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 ann z, a neural network-based photometric-redshift estimator. The rms photometric redshift accuracy obtained for an evaluation set selected from the 2SLAQ sample is  σ _z = 0.049  averaged over all galaxies, and  σ _z = 0.040  for a brighter subsample  ( i < 19.0)  . The catalogue is expected to contain ∼5 per cent stellar contamination. The ann z 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 .     

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 Å), and [O  ii ] (3727 Å) lines and the D4000 index. LRGs are luminous  ( L > 3 L *)  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 level of k+a and em+a activities is not sufficient to reconcile the predicted mass growth for massive early types in a hierarchical merging scenario.     

Triggered star formation in the LMC4/Constellation III region of the Large Magellanic Cloud

We present multicolour images of the hosts of three z  = 2 QSOs previously detected in the R band by our group. The luminosities, colours and sizes of the hosts overlap with those of actively star-forming galaxies in the nearby Universe. Surface brightness radial profiles over the outer resolved areas roughly follow either an r ^1/4 or an exponential law. These properties give support to the young host galaxy interpretation of the extended light around QSOs at high redshift. The rest-frame UV and UV–optical colours are inconsistent with the hypothesis of a scattered halo of light from the active nucleus by a simple optically thin scattering process produced by dust or hot electrons. If the UV light is indeed stellar, star formation rates of hundreds of solar masses per year are implied, an order of magnitude larger than in field galaxies at similar redshifts and above. This might indicate that the QSO phenomenon (at least the high-luminosity one) is preferentially accompanied by enhanced galactic activity at high redshifts.     

QSO lensing magnification associated with galaxy groups

We simulated both the matter and light (galaxy) distributions in a wedge of the Universe and calculated the gravitational lensing magnification caused by the mass along the line-of-sight of galaxies and galaxy groups identified in sky surveys. A large volume redshift cone containing cold dark matter particles mimics the expected cosmological matter distribution in a flat universe with low matter density and a cosmological constant. We generate a mock galaxy catalogue from the matter distribution and identify thousands of galaxy groups in the luminous sky projection. We calculate the expected magnification around galaxies and galaxy groups and then the induced quasi-stellar object (QSO)–lens angular correlation due to magnification bias. This correlation is observable and can be used both to estimate the average mass of the lens population and to make cosmological inferences. We also use analytical calculations and various analyses to compare the observational results with theoretical expectations for the cross-correlation between faint QSOs from the 2dF Survey and nearby galaxies and groups from the Automated Plate Measurement and Sloan Digital Sky Survey Early Data Release. The observed QSO–lens anticorrelations are stronger than the predictions for the cosmological model used. This suggests that there could be unknown systematic errors in the observations and data reduction, or that the model used is not adequate. If the observed signal is assumed to be solely due to gravitational lensing, then the lensing is stronger than expected, due to more massive galactic structures or more efficient lensing than simulated.     

A QSO survey via optical variability and zero proper motion in the M92 field. V. Completion of the QSO sample

We present results from the spectroscopic follow‐up observation of additional QSO candidates in the variability and proper motion (VPM) survey field around M92. In the previous studies, the VPMsurvey has shown to be an efficient way to select QSOs; the QSO sample was confined to the magnitude range B ≲ 19.8 with a completeness estimated to ∼ 90% for B ≲ 19.5. Here we study the faint end (B > 19.8) of the candidate list and check the completeness of the brighter (B < 19.7) QSO subsample. We first study an additional 34 brighter QSO candidates selected by FOCA UV brightness, radio detection, or optical colours. Only one new QSO was found. We show that a two‐colour search making use of FOCA 2 000 magnitudes efficiently selects QSOs, Seyfert galaxies, and NELGs. However, there is no QSO found from this method which would not have been selected by the VPM method as well. The main aim of this paper is to identify the VPM QSO candidates with 19.8 < B < 20.5. We select 53 candidates of which 50 were observed; 26 QSOs, 2 Sey1, and 2 NELGs were found. The fraction of QSOs and Sey1s among the faint high‐priority candidates amounts to 71%. The whole AGN sample comprises now 123 QSOs and 18 Sey1s. We find that the extended VPM QSO sample does not significantly differ from other optically selected samples with regard to their low‐resolution spectra and optical broad‐band colours.     

