Cosmology. Probing matter at the lowest densities.
Explore the source record for details and available documents.
Journal Article↗
Biomedical subjects
J Miralda-Escudé
Publications and source records attributed to J Miralda-Escudé.
Weak Lensing by Large-Scale Structure: A Dark Matter Halo Approach.
Weak gravitational lensing observations probe the spectrum and evolution of density fluctuations and the cosmological parameters that govern them, but they are currently limited to small fields and subject to selection biases. We show how the expected signal from large-scale structure arises from the contributions from and correlations between individual halos. We determine the convergence power spectrum as a function of the maximum halo mass and so provide the means to interpret results from surveys that lack high-mass halos either through selection criteria or small fields. Since shot noise from rare massive halos is mainly responsible for the sample variance below 10&arcmin;, our method should aid our ability to extract cosmological information from small fields.
Journal Article↗
Soft X-Ray Absorption by High-Redshift Intergalactic Helium.
The Lyalpha absorption from intergalactic, once-ionized helium (He ii) has been measured with the Hubble Space Telescope in four quasars over the last few years in the redshift range 2.4<z<3.2. These observations have indicated that the He ii reionization may not have been completed until z approximately 2.8 and that large fluctuations in the intensity of the He ii-ionizing background were present before this epoch. The detailed history of He ii reionization at higher redshifts is, however, model-dependent and difficult to determine from these observations, since the intergalactic medium (IGM) can be completely optically thick to Lyalpha photons when only a small fraction of the helium remains as He ii. In addition, finding quasars in which the He ii Lyalpha absorption can be observed becomes increasingly difficult at higher redshift owing to the large abundance of hydrogen Lyman limit systems. It is pointed out here that He ii in the IGM should also cause detectable continuum absorption in the soft X-rays. The spectrum of a high-redshift source seen behind the IGM when most of the helium was He ii should recover from the He ii Lyman continuum absorption at an observed energy of approximately 0.1 keV. Galactic absorption will generally be stronger, but not by a large factor; the intergalactic He ii absorption can be detected as an excess over the expected Galactic absorption from the 21 cm H i column density. In principle, this method allows a direct determination of the fraction of helium that was singly ionized as a function of redshift if the measurement is done on a large sample of high-redshift sources over a range of redshifts.
Journal Article↗