Cosmology

Abstract:

[Abridged] We use mock catalogues based on the GALICS model (Hatton et al. 03) to explore the nature of galaxy clustering observed in the SDSS. We measure low and high order angular clustering statistic from these mock catalogues, after selecting galaxies the same way as for observations, and compare them directly to estimates from SDSS data. Note that we also present measurements of S3-S5 on the SDSS DR1. We find that our model is in general good agreement with observations in the scale/luminosity range where we can trust the predictions. This range is found to be limited (i) by the size of the dark matter simulation used -- which introduces finite volume effects at large scales -- and by the mass resolution of this simulation -- which introduces incompleteness at apparent magnitudes fainter than $r\sim 20$. We then focus on the small scale clustering properties of galaxies and investigate the behaviour of three different prescriptions for positioning galaxies within haloes of dark matter. We show that galaxies are poor tracers both of DM particles or DM sub-structures, within groups and clusters. Instead, SDSS data tells us that the distribution of galaxies lies somewhat in between these two populations. This confirms the general theoretical expectation from numerical simulations and semi-analytic modelling.

Abstract:

Gamma-ray bursts (GRBs) and their afterglows are the most brilliant transient events in the Universe. Both the bursts themselves and their afterglows have been predicted to be visible out to redshifts of z ≈ 20, and therefore to be powerful probes of the early Universe^1,2. The burst GRB 000131, at z = 4.50, was hitherto the most distant such event identified³. Here we report the discovery of the bright near-infrared afterglow of GRB 050904 (ref. 4). From our measurements of the near-infrared afterglow, and our failure to detect the optical afterglow, we determine the photometric redshift of the burst to be z = 6.39-0.12^+0.11 (refs 5-7). Subsequently, it was measured⁸ spectroscopically to be z = 6.29 ± 0.01, in agreement with our photometric estimate. These results demonstrate that GRBs can be used to trace the star formation, metallicity, and reionization histories of the early Universe. © 2006 Nature Publishing Group.

Abstract:

We present the fundamental plane (FP) for 38 early-type galaxies in the two rich galaxy clusters RX J0152.7-1357 (z = 0.83) and RX J1226.9+3332 (z = 0.89), reaching a limiting magnitude of MB = -19.8 in the rest frame of the clusters. While the zero-point offset of the FP for these high-redshift clusters relative to our low-redshift sample is consistent with passive evolution with a formation redshift of zform ≈ 3.2, the FP for the high-redshift clusters is not only shifted as expected for a mass-independent zform but rotated relative to the low-redshift sample. Expressed as a relation between the galaxy masses and the mass-to-light ratios, the FP is significantly steeper for the high-redshift clusters than for our low-redshift sample. We interpret this as a mass dependency of the star formation history, as has been suggested by other recent studies. The low-mass galaxies (10 ^10.3 M⊙) have experienced star formation as recently as z ≈ 1.35 (1.5 Gyr prior to their look-back time), while galaxies with masses larger than 10^11.3 M⊙ had their last major star formation episode at z > 4.5. © 2006, The American Astronomical Society. All rights reserved.