arXiv subjects

T. Shanks

Publications and source records attributed to T. Shanks.

At least 145 records · Page 8Linked to original sources

arXiv1998

The 2dF QSO Redshift Survey

We present preliminary results from the 2-degree Field (2dF) QSO Redshift Survey currently under way at the Anglo-Australian Telescope. This survey aims to determine the redshifts of >25000 QSOs over a redshift range of 0.3<z<3.0 with the primary goal of investigating large-scale structure in the Universe to high redshift and at very large scales (~1000h-1Mpc). We describe the photometric procedure used to select QSO candidates for spectroscopic observation. We then describe results from our first 2dF observations, which have so far measured the redshifts for over 1000 QSOs. We already find a significant detection of clustering and have also found one close pair of QSOs (separation 17'') which are gravitational lens candidates. To keep up to date with the current progress of the survey see: http://msowww.anu.edu.au/~rsmith/QSO_Survey/qso_surv.html

astro-ph↗

arXiv1998

Discovery of Radio-loud Broad Absorption Line Quasars Using Ultraviolet Excess and Deep Radio Selection

We report the discovery of five broad-absorption-line (BAL) QSOs in a complete sample of 111 ultraviolet excess (UVX) QSO candidates also detected in the NRAO VLA Sky Survey. All five BAL QSOs, which include two high-ionization BAL QSOs and three low-ionization BAL QSOs, are formally radio-loud. Of QSOs with z > 0.4, 3+/-2% show low-ionization BALs, and of QSOs with z > 1.5, all radio-loud, 9+/-5% show BALs; these frequencies are consistent with those of optical surveys. While the first reported radio-loud BAL QSO, FIRST J155633.8+351758, is likely to be heavily dust reddened and thus less radio-loud than indicated by its observed radio-to-optical luminosity, these QSOs are UVX selected and probably free of significant dust along the line of sight. We point out unusual features in two of the BAL QSOs and discuss the significance finding these heretofore rare objects.

astro-ph↗

arXiv1998

QSO clustering and the AAT 2dF redshift survey

We review previous results on the clustering and environments of QSOs. We show that the correlation length for QSOs derived from existing surveys is r~5/h Mpc, similar to the observed correlation length for field galaxies at the present epoch. The galaxy environment for z<1 radio-quiet QSOs is also consistent with field galaxies. The evolution of the QSO correlation length with redshift is currently uncertain, largely due to the small numbers of QSOs (~2000) in surveys suitable for clustering analysis. We report on intial progress with the AAT 2dF QSO redshift survey, which, once completed will comprise almost 30000 QSOs. With over 1000 QSOs already observed, it is already the largest single homogeneous QSO survey. We discuss prospects for deriving limits on cosmological parameters from this survey, and on the evolution of large-scale structure in the Universe.

astro-ph↗

arXiv1998

ASCA observations of Deep ROSAT fields - III. The Discovery of an Obscured Type 2 AGN at z=0.67

We report on the discovery of a narrow-emission-line object at z=0.672 detected in a deep ASCA survey. The object, AXJ0341.4-4453, has a flux in the 2--10keV band of 1.1+/-0.27 E-13 erg/s/cm**2, corresponding to a luminosity of 1.8E44 erg/s (q0=0.5, $H0 = 50km/s/Mpc. It is also marginally detected in the ROSAT 0.5-2keV band with a flux 5.8E-15 erg/s/cm**2. Both the ASCA data alone and the combined ROSAT/ASCA data show a very hard X-ray spectrum, consistent with either a flat power law (alpha < 0.1) or photoelectric absorption with a column of nH > 4E22 cm**-2 (alpha = 1). The optical spectrum shows the high-ionisation, narrow emission lines typical of a Seyfert 2 galaxy. We suggest that this object may be typical of the hard sources required to explain the remainder of the X-ray background at hard energies.

