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Taking advantage of ∼4700 deg2 optical coverage of the Southern sky offered by the VST ATLAS survey, we construct a new catalogue of photometrically selected galaxy groups and clusters using the orca cluster detection algorithm. The catalogue contains ∼22 000 detections with N 200 > 10 and ∼9000 with N 200 > 20. We estimate the photometric redshifts of the clusters using machine learning and find the redshift distribution of the sample to extend to z ∼ 0.7, peaking at z ∼ 0.25. We calibrate the ATLAS cluster mass-richness scaling relation using masses from the MCXC, Planck, ACT DR5, and SDSS redMaPPer cluster samples. We estimate the ATLAS sample to be |$\gt 95~{{\ \rm per\ cent}}$| complete and |$\gt 85~{{\ \rm per\ cent}}$| pure at z < 0.35 and in the M 200m> |$1\times 10^{14}h^{-1}\, \mbox{M}_\odot$| mass range. At z < 0.35, we also find the ATLAS sample to be more complete than redMaPPer, recovering a |$\sim 40~{{\ \rm per\ cent}}$| higher fraction of Abell clusters. This higher sample completeness places the amplitude of the z < 0.35 ATLAS cluster mass function closer to the predictions of a ΛCDM model with parameters based on the Planck CMB analyses, compared to the mass functions of the other cluster samples. However, strong tensions between the observed ATLAS mass functions and models remain. We shall present a detailed cosmological analysis of the ATLAS cluster mass functions in paper II. In the future, optical counterparts to X-ray-detected eROSITA clusters can be identified using the ATLAS sample. The catalogue is also well suited for auxiliary spectroscopic target selection in 4MOST. The ATLAS cluster catalogue is publicly available at http://astro.dur.ac.uk/cosmology/vstatlas/cluster_catalogue/. [ABSTRACT FROM AUTHOR]

The integrated Sachs–Wolfe (ISW) effect probes the late-time expansion history of the Universe, offering direct constraints on dark energy. Here, we present our measurements of the ISW signal at redshifts of |$\bar{z}=0.35$|⁠ , 0.55, and 0.68, using the cross-correlation of the Planck cosmic microwave background temperature map with ∼0.5 million luminous red galaxies (LRGs) selected from the VST ATLAS survey. We then combine these with previous measurements based on WMAP and similar SDSS LRG samples, providing a total sample of ∼2.1 million LRGs covering ∼12 000 deg2 of sky. At |$\bar{z}=0.35$| and |$\bar{z}=0.55$|⁠ , we detect the ISW signal at 1.2σ and 2.3σ (or 2.6σ combined), in agreement with the predictions of lambda cold dark matter (ΛCDM). We verify these results by repeating the measurements using the BOSS LOWZ and CMASS, spectroscopically confirmed LRG samples. We also detect the ISW effect in three magnitude limited ATLAS + SDSS galaxy samples extending to z ≈ 0.4 at ∼2σ per sample. However, we do not detect the ISW signal at |$\bar{z}=0.68$| when combining the ATLAS and SDSS results. Further tests using spectroscopically confirmed eBOSS LRGs at this redshift remain inconclusive due to the current low sky coverage of the survey. If the ISW signal is shown to be redshift dependent in a manner inconsistent with the predictions of ΛCDM, it could open the door to alternative theories such as modified gravity. It is therefore important to repeat the high-redshift ISW measurement using the completed eBOSS sample, as well as deeper upcoming surveys such as DESI and LSST. [ABSTRACT FROM AUTHOR]

