Observations of high-redshift type Ia supernovae indicate that the universe is accelerating, fueled perhaps by a cosmological constant or by a self-interacting scalar field. In this letter, we develop a model-independent method for estimating the form of the scalar field potential V (φ) and the associated equation of state w φ ≡ p/ε φ . Our method is based on a simple yet powerful analytical form for the luminosity distance DL which is optimized to fit observed distances to distant extragalactic supernovae, and then differentiated to yield V (φ) and w φ . Our results favor w φ −1 at the present epoch, steadily increasing with redshift. However, a cosmological constant is consistent with our results. A model-independent way of obtaining the age of the universe is also proposed.PACS numbers: 98.80. Es, 98.80.Cq, 98.80.Hw, 97.60.Bw The relation between luminosity distance and redshift for extragalactic Type Ia Supernovae (SNe) appears to favor an accelerating Universe, where almost two-thirds of the critical energy density may be in the form of a component with negative pressure [1,2]. On the other hand, several studies of large-scale structure, including those of the abundances of rich galaxy clusters [3] and clustering of galaxies [4] and Lyman-α clouds [5] (for recent reviews, see [6]) indicate low baryonic and matterThis consistency is encouraging since it is well-known that a flat Cold Dark matter Universe with Ω M < 1 and a Cosmological Constant Λ > 0 fits observations of large-scale structure [6,7] better than any other theoretical model.Although Λ = 0 does agree well with the recent SNe observations, it is clear that at a theoretical level a constant Λ runs into serious difficulties, since the present value of Λ is ∼10 123 times smaller than predicted by most particle physics models [7].A time-dependent Λ-like term, which considerably alleviates this fine-tuning problem, can be described in a simple and natural way in terms of a scalar field (referred to here as the Λ-field) with a self-interaction potential V (φ) which is minimally coupled to the Einstein gravity field, and has little or no coupling to any other known physical field. Actually, this model mimics the simplest variant of the inflationary scenario of the early Universe. Since we have not yet got a definite prediction for the form of V (φ) from theoretical considerations, it has to be reconstructed from present-day observations.The aim of the present letter is to go from observations to theory, i.e. from D L (z) to V (φ), following the prescription outlined by Starobinsky [8] (see also [9]). This is the first attempt at reconstructing V (φ) from real observational data without resorting to specific models (e.g. cosmological constant, quintessence etc.).Since the spatially flat Universe (Ω φ + Ω M = 1) is both predicted by the simplest inflationary models and agrees well with observational evidence, we will not consider spatially curved Friedmann-Robertson-Walker (FRW) cosmological models. In a flat FRW cosmology, the luminosity distance D L ...

Ongoing or recent star formation in galaxies is known to increase with increasing projected distance from the centre of a cluster out to several times its virial radius (Rv). Using a complete sample (Mr≤−20.5, 0.02 ≤z≤ 0.15) of galaxies in and around 268 clusters from the Sloan Digital Sky Survey's Fourth Data Release, we investigate how, at a given projected radius from the cluster centre, the stellar mass and star formation properties of a galaxy depend on its absolute line‐of‐sight velocity in the cluster rest frame, |vLOS|. We find that for projected radii R < 0.5 Rv, the fraction of high‐mass non‐brightest cluster galaxies increases towards the centre for low |vLOS|, which may be the consequence of the faster orbital decay of massive galaxies by dynamical friction. At a given projected radius, the fraction of Galaxies with Ongoing or Recent (<1–3 Gyr) Efficient Star Formation [GORES; with EW(Hδ) > 2 Å & Dn4000 > 1.5] is slightly but significantly lower for low |vLOS| galaxies than for their high‐velocity counterparts. We study these observational trends with the help of a dark matter (DM) cosmological simulation. We classify DM particles as virial, infall and backsplash according to their present positions in (r, vr) radial phase space and measure the frequencies of each class in cells of (R, |vLOS|) projected phase space. As expected, the virial class dominates at projected radii R < Rv, while the infall particles dominate outside, especially at high |vLOS|. However, the backsplash particles account for at least one‐third (half) of all particles at projected radii slightly greater than the virial radius and |vLOS| < σv (|vLOS| ≪σv). The deprojection of the GORES fraction leads to a saturated linear increase with radius. We fit simple models of the fraction of GORES as a function of class only or class and distance to the cluster centre (as in our deprojected fraction). While GORES account for 18 ± 1 per cent of all galaxies within the virial cylinder, in our best‐fitting model, they account for 13 ± 1 per cent of galaxies within the virial sphere, 11 ± 1 per cent of the virial population, 34 ± 1 per cent of the distant (for projected radii R < 2 Rv) infall population and 19 ± 4 per cent of the backsplash galaxies. Also, 44 ± 2 per cent of the GORES within the virial cylinder are outside the virial sphere. These fractions are very robust to the precise good‐fitting model and to our scheme for assigning simulation particle classes according to their positions in radial phase space (except for two of our models, where the fraction of GORES reaches 27 ± 4 per cent). Given the 1–3 Gyr lookback time of our GORES indicators, these results suggest that star formation in a galaxy is almost completely quenched in a single passage through the cluster.

