arXiv subjects

S. J. Maddox

Publications and source records attributed to S. J. Maddox.

71 records · Page 4Linked to original sources

arXiv1999

Galaxy and Cluster Biasing from Local Group Dynamics

Comparing the gravitational acceleration induced on the Local Group of galaxies by different tracers of the underline density field we estimate, within the linear gravitational instability theory and the linear biasing ansatz, their relative bias factors. Using optical SSRS2 galaxies, IRAS (PSCz) galaxies and Abell/ACO clusters, we find b_{O,I} ~ 1.21 +- 0.06 and b_{C,I} ~ 4.3 +- 0.8, in agreement with other recent studies. Finally, there is an excellent one-to-one correspondence of the PSCz and Abell/ACO cluster dipole profiles, once the latter is rescaled by b_{C,I}, out to at least ~150 h^{-1} Mpc.

astro-ph↗

arXiv1999

The Observed Evolution of Galaxy Clustering Vs. Epoch-Dependent Biasing Models

We study the observed evolution of galaxy clustering as a function of redshift. We find that the clustering of galaxies decreases as we go from observations of the local Universe to $z \sim 2$. On the other hand, clustering of the Lyman break galaxies at $z \sim 3$ is very strong, comparable to the clustering of present day galaxies. However there are three major factors to take into account while comparing clustering measurements coming from various surveys: the so-called ``scale-dependence'' effect, due to measurements being made at different scales; the ``type-selection'' effect introduced by the fact that different galaxy surveys select different populations which do not have the same clustering amplitudes; and the Malmquist bias. We correct for the first two effects and discuss the implications of Malmquist bias on the interpretation of the data at different $z$. Then we compare the observed galaxy clustering with models for the evolution of clustering in some fixed cosmologies. Correcting for the scale-dependence effect significantly reduces the discrepancies amongst different measurements. We interpret the observed clustering signal at high redshift as coming from objects which are highly biased with respect to the underlying distribution of mass; this is not the case for $z\simlt 2$ where measurements are compatible with the assumption of a much lower biasing level which only shows a weak dependence on $z$. Finally we propose a model-independent test that can be used to place a lower limit on the density parameter $Ω_0$.

astro-ph↗

arXiv1999

The environments of intermediate-redshift QSOs: 0.3 < z < 0.7

An angular correlation of low significance (2 sigma) is observed between 0.3 < z < 0.5 QSOs and V < 23 galaxies. Overall, the cross-correlation function between 82 intermediate-redshift (0.3 < z < 0.7), X-ray selected QSOs and V < 24 galaxies is investigated, but no signal is detected for the z > 0.5 QSOs. After converting to an excess of galaxies physically associated with the QSO, this lack of strong correlation is shown to be consistent with the clustering of normal galaxies at the same moderate redshifts. Combined with previous observations, these results imply that the environments of radio-quiet QSOs do not undergo significant evolution with respect to the galaxy population over a wide range of redshifts (0 < z < 1.5). This is in marked contrast to the rapid increase in the richness of the environments associated with radio-loud QSOs over the same redshift range.

astro-ph↗

arXiv1999

The 2dF Galaxy Redshift Survey: Spectral Types and Luminosity Functions

We describe the 2dF Galaxy Redshift Survey (2dFGRS), and the current status of the observations. In this exploratory paper, we apply a Principal Component Analysis to a preliminary sample of 5869 galaxy spectra and use the two most significant components to split the sample into five spectral classes. These classes are defined by considering visual classifications of a subset of the 2dF spectra, and also by comparing to high quality spectra of local galaxies. We calculate a luminosity function for each of the different classes and find that later-type galaxies have a fainter characteristic magnitude, and a steeper faint-end slope. For the whole sample we find M*=-19.7 (for Omega=1, H_0=100 km/sec/Mpc), alpha=-1.3, phi*=0.017. For class 1 (`early-type') we find M*=-19.6, alpha=-0.7, while for class 5 (`late-type') we find M*=-19.0, alpha=-1.7. The derived 2dF luminosity functions agree well with other recent luminosity function estimates.

