Observed Redshift Range Research Articles

Neural networks and standard cosmography with newly calibrated high redshift GRB observations

Gamma-ray bursts (GRBs) detected at high redshift can be used to trace the cosmic expansion history. However, the calibration of their luminosity distances is not an easy task in comparison to Type Ia Supernovae (SNeIa). To calibrate these data, correlations between their luminosity and other observed properties of GRBs need to be identified, and we must consider the validity of our assumptions about these correlations over their entire observed redshift range. In this work, we propose a new method to calibrate GRBs as cosmological distance indicators using SNeIa observations with a machine learning architecture. As well we include a new data GRB calibrated sample using extended cosmography in a redshift range above z > 3.6. An overview of this machine learning technique was developed in [1] to study the evolution of dark energy models at high redshift. The aim of the method developed in this work is to combine two networks: a Recurrent Neural Network (RNN) and a Bayesian Neural Network (BNN). Using this computational approach, denoted RNN+BNN, we extend the network's efficacy by adding the computation of covariance matrices to the Bayesian process. Once this is done, the SNeIa distance-redshift relation can be tested on the full GRB sample and therefore used to implement a cosmographic reconstruction of the distance-redshift relation in different regimes. Thus, our newly-trained neural network is used to constrain the parameters describing the kinematical state of the Universe via a cosmographic approach at high redshifts (up to z ≈ 10), wherein we require a very minimal set of assumptions on the deep learning arquitecture itself that do not rely on dynamical equations for any specific theory of gravity.

Evolution of the Stellar Mass–Metallicity Relation. II. Constraints on Galactic Outflows from the Mg Abundances of Quiescent Galaxies

Abstract We present the stellar mass–[Fe/H] and mass–[Mg/H] relation of quiescent galaxies in two galaxy clusters at z ∼ 0.39 and z ∼ 0.54. We derive the age, [Fe/H], and [Mg/Fe] for each individual galaxy using a full-spectrum fitting technique. By comparing with the relations for z ∼ 0 Sloan Digital Sky Survey galaxies, we confirm our previous finding that the mass–[Fe/H] relation evolves with redshift. The mass–[Fe/H] relation at higher redshift has lower normalization and possibly steeper slope. However, based on our sample, the mass–[Mg/H] relation does not evolve over the observed redshift range. We use a simple analytic chemical evolution model to constrain the average outflow that these galaxies experience over their lifetime, via the calculation of mass-loading factor. We find that the average mass-loading factor η is a power-law function of galaxy stellar mass, . The measured mass-loading factors are consistent with the results of other observational methods for outflow measurements and with the predictions where outflow is caused by star formation feedback in turbulent disks.

A comparison of the R_{mathrm{h}}=ct and varLambda CDM cosmologies based on the observed halo mass function

The growth of structure may be traced via the redshift-dependent halo mass function. This quantity probes the re-ionization history and quasar abundance in the Universe, constituting an important probe of the cosmological predictions. Halos are not directly observable, however, so their mass and evolution must be inferred indirectly. The most common approach is to presume a relationship with galaxies and halos. Studies based on the assumption of a constant halo to stellar mass ratio M_h/M_* (extrapolated from z lesssim 4) reveal significant tension with varLambda CDM – a failure known as “The Impossibly Early Galaxy Problem”. But whether this ratio evolves or remains constant through redshift 4 lesssim z lesssim 10 is still being debated. To eliminate the tension with varLambda CDM, it would have to change by about 0.8 dex over this range, an issue that may be settled by upcoming observations with the James Webb Space Telescope. In this paper, we explore the possibility that this major inconsistency may instead be an indication that the cosmological model is not completely correct. We study this problem in the context of another Friedmann–Lemaître–Robertson–Walker (FLRW) model known as the R_{mathrm{h}}=ct universe, and use our previous measurement of sigma _8 from the cosmological growth rate, together with new solutions to the Einstein–Boltzmann equations, to interpret these recent halo measurements. We demonstrate that the predicted mass and redshift dependence of the halo distribution in R_{mathrm{h}}=ct is consistent with the data, even assuming a constant M_h/M_* throughout the observed redshift range (4lesssim zlesssim 10), contrasting sharply with the tension in varLambda CDM. We conclude that – if M_h/M_* turns out to be constant – the massive galaxies and their halos must have formed earlier than is possible in varLambda CDM.

