arXiv subjects

Ofer Lahav

Publications and source records attributed to Ofer Lahav.

At least 199 records · Page 11Linked to original sources

arXiv2002

Could Dark Energy be Measured from Redshift Surveys ?

We review the ability of redshift surveys to provide constraints on the Dark Energy content of the Universe. The matter power spectrum and dynamics at the present epoch are nearly `blind' to Dark Energy, but combined with the CMB they can provide a constraint on the Equation of State parameter w. A representative result from the 2dF galaxy redshift survey combined with the CMB is w <-0.52 (95 % CL; with a prior of w >= -1), consistent with Einstein's Cosmological Constant model (w=-1). More complicated forms of Quintessence (e.g. epoch-dependent w or w<-1) are not yet ruled out. At higher redshifts, the abundance of galaxies and clusters of galaxies, variants of the Alcock-Paczynski curvature test and cross correlation of the CMB with radio sources look potentially promising, but they suffer from degeneracy with other parameters such as the matter density and galaxy biasing.

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arXiv2002

`Hyper Parameters' Approach to Joint Estimation: Applications to Cepheid-Calibrated Distances and X-Ray Clusters

We use a generalised procedure for the combined likelihood analysis of different cosmological probes, the `Hyper-Parameters' method, that allows freedom in the relative weights of the raw measurements. We perform a joint analysis of the cepheid-calibrated data from the Hubble Space Telescope Key Project and the baryon mass fraction in clusters to constrain the total matter density of the universe, Omega_m, and the Hubble parameter, h. We compare the results obtained using Hyper-Parameters method with the estimates from standard chi^2 analysis. We assume that the universe is spatially flat, with a cosmological constant. We adopt the Big-Bang nucleosynthesis constraint for the baryon density, assuming the uncertainty is Gaussian distributed. Using this and the cluster baryon fraction data, we find that the matter density and the Hubble constant are correlated, Omega_m h^0.5~0.25, with preference for a very high h. To break the degeneracy, we add in the cepheid-calibrated data and find the best fit values (Omega_m, h) = (0.26 (+0.06,-0.06),0.72 (+0.04,-0.02)) (68 per cent confindence limits) using the Hyper-Parameters approach. We use the derived Hyper-Parameters to `grade' the 6 different data sets we analyse. Although our analysis is free of assumptions about the power spectrum of fluctuations, our results are in agreement with the Lambda-Cold Dark Matter `concordance' parameters derived from the Cosmic Microwave Background anisotropies combined with Supernovae Ia, redshift surveys and other probes.

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arXiv2002

Estimating photometric redshifts with artificial neural networks

A new approach to estimating photometric redshifts - using Artificial Neural Networks (ANNs) - is investigated. Unlike the standard template-fitting photometric redshift technique, a large spectroscopically-identified training set is required but, where one is available, ANNs produce photometric redshift accuracies at least as good as and often better than the template-fitting method. The Bayesian priors on the underlying redshift distribution are automatically taken into account. Furthermore, inputs other than galaxy colours - such as morphology, angular size and surface brightness - may be easily incorporated, and their utility assessed. Different ANN architectures are tested on a semi-analytic model galaxy catalogue and the results are compared with the template-fitting method. Finally the method is tested on a sample of ~ 20000 galaxies from the Sloan Digital Sky Survey. The r.m.s. redshift error in the range z < 0.35 is ~ 0.021.

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arXiv2002

Parameter constraints for flat cosmologies from CMB and 2dFGRS power spectra

We constrain flat cosmological models with a joint likelihood analysis of a new compilation of data from the cosmic microwave background (CMB) and from the 2dF Galaxy Redshift Survey (2dFGRS). Fitting the CMB alone yields a known degeneracy between the Hubble constant h and the matter density Omega_m, which arises mainly from preserving the location of the peaks in the angular power spectrum. This `horizon-angle degeneracy' is considered in some detail and shown to follow a simple relation Omega_m h^{3.4} = constant. Adding the 2dFGRS power spectrum constrains Omega_m h and breaks the degeneracy. If tensor anisotropies are assumed to be negligible, we obtain values for the Hubble constant h=0.665 +/- 0.047, the matter density Omega_m=0.313 +/- 0.055, and the physical CDM and baryon densities Omega_c h^2 = 0.115 +/- 0.009, Omega_b h^2 = 0.022 +/- 0.002 (standard rms errors). Including a possible tensor component causes very little change to these figures; we set a upper limit to the tensor-to-scalar ratio of r<0.7 at 95% confidence. We then show how these data can be used to constrain the equation of state of the vacuum, and find w<-0.52 at 95% confidence. The preferred cosmological model is thus very well specified, and we discuss the precision with which future CMB data can be predicted, given the model assumptions. The 2dFGRS power-spectrum data and covariance matrix, and the CMB data compilation used here, are available from http://www.roe.ac.uk/~wjp/