We present results from a survey of galaxies in the fields of six z ≥ 3 damped Lyman α (Lyα) systems (DLAs) using the Multi Unit Spectroscopic Explorer (MUSE) at the Very Large Telescope (VLT). We report a high detection rate of up to |${\approx } 80{{\ \rm per\ cent}}$| of galaxies within 1000 km s−1 from DLAs and with impact parameters between 25 and 280 kpc. In particular, we discovered five high-confidence Lyα emitters associated with three DLAs, plus up to nine additional detections across five of the six fields. The majority of the detections are at relatively large impact parameters (>50 kpc) with two detections being plausible host galaxies. Among our detections, we report four galaxies associated with the most metal-poor DLA in our sample (Z / Z ⊙ = −2.33 ± 0.22), which trace an overdense structure resembling a filament. By comparing our detections with predictions from the Evolution and Assembly of GaLaxies and their Environments (EAGLE) cosmological simulations and a semi-analytic model designed to reproduce the observed bias of DLAs at z > 2, we conclude that our observations are consistent with a scenario in which a significant fraction of DLAs trace the neutral regions within haloes with a characteristic mass of |$M_{\rm h} \approx 10^{11}-10^{12}~\rm M_\odot$|⁠ , in agreement with the inference made from the large-scale clustering of DLAs. We finally show how larger surveys targeting ≈25 absorbers have the potential of constraining the characteristic masses of haloes hosting high-redshift DLAs with sufficient accuracy to discriminate between different models. [ABSTRACT FROM AUTHOR]

We compare our analysis of the baryon acoustic oscillations (BAO) feature in the correlation functions of SDSS BOSS DR12 LOWZ and CMASS galaxy samples with previous literature results. Using subsets of the data we obtain an empirical estimate of the errors on the correlation functions that are in agreement with the simulated errors of previous works. We find that the significance of BAO detection is the quantity most sensitive to the choice of the fitting range with the CMASS value decreasing from 8.0σ to 5.3σ as the fitting range is reduced. Although our measurements of DV (z) are in agreement with previous studies, we note that their CMASS 8.0σ (LOWZ 4.0σ) detection significance reduces to 4.7σ (2.8σ) in fits with their diagonal covariance terms only. We extend our BAO analysis to higher redshifts by fitting to the weighted mean of 2QDESp, SDSS DR5 UNIFORM, 2QZ, and 2SLAQ quasar correlation functions, obtaining a 7.6 per cent measurement compared to 3.9 per cent achieved by eBOSS DR14. Unlike for the LRG surveys, the larger error on quasar correlation functions implies a smaller role for nuisance parameters (accounting for scale-dependent clustering) in providing a good fit to the fiducial Λ cold dark matter model. Again we find that the eBOSS peak significance reduces from 2.8 to 1.4σ if we ignore the off-diagonal covariance matrix terms in our fitting. We conclude that for both LRGs and quasars, the reported BAO peak significances from the SDSS surveys depend sensitively on the accuracy of the covariance matrix at large separations. [ABSTRACT FROM AUTHOR]

We present new observations acquired with the Multi Unit Spectroscopic Explorer instrument on the Very Large Telescope in a quasar field that hosts a high column density damped Lyα absorber (DLA) at z ≈ 3.25. We detect Lyα emission from a nebula at the redshift of the DLA with line luminosity (27 ± 1) × 1041 erg s-1, which extends over 37 ± 1 kpc above a surface brightness limit of 6 × 10-19 erg s-1 cm-2 arcsec-2 at a projected distance of 30.5 ± 0.5 kpc from the quasar sightline. Two clumps lie inside this nebula, both with Lyα rest-frame equivalent width >50Å and with relative line-of-sight velocities aligned with two main absorption components seen in the DLA spectrum. In addition, we identify a compact galaxy at a projected distance of 19.1 ± 0.5 kpc from the quasar sightline. The galaxy spectrum is noisy but consistent with that of a star-forming galaxy at the DLA redshift.We argue that the Lyα nebula is ionized by radiation from star formation inside the two clumps, or by radiation from the compact galaxy. In either case, these data imply the presence of a structure with size ⪢50 kpc inside which galaxies are assembling, a picture consistent with galaxy formation in groups and filaments as predicted by cosmological simulations such as the EAGLE simulations. [ABSTRACT FROM AUTHOR]

We report the results of the 2dF-VST ATLAS Cold Spot galaxy redshift survey (2CSz) based on imaging from VST ATLAS and spectroscopy from 2dF AAOmega over the core of the CMB Cold Spot. We sparsely surveyed the inner 5° radius of the Cold Spot to a limit of iAB ≤ 19.2, sampling ~7000 galaxies at z < 0.4. We have found voids at z = 0.14, 0.26 and 0.30 but they are interspersed with small overdensities, and the scale of these voids is insufficient to explain the Cold Spot through the ΛCDM ISW effect. Combining with previous data out to z ~ 1, we conclude that the CMB Cold Spot could not have been imprinted by a void confined to the inner core of the Cold Spot. Additionally, we find that our 'control' field GAMA G23 shows a similarity in its galaxy redshift distribution to the Cold Spot. Since the GAMA G23 line of sight shows no evidence of a CMB temperature decrement, we conclude that the Cold Spot may have a primordial origin rather than being due to line-of-sight effects. [ABSTRACT FROM AUTHOR]

