15295782
Probing quintessence: reconstruction and parameter estimation from supernovae
We explore the prospects for using future supernova observations to probe the dark energy. We focus on quintessence, an evolving scalar field that has been suggested as a candidate for the dark energy. After simulating the observations that would be expected from the proposed SuperNova/Acceleration Probe satellite (SNAP), we investigate two methods for extracting information concerning quintessence from such data. First, by expanding the quintessence equation of state as w Q (z) = w Q (0) - a In(1 + z), to fit the data, it is possible to reconstruct the quintessence potential for a wide range of smoothly varying potentials. Secondly, it will be possible to test the basic properties of the dark energy by constraining the parameters Ω Q , w Q and a. We show that it may be possible, for example, to distinguish between quintessence and the cosmological constant in this way. Furthermore, when supernova data are combined with other planned cosmological observations, the precision of reconstructions and parameter constraints is significantly improved, allowing a wider range of dark energy models to be distinguished.
116986790
Phenomenological classification of inflationary potentials
We investigate the Lyth relationship between the tensor‐scalar ratio, r, and the variation of the inflaton field, Δφ, over the course of inflation. We compute r and Δφ for 106 single‐field inflationary models generated using the the Monte Carlo reconstruction method of Easther and Kinney. Despite a large scatter, our results show that the Lyth relation provides a useful classification of inflationary models since a detectable value of the tenser‐scalar ratio necessarily requires an inflaton field variation of the order of the Planck mass. Inflationary models that satisfy current observational constraints on the scalar spectral index and its first derivative satisfy a much tighter relation between r and Δφ. For example, all models with r ≳ 0.01 (roughly the limit at which lensing of E‐modes produces a B‐mode signal comparable to those generated by inflation), require a field variation Δφ ≳ 1.5mPl. Such a large variation probably requires that the underlying theory of inflation possesses a shift symmetry φ ...
119351553
Observational constraints on dark matter in the universe
Abstract Application of the cosmic virial theorem to galaxy redshift surveys suggests an open universe - if the dark material is distributed in the same way as galaxies. Here we investigate four redshift surveys. We present new results on the amplitudes of the two- and three-point galaxy correlation functions and we estimate the relative peculiar velocities in the deep redshift survey of Kirshner and co-workers (1983). Next, we review evidence which suggests that dark material is clustered on scales comparable to the optical radii of galaxies. In particular, we use spectroscopic and photometric observations to set constraints on the mass distribution in elliptical galaxies.
15478988
The two-point correlation function of rich clusters of galaxies: results from an extended APM cluster redshift survey
We present new estimates of the spatial two-point correlation function of rich clusters of galaxies selected from the APM Galaxy Survey. We have measured redshifts for a sample of $364$ clusters out to a depth of $\sim 450\hmpc$. The clusters have a mean space density of $\bar{n} = 3.4\times 10^{-5}\hmpccc$. The two-point correlation function, $\xi_{cc}$, for this sample is equal to unity at a pair-separation of $r_0 = 14.3\pm1.75\hmpc$ (2$\sigma$ errors), consistent with our earlier results from a smaller sample. The new observations provide an accurate determination of the shape of $\xi_{cc}$ to pair-separations of about $50\hmpc$. Our results show that $\xi_{cc}$ has a higher amplitude than expected according to the standard $\Omega=1$ cold dark matter (CDM) model on spatial scales $2\simlt s \simlt 50\hmpc$, but are in good agreement with scale-invariant fluctuations in either a low density CDM model or a critical density universe made up of a mixture of hot and cold dark matter. Our results provide strong constraints on so called `co-operative' models of galaxy formation in which the galaxy formation process introduces large-scale structure in the galaxy distribution.
