2dF-SDSS Luminous Red Galaxies Research Articles

Angular correlation function of 1.5 million luminous red galaxies: clustering evolution and a search for baryon acoustic oscillations

We present the angular correlation function measured from photometric samples comprising 1562 800 luminous red galaxies (LRGs). Three LRG samples were extracted from the Sloan Digital Sky Survey (SDSS) imaging data, based on colour-cut selections at redshifts, z≈ 0.35, 0.55 and 0.7 as calibrated by the spectroscopic surveys, SDSS-LRG, 2dF-SDSS LRG and QSO (quasi-stellar object) (2SLAQ) and the AAΩ-LRG survey. The galaxy samples cover ≈7600 deg2 of sky, probing a total cosmic volume of ≈5.5 h−3 Gpc3. The small- and intermediate-scale correlation functions generally show significant deviations from a single power-law fit with a well-detected break at ≈1 h−1 Mpc, consistent with the transition scale between the one- and two-halo terms in halo occupation models. For galaxy separations 1–20 h−1 Mpc and at fixed luminosity, we see virtually no evolution of the clustering with redshift and the data are consistent with a simple high peaks biasing model where the comoving LRG space density is constant with z. At fixed z, the LRG clustering amplitude increases with luminosity in accordance with the simple high peaks model, with a typical LRG dark matter halo mass 1013–1014h−1 M⊙. For r < 1 h−1 Mpc, the evolution is slightly faster and the clustering decreases towards high redshift consistent with a virialized clustering model. However, assuming the halo occupation distribution (HOD) and Λ cold dark matter (ΛCDM) halo merger frameworks, ∼2–3 per cent/Gyr of the LRGs are required to merge in order to explain the small scales clustering evolution, consistent with previous results. At large scales, our result shows good agreement with the SDSS-LRG result of Eisenstein et al. but we find an apparent excess clustering signal beyond the baryon acoustic oscillations (BAO) scale. Angular power spectrum analyses of similar LRG samples also detect a similar apparent large-scale clustering excess but more data are required to check for this feature in independent galaxy data sets. Certainly, if the ΛCDM model were correct then we would have to conclude that this excess was caused by systematics at the level of Δw≈ 0.001–0.0015 in the photometric AAΩ-LRG sample.

AN UPPER LIMIT TO THE DRY MERGER RATE AT ⟨z⟩ ∼ 0.55

We measure the fraction of Luminous Red Galaxies (LRGs) in dynamically close pairs (with projected separation less than 20 $h^{-1}$ kpc and velocity difference less than 500 km s$^{-1}$) to estimate the dry merger rate for galaxies with $-23 < M(r)_{k+e,z=0.2} +5 \log h < -21.5$ and $0.45 < z < 0.65$ in the 2dF-SDSS LRG and QSO (2SLAQ) redshift survey. For galaxies with a luminosity ratio of $1:4$ or greater we determine a $5\sigma$ upper limit to the merger fraction of 1.0% and a merger rate of $< 0.8 \times 10^{-5}$ Mpc$^{-3}$ Gyr$^{-1}$ (assuming that all pairs merge on the shortest possible timescale set by dynamical friction). This is significantly smaller than predicted by theoretical models and suggests that major dry mergers do not contribute to the formation of the red sequence at $z < 0.7$.

The 2dF-SDSS LRG and QSO Survey: the spectroscopic QSO catalogue

We present the final spectroscopic QSO catalogue from the 2dF-SDSS LRG (luminous red galaxy) and QSO (2SLAQ) survey. This is a deep, 18 < g < 21.85 (extinction corrected), sample aimed at probing in detail the faint end of the broad line active galactic nuclei luminosity distribution at z 2.6. The candidate QSOs were selected from SDSS photometry and observed spectroscopically with the 2dF spectrograph on the Anglo-Australian Telescope. This sample covers an area of 191.9 deg 2 and contains new spectra of 16 326 objects, of which 8764 are QSOs and 7623 are newly discovered [the remainder were previously identified by the 2dF QSO Redshift Survey (2QZ) and SDSS]. The full QSO sample (including objects previously observed in the SDSS and 2QZ surveys) contains 12 702 QSOs. The new 2SLAQ spectroscopic data set also contains 2343 Galactic stars, including 362 white dwarfs, and 2924 narrow emission-line galaxies with a median redshift of z = 0.22. We present detailed completeness estimates for the survey, based on modelling of QSO colours, including host-galaxy contributions. This calculation shows that at g � 21.85 QSO colours are significantly affected by the presence of a host galaxy up to redshift z ∼ 1i n the SDSS ugriz bands. In particular, we see a significant reddening of the objects in g − i towards the fainter g-band magnitudes. This reddening is consistent with the QSO host galaxies being dominated by a stellar population of age at least 2–3 Gyr.