With the help of a statistical parameter derived from optical spectra, we show that the current star formation rate of a galaxy, falling into a cluster along a supercluster filament, is likely to undergo a sudden enhancement before the galaxy reaches the virial radius of the cluster. From a sample of 52 supercluster‐scale filaments of galaxies joining a pair of rich clusters of galaxies within the two‐degree Field Redshift Survey region, we find a significant enhancement of star formation, within a narrow range between ∼2 and 3 h−170 Mpc of the centre of the cluster into which the galaxy is falling. This burst of star formation is almost exclusively seen in the fainter dwarf galaxies (MB≥−20). The relative position of the peak does not depend on whether the galaxy is a member of a group or not, but non‐group galaxies have on average a higher rate of star formation immediately before falling into a cluster. From the various trends, we conclude that the predominant process responsible for this rapid burst is the close interaction with other galaxies falling into the cluster along the same filament, if the interaction occurs before the gas reservoir of the galaxy gets stripped off due to the interaction with the intracluster medium.

The evolution of present‐day fossil galaxy groups is studied in the Millennium simulation. Using the corresponding Millennium gas simulation and semi‐analytic galaxy catalogues, we select fossil groups at redshift zero according to the conventional observational criteria, and trace the haloes corresponding to these groups backwards in time, extracting the associated dark matter, gas and galaxy properties. The space density of the fossils from this study is remarkably close to the observed estimates and various possibilities for the remaining discrepancy are discussed. The fraction of X‐ray bright systems which are fossils appears to be in reasonable agreement with observations, and the simulations predict that fossil systems will be found in significant numbers (3–4 per cent of the population) even in quite rich clusters. We find that fossils assemble a higher fraction of their mass at high redshifts, compared to non‐fossil groups, with the ratio of the currently assembled halo mass to final mass, at any epoch, being about 10–20 per cent higher for fossils. This supports the paradigm whereby fossils represent undisturbed, early‐forming systems in which large galaxies have merged to form a single dominant elliptical.

Context. The largest uncertainty for cosmological studies using clusters of galaxies is introduced by our limited knowledge of the statistics of galaxy cluster structure, and of the scaling relations between observables and cluster mass. Aims. To improve on this situation we have started an XMM-Newton Large Programme for the in-depth study of a representative sample of 33 galaxy clusters, selected in the redshift range z = 0.055 to 0.183 from the REFLEX Cluster Survey, having X-ray luminosities above 0.4 × 10 44 h −2 70 erg s −1 in the 0.1−2.4 keV band. This paper introduces the sample, compiles properties of the clusters, and provides detailed information on the sample selection function. Methods. We describe the selection of a nearby galaxy cluster sample that makes optimal use of the XMM-Newton field-of-view, and provides nearly homogeneous X-ray luminosity coverage for the full range from poor clusters to the most massive objects in the Universe. Results. For the clusters in the sample, X-ray fluxes are derived and compared to the previously obtained fluxes from the ROSAT All-Sky Survey. We find that the fluxes and the flux errors have been reliably determined in the ROSAT All-Sky Survey analysis used for the REFLEX Survey. We use the sample selection function documented in detail in this paper to determine the X-ray luminosity function, and compare it with the luminosity function of the entire REFLEX sample. We also discuss morphological peculiarities of some of the sample members. Conclusions. The sample and some of the background data given in this introductory paper will be important for the application of these data in the detailed studies of cluster structure, to appear in forthcoming publications.