Ruling out 3 keV warm dark matter using 21 cm EDGES data

Weakly interacting cold dark matter (CDM) particles, which are otherwise extremely successful in explaining various cosmological observations, exhibit a number of problems on small scales. One possible way of solving these problems is to invoke (so-called) warm dark matter (WDM) particles with masses $m_x \sim$ keV. Since the formation of structure is delayed in such WDM models, it is natural to expect that they can be constrained using observations related to the first stars, e.g., the 21 cm signal from cosmic dawn. In this work, we use a detailed galaxy formation model, Delphi, to calculate the 21 cm signal at high-redshifts and compare this to the recent EDGES observations. We find that while CDM and 5 keV WDM models can obtain a 21 cm signal within the observed redshift range, reproducing the amplitude of the observations requires the introduction of an excess radio background. On the other hand, WDM models with $m_x \leq 3$ keV can be ruled out since they are unable to match either the redshift range or the amplitude of the EDGES signal, irrespective of the parameters used. Comparable to values obtained from the low-redshift Lyman Alpha forest, our results extend constraints on the WDM particle to an era inaccessible by any other means; additional forthcoming 21 cm data from the era of cosmic dawn will be crucial in refining such constraints.

Spectroscopy of Galaxies: Evolution of Escape Fractions, Metallicity Gradients and Stellar Metallicity

We use spectroscopic observations to investigate galaxies from the age of reionization to the peak of star formation, and the local universe. This thesis presents three projects to better understand characteristics of galactic feedback, i.e., how it regulates galactic gas flows. We used deep absorption line spectroscopy to estimate the escape fractions fesc of star-forming gravitationally-lensed galaxies at z ≃ 5. The approach is to measure the covering fraction of neutral hydrogen in a galaxy from the amount of non-ionizing UV radiation absorbed by low-ionization metal species. With the boost of signal by gravitational lensing, we observed four 4 Spatially resolved spectroscopic observations of galaxies at the peak era of star-formation activities are effective in providing insight into the primitive disks. Specifically, we used metallicity gradients to provide constraints on the amount and extent of feedback produced in star-forming galaxies. We observed 15 star-forming galaxies at z ~ 2 with OSIRIS to obtain their kinematic properties and gas-phase metallicity gradients. With helps from AO correction and gravitational lensing, the typical spatial resolution in our study is less than a half-light radius of a typical L* galaxies at z ≃ 2. Combining with the sample in Jones et al., 2013, we approximately tripled the existing metallicity gradient measurements. We found a lower fraction of rotationally-supported systems than reported from larger kinematic surveys with coarser spatial resolution, which might be partially due to a our improved spatial resolution. We demonstrated that a high spatial resolution is crucial for an accurate diagnosis of the kinematic properties and dynamical maturity of z ≃ 2 galaxies. As for metallicity gradients, we found a much higher fraction of z ≃ 2 galaxies having weak or flat metallicity gradients than in previous studies. We correlated the metallicity gradient with the total metallicity and found that all galaxies with low total metallicities have flat gradients ( 0.1), there is a divergence between isolated or rotationally-supported and dynamically-immature systems with the latter showing zero gradients irrespective of the integrated metallicity. The results indicate that relatively strong feedback (e.g. high mass loading factors or high SN energy output) is required in order to explain the majority of the observed flat gradients. In the second part of the thesis, we observed quiescent galaxies at z Lastly, we measured magnesium (Mg) abundances and extended the observed redshift to z ~ 0.55. We found that while the mass-[Fe/H] relation evolves significantly over the observed redshift range, the mass-[Mg/H] relation does not. This is due to the shorter star formation histories of quiescent galaxies at higher redshifts. Fe is mainly produced in Type Ia SN. It has a longer recycling time than Mg, which is mainly produced in core-collapse SN. Using core-collapse SN elements as a metal indicator lessens the complication of delayed recycling time and allows us to effectively use galactic chemical models with instantaneous recycling to quantify average outflows that these galaxies experience over their lifetime. We found that the average mass-loading factor η is a power-law function of galaxy stellar mass, η ∝ M*-0.21±0.09, consistent with the results of other observational methods and with the predictions where outflow is caused by star formation feedback in turbulent disks.