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arXiv2002

Moriond Conference Summary: The Cosmological Model(s)

The XXXVIIth Rencontres de Moriond on "The Cosmological Model" is briefly summarized. Almost none of the current observations argues against the popular Cold Dark Matter + Lambda concordance model. However, it remains to be tested how astrophysical uncertainties involved in the interpretation of the different data sets affect the derived cosmological parameters. Independent tests are still required to establish if the Cold Dark Matter and Dark Energy components are `real', or just `epicycles' that happen to fit the current data sets well.

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arXiv2002

Spectroscopic detection of quasars in the 2dF Galaxy Redshift Survey

The 100,000 spectra from the 2 degree Field Galaxy Redshift Survey (2dFGRS) in the 100k Public Data Release represent the largest single compilation of galaxy spectra available. By virtue of its sheer size and the properties of the photometric catalogue that defines the sample, the 2dFGRS is expected to contain a number of potentially interesting objects other than galaxies. A search of the spectra in the 100k Data Release results in a census of 55 candidate high-redshift (z > 0.3) quasars. One additional 2dFGRS spectrum of a low-redshift galaxy shows an apparent anomalous broad emission feature perhaps indicating the presence of a gravitationally lensed quasar. These objects have been identified primarily using two automated routines that we have developed specifically for this task, one of which uses a matched filter and the other a wavelet transform. A number of the quasar images possess complicated morphologies, suggesting the presence of either diffuse foreground objects along the line-of-sight or very nearby point sources. The quasar catalogue will form a target list for future absorption and lensing studies, as well as providing an assessment of the loss of quasars with non-stellar images from the companion 2dF QSO Redshift Survey.

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arXiv2002

The 2dF Galaxy Redshift Survey: Cosmological Parameters and Galaxy Biasing

The 2dF Galaxy Redshift Survey (2dFGRS) has already measured over 220,000 redshifts of nearby (z ~0.1) galaxies. It allows us to estimate fundamental cosmological parameters and to subdivide the survey into subsets according to the galaxy intrinsic properties. The large-scale structure analysis of the survey has already yielded several significant results: (i) the shape of the power spectrum of fluctuations is consistent with the Lambda - Cold Dark Matter concordance model; (ii) from joint analysis of the 2dFGRS data with the Cosmic Microwave Background anisotropies the linear-theory rms mass fluctuations is sigma_8m ~ 0.73, lower than the COBE-alone normalization and previous estimates from cluster abundance and cosmic shear; (iii) The biasing parameter of bright galaxies on scales > 10 Mpc/h is nearly unity; and (iv) on scales smaller than < 10 Mpc/h red galaxies are more strongly clustered than blue galaxies.

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arXiv2002

The 2dF Galaxy Redshift Survey: The amplitudes of fluctuations in the 2dFGRS and the CMB, and implications for galaxy biasing

We compare the amplitudes of fluctuations probed by the 2dF Galaxy Redshift Survey and by the latest measurements of the Cosmic Microwave Background anisotropies. By combining the 2dFGRS and CMB data we find the linear-theory rms mass fluctuations in 8 Mpc/h spheres to be sigma_8 = 0.73 +-0.05 (after marginalization over the matter density parameter Omega_m and three other free parameters). This normalization is lower than the COBE normalization and previous estimates from cluster abundance, but it is in agreement with some revised cluster abundance determinations. We also estimate the scale-independent bias parameter of present-epoch L_s = 1.9L_* APM-selected galaxies to be b(L_s,z=0) = 1.10 +- 0.08 on comoving scales of 0.02 < k < 0.15 h/Mpc. If luminosity segregation operates on these scales, L_* galaxies would be almost un-biased, b(L_*,z=0) = 0.96. These results are derived by assuming a flat Lambda-CDM Universe, and by marginalizing over other free parameters and fixing the spectral index n=1 and the optical depth due to reionization tau=0. We also study the best fit pair (Omega_m,b), and the robustness of the results to varying n and tau. Various modelling corrections can each change the resulting b by 5-15 per cent. The results are compared with other independent measurements from the 2dFGRS itself, and from the SDSS, cluster abundance and cosmic shear.

