Is the Redshift Clustering of Long-Duration Gamma-Ray Bursts Significant?

The 26 long-duration gamma-ray bursts (GRBs) with known redshifts form a distinct cosmological set, selected differently than other cosmological probes such as quasars and galaxies. Since the progenitors are now believed to be connected with active star-formation and since burst emission penetrates dust, one hope is that with a uniformly-selected sample, the large-scale redshift distribution of GRBs can help constrain the star-formation history of the Universe. However, we show that strong observational biases in ground-based redshift discovery hamper a clean determination of the large-scale GRB rate and hence the connection of GRBs to the star formation history. We then focus on the properties of the small-scale (clustering) distribution of GRB redshifts. When corrected for heliocentric motion relative to the local Hubble flow, the observed redshifts appear to show a propensity for clustering: 8 of 26 GRBs occurred within a recession velocity difference of 1000 km/s of another GRB. That is, 4 pairs of GRBs occurred within 30 h_65^-1 Myr in cosmic time, despite being causally separated on the sky. We investigate the significance of this clustering. Comparison of the numbers of close redshift pairs expected from the simulation with that observed shows no significant small-scale clustering excess in the present sample; however, the four close pairs occur only in about twenty percent of the simulated datasets (the precise significance of the clustering is dependent upon the modeled biases). We conclude with some impetuses and suggestions for future precise GRB redshift measurements.Comment: Published in the Astronomical Journal, June 2003: see http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2003AJ....125.2865

Cluster versus POTENT Density and Velocity Fields: Cluster Biasing and Omega

The density and velocity fields as extracted from the Abell/ACO clusters are compared to the corresponding fields recovered by the POTENT method from the Mark~III peculiar velocities of galaxies. In order to minimize non-linear effects and to deal with ill-sampled regions we smooth both fields using a Gaussian window with radii ranging between 12 - 20\hmpc. The density and velocity fields within 70\hmpc exhibit similarities, qualitatively consistent with gravitational instability theory and a linear biasing relation between clusters and mass. The random and systematic errors are evaluated with the help of mock catalogs. Quantitative comparisons within a volume containing $\sim\!12$ independent samples yield \betac\equiv\Omega^{0.6}/b_c=0.22\pm0.08, where $b_c$ is the cluster biasing parameter at 15\hmpc. If $b_c \sim 4.5$, as indicated by the cluster correlation function, our result is consistent with $\Omega \sim 1$.Comment: 18 pages, latex, 2 ps figures 6 gif figures. Accepted for pubblications in MNRA

The dwarf low surface brightness population in different environments of the Local Universe

The nature of the dwarf galaxy population as a function of location in the cluster and within different environments is investigated. We have previously described the results of a search for low surface brightness objects in data drawn from an East-West strip of the Virgo cluster (Sabatini et al., 2003) and have compared this to a large area strip outside of the cluster (Roberts et al., 2004). In this talk I compare the East-West data (sampling sub-cluster A and outward) to new data along a North-South cluster strip that samples a different region (part of sub-cluster A, and the N,M clouds) and with data obtained for the Ursa Major cluster and fields around the spiral galaxy M101. The sample of dwarf galaxies in different environments is obtained from uniform datasets that reach central surface brightness values of ~26 B mag/arcsec^2 and an apparent B magnitude of 21 (M_B=-10 for a Virgo Cluster distance of 16 Mpc). We discuss and interpret our results on the properties and distribution of dwarf low surface brightness galaxies in the context of variuos physical processes that are thought to act on galaxies as they form and evolve.Comment: 10 pages, 3 figures, to appear in "Dark Galaxies and Lost Baryons", IAU244 conference proceeding

Cosmological Parameters from Velocities, CMB and Supernovae

We compare and combine likelihood functions of the cosmological parameters Omega_m, h and sigma_8, from peculiar velocities, CMB and type Ia supernovae. These three data sets directly probe the mass in the Universe, without the need to relate the galaxy distribution to the underlying mass via a "biasing" relation. We include the recent results from the CMB experiments BOOMERANG and MAXIMA-1. Our analysis assumes a flat Lambda CDM cosmology with a scale-invariant adiabatic initial power spectrum and baryonic fraction as inferred from big-bang nucleosynthesis. We find that all three data sets agree well, overlapping significantly at the 2 sigma level. This therefore justifies a joint analysis, in which we find a joint best fit point and 95 per cent confidence limits of Omega_m=0.28 (0.17,0.39), h=0.74 (0.64,0.86), and sigma_8=1.17 (0.98,1.37). In terms of the natural parameter combinations for these data sigma_8 Omega_m^0.6 = 0.54 (0.40,0.73), Omega_m h = 0.21 (0.16,0.27). Also for the best fit point, Q_rms-ps = 19.7 muK and the age of the universe is 13.2 Gyr.Comment: 8 pages, 5 figures. Submitted to MNRA