astro-ph↗

arXiv1999

Constraints on the Clustering, Biasing and Redshift Distribution of Radio Sources

We discuss how different theoretical predictions for the variance $σ^2$ of the distribution of radio sources can be matched to measurements from the FIRST survey at different flux limits. The predictions are given by the integration of models for the angular correlation function $w(θ)$ for three different functional forms of the redshift distribution $N(z)$, different spatial correlation functions and by different evolutions of the bias $b(z)$ with redshift. We also consider the two cases of open and flat Universes. Although the predicted $w(θ)$ show substantial differences due to differences in the $N(z)$'s, these differences are not significant compared to the uncertainties in the current observations. It turns out that the best fit is provided by models with constant biasing at all times, although the difference between models with epoch-independent bias and models with bias that evolves linearly with redshift is not very large. All models with strong evolution of bias with epoch are ruled out. As a further step we directly calculated $w_{obs}(θ)$ at 3mJy from the catalogue and matched it with our models for the angular correlation function in the hypothesis that the clustering signal comes from two different populations, namely AGN-powered sources and starbursting galaxies. The results are consistent with a scenario for hierarchical clustering where the fainter starbursting galaxies trace the mass at all epochs, while brighter AGN's are strongly biased, with $b(z)$ evolving linearly with redshift, as suggested by some theories of galaxy formation and evolution.

astro-ph↗

arXiv1999

The Redshift Evolution of Clustering in the HDF

We present a correlation function analysis for the catalogue of photometric redshifts obtained from the Hubble Deep Field image by Fernandez-Soto et al., 1998. By dividing the catalogue into redshift bins of width $Δz=0.4$ we measured the angular correlation function $w(θ)$ as a function of redshift up to $z\sim 4.8$. From these measurements we derive the trend of the correlation length $r_0$. We find that $r_0(z)$ is roughly constant with look-back time up to $z \simeq 2$, and then increases to higher values at $z\simgt 2.4$. We estimate the values of $r_0$, assuming $ξ(r,z)=(r/r_0(z))^{-γ}$, $γ=1.8$ and different geometries. For $Ω_0=1$ we find $r_0(z=3)\simeq 7.00\pm 4.87 h^{-1}$ Mpc, in good agreement with the values obtained from analysis of the Lyman Break Galaxies.

astro-ph↗

arXiv1999

Spherical Harmonic Analysis of the PSCz Galaxy Catalogue: Redshift distortions and the real-space power spectrum

We apply the formalism of spherical harmonic decomposition to the galaxy density field of the IRAS PSCz redshift survey. The PSCz redshift survey has almost all-sky coverage and includes IRAS galaxies to a flux limit of 0.6 Jy. Using maximum likelihood methods to examine (to first order) the distortion of the galaxy pattern due to redshift coordinates, we have measured the parameter β= Ω^{0.6}/b. We also simultaneously measure (a) the undistorted amplitude of perturbations in the galaxy distribution when a parameterised power spectrum is assumed, or (b) the shape and amplitude of the real-space power spectrum if the band-power in a set of passbands is measured in a step- wise fashion. These methods are extensively tested on a series of CDM, ΛCDM and MDM simulations and are found to be unbiased. We obtain consistent results for the subset of the PSCz catalogue with flux above 0.75 Jy but inclusion of galaxies to the formal flux limit of the catalogue gives variations which are larger than our internal errors. For the 0.75 Jy catalogue we find, in the case of a parameterised power spectrum, β=0.58\pm0.26 and the amplitude of the real space power measured at wavenumber k=0.1 h/Mpc is Δ_{0.1}=0.42 \pm 0.03. Freeing the shape of the power spectrum we find that β=0.47\pm 0.16 (conditional error), and Δ_{0.1}=0.47 \pm 0.03. The shape of the real-space power spectrum is consistent with a Γ= 0.2 CDM-like shape parameter, but does not strongly rule out a number of other models. Finally by combining our estimate of the amplitude of galaxy clustering and the distortion parameter we find the amplitude of mass fluctuations on a scale k=0.1 \hmpc is Δ_ρ= 0.24 Ω_0^{-0.6}$, with an uncertainty of 50%.