astro-ph↗

arXiv1998

SBF Distances to Leo and Virgo using the HST

We have used archive HST WFPC2 data for three elliptical galaxies (NGC 3379 in the Leo I group, and NGC 4472 and NGC 4406 in the Virgo cluster) to determine their distances using the Surface Brightness Fluctuation (SBF) method as described by Tonry and Schneider (1988). A comparison of the HST results with the SBF distance moduli of Ciardullo et al (1993) shows significant disagreement and suggests that the r.m.s. error on these ground-based distance moduli is actually as large as +-0.25 mag. The agreement is only slightly improved when we compare our results with the HST and ground-based SBF distances from Ajhar et al (1997) and Tonry et al (1997); the comparison suggests that a lower limit on the error of the HST SBF distance moduli is +-0.17 mag. Overall, these results suggest that previously quoted measurement errors may underestimate the true error in SBF distance moduli by at least a factor of 2-3.

astro-ph↗

arXiv1998

Galaxy Counts, Sizes, Colours and Redshifts in the Hubble Deep Field

We compare the galaxy evolution models of Bruzual & Charlot (1993) with the faint galaxy count, size and colour data from the Hubble and Herschel Deep Fields (Metcalfe et al 1996). For qo=0.05, we find that models where the SFR increases exponentially out to z>2 are consistent with all of the observational data. For qo=0.5, such models require an extra population of galaxies which are only seen at high redshift and then rapidly fade or disappear. We find that, whatever the cosmology, the redshift of the faint blue galaxies and hence the epoch of galaxy formation is likely to lie at z>2. We find no implied peak in the SFR at z=1 and we suggest that the reasons for this contradiction with the results of Madau et al (1996) include differences in faint galaxy photometry, in the treatment of spiral dust and in the local galaxy count normalisation.

astro-ph↗

arXiv1997

ASCA observations of Deep ROSAT fields - II. The 2-10keV AGN luminosity function

We present additional optical spectroscopic identifications of sources identified in three deep ASCA GIS fields which also form part of a deep ROSAT survey. In total, 26 ASCA sources have been detected down to a 2-10keV flux limit of S = 5e-14 erg/cm/s. LDSS observations have increased the spectroscopic completeness of the survey to between 65 and 85 per cent, with identifications for up to 13 QSOs with broad emission lines and 6 objects with narrow emission lines. Combining these objects with the AGN identified in the LMA/HEAO-1 sample by Grossan, we find evidence for significant cosmological evolution in the 2-10keV band ( =0.63+/-0.03), fit by a pure luminosity evolution model; L \propto (1+z)^k, k=2.04^{+0.16}_{-0.22}$. The present-epoch 2-10keV AGN X-ray luminosity function, Φ(L_X), is best represented by a two power law function: Φ(L_X) \propto L^{-3.0}, L*>10^{44.1}erg/s; $Φ(L_X) \propto L^{-1.9}$, L*<10^{44.1}erg/s. Depending on the extent to which we extrapolate the z=0 AGN luminosity function, we predict a total contribution to the 2-10keV X-ray background from QSOs and narrow-emission-line galaxies which ranges from 48 per cent (for AGN with L>10^{42}erg/s) to 80 per cent (L>10^{39}erg/s).

astro-ph↗

arXiv1997

Faint galaxies and the X-ray background

We summarise our recent work on the faint galaxy contribution to the cosmic X-ray background (XRB). At bright X-ray fluxes (in the ROSAT pass band), broad line QSOs dominate the X-ray source population, but at fainter fluxes there is evidence for a significant contribution from emission-line galaxies. Here we present statistical evidence that these galaxies can account for a large fraction of the XRB. We also demonstrate that these galaxies have significantly harder X-ray spectra than QSOs. Finally we present preliminary findings from infra-red spectroscopy on the nature of this X-ray emitting galaxy population. We conclude that a hybrid explanation consisting of obscured/Type 2 AGN surrounded by starburst activity can explain the properties of these galaxies and perhaps the origin of the entire XRB.

astro-ph↗

arXiv1997

The AAO 2dF QSO Redshift Survey

We describe the aims, strategy and status of the AAO 2dF QSO redshift survey. This goal of the survey is to obtain redshifts for 30000 QSOs in a homogeneous magnitude limited (B<21) survey. The survey began in early 1997 and should be complete by the end of 1999.