The 2dF-SDSS LRG and QSO Survey: evolution of the clustering of luminous red galaxies sincez= 0.6

We present an analysis of the small-to-intermediate scale clustering of samples of luminous red galaxies (LRGs) from the Sloan Digital Sky Survey (SDSS) and the 2dF-SDSS LRG and QSO Survey (2SLAQ) survey carefully matched to have the same rest-frame colours and luminosity. We study the spatial two-point autocorrelation function in both redshift space [xi(s)] and real space [xi(r)] of a combined sample of over 10 000 LRGs, which represent the most massive galaxies in the universe with stellar masses > 10(11) h(-1) M-circle dot and space densities similar or equal to 10(-4) h(3) Mpc(-3). We find no significant evolution in the amplitude (r(0)) of the correlation function with redshift, but do see a slight decrease in the slope (gamma) with increasing redshift over 0.19 99.9 per cent significance) with a passive model whereby the LRGs do not merge with one another; a model with a merger rate of 7.5 +/- 2.3 per cent from z = 0.55 to 0.19 (i.e. an average rate of 2.4 per cent Gyr(-1)) provides a better fit to our observations. Our clustering and number density measurements are consistent with the hypothesis that the merged LRGs were originally central galaxies in different haloes which, following the merger of these haloes, merged to create a single brightest cluster galaxy. In addition, we show that the small-scale clustering signal constrains the scatter in halo merger histories. When combined with measurements of the luminosity function, our results suggest that this scatter is sub-Poisson. While this is a generic prediction of hierarchical models, it has not been tested before.

Radio galaxies in the 2SLAQ Luminous Red Galaxy survey - II. The stellar populations of radio-loud and radio-quiet LRGs

We present an analysis of the optical spectra of a volume-limited sample of 375 radio galaxies at redshift 0.4<z<0.7 from the 2dF-SDSS Luminous Red Galaxy and QSO (2SLAQ) redshift survey. We investigate the evolution of the stellar populations and emission-line properties of these galaxies. By constructing composite spectra and comparing with a matched sample of radio-quiet sources from the same survey, we also investigate the effect on the galaxy of the presence of an active nucleus. The composite spectra, binned by redshift and radio luminosity, all require two components to describe them, which we interpret as an old and a younger population. We found no evolution with redshift of the age of the younger population in radio galaxies, nor were they different from the radio-quiet comparison sample. Similarly, there is no correlation with radio power, with the exception that the most powerful radio sources (P(1.4) > 10^26 W/Hz) have younger stars and stronger emission lines than the less powerful sources. This suggests that we have located the threshold in radio power where strong emission lines switch on, at radio powers of around 10^26 W/Hz. Except for the very powerful radio galaxies, the presence of a currently-active radio AGN does not appear to be correlated with any change in the observed stellar population of a luminous red galaxy at z~0.5.

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

We combine the quasi-stellar object (QSO) samples from the 2dF QSO Redshift Survey (2QZ) and the 2dF-Sloan Digital Sky Survey luminous red galaxy (LRG) and QSO Survey (2dF-SDSS LRG and QSO, hereafter 2SLAQ) in order to investigate the clustering of z∼ 1.5 QSOs and measure the correlation function (ξ). The clustering signal in redshift-space and projected along the sky direction is similar to that previously obtained from the 2QZ sample alone. By fitting functional forms for ξ(σ, π), the correlation function measured along and across the line of sight, we find, as expected, that β, the dynamical infall parameter and Ω0m , the cosmological density parameter, are degenerate. However, this degeneracy can be lifted by using linear theory predictions under different cosmological scenarios. Using the combination of the 2QZ and 2SLAQ QSO data, we obtain: βQSO(z= 1.5) = 0.60+0.14−0.11, Ω0m= 0.25+0.09−0.07 which imply a value for the QSO bias, b(z= 1.4) = 1.5 ± 0.2 . The combination of the 2QZ with the fainter 2SLAQ QSO sample further reveals 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. By assuming ellipsoidal models for the collapse of density perturbations, we estimate the mass of the dark matter haloes which the QSOs inhabit as ∼3 × 1012 h−1 M⊙ . We find that this halo mass does not evolve strongly with redshift nor depend on QSO luminosity. Assuming a range of relations which relate halo to black hole mass, we 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.

