Showing papers by "Robert C. Nichol published in 2006"

Cosmological constraints from the SDSS luminous red galaxies

Abstract: We measure the large-scale real-space power spectrum P(k) using luminous red galaxies (LRGs) in the Sloan Digital Sky Survey (SDSS) and use this measurement to sharpen constraints on cosmological parameters from the Wilkinson Microwave Anisotropy Probe (WMAP). We employ a matrix-based power spectrum estimation method using Pseudo-Karhunen-Loeve eigenmodes, producing uncorrelated minimum-variance measurements in 20 k-bands of both the clustering power and its anisotropy due to redshift-space distortions, with narrow and well-behaved window functions in the range 0.01h/Mpc 0.1h/Mpc and associated nonlinear complications, yet agree well with more aggressive published analyses where nonlinear modeling is crucial.

The fourth data release of the Sloan Digital Sky Survey

Abstract: This paper describes the Fourth Data Release of the Sloan Digital Sky Survey (SDSS), including all survey-quality data taken through 2004 June. The data release includes five-band photometric data for 180 million objects selected over 6670 deg2 and 673,280 spectra of galaxies, quasars, and stars selected from 4783 deg2 of those imaging data using the standard SDSS target selection algorithms. These numbers represent a roughly 27% increment over those of the Third Data Release; all the data from previous data releases are included in the present release. The Fourth Data Release also includes an additional 131,840 spectra of objects selected using a variety of alternative algorithms, to address scientific issues ranging from the kinematics of stars in the Milky Way thick disk to populations of faint galaxies and quasars.

Galaxy bimodality versus stellar mass and environment

Abstract: We analyse a z < 0.1 galaxy sample from the Sloan Digital Sky Survey focusing on the variation in the galaxy colour bimodality with stellar mass M and projected neighbour density Σ, and on measurements of the galaxy stellar mass functions. The characteristic mass increases with environmental density from about 10 10. 6 to 10 10.9 M ⊙ (Kroupa initial mass function, H 0 = 70) for Σ in the range 0.1-10 Mpc -2 . The galaxy population naturally divides into a red and blue sequence with the locus of the sequences in colour-mass and colour-concentration indices not varying strongly with environment. The fraction of galaxies on the red sequence is determined in bins of 0.2 in log Σ and log M (12 x 13 bins). The red fraction f r generally increases continuously in both Σ and M such that there is a unified relation: f, = F(Σ, M). Two simple functions are proposed which provide good fits to the data. These data are compared with analogous quantities in semi-analytical models based on the Millennium N-body simulation: the Bower et al. and Croton et al. models that incorporate active galactic nucleus feedback. Both models predict a strong dependence of the red fraction on stellar mass and environment that is qualitatively similar to the observations. However, a quantitative comparison shows that the Bower et al. model is a significantly better match; this appears to be due to the different treatment of feedback in central galaxies.

The Clustering of Luminous Red Galaxies in the Sloan Digital Sky Survey Imaging Data

Abstract: We present the 3D real space clustering power spectrum of a sample of \~600,000 luminous red galaxies (LRGs) measured by the Sloan Digital Sky Survey (SDSS), using photometric redshifts. This sample of galaxies ranges from redshift z=0.2 to 0.6 over 3,528 deg^2 of the sky, probing a volume of 1.5 (Gpc/h)^3, making it the largest volume ever used for galaxy clustering measurements. We measure the angular clustering power spectrum in eight redshift slices and combine these into a high precision 3D real space power spectrum from k=0.005 (h/Mpc) to k=1 (h/Mpc). We detect power on gigaparsec scales, beyond the turnover in the matter power spectrum, on scales significantly larger than those accessible to current spectroscopic redshift surveys. We also find evidence for baryonic oscillations, both in the power spectrum, as well as in fits to the baryon density, at a 2.5 sigma confidence level. The statistical power of these data to constrain cosmology is ~1.7 times better than previous clustering analyses. Varying the matter density and baryon fraction, we find \Omega_M = 0.30 \pm 0.03, and \Omega_b/\Omega_M = 0.18 \pm 0.04, The detection of baryonic oscillations also allows us to measure the comoving distance to z=0.5; we find a best fit distance of 1.73 \pm 0.12 Gpc, corresponding to a 6.5% error on the distance. These results demonstrate the ability to make precise clustering measurements with photometric surveys (abridged).