Results from EDGES High-band. I. Constraints on Phenomenological Models for the Global 21 cm Signal

Abstract We report constraints on the global 21 cm signal due to neutral hydrogen at redshifts 14.8 ≥ z ≥ 6.5 . We derive our constraints from low-foreground observations of the average sky brightness spectrum conducted with the EDGES High-band instrument between 2015 September 7 and October 26. Observations were calibrated by accounting for the effects of antenna beam chromaticity, antenna and ground losses, signal reflections, and receiver parameters. We evaluate the consistency between the spectrum and phenomenological models for the global 21 cm signal. For tanh-based representations of the ionization history during the epoch of reionization, we rule out, at ≥ 2 σ significance, models with duration of up to Δ z = 1 at z ≈ 8.5 and higher than Δ z = 0.4 across most of the observed redshift range under the usual assumption that the 21 cm spin temperature is much larger than the temperature of the cosmic microwave background during reionization. We also investigate a “cold” intergalactic medium (IGM) scenario that assumes perfect Lyα coupling of the 21 cm spin temperature to the temperature of the IGM, but that the latter is not heated by early stars or stellar remants. Under this assumption, we reject tanh-based reionization models of duration Δ z ≲ 2 over most of the observed redshift range. Finally, we explore and reject a broad range of Gaussian models for the 21 cm absorption feature expected in the First Light era. As an example, we reject 100 mK Gaussians with duration (full width at half maximum) Δ z ≤ 4 over the range 14.2 ≥ z ≥ 6.5 at ≥ 2 σ significance.

The VLA-COSMOS 3 GHz Large Project: Cosmic star formation history sincez~ 5

We make use of the deep Karl G. Jansky Very Large Array (VLA) COSMOS radio observations at 3 GHz to infer radio luminosity functions of star-forming galaxies up to redshifts of z~5 based on approximately 6000 detections with reliable optical counterparts. This is currently the largest radio-selected sample available out to z~5 across an area of 2 square degrees with a sensitivity of rms=2.3 ujy/beam. By fixing the faint and bright end shape of the radio luminosity function to the local values, we find a strong redshift trend that can be fitted with a pure luminosity evolution L~(1+z)^{(3.16 +- 0.2)-(0.32 +- 0.07) z}. We estimate star formation rates (SFRs) from our radio luminosities using an infrared (IR)-radio correlation that is redshift dependent. By integrating the parametric fits of the evolved luminosity function we calculate the cosmic SFR density (SFRD) history since z~5. Our data suggest that the SFRD history peaks between 2<z<3 and that the ultraluminous infrared galaxies (ULIRGs; 100 Msol/yr<SFR<1000 Msol/yr) contribute up to ~25% to the total SFRD in the same redshift range. Hyperluminous infrared galaxies (HyLIRGs; SFR>1000 Msol/yr) contribute an additional <2% in the entire observed redshift range. We find evidence of a potential underestimation of SFRD based on ultraviolet (UV) rest-frame observations of Lyman break galaxies (LBGs) at high redshifts (z>4) on the order of 15-20%, owing to appreciable star formation in highly dust-obscured galaxies, which might remain undetected in such UV observations.

A cosmological exclusion plot: towards model-independent constraints on modified gravity from current and future growth rate data