astro-ph↗

arXiv1997

A deep ROSAT Survey - XIV. X-ray emission from faint galaxies

We present a cross-correlation analysis to constrain the faint galaxy contribution to the cosmic X-ray background (XRB). Cross-correlating faint optical galaxy catalogues with unidentified X-ray sources from 3 deep ROSAT fields we find that B < 23 galaxies account for 20+-7 % of all X-ray sources to a flux limit of S(0.5-2.0 keV)=4x10^{-15} erg s-1 cm-2. To probe deeper, galaxies are then cross-correlated with the remaining unresolved X-ray images. A highly significant signal is obtained on each field. Allowing for the effect of the ROSAT point-spread function, and deconvolving the effect of galaxy clustering, we find that faint B < 23 galaxies directly account for 23+-3% of the unresolved XRB at 1keV. Using the optical magnitude of faint galaxies as probes of their redshift distribution, we find evidence for strong evolution in their X-ray luminosity, parameterised with the form L_x \propto (1+z)^3.2+-1.0. Extrapolation to z=2 will account for 40+-10% of the total XRB at 1keV. The nature of the emitting mechanism in these galaxies remains unclear, but we argue that obscured and/or low luminosity AGN provide the most natural explanation.

astro-ph↗

arXiv1997

ASCA observations of deep ROSAT fields I. the nature of the X-ray source populations

We present ASCA GIS observations (total exposure 100-200 ksec) of three fields which form part of our deep ROSAT survey. We detect 26 sources down to a limiting flux (2-10 keV) of 5x10^{-14} erg cm-2 s-1. Sources down to this flux level contribute 30 per cent of the 2-10 keV X-ray background. The number-count distribution, logN-logS, is a factor of three above the ROSAT counts, assuming a photon spectral index of 2 for the ROSAT sources. This suggests the presence at hard energies of a population other than the broad-line AGN which contribute to the ROSAT counts. This is supported by spectroscopic observations that show a large fraction of sources that are not obvious broad-line AGN. The average 1-10 keV spectral index of these sources is flat (0.92+-0.16), significantly different than that of the broad-line AGN (1.78+-0.16). Although some of the Narrow Emission Line Galaxies which are detected with ROSAT are also detected here, the nature of the flat spectrum sources remains as yet unclear.

astro-ph↗

arXiv1997

The Durham/UKST Galaxy Redshift Survey -III. Large Scale Structure via the 2-Point Correlation Function

We have investigated the statistical clustering properties of galaxies by calculating the 2-point galaxy correlation function from the Durham/UKST Galaxy Redshift Survey. This survey is magnitude limited to bj = 17, contains 2500 galaxies sampled at a rate of one in three and surveys a 4 \times 10^6 (h^-1Mpc)^3 volume of space. We have empirically determined the optimal method of estimating the 2-point correlation function from just such a magnitude limited survey. Using this method, our correlation function results confirm the previously claimed detections of large scale power out to 40h^-1 Mpc scales. We compare with two common models of cosmological structure formation and find that our 2-point correlation function has power significantly in excess of the standard cold dark matter model in the 10-30h^-1 Mpc region, supporting the observational results of the APM galaxy survey.

astro-ph↗

arXiv1997

The Durham/UKST Galaxy Redshift Survey - IV. Redshift Space Distortions via the 2-Point Correlation Function

We have investigated the redshift space distortions in the optically selected Durham/UKST Galaxy Redshift Survey using the 2-point galaxy correlation function perpendicular and parallel to the observer's line of sight. We present results for the real space 2-point correlation function, by inverting the optimally estimated projected correlation function and find good agreement with other real space estimates. On small, non-linear scales we observe an elongation of the z-space corelation function contours in the line of sight direction. Our result for the 1-D pairwise rms velocity dispersion is sigma = 416 +- 36 kms^-1 which is consistent with those from recent redshift surveys and canonical values, but inconsistent with SCDM or LCDM models. On larger, linear scales we observe a compression of the z-space correlation function contours in the line of sight direction. This is due to the infall of galaxies into overdense regions and the Durham/UKST data favours a value of Omega^0.6/b = 0.5, where Omega is the mean mass density of the Universe and b is the linear bias factor which relates the galaxy and mass distributions. Comparison with other optical estimates yield consistent results, with the conclusion that the data does not favour an unbiased critical-density universe.

astro-ph↗

arXiv1997

The Durham/UKST Galaxy Redshift Survey - II. The Field Galaxy Luminosity Function