The shape of the SDSS DR5 galaxy power spectrum

Abstract: We present a Fourier analysis of the clustering of galaxies in the combined Main galaxy and Luminous Red Galaxy (LRG) Sloan Digital Sky Survey (SDSS) Data Release 5 (DR5) sample. The aim of our analysis is to consider how well we can measure the cosmological matter density using the signature of the horizon at matter-radiation equality embedded in the large-scale power spectrum. The new data constrains the power spectrum on scales 100--600h^-1Mpc with significantly higher precision than previous analyses of just the SDSS Main galaxies, due to our larger sample and the inclusion of the LRGs. This improvement means that we can now reveal a discrepancy between the shape of the measured power and linear CDM models on scales 0.01

The 2dF-SDSS LRG and QSO Survey: QSO clustering and the L-z degeneracy

Abstract: We combine the QSO samples from the 2dF QSO Redshift Survey (2QZ) and the 2dF-SDSS LRG and QSO Survey (2SLAQ) in order to investigate the clustering of z~1.4 QSOs and measure the correlation function. The clustering signal in z-space, projected along the sky direction, is similar to that previously obtained from 2QZ alone. By fitting the z-space correlation function and lifting the degeneracy between beta and Omega_m_0 by using linear theory predictions, we obtain beta(z=1.4) = 0.60+-0.12 and Omega_m_0=0.25+-0.08, implying a value for the QSO bias, b(z=1.4)=1.5+-0.2. We further find that QSO clustering does not depend strongly on luminosity at fixed redshift. This result is inconsistent with the expectation of simple `high peaks' biasing models where more luminous, rare QSOs are assumed to inhabit higher mass haloes. The data are more consistent with models which predict that QSOs of different luminosities reside in haloes of similar mass. We find that halo mass does not evolve strongly with redshift nor depend on QSO luminosity. We finally investigate how black hole mass correlates with luminosity and redshift and ascertain the relation between Eddington efficiency and black hole mass. Our results suggest that QSOs of different luminosities may contain black holes of similar mass.

First measurement of the clustering evolution of photometrically classified quasars

Abstract: We present new measurements of the quasar angular autocorrelation function from a sample of ~80,000 photometrically classified quasars taken from the First Data Release of the Sloan Digital Sky Survey. We find a best-fit model of ω(θ) = (0.066)θ-(0.98±0.15) for the angular correlation function, consistent with estimates of the slope from spectroscopic quasar surveys. We show that only models with little or no evolution in the clustering of quasars in comoving coordinates since a median redshift of z ~ 1.4 can recover a scale length consistent with local galaxies and active galactic nuclei (AGNs). A model with little evolution of quasar clustering in comoving coordinates is best explained in the current cosmological paradigm by rapid evolution in quasar bias. We show that quasar biasing must have changed from bQ ~ 3 at a (photometric) redshift of phot = 2.2 to bQ ~ 1.2-1.3 by phot = 0.75. Such a rapid increase with redshift in biasing implies that quasars at z ~ 2 cannot be the progenitors of modern L* objects; rather they must now reside in dense environments, such as clusters. Similarly, the duration of the UVX (ultraviolet-excess) quasar phase must be short enough to explain why local UVX quasars reside in essentially unbiased structures. Our estimates of bQ are in good agreement with recent spectroscopic results (Croom et al. 2005), which demonstrate that the implied evolution in bQ is consistent with quasars inhabiting halos of similar mass at every redshift. Treating quasar clustering as a bivariate function of both redshift and luminosity, we find no evidence for luminosity dependence in quasar clustering, and that redshift evolution thus affects quasar clustering more than changes in quasars' luminosity. Our results are robust against a range of systematic uncertainties. We provide a new method for quantifying stellar contamination in photometrically classified quasar catalogs via the correlation function.