Most cosmological constraints on modified gravity are obtained assuming that the cosmic evolution was standard ΛCDM in the past and that the present matter density and power spectrum normalization are the same as in a ΛCDM model. Here we examine how the constraints change when these assumptions are lifted. We focus in particular on the parameter Y (also called Geff) that quantifies the deviation from the Poisson equation. This parameter can be estimated by comparing with the model-independent growth rate quantity fσ8(z) obtained through redshift distortions. We reduce the model dependency in evaluating Y by marginalizing over σ8 and over the initial conditions, and by absorbing the degenerate parameter Ωm,0 into Y. We use all currently available values of fσ8(z). We find that the combination Ŷ=YΩm,0, assumed constant in the observed redshift range, can be constrained only very weakly by current data, Ŷ=0.28−0.23+0.35 at 68% c.l. We also forecast the precision of a future estimation of Ŷ in a Euclid-like redshift survey. We find that the future constraints will reduce substantially the uncertainty, Ŷ=0.30−0.09+0.08 , at 68% c.l., but the relative error on Ŷ around the fiducial remains quite high, of the order of 30%. The main reason for these weak constraints is that Ŷ is strongly degenerate with the initial conditions, so that large or small values of Ŷ are compensated by choosing non-standard initial values of the derivative of the matter density contrast.Finally, we produce a forecast of a cosmological exclusion plot on the Yukawa strength and range parameters, which complements similar plots on laboratory scales but explores scales and epochs reachable only with large-scale galaxy surveys. We find that future data can constrain the Yukawa strength to within 3% of the Newtonian one if the range is around a few Megaparsecs. In the particular case of f(R) models, we find that the Yukawa range will be constrained to be larger than 80 Mpc/h or smaller than 2 Mpc/h (95% c.l.), regardless of the specific f(R) model.

Model-independent constraints on the cosmological anisotropic stress

The effective anisotropic stress or gravitational slip $\eta=-\Phi/\Psi$ is a key variable in the characterisation of the physical origin of the dark energy, as it allows to test for a non-minimal coupling of the dark sector to gravity in the Jordan frame. It is however important to use a fully model-independent approach when measuring $\eta$ to avoid introducing a theoretical bias into the results. In this paper we forecast the precision with which future large surveys can determine $\eta$ in a way that only relies on directly observable quantities. In particular, we do not assume anything concerning the initial spectrum of perturbations, nor on its evolution outside the observed redshift range, nor on the galaxy bias. We first leave $\eta$ free to vary in space and time and then we model it as suggested in Horndeski models of dark energy. Among our results, we find that a future large scale lensing and clustering survey can constrain $\eta$ to within 10% if $k$-independent, and to within 60% or better at $k=0.1 h/$Mpc if it is restricted to follow the Horndeski model.

On the nature of dark energy: the lattice Universe

There is something unknown in the cosmos. Something big. Which causes the acceleration of the Universe expansion, that is perhaps the most surprising and unexpected discovery of the last decades, and thus represents one of the most pressing mysteries of the Universe. The current standard $\Lambda$CDM model uses two unknown entities to make everything fit: dark energy and dark matter, which together would constitute more than 95% of the energy density of the Universe. A bit like saying that we have understood almost nothing, but without openly admitting it. Here we start from the recent theoretical results that come from the extension of general relativity to antimatter, through CPT symmetry. This theory predicts a mutual gravitational repulsion between matter and antimatter. Our basic assumption is that the Universe contains equal amounts of matter and antimatter, with antimatter possibly located in cosmic voids, as discussed in previous works. From this scenario we develop a simple cosmological model, from whose equations we derive the first results. While the existence of the elusive dark energy is completely replaced by gravitational repulsion, the presence of dark matter is not excluded, but not strictly required, as most of the related phenomena can also be ascribed to repulsive-gravity effects. With a matter energy density ranging from $\sim5%$ (baryonic matter alone, and as much antimatter) to $\sim25%$ of the so-called critical density, the present age of the Universe varies between about 13 and $15\rm\,Gyr$. The SN Ia test is successfully passed, with residuals comparable with those of the $\Lambda$CDM model in the observed redshift range, but with a clear prediction for fainter SNe at higher $z$. Moreover, this model has neither horizon nor coincidence problems, and no initial singularity is requested. In conclusion, we have replaced all the tough problems of the current

Can galactic outflows explain the properties of Ly α emitters?