The Sloan Digital Sky Survey (SDSS) has created a knowledge gap between the Northern and the Southern hemispheres, which is very marked for white dwarfs: Only ≃15 per cent of the known white dwarfs are south of the equator. Here, we make use of the VLT Survey Telescope (VST) ATLAS survey, one of the first surveys obtaining deep, optical, multiband photometry over a large area of the southern skies, to remedy this situation. Applying the colour and proper-motion selection developed in our previous work on SDSS to the most recent internal data release (2016 April 25) of VST ATLAS, we created a catalogue of ≃4200 moderately bright (g ≤ 19), high-confidence southern white dwarf candidates, which can be followed up individually with both the large array of southern telescopes or in bulk with ESO’s forthcoming multi-object spectrograph 4MOST. [ABSTRACT FROM AUTHOR]

We present new results on the auto- and cross-correlation functions of galaxies and OVI absorbers in a ~18 Gpc3 comoving volume at z < 1. We use a sample of 51 296 galaxies and 140 OVI absorbers in the column density range 13 ≲ logN ≲ 15 to measure twopoint correlation functions in the two dimensions transverse and orthogonal to the line of sight ξ (r⊥, r‖). We furthermore infer the corresponding 'real-space' correlation functions, ξ (r), by projecting ξ (r⊥, r‖) along r‖, and assuming a power-law form, ξ (r) = (r/r0)-γ. Comparing the results from the absorber-galaxy cross-correlation function, ξag, the galaxy autocorrelation function, ξgg, and the absorber autocorrelation function, ξaa, we constrain the statistical connection between galaxies and the metal-enriched intergalactic medium as a function of star formation activity. We also compare these results to predictions from the EAGLE cosmological hydrodynamical simulation and find a reasonable agreement. We find that: (i) OVI absorbers show very little velocity dispersion with respect to galaxies on ~ Mpc scales, likely ≲100 km s-1; (ii)OVI absorbers are less clustered, and potentially more extended around galaxies than galaxies are around themselves; (iii) on ≳100 kpc scales, the likelihood of finding OVI absorbers around star-forming galaxies is similar to the likelihood of finding OVI absorbers around non-star-forming galaxies; and (iv) OVI absorbers are either not ubiquitous to galaxies in our sample, or their distribution around them is patchy on scales ≲100 kpc (or both), at least for the column densities at which most are currently detected. [ABSTRACT FROM AUTHOR]

We report the serendipitous detection of a 0.2 L*, Lyα emitting galaxy at redshift 2.5 at an impact parameter of 50 kpc from a bright background QSO sightline. A high-resolution spectrum of the QSO reveals a partial Lyman-limit absorption system (${\it N}_\mathrm{H\,\small {i}}=10^{16.94\pm 0.10}$ cm−2) with many associated metal absorption lines at the same redshift as the foreground galaxy. Using photoionization models that carefully treat measurement errors and marginalize over uncertainties in the shape and normalization of the ionizing radiation spectrum, we derive the total hydrogen column density ${\it N}_\mathrm{H}=10^{19.4\pm 0.3}\,\rm cm^{-2}$, and show that all the absorbing clouds are metal enriched, with Z = 0.1–0.6 Z⊙. These metallicities and the system's large velocity width (436 km s− 1) suggest the gas is produced by an outflowing wind. Using an expanding shell model we estimate a mass outflow rate of ∼5 M⊙ yr−1. Our photoionization model yields extremely small sizes (<100–500 pc) for the absorbing clouds, which we argue is typical of high column density absorbers in the circumgalactic medium (CGM). Given these small sizes and extreme kinematics, it is unclear how the clumps survive in the CGM without being destroyed by hydrodynamic instabilities. The small cloud sizes imply that even state-of-the-art cosmological simulations require more than a 1000-fold improvement in mass resolution to resolve the hydrodynamics relevant for cool gas in the CGM. [ABSTRACT FROM AUTHOR]