Evolution and Environment of Early-Type Galaxies

Abstract: Chemical abundance indicators are studied using composite spectra, which we provide in tabular form. Tables of line strengths measured from these spectra and parameters derived from these line strengths are also provided. From these we find that at fixed luminosity, early-type galaxies in low-density environments are slightly bluer, with stronger O II emission and stronger Hδ and Hγ Balmer absorption lines, indicative of star formation in the not very distant past. These galaxies also tend to have systematically weaker D4000 indices. The Lick indices and α-element abundance indicators correlate weakly but significantly with environment. For example, at fixed velocity dispersion, Mg is weaker in early-type galaxies in low-density environments by 30% of the rms scatter across the full sample, whereas most Fe indicators show no significant environmental dependence. The galaxies in our sample span a redshift range that corresponds to look-back times of ~1 Gyr. We see clear evidence for evolution of line-index strengths over this time. Since the low-redshift population is almost certainly a passively aged version of the more distant population, age is likely the main driver for any observed evolution. We use the observed redshift evolution as a model-independent clock to identify indicators that are more sensitive to age than to other effects such as metallicity. In principle, for a passively evolving population, comparison of the trends with redshift and environment constrain how strongly the luminosity-weighted ages and metallicities depend on environment. We develop a method for doing this that does not depend on the details of stellar population synthesis models. Our analysis suggests that the galaxies that populate the densest regions in our sample are older by ~1 Gyr than objects of the same luminosity in the least dense regions, and that metallicity differences are negligible. We also use single-burst stellar population synthesis models, which allow for nonsolar α-element abundance ratios, to interpret our data. The combination of Hβ, Mg b, and Fe lines suggests that age, metallicity, and α-enhancement all increase with velocity dispersion. The objects at lower redshifts are older but have the same metallicities and α-enhancements as their counterparts of the same σ at higher redshifts, as expected if the low-redshift sample is a passively aged version of the sample at higher redshifts. In addition, objects in dense environments are less than 1 Gyr older and α-enhanced by ~0.02 relative to their counterparts of the same velocity dispersion in less dense regions, but the metallicities show no dependence on environment. This suggests that in dense regions, the stars in early-type galaxies formed at slightly earlier times and on a slightly shorter timescale than in less dense regions. Using HγF instead of Hβ leads to slightly younger ages but the same qualitative differences between environments. In particular, we find no evidence that objects in low-density regions are more metal-rich.

The XMM Cluster Survey: A Massive Galaxy Cluster at z = 1.45

Abstract: We report the discovery of XMMXCS J2215.9-1738, a massive galaxy cluster at z=1.45, which was found in the XMM Cluster Survey. The cluster candidate was initially identified as an extended X-ray source in archival XMM data. Optical spectroscopy shows that six galaxies within a ~60" diameter region lie at z=1.45+/-0.01. Model fits to the X-ray spectra of the extended emission yield kT=7.4+2.7-1.8 keV (90% confidence); if there is an undetected central X-ray point source, then kT=6.5+2.6-1.8 keV. The bolometric X-ray luminosity is LX=4.4+0.8-0.6C 1044 ergs s-1 over a 2 Mpc radial region. The measured TX, which is the highest for any known cluster at z>1, suggests that this cluster is relatively massive for such a high redshift. The redshift of XMMXCS J2215.9-1738 is the highest currently known for a spectroscopically confirmed cluster of galaxies.

The 2dF-SDSS LRG and QSO Survey: The LRG 2-Point Correlation Function and Redshift-Space Distortions

Abstract: We present a clustering analysis of Luminous Red Galaxies (LRGs) using nearly 9 000 objects from the final catalogue of the 2dF-SDSS LRG And QSO (2SLAQ) Survey. We measure the redshift-space two-point correlation function, xi(s), at the mean LRG redshift of z=0.55. A single power-law fits the deprojected correlation function, xi(r), with a correlation length of r_0=7.45+-0.35 Mpc and a power-law slope of gamma=1.72+-0.06 in the 0.4

The 2dF-SDSS LRG and QSO (2SLAQ) luminous red galaxy survey

Abstract: We present a spectroscopic survey of almost 15 000 candidate intermediate-redshift luminous red galaxies (LRGs) brighter than i = 19.8, observed with 2dF on the Anglo-Australian Telescope. The targets were selected photometrically from the Sloan Digital Sky Survey (SDSS) and lie along two narrow equatorial strips covering 180 deg 2 . Reliable redshifts were obtained for 92 per cent of the targets and the selection is very efficient: over 90 per cent have 0.45 < z < 0.8. More than 80 per cent of the ∼11 000 red galaxies have pure absorption-line spectra consistent with a passively evolving old stellar population. The redshift, photometric and spatial distributions of the LRGs are described. The 2SLAQ data will be released publicly from mid-2006, providing a powerful resource for observational cosmology and the study of galaxy evolution.