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arXiv2002

The 2dF Galaxy Redshift Survey: The environmental dependence of galaxy star formation rates near clusters

We have measured the equivalent width of the H-alpha emission line for 11006 galaxies brighter than M_b=-19 (LCDM) at 0.05<z<0.1 in the 2dF Galaxy Redshift Survey (2dF), in the fields of seventeen known galaxy clusters. The limited redshift range ensures that our results are insensitive to aperture bias, and to residuals from night sky emission lines. We use these measurements to trace mustar, the star formation rate normalized to Lstar, as a function of distance from the cluster centre, and local projected galaxy density. We find that the distribution of mustar steadily skews toward larger values with increasing distance from the cluster centre, converging to the field distribution at distances greater than ~3 times the virial radius. A correlation between star formation rate and local projected density is also found, which is independent of cluster velocity dispersion and disappears at projected densities below ~1 galaxy (brighter than M_b=-19) per Mpc^2. This characteristic scale corresponds approximately to the mean density at the cluster virial radius. The same correlation holds for galaxies more than two virial radii from the cluster centre. We conclude that environmental influences on galaxy properties are not restricted to cluster cores, but are effective in all groups where the density exceeds this critical value. The present day abundance of such systems, and the strong evolution of this abundance, makes it likely that hierarchical growth of structure plays a significant role in decreasing the global average star formation rate. Finally, the low star formation rates well beyond the virialised cluster rule out severe physical processes, such as ram pressure stripping of disk gas, as being completely responsible for the variations in galaxy properties with environment.

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arXiv2002

The 2dF Galaxy Redshift Survey: The Number and Luminosity Density of Galaxies

We present the bivariate brightness distribution (BBD) for the 2dF Galaxy Redshift Survey (2dFGRS) based on a preliminary subsample of 45,000 galaxies. The BBD is an extension of the galaxy luminosity function incorporating surface brightness information. It allows the measurement of the local luminosity density, j_B, and the galaxy luminosity and surface brightness distributions while accounting for surface brightness selection biases. The recovered 2dFGRS BBD shows a strong surface brightness-luminosity relation (M_B~2.4μ_e). The luminosity-density is dominated by normal galaxies and the luminosity-density peak lies away from the selection boundaries implying that the 2dFGRS is complete and that luminous low surface brightness galaxies are rare. The final value we derive for the local luminosity-density, inclusive of surface brightness corrections, is: j_B=2.49+/-0.20x10^8 h L_solar Mpc^-3. Representative Schechter function parameters are: M*=-19.75+/-0.05, phi*=2.02+/-0.02x10^-2 and alpha=-1.09+/-0.03. Extending the conventional methodology to incorporate surface brightness selection effects has resulted in an increase in the luminosity-density of 37%.

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arXiv2001

Observational Tests of FRW World Models

Observational tests for the Cosmological Principle are reviewed. Assuming the FRW metric we then summarize estimates of cosmological parameters from various data sets, in particular the Cosmic Microwave Background and the 2dF galaxy redshift survey. These and other analyses suggest a best-fit Lambda-Cold Dark Matter model with Omega_m = 1 - Omega_lambda = 0.3 and H_0 = 70 km/sec/Mpc. It is remarkable that different measurements converge to this `concordance model', although it remains to be seen if the two main components of this model, the dark matter and the dark energy, are real entities or just `epicycles'. We point out some open questions related to this fashionable model.

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arXiv2001

The 2dF Galaxy Redshift Survey: Near Infrared Galaxy Luminosity Functions

We combine the 2MASS extended source catalogue and the 2dFGRS to produce an IR selected galaxy catalogue with 17,173 measured redshifts. We use this extensive dataset to estimate the J and K-band galaxy luminosity functions. The LFs are fairly well fit by Schechter functions with J: M*-5log h= -22.36+/-0.02, alpha= -0.93+/-0.04, Phi=0.0104+/-0.0016 h^3/Mpc^3 and K: M*-5log h= -23.44+/-0.03, alpha=-0.96+/-0.05, Phi=0.0108+/-0.0016 h^3/Mpc^3 (2MASS Kron magnitudes). These parameters assume a cosmological model with Omega=0.3 and Lambda=0.7. With datasets of this size, systematic rather than random errors are the dominant source of uncertainty in the determination of the LF. We carry out a careful investigation of possible systematic effects in our data. The surface brightness distribution of the sample shows no evidence that significant numbers of low surface brightness or compact galaxies are missed by the survey. We estimate the present-day distributions of B-K and J-K colours as a function of absolute magnitude and use models of the galaxy stellar populations, constrained by the observed optical and infrared colours, to infer the galaxy stellar mass function. Integrated over all galaxy masses, this yields a total mass fraction in stars (in units of the critical mass density) of Omega_*.h= (1.6+/-0.24)/10^3 for a Kennicutt IMF and Omega_*.h= (2.9+/-0.43)/10^3 for a Salpeter IMF. These values agree with those inferred from observational estimates of the star formation history of the universe provided that dust extinction corrections are modest.