We present the results of a project to develop a proof of concept for a novel hyperspectral imager based on the use of advanced micro-optics technology. The technology gives considerably more spatial elements than a classic pushbroom which translates into far more light being integrated per unit of time. This permits us to observe at higher spatial and/or spectral resolution, darker targets and under lower illumination, as in the early morning. Observations of faint glow at night should also be possible but need further studies. A full instrument for laboratory demonstration and field tests has now been built and tested. It has about 10,000 spatial elements and spectra 150 pixel long. It is made of a set of cylindrical fore-optics followed by a new innovative optical system called a microslice Integral Field Unit (IFU) which is itself followed by a standard spectrograph. The fore-optics plus microslice IFU split the field into a large number of small slit-like images that are dispersed in the spectrograph. Our goal is to build instruments with at least hundreds of thousands of spatial elements. [ABSTRACT FROM AUTHOR]

Simple pure luminosity evolution (PLE) models, in which galaxies brighten at high redshift due to increased star formation rates (SFRs), are known to provide a good fit to the colours and number counts of galaxies throughout the optical and near-infrared. We show that optically defined PLE models, where dust reradiates absorbed optical light into infrared spectra composed of local galaxy templates, fit galaxy counts and colours out to 8 m and to at least . At 24-70 m, the model is able to reproduce the observed source counts with reasonable success if 16 per cent of spiral galaxies show an excess in mid-IR flux due to a warmer dust component and a higher SFR, in line with observations of local starburst galaxies. There remains an underprediction of the number of faint-flux, high- sources at 24 m, so we explore how the evolution may be altered to correct this. At 160 m and longer wavelengths, the model fails, with our model of normal galaxies accounting for only a few percent of sources in these bands. However, we show that a PLE model of obscured AGN, which we have previously shown to give a good fit to observations at 850 m, also provides a reasonable fit to the Herschel/BLAST number counts and redshift distributions at 250-500 m. In the context of a CDM cosmology, an AGN contribution at 250-870 m would remove the need to invoke a top-heavy IMF for high-redshift starburst galaxies. [ABSTRACT FROM AUTHOR]

ABSTRACT We find that X-ray sources in the Extended Chandra Deep Field-South are strongly spatially correlated with Large APEX Bolometer Camera (LABOCA) 870 μm sources. We investigate the dependence of this correlation on X-ray flux, hardness ratio and column density, finding that specifically faint and absorbed X-ray sources are significant sub-mm emitters. In the X-ray source redshift subsample we confirm the previous result that higher luminosity sources ( LX > 1044 erg s−1) have greater 870 μm fluxes but we also find that this subsample selects against absorbed sources, faint in X-ray flux. Overall, we find that X-ray sources contribute 1.5 ± 0.1 Jy deg−2 to the sub-mm background, ≈3 per cent of the total, in agreement with the prediction of an obscured active galactic nuclei (AGN) model which also gives a reasonable fit to the bright sub-mm source counts. This non-unified model also suggests that when Compton-thick, X-ray-undetected sources are included, then the fractional AGN contribution to the sub-mm background would rise from ≈3 per cent to a total of 25-40 per cent, although in a unified model the AGN contribution would only reach ≈13 per cent, because the sub-mm flux of the X-ray sources is then more representative of the whole AGN population. Measurements of the dependence of sub-mm flux on X-ray flux, luminosity and column density all agree well with the predictions of the non-unified AGN model. Heavily absorbed, X-ray-undetected AGN could explain the further cross-correlation we find between sub-mm sources and z > 0.5 red galaxies. We conclude that sub-mm galaxies may contain the long-sought absorbed AGN population needed to explain the X-ray background. [ABSTRACT FROM AUTHOR]