An excess of damped Lyman α galaxies near quasi-stellar objects

We present a sample of 33 damped Lyman α systems (DLAs) discovered in the Sloan Digital Sky Survey (SDSS) whose absorption redshifts ( z _abs) are within 6000 km s⁻¹ of the quasi-stellar object's (QSO) systemic redshift ( z _sys). Our sample is based on  731 2.5 < z _sys < 4.5  non-broad absorption line (non-BAL) QSOs from Data Release 3 (DR3) of the SDSS. We estimate that our search is ≈100 per cent complete for absorbers with N (H  i )  ≥ 2 × 10²⁰ cm⁻²  . The derived number density of DLAs per unit redshift, n ( z ), within  Δ v < 6000 km s⁻¹  is higher (3.5σ significance) by almost a factor of 2 than that of intervening absorbers observed in the SDSS DR3, i.e. there is evidence for an overdensity of galaxies near the QSOs. This provides a physical motivation for excluding DLAs at small velocity separations in surveys of intervening 'field' DLAs. In addition, we find that the overdensity of proximate DLAs is independent of the radio-loudness of the QSO, consistent with the environments of radio-loud and radio-quiet QSOs being similar.     

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

An angular correlation of low significance (2 σ ) 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.     

Compact groups in theory and practice – III. Compact groups of galaxies in the Sixth Data Release of the Sloan Digital Sky Survey

We present the largest publicly available catalogue of compact groups (CGs) of galaxies identified using the original selection criteria of Hickson, selected from the Sixth Data Release of the Sloan Digital Sky Survey (SDSS DR6). We identify 2297 CGs down to a limiting magnitude of   r = 18 (∼0.24 groups  deg⁻²), and 74 791 CGs down to a limiting magnitude of   r = 21 (∼6.7 groups  deg⁻²). This represents 0.9 per cent of all galaxies in the SDSS DR6 at these magnitude levels. Contamination due to gross photometric errors has been removed from the bright sample of groups, and we estimate it is present in the large sample at the 14 per cent level. Spectroscopic information is available for 4131 galaxies in the bright catalogue (43 per cent completeness), and we find that the median redshift of these groups is   z _med= 0.09  . The median line-of-sight (LOS) velocity dispersion within the CGs from the bright catalogue is  σ_LOS≃ 230 km s⁻¹  , and their typical intergalactic separations are of the order of 50–100 kpc. We show that the fraction of groups with interloping galaxies identified as members is in good agreement with the predictions from our previous study of a mock galaxy catalogue, and we demonstrate how to select CGs such that the interloper fraction is well defined and minimized. This observational data set is ideal for large statistical studies of CGs, the role of environment on galaxy evolution and the effect of galaxy interactions in determining galaxy morphology.     

Estimating the redshift distribution of photometric galaxy samples – II. Applications and tests of a new method

In Lima et al. we presented a new method for estimating the redshift distribution,   N ( z )  , of a photometric galaxy sample, using photometric observables and weighted sampling from a spectroscopic subsample of the data. In this paper, we extend this method and explore various applications of it, using both simulations and real data from the Sloan Digital Sky Survey (SDSS). In addition to estimating the redshift distribution for an entire sample, the weighting method enables accurate estimates of the redshift probability distribution,   p ( z )  , for each galaxy in a photometric sample. Use of   p ( z )  in cosmological analyses can substantially reduce biases associated with traditional photometric redshifts, in which a single redshift estimate is associated with each galaxy. The weighting procedure also naturally indicates which galaxies in the photometric sample are expected to have accurate redshift estimates, namely those that lie in regions of photometric-observable space that are well sampled by the spectroscopic subsample. In addition to providing a method that has some advantages over standard photo- z estimates, the weights method can also be used in conjunction with photo- z estimates e.g. by providing improved estimation of   N ( z )  via deconvolution of   N ( z _phot)  and improved estimates of photo- z scatter and bias. We present a publicly available   p ( z )  catalogue for ∼78 million SDSS DR7 galaxies.     