We study the properties of Ly-alpha emitters in a cosmological framework by computing the escape of Ly-alpha photons through galactic outflows. We combine the GALFORM semi-analytical model of galaxy formation with a Monte Carlo Ly-alpha radiative transfer code. The properties of Ly-alpha emitters at 0<z<7 are predicted using two outflow geometries: a Shell of neutral gas and a Wind ejecting material, both expanding at constant velocity. We characterise the differences in the Ly-alpha line profiles predicted by the two outflow geometries in terms of their width, asymmetry and shift from the line centre for a set of outflows with different hydrogen column densities, expansion velocities and metallicities. In general, the Ly-alpha line profile of the Shell geometry is broader and more asymmetric, and the Ly-alpha escape fraction is lower than with the Wind geometry for the same set of parameters. In order to implement the outflow geometries in the semi-analytical model GALFORM, a number of free parameters in the outflow model are set by matching the luminosity function of Ly-alpha emitters over the whole observed redshift range. The models are consistent with the observationally inferred Ly-alpha escape fractions, equivalent width distributions and with the shape of the Ly-alpha line from composite spectra. Interestingly, our predicted UV luminosity function of Ly-alpha emitters and the fraction of Ly-alpha emitters in Lyman-break galaxy samples at high redshift are in partial agreement with observations. Attenuation of the Ly-alpha line by the presence of a neutral intergalactic medium at high redshift could be responsible for this disagreement. We predict that Ly-alpha emitters constitute a subset of the galaxy population with lower metallicities, lower instantaneous star formation rates and larger sizes than the overall population at the same UV luminosity.

THE GREATER IMPACT OF MERGERS ON THE GROWTH OF MASSIVE GALAXIES: IMPLICATIONS FOR MASS ASSEMBLY AND EVOLUTION SINCEz≃ 1

Using deep infrared observations conducted with the MOIRCS on the Subaru Telescope in GOODS-N combined with public surveys in GOODS-S, we investigate the dependence on stellar mass, M_*, and galaxy type of the close pair fraction (5 kpc < r < 20 kpc) and implied merger rate. In common with some recent studies we find that the fraction of paired systems that could result in major mergers is low (~4%) and does not increase significantly with redshift to z~1.2, with (1+z)^{1.6 \pm 1.6}. Our key finding is that massive galaxies with M_* > 1E11 Msun are more likely to host merging companions than less massive systems (M_* ~ 1E10 Msun). We find evidence for a higher pair fraction for red, spheroidal hosts compared to blue, late-type systems, in line with expectations based on clustering at small scales. So-called "dry" mergers between early-type galaxies represent nearly 50% of close pairs with M_* > 3E10 Msun at z~0.5, but less than 30% at z~1. This result can be explained by the increasing abundance of red, early-type galaxies at these masses. We compare the volumetric merger rate of galaxies with different masses to mass-dependent trends in galaxy evolution, finding that major mergers cannot fully account for the formation of spheroidal galaxies since z~1. In terms of mass assembly, major mergers contribute little to galaxy growth below M_* ~ 3E10 Msun but are more significant among galaxies with M_* > 1E11 Msun, 30% of which have undergone mostly dry mergers over the observed redshift range. Overall, the relatively more rapid coalescence of high mass galaxies mirrors the expected hierarchical growth of halos and is consistent with recent model predictions, even if the downsizing of star formation and morphological evolution involves additional physical processes.

The star formation history of intermediate-redshift late-type galaxies

We combine the latest observations of disc galaxy photometry and rotation curves at moderate redshift from the FORS Deep Field (FDF) with simple models of chemical enrichment. Our method describes the build-up of the stellar component through infall of gas and allows for gas and metal outflows. In this framework, we keep a minimum number of constraints and we search a large volume of parameter space, looking for the models that best reproduce the photometric observations in the observed redshift range (0.5 < z < 1). We find that star formation efficiency correlates well with v max ,s othat massive discs are more efficient in the formation of stars and have a smaller spread in stellar ages. This trend presents a break at around v max ∼ 140 km s −1 . Galaxies on either side of this threshold have significantly different age distributions. This break has been already suggested by several authors in connection with the contribution from either gravitational instabilities or supernova-driven turbulence to star formation. The gas infall time-scale and gas outflows also present a correlation with galaxy mass, so that massive discs have shorter infall time-scales and smaller outflow fractions. The model presented in this paper suggests that massive discs have formation histories resembling those of early-type galaxies, with highly efficient and short-lived bursts, in contrast with low-mass discs, which have a more extended star formation history. The ages correlate well with galaxy mass or luminosity, and the predicted gas-phase metallicities are consistent with the observations of local and moderate-redshift galaxies. One option to explain the observed shallow slope of the Tully‐Fisher relation at intermediate redshift could be small episodes of star formation in low-mass discs. Ke yw ords: galaxies: evolution ‐ galaxies: formation ‐ galaxies: spiral ‐ galaxies: stellar content.