We present the quasi-stellar object (QSO) luminosity function (LF) of the completed 2dF–SDSS LRG and QSO (2SLAQ) survey, based on QSOs photometrically selected from Sloan Digital Sky Survey (SDSS) imaging data and then observed spectroscopically using the 2dF instrument on the Anglo-Australian Telescope. We analyse 10 637 QSOs in the redshift range to a g-band flux limit of 21.85 (extinction-corrected) and an absolute continuum magnitude of . This sample covers an area of 191.9 deg2. The binned QSO LF agrees with that of the brighter SDSS main QSO sample, but extends mag fainter, clearly showing the flattening of the LF towards faint absolute magnitudes. 2SLAQ finds an excess of QSOs compared to the 2dF QSO Redshift Survey at , as found previously by Richards et al. The LF is consistent with other previous, much smaller, samples produced to the depth of 2SLAQ. By combining the 2SLAQ and SDSS QSO samples, we produce a QSO LF with an unprecedented combination of precision and dynamic range. With this we are able to accurately constrain both the bright and faint ends of the QSO LF. While the overall trends seen in the evolution of the QSO LF appear similar to pure luminosity evolution, the data show very significant departures from such a model. Most notably we see clear evidence that the number density of faint QSOs peaks at lower redshift than bright QSOs: QSOs with have space densities which peak at , while QSOs at peak at . By fitting simple LF models in narrow intervals, we find that this downsizing is significant at the 99.98 per cent level. We show that LF models which follow the pure luminosity evolution form [i.e. ], but with a redshift-dependent bright-end slope and an additional density evolution term, , provide a much improved fit to the data. The bright-end slope, α, steepens from at to at . This steepening is significant at the 99.9 per cent level. We find a decline in from to 2.5 which is significant at the 94 per cent level. [ABSTRACT FROM AUTHOR]

We present the final spectroscopic QSO catalogue from the 2dF-SDSS LRG (luminous red galaxy) and QSO (2SLAQ) survey. This is a deep, (extinction corrected), sample aimed at probing in detail the faint end of the broad line active galactic nuclei luminosity distribution at . 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 deg2 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 . We present detailed completeness estimates for the survey, based on modelling of QSO colours, including host-galaxy contributions. This calculation shows that at QSO colours are significantly affected by the presence of a host galaxy up to redshift in the SDSS ugriz bands. In particular, we see a significant reddening of the objects in 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 full catalogue, including completeness estimates, is available on-line at . [ABSTRACT FROM AUTHOR]

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 and real space of a combined sample of over 10 000 LRGs, which represent the most massive galaxies in the universe with stellar masses and space densities . We find no significant evolution in the amplitude ( r0) of the correlation function with redshift, but do see a slight decrease in the slope (γ) with increasing redshift over and scales of . We compare our measurements with the predicted evolution of dark matter clustering and use the halo model to interpret our results. We find that our clustering measurements are inconsistent (>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 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. [ABSTRACT FROM AUTHOR]

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 . 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 , consistent with pure luminosity evolution of the form , where . 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 (the median redshift of the 2SLAQ LRG sample) than in the local universe. [ABSTRACT FROM AUTHOR]

We describe the construction of MegaZ-LRG, a photometric redshift catalogue of over one million luminous red galaxies (LRGs) in the redshift range with limiting magnitude . 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 ofannz, a neural network-based photometric-redshift estimator. The rms photometric redshift accuracy obtained for an evaluation set selected from the 2SLAQ sample is averaged over all galaxies, and for a brighter subsample . The catalogue is expected to contain ∼5 per cent stellar contamination. Theannz 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 . [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We discuss the construction of a photometric redshift catalogue of luminous red galaxies (LRGs) from the Sloan Digital Sky Survey (SDSS), emphasizing the principal steps necessary for constructing such a catalogue: (i) photometrically selecting the sample, (ii) measuring photometric redshifts and their error distributions, and (iii) estimating the true redshift distribution. We compare two photometric redshift algorithms for these data and find that they give comparable results. Calibrating against the SDSS and SDSS–2dF (Two Degree Field) spectroscopic surveys, we find that the photometric redshift accuracy isfor redshifts less than 0.55 and worsens at higher redshift (∼ 0.06 for). These errors are caused by photometric scatter, as well as systematic errors in the templates, filter curves and photometric zero-points. We also parametrize the photometric redshift error distribution with a sum of Gaussians and use this model to deconvolve the errors from the measured photometric redshift distribution to estimate the true redshift distribution. We pay special attention to the stability of this deconvolution, regularizing the method with a prior on the smoothness of the true redshift distribution. The methods that we develop are applicable to general photometric redshift surveys. [ABSTRACT FROM AUTHOR]