High redshift detection of the integrated Sachs-Wolfe effect

Abstract: We present evidence of a large angle correlation between the cosmic microwave background measured by WMAP and a catalog of photometrically detected quasars from the SDSS. The observed cross correlation is $0.30\ifmmode\pm\else\textpm\fi{}0.14\text{ }\text{ }\ensuremath{\mu}\mathrm{K}$ at zero lag, with a shape consistent with that expected for correlations arising from the integrated Sachs-Wolfe effect. The photometric redshifts of the quasars are centered at $z\ensuremath{\sim}1.5$, making this the deepest survey in which such a correlation has been observed. Assuming this correlation is due to the ISW effect, this constitutes the earliest evidence yet for dark energy and it can be used to constrain exotic dark energy models.

The XMM Cluster Survey: A Massive Galaxy Cluster at z=1.45

Abstract: We report the discovery of XMMXCS J2215.9-1738, a massive galaxy cluster at z =1.45, which was found in the XMM Cluster Survey. The cluster candidate was initially identified as an extended X-ray source in archival XMM data. Optical spectroscopy shows that 6 galaxies within a 60 arcsec diameter region lie at z = 1.45 +/- 0.01. Model fits to the X-ray spectra of the extended emission yield kT = 7.4 (+2.7,-1.8) keV (90 % confidence); if there is an undetected central X-ray point source then kT = 6.5 (+2.6,-1.8) keV. The bolometric X-ray luminosity is Lx = 4.4 (+0.8,-0.6) x 10^44 ergs/s over a 2 Mpc radial region. The measured Tx, which is the highest known for a cluster at z > 1, suggests that this cluster is relatively massive for such a high redshift. The redshift of XMMXCS J2215.9-1738 is the highest currently known for a spectroscopically-confirmed cluster of galaxies.

The 2df SDSS LRG and QSO survey: evolution of the luminosity function of luminous red galaxies to z= 0.6

Abstract: We present new measurements of the luminosity function (LF) of luminous red galaxies (LRGs) from the Sloan Digital Sky Survey (SDSS) and the 2dF SDSS LRG and Quasar (2SLAQ) survey. We have carefully quantified, and corrected for, uncertainties in the K and evolutionary corrections, differences in the colour selection methods, and the effects of photometric errors, thus ensuring we are studying the same galaxy population in both surveys. Using a limited subset of 6326 SDSS LRGs (with 0.17 < z < 0.24) and 1725 2SLAQ LRGs (with 0.5 < z < 0.6), for which the matching colour selection is most reliable, we find no evidence for any additional evolution in the LRG LF, over this redshift range, beyond that expected from a simple passive evolution model. This lack of additional evolution is quantified using the comoving luminosity density of SDSS and 2SLAQ LRGs, brighter than M0.2r − 5 log h0.7 = −22.5, which are 2.51 ± 0.03 × 10−7 L Mpc−3 and 2.44 ± 0.15 × 10−7 L Mpc−3, respectively (<10 per cent uncertainty). We compare our LFs to the COMBO-17 data and find excellent agreement over the same redshift range. Together, these surveys show no evidence for additional evolution (beyond passive) in the LF of LRGs brighter than M0.2r − 5 log h0.7 = −21 (or brighter than L*). We test our SDSS and 2SLAQ LFs against a simple ‘dry merger’ model for the evolution of massive red galaxies and find that at least half of the LRGs at z 0.2 must already have been well assembled (with more than half their stellar mass) by z 0.6. This limit is barely consistent with recent results from semi-analytical models of galaxy evolution.

The 2dF-SDSS LRG and QSO (2SLAQ) Luminous Red Galaxy Survey

Abstract: We present a spectroscopic survey of almost 15,000 candidate intermediate-redshift Luminous Red Galaxies (LRGs) brighter than i=19.8, observed with 2dF on the Anglo-Australian Telescope. The targets were selected photometrically from the Sloan Digital Sky Survey (SDSS) and lie along two narrow equatorial strips covering 180 sq deg. Reliable redshifts were obtained for 92% of the targets and the selection is very efficient: over 90% have redshifts between 0.45 and 0.8. More than 80% of the ~11,000 red galaxies have pure absorption-line spectra consistent with a passively-evolving old stellar population. The redshift, photometric and spatial distributions of the LRGs are described. The 2SLAQ data will be released publicly from mid-2006, providing a powerful resource for observational cosmology and the study of galaxy evolution.