The Cosmological Evolution of Metal Enrichment in Quasar Host Galaxies

We study the gas metallicity of quasar hosts using cosmological hydrodynamic simulations of the ?CDM model. Galaxy formation in the simulations is coupled with a prescription for black hole activity, enabling us to study the evolution of the metal enrichment in quasar hosts and hence explore the relationship between star/spheroid formation and black hole growth/activity. In order to assess effects of numerical resolution, we compare simulations with different particle numbers and box sizes. We find a steep radial metallicity gradient in quasar host galaxies, with gas metallicities close to solar values in the outer parts but becoming supersolar in the center. The hosts of the rare bright quasars at z ~ 5-6 have star formation rates of several hundred M? yr-1 and halo masses of order ~1012 M?. Already at these redshifts they have supersolar (Z/Z? ~ 2-3) central metallicities, with a mild dependence of metallicity on luminosity, consistent with observed trends. The mean value of metallicity is sensitive to the assumed quasar lifetime, providing a useful new probe of this parameter. We find that lifetimes from 107 to 4 ? 107 yr are favored by comparison with observational data. In both the models and observations, the rate of evolution of the mean quasar metallicity as a function of redshift is generally flat out to z ~ 4-5. Beyond the observed redshift range and out to redshift z = 6-8, we predict a slow decline of the mean central metallicity toward solar and slightly subsolar values (Z/Z? ~ 0.4-1) as we approach the epoch of the first significant star formation activity.

The Evolution of Radio Galaxies at Intermediate Redshift

We describe a new estimate of the radio galaxy 1.4 GHz luminosity function and its evolution at intermediate redshifts (z~0.4). Photometric redshifts and color selection have been used to select Bj<23.5 early-type galaxies from the Panoramic Deep Fields, a multicolor survey of two 25 sq deg fields. Approximately 230 radio galaxies have then been selected by matching early-type galaxies with NVSS radio sources brighter than 5 mJy. Estimates of the 1.4 GHz luminosity function of radio galaxies measure significant evolution over the observed redshift range. For an Omega_M=1 cosmology the evolution of the radio power is consistent with luminosity evolution where P(z)=P(0)(1+z)^{k_L} and 3<k_L<5. The observed evolution is similar to that observed for UVX and X-ray selected AGN and is consistent with the same physical process being responsible for the optical and radio luminosity evolution of AGN.

Evolution of Lyman Break Galaxies beyond z = 4.

The formation rate of luminous galaxies seems to be roughly constant from z approximately 2 to approximately 4 from the recent observations of Lyman break galaxies (LBGs). The abundance of luminous quasars, on the other hand, appears to drop off by a factor of more than 20 from z approximately 2 to z approximately 5. The difference in evolution between the two classes of objects in the overlapping, observed redshift range (z=2-4) can be explained naturally if we assume that quasar activity is triggered by mergers of luminous LBGs and one quasar lifetime is approximately 107-108 yr. If this merger scenario holds at higher redshift, for the evolutions of these two classes of objects to be consistent at z>4, the formation rate of luminous LBGs is expected to drop off at least as rapidly as exp-&parl0;z-4&parr0;6&solm0;5 at z>4.