Using published Hubble Space Telescope ( HST) Cepheid data from 25 galaxies, we have found a correlation between the dispersion in the Cepheid period–luminosity ( P– L) relation and host galaxy metallicity, which is significant at the ≈3σ level in the V band. In the I band the correlation is less significant, although the tighter intrinsic dispersion of the P– L relation in I may make it harder to detect such a correlation in the HST sample. One possibility is that low metallicity galaxies have smaller metallicity gradients than high metallicity galaxies; if the Cepheid P– L relation has a significant dependence on metallicity then this might explain the higher P– L dispersion in the higher metallicity galaxies. A second possibility is that the increased P– L dispersion is driven by metallicity dispersion but now due to a relation between metallicity and Cepheid colour rather than luminosity. A third possibility is that the increased P– L dispersion is caused by an increase in the width of the instability strip with metallicity. Whatever the explanation, the high observed dispersions in the HST Cepheid P– L relations have the important consequence that the bias due to incompleteness in the P– L relation at faint magnitudes is more significant than previously thought. Using a maximum likelihood technique which takes into account the effect on the P– L relations of truncation by consistently defined magnitude completeness limits, we rederive the Cepheid distances to the 25 galaxies. In the case of the galaxies with the highest P– L dispersion at the largest distances, we find that the published distance modulus underestimates the true distance modulus by up to ≈0.5 mag. When both metallicity and magnitude incompleteness corrections are made, a scale error in the published Cepheid distances is seen in the sense that the published distance moduli are increasingly underestimated at larger distances. This results in the average distance modulus to the four galaxies in the Virgo cluster core increasing from to if the metallicity correction of Kennicutt et al. is assumed. For the 18 HST galaxies with good Tully–Fisher (TF) distances and the Cepheid–TF distance modulus average residual increases from to with . This indicates a significant scale error in TF distances, which reduces the previous Pierce & Tully TF estimate of to , assuming and a still uncertain Virgo infall model. Finally, for the eight HST galaxies with Type Ia supernovae (SNIa), the metallicity and incompleteness corrected Cepheid distances marginally suggest there may be a metallicity dependence of SNIa peak luminosity in the sense that metal-poor hosts have lower luminosity SNIa. Thus, SNIa Hubble diagram estimates of both H0 and q0 may therefore also require significant corrections for metallicity, once the exact sizes of the Cepheid metallicity corrections become better established. [ABSTRACT FROM AUTHOR]

We investigate the reddening and main-sequence-fitted distances to 11 young, Galactic open clusters that contain Cepheids. Each cluster contains or is associated with at least one Cepheid variable star. Reddening to the clusters is estimated using the colours of the OB stars and the distance modulus to the cluster is estimated via and colour–magnitude diagrams. Our main-sequence fitting assumes that the solar-metallicity zero-age main sequence of Allen applies universally to all the open clusters, although this point is controversial at present. In this way we proceed to calibrate the Cepheid period–luminosity (PL) relation and find and absolute distance moduli to the Large Magellanic Cloud (LMC) of from the V-band and from the K-band giving an overall distance modulus to the LMC of . This is in good agreement with the previous Cepheid PL-K result of Laney & Stobie at and with the Hipparcos parallax-calibrated Cepheid PL- K estimate of Feast & Catchpole at when no account is taken of the LMC metallicity. We also find that the two-colour diagrams of two important clusters are not well fitted by the standard main-sequence line. In one case, NGC 7790, we find that the F stars show a UV excess and in the second case, NGC 6664, they are too red in . Previous spectroscopic estimates of the metallicity of the Cepheids in these clusters appear to suggest that the effects are not due to metallicity variations. Other possible explanations for these anomalies are positional variations in the dust reddening law and contamination by foreground or background stars. [ABSTRACT FROM AUTHOR]

In the first of a pair of articles, Utane Sawangwit and Tom Shanks play devil's advocate, wondering if the standard cosmological model is missing something. [ABSTRACT FROM AUTHOR]