The evolution of [O ii] emission from cluster galaxies

We investigate the evolution of the star formation rate in cluster galaxies. We complement data from the Canadian Network for Observational Cosmology 1 (CNOC1) cluster survey  (0.15 < z < 0.6)  with measurements from galaxy clusters in the Two-degree Field (2dF) galaxy redshift survey  (0.05 < z < 0.1)  and measurements from recently published work on higher-redshift clusters, up to almost   z = 1  . We focus our attention on galaxies in the cluster core, i.e. galaxies with   r < 0.7  h ⁻¹₇₀ Mpc  . Averaging over clusters in redshift bins, we find that the fraction of galaxies with strong [O  ii ] emission is ≲20 per cent in cluster cores, and the fraction evolves little with redshift. In contrast, field galaxies from the survey show a very strong increase over the same redshift range. It thus appears that the environment in the cores of rich clusters is hostile to star formation at all the redshifts studied. We compare this result with the evolution of the colours of galaxies in cluster cores, first reported by Butcher and Oemler. Using the same galaxies for our analysis of the [O  ii ] emission, we confirm that the fraction of blue galaxies, which are defined as galaxies 0.2 mag bluer in the rest-frame B – V than the red sequence of each cluster, increases strongly with redshift. Because the colours of galaxies retain a memory of their recent star formation history, while emission from the [O  ii ] line does not, we suggest that these two results can best be reconciled if the rate at which the clusters are being assembled is higher in the past, and the galaxies from which it is being assembled are typically bluer.     

Luminous early-type field galaxies at z∼0.4– I. Observations and redshift catalogue of 581 galaxies

We have compiled a sample of ∼ 9600 bright, i ≤18.95 , red, b _J− r >2 , candidate galaxies in an area of 220 deg². These are luminous, L > L * , field early-type galaxies with redshifts 0.3≲ z ≲0.6 . We present a redshift catalogue of a subsample of 581 targets. The galaxies were selected according to their broad-band b _J ri colours from United Kingdom Schmidt Telescope plates, and have a surface density on the sky of only ∼ 50 deg⁻². Such luminous field galaxies are virtually absent from published redshift surveys and the catalogue provides a large sample of the most luminous normal galaxies, at cosmological distances. The statistical properties of the galaxy spectra, including absorption-line and emission-line measures, are presented and a composite spectrum constructed. The nature of the sample, combined with the relatively bright apparent magnitudes, makes the galaxies suitable targets for several key investigations in galaxy evolution and cosmology.     

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 (Sloan Digital Sky Survey) Luminous Red Galaxy (LRG) and QSO (quasi-stellar object) (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 > 10²⁶  W Hz⁻¹) 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 10²⁶ W Hz⁻¹. Except for the very powerful radio galaxies, the presence of a currently active radio active galactic nucleus (AGN) does not appear to be correlated with any change in the observed stellar population of a luminous red galaxy at   z ∼ 0.5  .     

A photometric study of the field around the candidate recoiling/binary black hole SDSS J092712.65+294344.0

We present a photometric far-ultraviolet (FUV) to K _s-band study of the field around quasar SDSS J092712.65+294344.0. The SDSS spectrum of this object shows various emission lines with two distinct redshifts, at   z = 0.699  and 0.712. Because of this peculiar spectroscopic feature, this source has been proposed as a candidate recoiling or binary black hole. A third alternative model involves two galaxies moving in the centre of a rich galaxy cluster. Here, we present a study addressing the possible presence of such a rich cluster of galaxies in the SDSS J092712.65+294344.0 field. We observed the  3.6 × 2.6  arcmin² field in the K _s band and matched the near-infrared data with the FUV and near-ultraviolet images in the Galaxy Evolution Explorer archive and the ugriz observations in the SDSS. From various colour–colour diagrams, we were able to classify the nature of 32 sources, only 6–11 of which have colours consistent with galaxies at   z ≈ 0.7  . We compare these numbers with the surface density of galaxies, stars and quasars and the expectations for typical galaxy clusters both at low and high redshift. Our study shows that the galaxy cluster scenario is in clear disagreement with the new observations.     