The 2dF-SDSS LRG and QSO survey: Evolution of the Luminosity Function of Luminous Red Galaxies to z=0.6

Abstract: We present new measurements of the luminosity function (LF) of Luminous Red Galaxies (LRGs) from the Sloan Digital Sky Survey (SDSS) and the 2dF-SDSS LRG and Quasar (2SLAQ) survey. We have carefully quantified, and corrected for, uncertainties in the K and evolutionary corrections, differences in the colour selection methods, and the effects of photometric errors, thus ensuring we are studying the same galaxy population in both surveys. Using a limited subset of 6326 SDSS LRGs (with 0.17

The Effect of Large-Scale Structure on the SDSS Galaxy Three-Point Correlation Function

Abstract: We present measurements of the normalized redshift-space three-point correlation function (3PCF) (Q(z)) of galaxies from the Sloan Digital Sky Survey (SDSS) main galaxy sample. These measurements were possible because of a fast new N-point correlation function algorithm (called npt) based on multiresolutional k-d trees. We have applied npt to both a volume-limited (36 738 galaxies with 0.05 = 10 h(-1)Mpc). If we exclude this supercluster, our observed Q(z) is in better agreement with that obtained from the 2-degree Field Galaxy Redshift Survey (2dFGRS) by other authors, thus demonstrating the sensitivity of these higher order correlation functions to large-scale structures in the Universe. This analysis highlights that the SDSS data sets used here are not 'fair samples' of the Universe for the estimation of higher order clustering statistics and larger volumes are required. We study the shape dependence of Q(z)(s, q, theta) as one expects this measurement to depend on scale if the large-scale structure in the Universe has grown via gravitational instability from Gaussian initial conditions. On small scales (s 10 h(-1)Mpc, we see considerable shape dependence in Q(z). However, larger samples are required to improve the statistical significance of these measurements on all scales.

Universal fitting formulae for baryon oscillation surveys

Abstract: The next generation of galaxy surveys will attempt to measure the baryon oscillations in the clustering power spectrum with high accuracy. These oscillations encode a preferred scale which may be used as a standard ruler to constrain cosmological parameters and dark energy models. In this paper we present simple analytical fitting formulae for the accuracy with which the preferred scale may be determined in the tangential and radial directions by future spectroscopic and photometric galaxy redshift surveys. We express these accuracies as a function of survey parameters such as the central redshift, volume, galaxy number density and (where applicable) photometric redshift error. These fitting formulae should greatly increase the efficiency of optimizing future surveys, which requires analysis of a potentially vast number of survey configurations and cosmological models. The formulae are calibrated using a grid of Monte Carlo simulations, which are analysed by dividing out the overall shape of the power spectrum before fitting a simple decaying sinusoid to the oscillations. The fitting formulae reproduce the simulation results with a fractional scatter of 7 per cent (10 per cent) in the tangential (radial) directions over a wide range of input parameters. We also indicate how sparse-sampling strategies may enhance the effective survey area if the sampling scale is much smaller than the projected baryon oscillation scale.

A search for the most massive galaxies: double trouble?

Abstract: We describe the results of a search for galaxies with large (≥350 km s-1) velocity dispersions. The largest systems we have found appear to be the extremes of the early-type galaxy population: compared to other galaxies with similar luminosities, they have the largest velocity dispersions and the smallest sizes. However, they are not distant outliers from the fundamental plane and mass-to-light scaling relations defined by the bulk of the early-type galaxy population. They may host the most massive black holes in the universe, and their abundance and properties can be used to constrain galaxy formation models. Clear outliers from the scaling relations tend to be objects in superposition (angular separations smaller than 1''), evidence for which comes sometimes from the spectra, sometimes from the images, and sometimes from both. The statistical properties of the superposed pairs, e.g., the distribution of pair separations and velocity dispersions, can be used to provide useful information about the expected distribution of image multiplicities, separations, and flux ratios due to gravitational lensing by multiple lenses, and may also constrain models of their interaction rates.

The 2dF-SDSS LRG and QSO Survey: the star formation histories of luminous red galaxies