12209082
Measuring the equation of state of the intergalactic medium
Numerical simulations indicate that the smooth, photoionized intergalactic medium (IGM) responsible for the low column density Lyα forest follows a well-defined temperature–density relation, which is well described by a power law . We demonstrate that such an equation of state results in a power-law cut-off in the distribution of linewidths (b-parameters) as a function of column density (N) for the low column density (N≲1014.5 cm−2) absorption lines. This explains the existence of the lower envelope that is clearly seen in scatter plots of the b(N) distribution in observed QSO spectra. Even a strict power-law equation of state will not result in an absolute cut-off because of line blending and contamination by unidentified metal lines. We develop an algorithm to determine the cut-off, which is insensitive to these narrow lines. We show that the parameters of the cut-off in the b(N) distribution are strongly correlated with the parameters of the underlying equation of state. We use simulations to determine these relations, which can then be applied to the observed cut-off in the b(N) distribution to measure the equation of state of the IGM. We show that systematics that change the b(N) distribution, such as cosmology (for a fixed equation of state), peculiar velocities, the intensity of the ionizing background radiation and variations in the signal-to-noise ratio, do not affect the measured cut-off. We argue that physical processes that have not been incorporated in the simulations, e.g. feedback from star formation, are unlikely to affect the results. Using Monte Carlo simulations of Keck spectra at z=3, we show that determining the slope of the equation of state will be difficult, but that the amplitude can be determined to within 10 per cent, even from a single QSO spectrum. Measuring the evolution of the equation of state with redshift will allow us to put tight constraints on the reionization history of the Universe.
53696073
The contribution of star-forming galaxies to fluctuations in the cosmic background light
Star-forming galaxies which are too faint to be detected individually produce intensity fluctuations in the cosmic background light. This contribution needs to be taken into account as a foreground when using the primordial signal to constrain cosmological parameters. The extragalactic fluctuations are also interesting in their own right as they depend on the star formation history of the Universe and the way in which this connects with the formation of cosmic structure. We present a new framework which allows us to predict the occupation of dark matter haloes by star-forming galaxies and uses this information, in conjunction with an N-body simulation of structure formation, to predict the power spectrum of intensity fluctuations in the infrared background. We compute the emission from galaxies at far-infrared, millimetre and radio wavelengths. Our method gives accurate predictions for the clustering of galaxies for both the one-halo and two-halo terms. We illustrate our new framework using a previously published model which reproduces the number counts and redshift distribution of galaxies selected by their emission at Graphicm. Without adjusting any of the model parameters, the predictions show encouraging agreement at high frequencies and on small angular scales with recent estimates of the extragalactic fluctuations in the background made from early data analysed by the Planck Collaboration. There are, however, substantial discrepancies between the model predictions and observations on large angular scales and at low frequencies, which illustrates the usefulness of the intensity fluctuations as a constraint on galaxy formation models.
28381668
QUANTITATIVE SPECTROSCOPY OF BLUE SUPERGIANT STARS IN THE DISK OF M81: METALLICITY, METALLICITY GRADIENT, AND DISTANCE
The quantitative spectral analysis of low-resolution (~5 ?) Keck LRIS spectra of blue supergiants in the disk of the giant spiral galaxy M81 is used to determine stellar effective temperatures, gravities, metallicities, luminosities, interstellar reddening, and a new distance using the flux-weighted gravity-luminosity relationship. Substantial reddening and extinction are found with E(B ? V) ranging between 0.13 and 0.38?mag and an average value of 0.26?mag. The distance modulus obtained after individual reddening corrections is 27.7 ? 0.1?mag. The result is discussed with regard to recently measured tip of the red giant branch and Cepheid distances. The metallicities (based on elements such as iron, titanium, magnesium) are supersolar (0.2?dex) in the inner disk (R? 5?kpc) and slightly subsolar ( ? 0.05?dex) in the outer disk (R? 10?kpc) with a shallow metallicity gradient of 0.034?dex?kpc?1. The comparison with published oxygen abundances of planetary nebulae and metallicities determined through fits of Hubble Space Telescope color-magnitude diagrams indicates a late metal enrichment and a flattening of the abundance gradient over the last 5?Gyr. This might be the result of gas infall from metal-rich satellite galaxies. Combining these M81 metallicities with published blue supergiant abundance studies in the Local Group and the Sculptor Group, a galaxy mass-metallicity relationship based solely on stellar spectroscopic studies is presented and compared with recent studies of Sloan Digital Sky Survey star-forming galaxies.