Intergalactic Helium Absorption in Cold Dark Matter Models

Observations from the Hopkins Ultraviolet Telescope and the Hubble Space Telescope have recently detected He II absorption along the lines of sight to two high-redshift quasars. We use cosmological simulations with gas dynamics to investigate He II absorption in the cold dark matter (CDM) theory of structure formation. We consider two Ω = 1 CDM models with different normalizations and one open universe (Ω0 = 0.4) CDM model. The simulations incorporate the photoionizing UV background spectrum computed by Haardt & Madau, which is based on the output of observed quasars and reprocessing by the Lyα forest. The simulated gas distribution, combined with the Haardt & Madau spectral shape, accounts for the relative observed values of H I and He II, the effective mean optical depths for H I and He II absorption. If the background intensity is as high as Haardt & Madau predict, then matching the absolute observed values of H I and He II requires a baryon abundance larger (by factors between 1.5 and 3 for the various CDM models) than our assumed value of Ωbh2 = 0.0125. The simulations reproduce the evolution of He II over the observed redshift range, 2.2 z 3.3, if the He II photoionization rate remains roughly constant. He II absorption in the CDM simulations is produced by a diffuse, fluctuating, intergalactic medium, which also gives rise to the H I Lyα forest. Much of the He II opacity arises in underdense regions where the H I optical depth is very low. We compute statistical properties of the He II and H I absorption that can be used to test the CDM models and distinguish them from an alternative scenario in which the He II absorption is caused by discrete, compact clouds. The CDM scenario predicts that a substantial amount of baryonic material resides in underdense regions at high redshift. He II absorption is the only sensitive observational probe of such extremely diffuse, intergalactic gas, so it can provide a vital test of this fundamental prediction.

Evolution of Cluster and Field Elliptical Galaxies at 0.2 &lt; [ITAL]z[/ITAL] &lt; 0.6 in the CNOC Cluster Survey

Two-dimensional surface photometry has been done for 166 early-type galaxies (bulge/total luminosity B/T > 0.6) in three fields of the Canadian Network for Observational Cosmology (CNOC) cluster survey. These galaxies are either spectroscopically confirmed members of clusters at z = 0.23 (45 galaxies), 0.43 (22), and 0.55 (16) or field galaxies in the same redshift range. An additional 51 early-type galaxies in the rich cluster Abell 2256 at z = 0.06 were analyzed with the same technique. The resulting structural and surface brightness measurements show that, in the plane of absolute magnitude MAB(B) versus log Re (half-light radius), the locus of cluster elliptical galaxies shifts monotonically with redshift so that at redshifts of (0.23, 0.43, 0.55), galaxies of a given size are more luminous by -0.25 ± 0.10, -0.55 ± 0.12, and -0.74 ± 0.21 mag with respect to the same relation measured at z = 0.06 (adopting q0 = 0.5). There is no evidence that early-type galaxies in the field evolve differently from those in clusters, although we emphasize that our cluster sample is dominated by galaxies far from the dense cluster core. If dynamical processes do not substantially modify the size-luminosity relation for early-type galaxies over the observed redshift range, then these galaxies have undergone significant luminosity evolution over the past half of the age of the universe. The amount of brightening is consistent with passive evolution models of old, single-burst stellar populations.

The redshift-distance relation.

Key predictions of the Hubble law are inconsistent with direct observations on equitable complete samples of extragalactic sources in the optical, infrared, and x-ray wave bands-e.g., the predicted dispersion in apparent magnitude is persistently greatly in excess of its observed value, precluding an explanation via hypothetical perturbations or irregularities. In contrast, the predictions of the Lundmark (homogeneous quadratic) law are consistent with the observations. The Lundmark law moreover predicts the deviations between Hubble law predictions and observation with statistical consistency, while the Hubble law provides no explanation for the close fit of the Lundmark law. The flux-redshift law F [symbol, see text] (1 + z)/z appears consistent with observations on equitable complete samples in the entire observed redshift range, when due account is taken of flux limits by an optimal statistical method. Under the theoretical assumption that space is a fixed sphere, as in the Einstein universe, this law implies the redshift-distance relation z = tan2(r/2R), where R is the radius of the spherical space. This relation coincides with the prediction of chronometric cosmology, which estimates R as 160 +/- 40 Mpc (1 parsec = 3.09 x 10(16) m) from the proper motion to redshift relation of superluminal sources. Tangential aspects, including statistical methodology, fundamental physical theory, bright cluster galaxy samples, and proposed luminosity evolution, are briefly considered.