Abstract: We present a detailed investigation into the recent star formation histories of 5697 luminous red galaxies (LRGs) based on the Hδ (4101 A), and [O II] (3727 A) lines and the D4000 index. LRGs are luminous (L > 3L ∗ ) galaxies which have been selected to have photometric properties consistent with an old, passively evolving stellar population. For this study, we utilize LRGs from the recently completed 2dF-SDSS LRG and QSO Survey (2SLAQ). Equivalent widths of the Hδ and [O II] lines are measured and used to define three spectral types, those with only strong Hδ absorption (k+a), those with strong [O II] in emission (em) and those with both (em+a). All other LRGs are considered to have passive star formation histories. The vast majority of LRGs are found to be passive (∼80 per cent); however, significant numbers of k+a (2.7 per cent), em+a (1.2 per cent) and em LRGs (8.6 per cent) are identified. An investigation into the redshift dependence of the fractions is also performed. A sample of SDSS MAIN galaxies with colours and luminosities consistent with the 2SLAQ LRGs is selected to provide a low-redshift comparison. While the em and em+a fractions are consistent with the low-redshift SDSS sample, the fraction of k+a LRGs is found to increase significantly with redshift. This result is interpreted as an indication of an increasing amount of recent star formation activity in LRGs with redshift. By considering the expected lifetime of the k+a phase, the number of LRGs which will undergo a k+a phase can be estimated. A crude comparison of this estimate with the predictions from semi-analytic models of galaxy formation shows that the predicted

Chandra observations of sdss j1004+4112: constraints on the lensing cluster and anomalous x-ray flux ratios of the quadruply imaged quasar

Abstract: We present results from Chandra observations of SDSS J1004+4112, a strongly lensed quasar system with a maximum image separation of 15''. All four bright images of the quasar, as well as resolved X-ray emission originating from the lensing cluster, are clearly detected. The emission from the lensing cluster extends out to approximately 15. We measure the bolometric X-ray luminosity and temperature of the lensing cluster to be 4.7 × 1044 ergs s-1 and 6.4 keV, consistent with the luminosity-temperature relation for distant clusters. The mass estimated from the X-ray observation shows excellent agreement with the mass derived from gravitational lensing. The X-ray flux ratios of the quasar images differ markedly from the optical flux ratios, and the combined X-ray spectrum of the images possesses an unusually strong Fe Kα emission line, both of which are indicative of microlensing.

Examining the effect of the map‐making algorithm on observed power asymmetry in WMAP data

Abstract: We analyze first-year data of WMAP to determine the significance of asymmetry in summed power between arbitrarily defined opposite hemispheres. We perform this analysis on maps that we create ourselves from the time-ordered data, using software developed independently of the WMAP team. We find that over the multipole range l = [2, 64], the significance of asymmetry is ~10-4, a value insensitive to both frequency and power spectrum. We determine the smallest multipole ranges exhibiting significant asymmetry and find 12, including l = [2, 3] and [6, 7], for which the significance → 0. Examination of the 12 ranges indicates both an improbable association between the direction of maximum significance and the ecliptic plane (significance ~0.01) and that contours of least significance follow great circles inclined relative to the ecliptic at the largest scales. The great circle for l = [2, 3] passes over previously reported preferred axes and is insensitive to frequency, while the great circle for l = [6, 7] is aligned with the ecliptic poles. We examine how changing map-making parameters, e.g., foreground masking, affects asymmetry. Only one change appreciably reduces asymmetry: asymmetry at large scales (l ≤ 7) is rendered insignificant if the magnitude of the WMAP dipole vector (368.11 km s-1) is increased by ≈1-3 σ (≈2-6 km s-1). While confirmation of this result requires the recalibration of the time-ordered data, such a systematic change would be consistent with observations of frequency-independent asymmetry. We conclude that the use of an incorrect dipole vector, in combination with a systematic or foreground process associated with the ecliptic, may help to explain the observed power asymmetry.

The 2dF-SDSS LRG and QSO Survey: The Star Formation Histories of Luminous Red Galaxies

Abstract: We present a detailed investigation into the recent star formation histories of 5,697 Luminous Red Galaxies (LRGs) based on the Hdelta (4101A) and [OII] (3727A) lines. LRGs are luminous (L>3L*), galaxies which have been selected to have photometric properties consistent with an old, passively evolving stellar population. For this study we utilise LRGs from the recently completed 2dF-SDSS LRG and QSO survey (2SLAQ). Equivalent widths of the Hdelta and [OII] lines are measured and used to define three spectral types, those with only strong Hdelta absorption (k+a), those with strong [OII] in emission (em) and those with both (em+a). All other LRGs are considered to have passive star formation histories. The vast majority of LRGs are found to be passive (~80 per cent), however significant numbers of k+a (2.7 per cent), em+a (1.2 per cent) and em LRGs (8.6 per cent) are identified. An investigation into the redshift dependence of the fractions is also performed. A sample of SDSS MAIN galaxies with colours and luminosities consistent with the 2SLAQ LRGs is selected to provide a low redshift comparison. While the em and em+a fractions are consistent with the low redshift SDSS sample, the fraction of k+a LRGs is found to increase significantly with redshift. This result is interpreted as an indication of an increasing amount of recent star formation activity in LRGs with redshift. By considering the expected life time of the k+a phase, the number of LRGs which will undergo a k+a phase can be estimated. A crude comparison of this estimate with the predictions from semi-analytic models of galaxy formation shows that the predicted level of k+a and em+a activity is not sufficient to reconcile the predicted mass growth for massive early-types in a hierarchical merging scenario.