We present the results for the galaxy luminosity function as estimated from the Durham/UKST Galaxy Redshift Survey. This survey is magnitude limited to bj = 17, contains 2500 galaxies sampled at a rate of one in three and surveys a 4 x 10^6 Mpc^3 volume of space. The maximum likelihood parameters for a standard Schechter luminosity function are estimated to be M*bj = -19.72+-0.09, alpha = -1.14+-0.08 and phi* = (1.2 +- 0.2) x 10^-2 ( h^3Mpc^-3). A comparison with galaxy luminosity functions from other redshift surveys shows good agreement and the shape of the luminosity function now appears well-defined down to Mbj = -17. There are some discrepancies between the different surveys for galaxies fainter than this absolute magnitude. However, our estimate agrees well with that from the APM-Stromlo Galaxy Redshift Survey and we measure a fairly flat faint end slope.

astro-ph↗

arXiv1997

A Test of the Calibration of the Tully-Fisher Relation Using Cepheid and SNIa Distances

We make a direct test of Tully-Fisher distance estimates to eleven spiral galaxies with HST Cepheid distances and to twelve spiral galaxies with SNIa distances. The HST Cepheid distances come from the work of Freedman (1997), Sandage et al (1996) and Tanvir et al (1995). The SNIa distances come from Pierce (1994), calibrated using the Cepheid results of Sandage et al (1996). The Tully-Fisher distances mostly come from the work of Pierce (1994). The results show that the Tully-Fisher distance moduli are too short with respect to the Cepheid distances by 0.46+-0.14mag and too short with respect to the SNIa distances by 0.46+-0.19mag. Combining the HST Cepheid and SNIa data suggests that, overall, previous Tully-Fisher distances were too short by 0.46+-0.11mag, a result which is significant at the 4sigma level. These data therefore indicate that previous Tully-Fisher distances should be revised upwards by 24+-6% implying, for example, a Virgo distance of 19.3+-1.9Mpc. The value of Ho from Tully-Fisher estimates is correspondingly revised downwards from Ho=84+-10 to Ho=68+-8.

astro-ph↗

arXiv1996

Galaxy Formation at High Redshift

Sensitive optical surveys have revealed a large population of `faint blue galaxies' which are believed to be young galaxies, observed close to their time of formation. But there has been considerable uncertainty regarding the epochs at which these galaxies are observed, owing to the difficulties inherent in determining spectroscopic redshifts for very faint objects. Here, by modelling the counts and colours of galaxies at the faintest detection limits, we show that the faint blue galaxies are likely to lie at high redshift (z approx. 2). This conclusion holds regardless of whether the Universe is assumed to be open or at the critical density (flat). In an open universe, the data are consistent with galaxy models in which star formation rates decay exponentially with decreasing redshift, whereas the assumption of a flat universe requires the addition of a population of galaxies which are seen only at high redshift.

astro-ph↗

arXiv1996

Detection of Large Scale Structure in a $B < 17^{m}$ Galaxy Redshift Survey

We report on results from the Durham/UKST Galaxy Redshift Survey where we have found large scale ``cellular'' features in the galaxy distribution. These have spatial 2-point correlation function power significantly in excess of the predictions of the standard cold dark matter cosmological model$^{1}$, supporting the previous observational results from the APM survey$^{2,3}$. At smaller scales, the 1-D pairwise galaxy velocity dispersion is measured to be $\bf 387^{+96}_{-62}$ kms$^{-1}$ which is also inconsistent with the prediction of the standard cold dark matter model$^{1}$. Finally, the survey has produced the most significant detection yet of large scale redshift space distortions due to dynamical infall of galaxies$^{4}$. An estimate of $\bf Ω^{0.6}/b = 0.55 \pm 0.12$ is obtained which is consistent either with a low density Universe or a critical density Universe where galaxies are biased tracers of the mass.

astro-ph↗

arXiv1995

Determination of the Hubble constant from observations of Cepheid variables in the galaxy M96

New Hubble Space Telescope observations of Cepheid variable stars in the nearby galaxy M96 give a distance to the host galaxy group, Leo-I, of 11.6+/-0.8 Mpc. This value, used in conjunction with several reliable secondary indicators of relative distance, constrains the distances to more remote galaxy clusters, and yields a value of the Hubble constant (Ho=69+/-8 km/s/Mpc) that is independent of the velocity of the Leo-I group itself.

astro-ph↗