The colour–magnitude relation for galaxies in the Coma cluster

We present a new photometric catalogue of the Coma galaxy cluster in the Johnson U and V bands. We cover an area of 3360 arcmin² of sky, to a depth of     in a 13-arcsec diameter aperture, and produce magnitudes for ∼1400 extended objects in metric apertures from 8.8- to 26-arcsec diameters. The mean internal rms scatter in the photometry is 0.014 mag in V , and 0.026 mag in U , for     .
We place new limits on the levels of scatter in the colour–magnitude relation (CMR) in the Coma cluster, and investigate how the slope and scatter of the CMR depend on galaxy morphology, luminosity and position within the cluster. As expected, the lowest levels of scatter are found in the elliptical galaxies, while the late-type galaxies have the highest numbers of galaxies bluewards of the CMR. We investigate whether the slope of the CMR is an artefact of colour gradients within galaxies, and show that it persists when the colours are measured within a diameter that scales with galaxy size. Looking at the environmental dependence of the CMR, we find a trend of systematically bluer galaxy colours with increasing projected radius from the centre of the cluster. Surprisingly, this is accompanied by a decreased scatter of the CMR. We investigate whether this gradient could be caused by dust in the cluster potential, however the reddening required would produce too large a scatter in the colours of the central galaxies. The gradient appears to be better reproduced by a gradient in the mean galactic ages with projected radius.     

The 2dF QSO Redshift Survey – VIII. Absorption systems in the 10k catalogue

We examine the highest signal-to-noise ratio spectra from the 2dF QSO Redshift Survey (2QZ) 10k release and identify over 100 new low-ionization heavy-element absorbers: damped Lyman α (DLA) candidates suitable for higher-resolution follow-up observations. These absorption systems map the spatial distribution of high- z metals in exactly the same volumes as the foreground 2QZ QSOs themselves sample and hence the 2QZ gives us the unique opportunity to compare directly the two tracers of large-scale structure. We examine the cross-correlation of the two populations to see how they are relatively clustered, and, by considering the colour of the QSOs, detect a small amount of dust in these metal systems.     

The redshift-space two-point correlation function of ELAIS-S1 galaxies

We investigate the clustering properties of galaxies in the recently completed ELAIS-S1 redshift survey through their spatial two-point autocorrelation function. We used a subsample of the ELAIS-S1 catalogue covering approximately 4 deg² and consisting of 148 objects selected at 15 μm with a flux >0.5 mJy and a redshift   z < 0.5  . We detected a positive signal in the correlation function that in the range of separations  1–10  h ⁻¹ Mpc  is well approximated by a power law with a slope  γ= 1.4 ± 0.25  and a correlation length   s ₀= 5.4 ± 1.2  h ⁻¹ Mpc  , at the 90 per cent significance level. This result is in good agreement with the redshift-space correlation function measured in more local samples of mid-infrared-selected galaxies such as the IRAS Point Source Catalog (PSC z ) redshift survey. This suggests a lack of significant clustering evolution of infrared-selected objects out to   z = 0.5  that is further confirmed by the consistency found between the correlation functions measured in a local  ( z < 0.2)  and a distant  (0.2 < z < 0.5)  subsample of ELAIS-S1 galaxies. We also confirm that optically selected galaxies in the local redshift surveys, especially those of the SDSS sample, are significantly more clustered than infrared objects.     