Searching for modified gravity with baryon oscillations : From SDSS to wide field multiobject spectroscopy (WFMOS)

Abstract: We discuss how the baryon acoustic oscillation (BAO) signatures in the galaxy power spectrum can distinguish between modified gravity and the cosmological constant as the source of cosmic acceleration To this end we consider a model characterized by a parameter $n$, which corresponds to the Dvali-Gabadadze-Porrati (DGP) model if $n=2$ and reduces to the standard spatially flat cosmological constant concordance model for $n$ equal to infinity We find that the different expansion histories of the modified gravity models systematically shifts the peak positions of BAO A preliminary analysis using the current SDSS luminous red galaxy (LRG) sample indicates that the original DGP model is disfavored unless the matter density parameter exceeds 03 The constraints will be strongly tightened with future spectroscopic samples of galaxies at high redshifts We demonstrate that WFMOS, in collaboration with other surveys such as Planck, will powerfully constrain modified gravity alternatives to dark energy as the explanation of cosmic acceleration

The luminosities, sizes and velocity dispersions of Brightest Cluster Galaxies: Implications for formation history

Abstract: The size-luminosity relation of early-type Brightest Cluster Galaxies (BCGs), R_e ~ L^0.88, is steeper than that for the bulk of the early-type galaxy population, for which R_e ~ L^0.68. In addition, although BCGs are hardly offset from the Fundamental Plane defined by the bulk of the early-type population, they show considerably smaller scatter. The larger than expected sizes of BCGs, and the increased homogeneity, are qualitatively consistent with models which seek to explain the colors of the most massive galaxies by invoking dry dissipationless mergers, since dissipation tends to reduce the sizes of galaxies, and wet mergers which result in star formation would tend to increase the scatter in luminosity at fixed size and velocity dispersion. Furthermore, BCGs define the same g-r color-magnitude relation as the bulk of the early-type population. If BCGs formed from dry mergers, then BCG progenitors must have been red for their magnitudes, suggesting that they hosted older stellar populations than typical for their luminosities. Our findings have two other consequences. First, the R_e-L relation of the early-type galaxy population as a whole (i.e., normal plus BCG) exhibits some curvature. Some of this curvature must be a consequence of the fact that an increasing fraction of the most luminous galaxies are BCGs. The second consequence is suggested by the fact that, despite following a steeper size-luminosity relation, BCGs tend to define a tight relation between dynamical mass R_e sigma^2/G and luminosity. As consequence, we find that BCGs define a shallower sigma-L relation than the bulk of the early-type galaxy population.

Detection of Ultra High Energy Neutrinos via Coherent Radio Emission

Abstract: Since the 1960’s it has been predicted that cosmic ray protons of the highest energies must produce corresponding high energy neutrinos due to collisions with the cosmic microwave background. Both the origin and mechanism for production of these protons remain a mystery. Detection of the neutrinos, which remain unaffected by poorly-constrained galactic and inter-galactic magnetic fields, provide a useful new astronomical observation window. Moreover, Askyaryan pointed out that detection of these neutrinos via induced showers in solid, radio transparent media, would be observable at large distances due to the coherenence of the radio emission. However, more than four decades later none of these predicted events have been observed. This has been due in part to the need for enormous target volumes and cost-effective ways to instrument them. Without being able to either trigger or record such transient signals in an efficient manner, detection remains elusive. We present a high-performance, cost-effective and low-power solution to this detection problem. Low-power is needed for balloon-borne detection, and this work was done in the context of the Antarctic Impulsive Transient Antenna (ANITA) high altitude balloon project. However the compact and low-cost nature of the sampling ASIC and triggering technology opens opportunities for viable, large-scale terrestrial radio neutrino detector arrays.

