Non-Gravitational Contributions to the Clustering of Ly-alpha Selected Galaxies: Implications for Cosmological Surveys

We show that the dependence of Ly-alpha absorption on environment leads to significant non-gravitational features in the redshift space power-spectrum of Ly-alpha selected galaxies. We derive a physically motivated fitting formula that can be included in clustering analyses, and use this to discuss the predicted features in the Ly-alpha galaxy power-spectrum based on detailed models in which Ly-alpha absorption is influenced by gas infall and/or by strong galactic outflows. We show that power-spectrum measurements could be used to study the astrophysics of the galaxy-IGM connection, and to measure the properties of outflows from star-forming galaxies. Applying the modified redshift space power-spectrum to a Ly-alpha survey with parameters corresponding to the planned Hobby-Eberly Telescope Dark Energy Experiment (HETDEX), we find that the dependence of observed Ly-alpha flux on velocity gradient and ionising background may compromise the ability of Ly-alpha selected galaxy redshift surveys to constrain cosmology using information from the full power-spectrum. This is because the effects of fluctuating ionizing background and velocity gradients effect the shape of the observed power-spectrum in ways that are similar to the shape of the primordial power-spectrum and redshift space distortions respectively. We use the Alcock-Paczynski test to show that without prior knowledge of the details of Ly-alpha absorption in the IGM, the precision of line-of-sight and transverse distance measurements for HETDEX will be ~1.3-1.7%, decreased by a factor of ~1.5-2 relative to the best case precision of ~0.8% available in a traditional galaxy redshift survey. We specify the precision with which modelling of Ly-alpha radiative transfer must be understood in order for HETDEX to achieve distance measurements that are better than 1%.Comment: 24 pages, 13 figures. Submitted to MNRA

Measuring our universe from galaxy redshift surveys

Galaxy redshift surveys have achieved significant progress over the last couple of decades. Those surveys tell us in the most straightforward way what our local universe looks like. While the galaxy distribution traces the bright side of the universe, detailed quantitative analyses of the data have even revealed the dark side of the universe dominated by non-baryonic dark matter as well as more mysterious dark energy (or Einstein's cosmological constant). We describe several methodologies of using galaxy redshift surveys as cosmological probes, and then summarize the recent results from the existing surveys. Finally we present our views on the future of redshift surveys in the era of Precision Cosmology.Comment: 82 pages, 31 figures, invited review article published in Living Reviews in Relativity, http://www.livingreviews.org/lrr-2004-

Cosmology with the Square Kilometre Array

We argue that the Square Kilometre Array has the potential to make both redshift (HI) surveys and radio continuum surveys that will revolutionize cosmological studies, provided that it has sufficient instantaneous field-of-view that these surveys can cover a hemisphere in a timescale ~1 yr. Adopting this assumption, we focus on two key experiments which will yield fundamental new measurements in cosmology, characterizing the properties of the mysterious dark energy which dominates the dynamics of today's Universe. Experiment I will map out ~10^9 HI galaxies to redshift z~1.5, providing the premier measurement of the clustering power spectrum of galaxies: accurately delineating the acoustic oscillations and the `turnover'. Experiment II will quantify the cosmic shear distortion of ~10^10 radio continuum sources, determining a precise power spectrum of the dark matter, and its growth as a function of cosmic epoch. We contrast the performance of the SKA in precision cosmology with that of other facilities which will, probably or possibly, be available on a similar timescale. We conclude that data from the SKA will yield transformational science as the direct result of four key features: (i) the immense cosmic volumes probed, exceeding future optical redshift surveys by more than an order of magnitude; (ii) well-controlled systematic effects such as the narrow `k-space window function' for Experiment I and the accurately-known `point-spread function' for Experiment II; (iii) the ability to measure with high precision large-scale modes in the clustering power spectra, for which nuisance effects such as non-linear structure growth, peculiar velocities and `galaxy bias' are minimised; and (iv) different degeneracies between key parameters to those which are inherent in the CMB.Comment: 20 pages, 8 figures. To appear in "Science with the Square Kilometer Array", eds. C.Carilli and S.Rawlings, New Astronomy Reviews (Elsevier: Amsterdam

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