 model provides only a marginally less good fit. A model with Omega_{Lambda}(0)=1 is ruled out. The results are in agreement with those found by Outram et al. using a similar analysis in Fourier space. (Abridged)

astro-ph↗

arXiv2001

The 2dF QSO Redshift Survey - II. Structure and evolution at high redshift

We present a clustering analysis of QSOs over the redshift range z=0.3-2.9. We use a sample of 10558 QSOs taken from the preliminary catalogue of the 2dF QSO Redshift Survey (2QZ). The two-point redshift-space correlation function of QSOs is shown to follow a power law on scales s~1-35h-1Mpc. Fitting a power law to QSO clustering averaged over the redshift interval 0.3 99 per cent confidence level). A redshift dependent bias is required to reconcile QSO clustering observations with theory. A simple biasing model, in which QSOs have cosmologically long lifetimes (or alternatively form in peaks above a constant threshold in the density field) is acceptable in an EdS cosmology, but is only marginally acceptable if Omega_0=0.3 and lambda_0=0.7. Biasing models which assume QSOs form over a range in redshift, based on the Press-Schechter formalism are approximately consistent with QSO clustering evolution (abridged).

astro-ph↗

arXiv2001

ASCA observations of deep ROSAT fields V. The X-ray spectrum of hard X-ray selected QSOs

We present an analysis of the \rosat and \asca spectra of 21 broad line AGN (QSOs) with $z\sim 1$ detected in the 2-10 keV band with the \asca \gis. The summed spectrum in the \asca band is well described by a power-law with $Γ=1.56\pm0.18$, flatter that the average spectral index of bright QSOs and consistent with the spectrum of the X-ray background in this band. The flat spectrum in the \asca band could be explained by only a moderate absorption ($\sim 10^{22} \rm cm^{-2}$) assuming the typical AGN spectrum ie a power-law with $Γ$=1.9. This could in principle suggest that some of the highly obscured AGN, required by most X-ray background synthesis models, may be associated with normal blue QSOs rather than narrow-line AGN. However, the combined 0.5-8 keV \asca-\rosat spectrum is well fit by a power-law of $Γ=1.7\pm0.2$ with a spectral upturn at soft energies. It has been pointed out that such an upturn may be an artefact of uncertainties in the calibration of the ROSAT or ASCA detectors. Nevertheless if real, it could imply that the above absorption model suggested by the \asca data alone is ruled out. Then a large fraction of QSOs could have ``concave'' spectra ie they gradually steepen towards softer energies. This result is in agreement with the \bepposax hardness ratio analysis of $\sim$ 100 hard X-ray selected sources.

astro-ph↗

arXiv2001

The 2dF QSO Redshift Survey - V. The 10k catalogue

We present a catalogue comprising over 10000 QSOs covering an effective area of 289.6 sq. degrees, based on spectroscopic observations with the 2-degree Field instrument at the Anglo-Australian Telescope. This catalogue forms the first release of the 2-degree Field QSO Redshift Survey. QSO candidates with 18.25<b_J<20.85 were obtained from a single homogeneous colour-selected catalogue based on APM measurements of UK Schmidt photographic material. The final catalogue will contain approximately 25000 QSOs and will be released to the public at the end of 2002, one year after the observational phase is concluded.

astro-ph↗

arXiv2001

First Results from the 2dF QSO redshift survey

We present some initial results from the 2dF QSO redshift survey. The aim of the survey is to produce an optically-selected catalogue of 25000 QSOs over the redshift range 0<z<3 using the 2-degree field at the Anglo-Australian Telescope.

astro-ph↗

arXiv2001

The 2dF QSO Redshift Survey - 10K@2K!

With ~10000 QSO redshifts, the 2dF QSO Redshift Survey (2QZ) is already the biggest individual 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; we present new estimates of the QSO luminosity function and the QSO correlation function. We also present the first estimate of the QSO power spectrum from the 2QZ catalogue, probing the form of the fluctuation power-spectrum out to the \~1000h-1Mpc scales only previously probed by COBE. We find a power spectrum which is steeper than the prediction of standard CDM and more consistent with the prediction of Lambda-CDM. The best-fit value for the power spectrum shape parameter for a range of cosmologies is Gamma=0.1+-0.1. Finally, we discuss how the complete QSO survey will be able to constrain the value of Omega_Lambda by combining results from the evolution of QSO clustering and from a geometric test of clustering isotropy.

astro-ph↗

arXiv2001

Cepheid and SNIa Distance Scales

We first discuss why there remains continuing, strong motivation to investigate Hubble's Constant. Then we review new evidence from an investigation of the Galactic Open Clusters containing Cepheids by Hoyle et al. that the metallicity dependence of the Cepheid P-L relation is stronger than expected. This result is supported by a new analysis of mainly HST Distance Scale Key Project data which shows a correlation between host galaxy metallicity and the rms scatter around the Cepheid P-L relation. If Cepheids do have a significant metallicity dependence then an already existing scale error for Tully-Fisher distances becomes worse and the distances of the Virgo and Fornax clusters extend to more than 20Mpc, decreasing the value of H_0. Finally, if the Cepheids have a metallicity dependence then so do Type Ia Supernovae since the metallicity corrected Cepheid distances to eight galaxies with SNIa now suggests that the SNIa peak luminosity is fainter in metal poor galaxies. As well as having important implications for H_0, this would also imply that the evidence for a non-zero cosmological constant from the SNIa Hubble Diagram may be subject to corrections for metallicity which are as big as the effects of cosmology.

astro-ph↗

arXiv2000

Evidence for Galaxy Formation at High Redshift

We have shown previously that galaxy counts from the UV to the near-IR are well-fitted by simple evolutionary models where the space density of galaxies remains constant with look-back time while the star-formation rate rises exponentially. We now extend these results, first by using data from the Herschel Deep Field to show that these same models give detailed fits to the faint galaxy r-i:b-r colour-colour diagram. We then use these models to predict the number counts of high redshift galaxies detected by the Lyman break technique. At z~3 there is almost exact agreement between ou r prediction and the data, suggesting that the space density of galaxies at z~3 may be close to its local value. At z~4 the space density of bright galaxies remains unchanged; however, the space density of dwarf galaxies is significantly lower than it is locally, suggesting that we have detected an epoch of dwarf galaxy formation at z~4. Finally, significant numbers of Lyman-break galaxy candidates are also detected at z~6 in the Hubble and Herschel Deep Fields; taking this observation together with a number of recent detections of spectroscopically confirmed z~6 galaxies suggests that the space density of bright galaxies at z~6 remains comparable to the local space density and thus that the epoch of formation of bright galaxies may lie at yet higher redshift.

astro-ph↗