astro-ph↗

arXiv1998

Variance and Skewness in the FIRST survey

We investigate the large-scale clustering of radio sources in the FIRST 1.4-GHz survey by analysing the distribution function (counts in cells). We select a reliable sample from the the FIRST catalogue, paying particular attention to the problem of how to define single radio sources from the multiple components listed. We also consider the incompleteness of the catalogue. We estimate the angular two-point correlation function $w(θ)$, the variance $Ψ_2$, and skewness $Ψ_3$ of the distribution for the various sub-samples chosen on different criteria. Both $w(θ)$ and $Ψ_2$ show power-law behaviour with an amplitude corresponding a spatial correlation length of $r_0 \sim 10 h^{-1}$Mpc. We detect significant skewness in the distribution, the first such detection in radio surveys. This skewness is found to be related to the variance through $Ψ_3=S_3(Ψ_2)^α$, with $α=1.9\pm 0.1$, consistent with the non-linear gravitational growth of perturbations from primordial Gaussian initial conditions. We show that the amplitude of variance and skewness are consistent with realistic models of galaxy clustering.

astro-ph↗

arXiv1996

An Artificial Neural Network Approach to Classification of Galaxy Spectra

We present a method for automated classification of galaxies with low signal-to-noise (S/N) spectra typical of redshift surveys. We develop spectral simulations based on the parameters for the 2dF Galaxy Redshift Survey, and with these simulations we investigate the technique of Principal Component Analysis when applied specifically to spectra of low S/N. We relate the objective principal components to features in the spectra and use a small number of components to successfully reconstruct the underlying signal from the low quality spectra. Using the principal components as input, we train an Artificial Neural Network to classify the noisy simulated spectra into morphological classes, revealing the success of the classification against the observed $b_{\rm J}$ magnitude of the source, which we compare with alternative methods of classification. We find that more than 90\% of our sample of normal galaxies are correctly classified into one of five broad morphological classes for simulations at $b_{\rm J}$=19.7. By dividing the data into separate sets we show that a classification onto the Hubble sequence is only relevant for normal galaxies and that spectra with unusual features should be incorporated into a classification scheme based predominantly on their spectral signatures. We discuss how an Artificial Neural Network can be used to distinguish normal and unusual galaxy spectra, and discuss the possible application of these results to spectra from galaxy redshift surveys.

astro-ph↗

arXiv1996

The Stromlo-APM Redshift Survey III. Redshift Space Distortions, Omega and Bias

Galaxy redshift surveys provide a distorted picture of the universe due to the non-Hubble component of galaxy motions. By measuring such distortions in the linear regime one can constrain the quantity $β= Ω^{0.6}/b$ where $Ω$ is the cosmological density parameter and $b$ is the (linear) bias factor for optically-selected galaxies. In this paper we apply two techniques for estimating $β$ from the Stromlo-APM redshift survey --- (1) measuring the anisotropy of the redshift space correlation function in spherical harmonics and (2) comparing the amplitude of the direction-averaged redshift space correlation function to the real space correlation function. We test the validity of these techniques, particularly whether the assumption of linear theory is justified, using two sets of large $N$-body simulations. We find that the first technique is affected by non-linearities on scales up to $\sim 30 \hMpc$. The second technique is less sensitive to non-linear effects and so is more useful for existing redshift surveys. The Stromlo-APM survey data favours a low value for $β$, with $β\simlt 0.6$. A bias parameter $b \approx 2$ is thus required if $Ω\equiv 1$. However, higher-order correlations measured from the APM galaxy survey (Gaztañaga and Frieman 1994) indicate a low value for the bias parameter $b \approx 1$, requiring that $Ω\simlt 0.5$. We also measure the relative bias for samples of galaxies of various luminosity and morphological type and find that low-luminosity galaxies are roughly three times less biased than $L^*$ galaxies. For the galaxy population as a whole, we measure a real space variance of galaxy counts in $8 \hMpc$ spheres of $(σ^2_8)_g = 0.89 \pm 0.05$.