The 2dF-SDSS LRG and QSO Survey: the LRG 2-point correlation function and redshift-space distortions

We present a clustering analysis of luminous red galaxies (LRGs) using nearly 9000 objects from the final, three-year catalogue of the 2dF-SDSS LRG and QSO (2SLAQ) Survey. We measure the redshift-space two-point correlation function, ξ(s) and find that, at the mean LRG redshift of shows the characteristic downturn at small scales (1 h−1 Mpc) expected from line-of-sight velocity dispersion. We fit a double power law to ξ(s) and measure an amplitude and slope of s0 = 17.3+2.5−2.0 h−1 Mpc, γ = 1.03 ± 0.07 at small scales (s 4.5 h−1 Mpc). In the semiprojected correlation function, wp(σ), we find a simple power law with γ = 1.83 ± 0.05 and r0 = 7.30 ± 0.34 h−1 Mpc fits the data in the range 0.4 < σ < 50 h−1 Mpc, although there is evidence of a steeper power law at smaller scales. A single power law also fits the deprojected correlation function ξ(r), with a correlation length of r0 = 7.45 ± 0.35 h−1 Mpc and a power-law slope of γ = 1.72 ± 0.06 in the 0.4 < r < 50 h−1 Mpc range. But it is in the LRG angular correlation function that the strongest evidence for non-power-law features is found where a slope of γ = −2.17 ± 0.07 is seen at 1 < r < 10 h−1 Mpc with a flatter γ = −1.67 ± 0.07 slope apparent at r 1 h−1 Mpc scales. We use the simple power-law fit to the galaxy ξ(r), under the assumption of linear bias, to model the redshift-space distortions in the 2D redshift-space correlation function, ξ(σ, π). We fit for the LRG velocity dispersion, wz, the density parameter, Ωm and β(z), where β(z) = Ω0.6m/b and b is the linear bias parameter. We find values of wz = 330 km s−1, Ωm = 0.10+0.35−0.10 and β = 0.40 ± 0.05. The low values for wz and β reflect the high bias of the LRG sample. These high-redshift results, which incorporate the Alcock–Paczynski effect and the effects of dynamical infall, start to break the degeneracy between Ωm and β found in low-redshift galaxy surveys such as 2dFGRS. This degeneracy is further broken by introducing an additional external constraint, which is the value β(z = 0.1) = 0.45 from 2dFGRS, and then considering the evolution of clustering from z 0 to zLRG 0.55. With these combined methods we find Ωm(z = 0) = 0.30 ± 0.15 and β(z = 0.55) = 0.45 ± 0.05. Assuming these values, we find a value for b(z = 0.55) = 1.66 ± 0.35. We show that this is consistent with a simple ����high-peak’ bias prescription which assumes that LRGs have a constant comoving density and their clustering evolves purely under gravity.

Radio galaxies in the 2SLAQ Luminous Red Galaxy Survey – I. The evolution of low-power radio galaxies to z∼ 0.7