A pilot survey for KX QSOs in the UKIDSS Ultra Deep Survey Field

We have undertaken a pilot survey for faint quasi-stellar objects (QSOs) in the UKIRT Infrared Deep Survey (UKIDSS) Ultra Deep Survey (UDS) Field using the KX selection technique. These observations exploit the very deep near-infrared and optical imaging of this field from United Kingdom Infrared Telescope (UKIRT) and Subaru to select candidate QSOs based on their VJK colours and morphologies. We determined redshifts for 426 candidates using the AAOmega spectrograph on the Anglo-Australian Telescope in service time. We identify 17 QSOs  ( M _B ≲−23)  in this pilot survey at z = 1.57–3.29. We combine our sample with an X-ray-selected sample of QSOs in the same field (a large fraction of which also comply with our KX selection) to constrain the surface density of QSOs with K ≤ 20, deriving limits on the likely surface density of 85–150 deg⁻². We use the good image quality available from our near-infrared imaging to detect a spatially extended component of the QSO light which probably represents the host galaxies. We also use our sample to investigate routes to improve the selection of KX QSOs at faint limits in the face of the significant contamination by compact, foreground galaxies. The brightest examples from our combined QSO sample will be used in conjunction with a large Very Large Telescope VIMOS spectroscopic survey of high-redshift galaxies in this region to study the structures inhabited by gas, galaxies and growing supermassive black holes at high redshifts in the UKIDSS UDS.     

Further constraints on the evolution of Ks-selected galaxies in the GOODS/CDFS field

We have selected and analysed the properties of a sample of  2905 K_s < 21.5  galaxies in  ∼131 arcmin²  of the Great Observatories Origins Deep Survey (GOODS) Chandra Deep Field South (CDFS), to obtain further constraints on the evolution of K_s -selected galaxies with respect to the results already obtained in previous studies. We made use of the public deep multiwavelength imaging from the optical B through the infrared (IR) 4.5-μm bands, in conjunction with available spectroscopic and COMBO17 data in the CDFS, to construct an optimized redshift catalogue for our galaxy sample. We computed the K_s -band luminosity function and determined that its characteristic magnitude has a substantial brightening and a decreasing total density from   z = 0  to  〈 z 〉= 2.5  . We also analysed the colours and number density evolution of galaxies with different stellar masses. Within our sample, and in contrast to what is observed for less massive systems, the vast majority (∼85–90 per cent) of the most massive  ( M > 2.5 × 10¹¹ M_⊙)  local galaxies appear to be in place before redshift   z ∼ 1  . Around 65–70 per cent of the total assemble between redshifts   z = 1  and 3 and most of them display extremely red colours, suggesting that plausible star formation in these very massive systems should mainly proceed in obscured, short-time-scale bursts. The remaining fraction (up to ∼20 per cent) could be in place at even higher redshifts   z = 3–4  , pushing the first epoch of formation of massive galaxies beyond the limits of current near-IR surveys.     

STAGES: the Space Telescope A901/2 Galaxy Evolution Survey

We present an overview of the Space Telescope A901/2 Galaxy Evolution Survey (STAGES). STAGES is a multiwavelength project designed to probe physical drivers of galaxy evolution across a wide range of environments and luminosity. A complex multicluster system at   z ∼ 0.165  has been the subject of an 80-orbit F606W Hubble Space Telescope (HST) /Advanced Camera for Surveys (ACS) mosaic covering the full     span of the supercluster. Extensive multiwavelength observations with XMM–Newton , GALEX, Spitzer , 2dF, Giant Metrewave Radio Telescope and the 17-band COMBO-17 photometric redshift survey complement the HST imaging. Our survey goals include simultaneously linking galaxy morphology with other observables such as age, star formation rate, nuclear activity and stellar mass. In addition, with the multiwavelength data set and new high-resolution mass maps from gravitational lensing, we are able to disentangle the large-scale structure of the system. By examining all aspects of an environment we will be able to evaluate the relative importance of the dark matter haloes, the local galaxy density and the hot X-ray gas in driving galaxy transformation. This paper describes the HST imaging, data reduction and creation of a master catalogue. We perform the Sérsic fitting on the HST images and conduct associated simulations to quantify completeness. In addition, we present the COMBO-17 photometric redshift catalogue and estimates of stellar masses and star formation rates for this field. We define galaxy and cluster sample selection criteria, which will be the basis for forthcoming science analyses, and present a compilation of notable objects in the field. Finally, we describe the further multiwavelength observations and announce public access to the data and catalogues.