astro-ph↗

arXiv1996

The APM Galaxy Survey III: An Analysis of Systematic Errors in the Angular Correlation Function and Cosmological Implications

We present measurements of the angular two-point galaxy correlation function, $w(theta)$, from the APM Galaxy Survey. The performance of various estimators of $w$ is assessed using simulated galaxy catalogues and analytic arguments. Several error analyses show that residual plate-to-plate errors do not bias our estimates of $w$ by more than $10^{-3}$. Direct comparison between our photometry and external CCD photometry of over 13,000 galaxies from the Las Campanas Deep Redshift Survey shows that the rms error in the APM plate zero points lies in the range 0.04-0.05 magnitudes, in agreement with our previous estimates. We estimate the effects on $w$ of atmospheric extinction and obscuration by dust in our Galaxy and conclude that these are negligible. We use our best estimates of the systematic errors in the survey to calculate corrected estimates of $w$. Deep redshift surveys are used to determine the selection function of the APM Galaxy Survey, and this is applied in Limber's equation to compute how $w$ scales as a function of limiting magnitude. Our estimates of $w$ are in excellent agreement with the scaling relation, providing further evidence that systematic errors in the APM survey are small. We explicitly remove large-scale structure by applying filters to the APM galaxy maps and conclude that there is still strong evidence for more clustering at large scales than predicted by the standard scale-invariant cold dark matter (CDM) model. We compare the APM $w$ and the three dimensional power spectrum derived by inverting $w$, with the predictions of scale-invariant CDM models. We show that the observations require $Gamma=Omega_0 h$ in the range 0.2-0.3 and are incompatible with the value $Gamma=0.5$ of the standard CDM model.

astro-ph↗

arXiv1995

The environments of z<0.3 QSOs

We have carried out an investigation of the galaxy environments of low redshift ($z<0.3$) QSOs by cross-correlating the positions on the sky of X-ray-selected QSOs/AGN identified in the {\it Einstein} Medium Sensitivity Survey (EMSS) with those of $B_J<20.5$ galaxies in the APM galaxy catalogues. At $<5\,$arcmin, we find a significant ($5σ$) galaxy excess around $z<0.3$ QSOs. The amplitude of the low redshift ($z<0.3$) QSO-galaxy angular cross-correlation function is identical to that of the APM galaxy angular correlation function, implying that these (predominantly radio-quiet) QSOs inhabit environments similar to those of normal galaxies. No significant galaxy excess was found around a `control' sample of $z>0.3$ QSOs. Coupled with previous observations, these results imply that the environment of radio-quiet QSOs undergoes little evolution over a wide range in redshift ($0 less than z less than 1.5$). This is in marked contrast to the rapid increase in the richness of the environments associated with radio-loud QSOs over the same redshift range. The similarity between QSO-galaxy clustering and galaxy-galaxy clustering also suggests that QSOs are unbiased with respect to galaxies and make useful tracers of large-scale structure in the Universe.

astro-ph↗

arXiv1995

The environments of z<0.3 QSOs

Based on a cross-correlation analysis of X-ray selected QSOs with faint (B<20.5) galaxies, we find a 5sigma galaxy excess around low-redshift (z<0.3) QSOs with an amplitude identical to that of the galaxy angular correlation function. The similarity between QSO-galaxy clustering and galaxy-galaxy clustering suggests that QSOs are unbiased with respect to galaxies and make useful tracers of large-scale structure in the Universe.