Experiences Mirroring and Distributing the Sloan Digital Sky Survey

Abstract: The Sloan Digital Sky Survey (SDSS) project has gathered or produced about 40 Terabytes of data since it started in 2000. The data is made available on the Internet through the SkyServer web interface, for use by professional astronomical researchers and the general public. We first consider how the database?s design helps explore the Sloan data. Then we describe our experience mirroring the SDSS database. We also discuss our preliminary work with distributing the multi-terabyte database across different UK sites using the OGSA-DAI middleware.

Clustering Analyses of 300,000 Photometrically Classified Quasars--II. The Excess on Very Small Scales

Abstract: We study quasar clustering on small scales, modeling clustering amplitudes using halo-driven dark matter descriptions. From 91 pairs on scales 5 times higher at z > 2, than at z 2 could be consistent with reduced merger rates at z > 2 for the galaxies forming UVX quasars. Comparing our clustering at ~28 kpc/h to a $\xi(r)=(r/4.8\Mpch)^{-1.53}$ power-law, we find an upper limit on any excess of a factor of 4.3+/-1.3, which, noting some caveats, differs from large excesses recently measured for binary quasars, at $2.2\sigma$. We speculate that binary quasar surveys that are biased to z > 2 may find inflated clustering excesses when compared to models fit at z < 2. We provide details of 111 photometrically classified quasar pairs with separations <0.1'. Spectroscopy of these pairs could significantly constrain quasar dynamics in merging galaxies.

The Effect of Large-Scale Structure on the SDSS Galaxy Three-Point Correlation Function

Abstract: We present measurements of the normalised redshift-space three-point correlation function (Q_z) of galaxies from the Sloan Digital Sky Survey (SDSS) main galaxy sample. We have applied our "npt" algorithm to both a volume-limited (36738 galaxies) and magnitude-limited sample (134741 galaxies) of SDSS galaxies, and find consistent results between the two samples, thus confirming the weak luminosity dependence of Q_z recently seen by other authors. We compare our results to other Q_z measurements in the literature and find it to be consistent within the full jack-knife error estimates. However, we find these errors are significantly increased by the presence of the ``Sloan Great Wall'' (at z ~ 0.08) within these two SDSS datasets, which changes the 3-point correlation function (3PCF) by 70% on large scales (s>=10h^-1 Mpc). If we exclude this supercluster, our observed Q_z is in better agreement with that obtained from the 2dFGRS by other authors, thus demonstrating the sensitivity of these higher-order correlation functions to large-scale structures in the Universe. This analysis highlights that the SDSS datasets used here are not ``fair samples'' of the Universe for the estimation of higher-order clustering statistics and larger volumes are required. We study the shape-dependence of Q_z(s,q,theta) as one expects this measurement to depend on scale if the large scale structure in the Universe has grown via gravitational instability from Gaussian initial conditions. On small scales (s 10h^-1 Mpc, we see considerable shape-dependence in Q_z.

Clustering Analyses of 300,000 Photometrically Classified Quasars--I. Luminosity and Redshift Evolution in Quasar Bias

Abstract: Using ~300,000 photometrically classified quasars, by far the largest quasar sample ever used for such analyses, we study the redshift and luminosity evolution of quasar clustering on scales of ~50 kpc/h to ~20 Mpc/h from redshifts of z~0.75 to z~2.28. We parameterize our clustering amplitudes using realistic dark matter models, and find that a LCDM power spectrum provides a superb fit to our data with a redshift-averaged quasar bias of b_Q = 2.41+/-0.08 ($P_{ 99.6% using our data set alone, increasing to >99.9999% if stellar contamination is not explicitly parameterized. We measure the quasar classification efficiency across our full sample as a = 95.6 +/- ^{4.4}_{1.9}%, a star-quasar separation comparable with the star-galaxy separation in many photometric studies of galaxy clustering. We derive the mean mass of the dark matter halos hosting quasars as MDMH=(5.2+/-0.6)x10^{12} M_solar/h. At z~1.9 we find a $1.5\sigma$ deviation from luminosity-independent quasar clustering; this suggests that increasing our sample size by a factor of 1.8 could begin to constrain any luminosity dependence in quasar bias at z~2. Our results agree with recent studies of quasar environments at z 50 kpc/h. At z < 1.6, our analysis suggests that b_Q is constant with luminosity to within ~0.6, and that, for g < 21, angular quasar autocorrelation measurements are unlikely to have sufficient statistical power at z < 1.6 to detect any luminosity dependence in quasars' clustering.