Abstract We have combined optical data from the 2dF-SDSS (Sloan Digital Sky Survey) LRG (Luminous Red Galaxy) and QSO (quasi-stellar object) (2SLAQ) redshift survey with radio measurements from the 1.4 GHz VLA (Very Large Array) FIRST (Faint Images of the Radio Sky at Twenty-cm) and NVSS (NRAO VLA Sky Survey) surveys to identify a volume-limited sample of 391 radio galaxies at redshift 0.4 &amp;lt; z &amp;lt; 0.7. By determining an accurate radio luminosity function for luminous early-type galaxies in this redshift range, we can investigate the cosmic evolution of the radio-galaxy population over a wide range in radio luminosity. The low-power radio galaxies in our LRG sample (those with 1.4 GHz radio luminosities in the range 1024 to 1025 W Hz−1, corresponding to Fanaroff–Riley I (FR I) radio galaxies in the local Universe) undergo significant cosmic evolution over the redshift range 0 &amp;lt; z &amp;lt; 0.7, consistent with pure luminosity evolution of the form (1 +z)k, where k= 2.0 ± 0.3. Our results appear to rule out (at the 6–7σ level) models in which low-power radio galaxies undergo no cosmic evolution. The most powerful radio galaxies in our sample (with radio luminosities above 1026 W Hz−1) may undergo more rapid evolution over the same redshift range. The evolution seen in the low-power radio-galaxy population implies that the total energy input into massive early-type galaxies from active galactic nucleus (AGN) heating increases with redshift, and was at least 50 per cent higher at z∼ 0.55 (the median redshift of the 2SLAQ LRG sample) than in the local universe.

MegaZ-LRG: a photometric redshift catalogue of one million SDSS luminous red galaxies

We describe the construction of MegaZ-LRG, a photometric redshift catalogue of over one million luminous red galaxies (LRGs) in the redshift range 0.4 < z < 0.7 with limiting magnitude i < 20. The catalogue is selected from the imaging data of the Sloan Digital Sky Survey (SDSS) Data Release 4. The 2dF-SDSS LRG and Quasar (2SLAQ) spectroscopic redshift catalogue of 13 000 intermediate-redshift LRGs provides a photometric redshift training set, allowing use of annz, a neural network-based photometric-redshift estimator. The rms photometric redshift accuracy obtained for an evaluation set selected from the 2SLAQ sample is σz= 0.049 averaged over all galaxies, and σz= 0.040 for a brighter subsample (i < 19.0). The catalogue is expected to contain ∼5 per cent stellar contamination. The annz code is used to compute a refined star/galaxy probability based on a range of photometric parameters; this allows the contamination fraction to be reduced to 2 per cent with negligible loss of genuine galaxies. The MegaZ-LRG catalogue is publicly available on the World Wide Web from http://www.2slaq.info.

Cosmological baryonic and matter densities from 600 000 SDSS luminous red galaxies with photometric redshifts

We analyse MegaZ-LRG, a photometric-redshift catalogue of luminous red galaxies (LRGs) based on the imaging data of the Sloan Digital Sky Survey (SDSS) 4th Data Release. MegaZ-LRG, presented in a companion paper, contains >106 photometric redshifts derived with annz, an artificial neural network method, constrained by a spectroscopic subsample of ≈13 000 galaxies obtained by the 2dF-SDSS LRG and Quasar (2SLAQ) survey. The catalogue spans the redshift range 0.4 < z < 0.7 with an rms redshift error σz≈ 0.03(1 +z), covering 5914 deg2 to map out a total cosmic volume 2.5 h−3 Gpc3. In this study we use the most reliable 600 000 photometric redshifts to measure the large-scale structure using two methods: (1) a spherical harmonic analysis in redshift slices, and (2) a direct re-construction of the spatial clustering pattern using Fourier techniques. We present the first cosmological parameter fits to galaxy angular power spectra from a photometric-redshift survey. Combining the redshift slices with appropriate covariances, we determine best-fitting values for the matter density Ωm and baryon density Ωb of Ωmh= 0.195 ± 0.023 and Ωb/Ωm= 0.16 ± 0.036 (with the Hubble parameter h= 0.75 and scalar index of primordial fluctuations nscalar= 1 held fixed). These results are in agreement with and independent of the latest studies of the cosmic microwave background radiation, and their precision is comparable to analyses of contemporary spectroscopic-redshift surveys. We perform an extensive series of tests which conclude that our power spectrum measurements are robust against potential systematic photometric errors in the catalogue. We conclude that photometric-redshift surveys are competitive with spectroscopic surveys for measuring cosmological parameters in the simplest ‘vanilla’ models. Future deep imaging surveys have great potential for further improvement, provided that systematic errors can be controlled.

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

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