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arXiv2001

Galaxy Spectral Types and Their Applications in the 2dFGRS

The 2dF Galaxy Redshift Survey (2dFGRS) has already measured over 200,000 redshifts of nearby (median redshift z~0.1) galaxies. This is the single largest set of galaxy spectra ever collected. It allows us to subdivide the survey into subsets according to the galaxy intrinsic properties. We outline here a method (based on Principal Component Analysis) for spectrally classifying these galaxies in a robust and independent manner. In so doing we develop a continuous measure of spectral type, eta, which reflects the actual distribution of spectra in the observed galaxy population. We demonstrate the usefulness of this classification by estimating luminosity functions and clustering per spectral type.

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arXiv2001

Evidence for a non-zero Lambda and a low matter density from a combined analysis of the 2dF Galaxy Redshift Survey and Cosmic Microwave Background Anisotropies

We perform a joint likelihood analysis of the power spectra of the 2dF Galaxy Redshift Survey (2dFGRS) and the cosmic microwave background (CMB) anisotropies under the assumptions that the initial fluctuations were adiabatic, Gaussian and well described by power laws with scalar and tensor indices of n_s and n_t. On its own, the 2dFGRS sets tight limits on the parameter combination Omega_m h, but relatively weak limits on the fraction of the cosmic matter density in baryons Omega_b/Omega_m. The CMB anisotropy data alone set poor constraints on the cosmological constant and Hubble constant because of a `geometrical degeneracy' among parameters. Furthermore, if tensor modes are allowed, the CMB data allow a wide range of values for the physical densities in baryons and cold dark matter. Combining the CMB and 2dFGRS data sets helps to break both the geometrical and tensor mode degeneracies. The values of the parameters derived here are consistent with the predictions of the simplest models of inflation, with the baryon density derived from primordial nucleosynthesis and with direct measurements of the Hubble parameter. In particular, we find strong evidence for a positive cosmological constant with a pm 2sigma range of 0.65 < Omega_Lambda < 0.85, completely independently of constraints on Omega_Λderived from Type Ia supernovae.

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arXiv2001

Do globular clusters time the Universe?

Estimating the age of the Universe is an old problem. Rapid progress in observational cosmology in recent years has led to more accurate values of the fundamental parameters. The current most popular model is a flat Universe, with about 30% of the critical density in the form of matter (baryonic and non-baryonic) and 70% in the form of `dark energy'. These two densities, together with the Hubble constant (estimated to be about 70 km/s/Mpc), constrain the age of the Universe to be approximately 13 Gyr. This expansion age is uncomfortably close to the age of the oldest globular clusters (approximately 12.5 Gyr), in particular if they formed relatively recently. We review here proposed models of globular cluster formation and point out possible conflicts with cosmology.

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arXiv2001

The Age of the Universe from Joint Analysis of Cosmological Probes

Analyses of various cosmological probes, including the latest Cosmic Microwave Background anisotropies, the 2dF Galaxy Redshift Survey and Cepheid-calibrated distance indicators suggest that the age of expansion is $13 \pm 3$ Gyr. We discuss some statistical aspects of this estimation, and we also present results for joint analysis of the latest CMB and Cepheid data by utilizing `Hyper-Parameters'. The deduced age of expansion might be uncomfortably close to the age of the oldest Globular Clusters, in particular if they formed relatively recently.

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arXiv2001

Large Surveys in Cosmology: The Changing Sociology

Galaxy redshift surveys and Cosmic Microwave Background experiments are undertaken with larger and larger teams, in a fashion reminiscent of particle physics experiments and the human genome projects. We discuss the role of young researchers, the issue of multiple authorship, and ways to communicate effectively in teams of tens to hundreds of collaborators.

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arXiv2001

New Cosmological Data and the `Best-Fit' FRW Universe

Observational tests for the homogeneity of the Universe on large scales are reviewed. Assuming the Cosmological Principle we then estimate cosmological parameters by joint analysis of the Cosmic Microwave Background, Supernovae Ia, peculiar velocities, cluster abundance and redshift surveys. Our results are consistent with results obtained by other groups, suggesting a best-fit FRW Universe with $\omegam = 1 -λ\approx 0.3$ and $H_0 \approx 75$ km/sec/Mpc. We point out some potential problems with this currently popular model.

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