Nonlinear Peculiar-Velocity Analysis and PCA

We allow for nonlinear effects in the likelihood analysis of peculiar velocities, and obtain ~35%-lower values for the cosmological density parameter and for the amplitude of mass-density fluctuations. The power spectrum in the linear regime is assumed to be of the flat LCDM model (h=0.65, n=1) with only Om_m free. Since the likelihood is driven by the nonlinear regime, we "break" the power spectrum at k_b=0.2 h/Mpc and fit a two-parameter power-law at k>k_b. This allows for an unbiased fit in the linear regime. Tests using improved mock catalogs demonstrate a reduced bias and a better fit. We find for the Mark III and SFI data Om_m=0.35+-0.09$ with sigma_8*Om_m^0.6=0.55+-0.10 (90% errors). When allowing deviations from \lcdm, we find an indication for a wiggle in the power spectrum in the form of an excess near k~0.05 and a deficiency at k~0.1 h/Mpc --- a "cold flow" which may be related to a feature indicated from redshift surveys and the second peak in the CMB anisotropy. A chi^2 test applied to principal modes demonstrates that the nonlinear procedure improves the goodness of fit. The Principal Component Analysis (PCA) helps identifying spatial features of the data and fine-tuning the theoretical and error models. We address the potential for optimal data compression using PCA.Comment: 15 pages, LaTex, in Mining the Sky, July 31 - August 4, 2000, Garching, German

Cosmological Density and Power Spectrum from Peculiar Velocities: Nonlinear Corrections and PCA

We allow for nonlinear effects in the likelihood analysis of galaxy peculiar velocities, and obtain ~35%-lower values for the cosmological density parameter Om and the amplitude of mass-density fluctuations. The power spectrum in the linear regime is assumed to be a flat LCDM model (h=0.65, n=1, COBE) with only Om as a free parameter. Since the likelihood is driven by the nonlinear regime, we "break" the power spectrum at k_b=0.2 h/Mpc and fit a power law at k>k_b. This allows for independent matching of the nonlinear behavior and an unbiased fit in the linear regime. The analysis assumes Gaussian fluctuations and errors, and a linear relation between velocity and density. Tests using proper mock catalogs demonstrate a reduced bias and a better fit. We find for the Mark3 and SFI data Om_m=0.32+-0.06 and 0.37+-0.09 respectively, with sigma_8*Om^0.6 = 0.49+-0.06 and 0.63+-0.08, in agreement with constraints from other data. The quoted 90% errors include cosmic variance. The improvement in likelihood due to the nonlinear correction is very significant for Mark3 and moderately so for SFI. When allowing deviations from LCDM, we find an indication for a wiggle in the power spectrum: an excess near k=0.05 and a deficiency at k=0.1 (cold flow). This may be related to the wiggle seen in the power spectrum from redshift surveys and the second peak in the CMB anisotropy. A chi^2 test applied to modes of a Principal Component Analysis (PCA) shows that the nonlinear procedure improves the goodness of fit and reduces a spatial gradient of concern in the linear analysis. The PCA allows addressing spatial features of the data and fine-tuning the theoretical and error models. It shows that the models used are appropriate for the cosmological parameter estimation performed. We address the potential for optimal data compression using PCA.Comment: 18 pages, LaTex, uses emulateapj.sty, ApJ in press (August 10, 2001), improvements to text and figures, updated reference

Luminescence from highly excited nanorings: Luttinger liquid description

We study theoretically the luminescence from quantum dots of a ring geometry. For high excitation intensities, photoexcited electrons and holes form Fermi seas. Close to the emission threshold, the single-particle spectral lines aquire weak many-body satellites. However, away from the threshold, the discrete luminescence spectrum is completely dominated by many-body transitions. We employ the Luttinger liquid approach to exactly calculate the intensities of all many-body spectral lines. We find that the transition from single-particle to many-body structure of the emission spectrum is governed by a single parameter and that the distribution of peaks away from the threshold is universal.Comment: 10 pages including 2 figure

Molecular Hydrogen and Global Star Formation Relations in Galaxies

(ABRIDGED) We use hydrodynamical simulations of disk galaxies to study relations between star formation and properties of the molecular interstellar medium (ISM). We implement a model for the ISM that includes low-temperature (T<10^4K) cooling, directly ties the star formation rate to the molecular gas density, and accounts for the destruction of H2 by an interstellar radiation field from young stars. We demonstrate that the ISM and star formation model simultaneously produces a spatially-resolved molecular-gas surface density Schmidt-Kennicutt relation of the form Sigma_SFR \propto Sigma_Hmol^n_mol with n_mol~1.4 independent of galaxy mass, and a total gas surface density -- star formation rate relation Sigma_SFR \propto Sigma_gas^n_tot with a power-law index that steepens from n_tot~2 for large galaxies to n_tot>~4 for small dwarf galaxies. We show that deviations from the disk-averaged Sigma_SFR \propto Sigma_gas^1.4 correlation determined by Kennicutt (1998) owe primarily to spatial trends in the molecular fraction f_H2 and may explain observed deviations from the global Schmidt-Kennicutt relation.Comment: Version accepted by ApJ, high-res version available at http://kicp.uchicago.edu/~brant/astro-ph/molecular_ism/rk2007.pd