astro-ph↗

arXiv1994

The Stromlo-APM Redshift Survey II. Variation of Galaxy Clustering with Morphology and Luminosity

We present clustering measurements for samples of galaxies selected by morphological type and luminosity from the recently completed Stromlo-APM Redshift Survey. Early type galaxies are clustered more strongly by a factor 3.5--5.5, than late type galaxies. Low-luminosity galaxies are clustered more weakly by a factor of $\sim 2$ than $L^*$ and brighter galaxies on scales $\simgt 1\hMpc$. Also the slope of the correlation function is steeper for low-luminosity galaxies, so that the amplitude is a factor 4 lower at $ 10 \hMpc$. No difference, however, is seen between the clustering of $L^*$ and more luminous galaxies, an observation which may be hard to reconcile with some theories of biased galaxy formation. Both redshift-space and real-space clustering estimates show a similar dependence on luminosity.

astro-ph↗

arXiv1994

Deep Imaging of the Double Quasar 0957+561: New Constraints on H_0

We present new results from extremely deep, high-resolution images of the field around the double quasar QSO 0957+561. A possible gravitational arc system near the double quasar has recently been reported, which, if real, would set strong constraints on determinations of the Hubble constant from the time delay in the double quasar. We find that both the morphology and the colors of the claimed arc systems suggest that they are chance alignments of three and two different objects, and not gravitationally lensed arcs. Hence, the constraints on $H_0$-determinations from the arcs are not valid. Also, a small group of galaxies at $z=0.5$ near the line-of-sight which was required to have a very large mass in the physically interesting arc models, is most likely insignificant. From our deep images we are able to use weak lensing of faint background galaxies in the field to map the gravitational potential in the main cluster. This sets new constraints on determinations of $H_0$. We find that the Hubble constant is constrained to be less than 70km/(s Mpc), if the time delay between the two images of the QSO is equal to or larger than 1.1 years.

astro-ph↗

arXiv1994

The Two-Point Correlation Function of Rich Clusters of Galaxies: Results from an Extended APM Cluster Redshift Survey

We present new estimates of the spatial two-point correlation function of rich clusters of galaxies selected from the APM Galaxy Survey. We have measured redshifts for a sample of $364$ clusters out to a depth of $\sim 450\hmpc$. The clusters have a mean space density of $\bar{n} = 3.4\times 10^{-5}\hmpccc$. The two-point correlation function, $ξ_{cc}$, for this sample is equal to unity at a pair-separation of $r_0 = 14.3\pm1.75\hmpc$ (2$σ$ errors), consistent with our earlier results from a smaller sample. The new observations provide an accurate determination of the shape of $ξ_{cc}$ to pair-separations of about $50\hmpc$. Our results show that $ξ_{cc}$ has a higher amplitude than expected according to the standard $Ω=1$ cold dark matter (CDM) model on spatial scales $2\simlt s \simlt 50\hmpc$, but are in good agreement with scale-invariant fluctuations in either a low density CDM model or a critical density universe made up of a mixture of hot and cold dark matter. Our results provide strong constraints on so called `co-operative' models of galaxy formation in which the galaxy formation process introduces large-scale structure in the galaxy distribution.

astro-ph↗

arXiv1993

The Apm Galaxy Survey IV: Redshifts of Rich Clusters of Galaxies

We present redshifts for a sample of 229 clusters selected from the APM Galaxy Survey, 189 of which are new redshift determinations. Non-cluster galaxy redshifts have been rejected from this sample using a likelihood ratio test based on the projected and apparent magnitude distributions of the cluster fields. We test this technique using cluster fields in which redshifts have been measured for more than 10 galaxies. Our redshift sample is nearly complete and has been used in previous papers to study the three dimensional distribution of rich clusters of galaxies. 157 of the clusters in our sample are listed in the Abell catalogue or supplement, and the remainder are new cluster identifications.

astro-ph↗