  GRB 080913 at Redshift 6.7; ; et al in Astrophysical Journal (2009), 693 We report on the detection by Swift of GRB 080913, and subsequent optical/near-infrared follow-up observations by GROND, which led to the discovery of its optical/NIR afterglow and the recognition of its ... [more ▼] We report on the detection by Swift of GRB 080913, and subsequent optical/near-infrared follow-up observations by GROND, which led to the discovery of its optical/NIR afterglow and the recognition of its high-z nature via the detection of a spectral break between the i' and z' bands. Spectroscopy obtained at the ESO-VLT revealed a continuum extending down to λ = 9400 Å, and zero flux for 7500 Å<λ < 9400 Å, which we interpret as the onset of a Gunn-Peterson trough at z = 6.695± 0.025 (95.5% confidence level), making GRB 080913 the highest-redshift gamma-ray burst (GRB) to date, and more distant than the highest-redshift QSO. We note that many redshift indicators that are based on promptly available burst or afterglow properties have failed for GRB 080913. We report on our follow-up campaign and compare the properties of GRB 080913 with bursts at lower redshift. In particular, since the afterglow of this burst is fainter than typical for GRBs, we show that 2 m class telescopes can identify most high-redshift GRBs. [less ▲] Detailed reference viewed: 11 (0 ULg)Cosmological formation of low-mass objects; Thoul, Anne  ; in Astrophysical Journal (1996), 464(2), 523-538 We investigate the early formation of bound objects with masses comparable to the cosmological Jeans mass (similar to 10(5) M.). We follow the growth of isolated spherically symmetric density peaks ... [more ▼] We investigate the early formation of bound objects with masses comparable to the cosmological Jeans mass (similar to 10(5) M.). We follow the growth of isolated spherically symmetric density peaks starting from the linear perturbative regime. The initial parameters correspond to density peaks of various widths and heights in a cold dark matter cosmology. We use a one-dimensional spherical Lagrangian hydrodynamics code to follow the dynamical, thermal, and nonequilibrium chemical evolution of the gas. The system includes a collisionless dark matter component and a baryonic component composed of the nine species H, H-, H+, He, He+, He++, H-2, H-2(+), and e(-). All relevant chemical reactions between these species and their cooling mechanisms are included in the calculations. We explore the dependence of the dynamical evolution of the gas on two parameters: the initial mass scale and the initial overdensity of the system. We follow the evolution of the density, temperature, and abundance profiles within the cloud, assuming two types of central boundary conditions for the collisionless component: in one the infalling dark matter virializes through a reflection from a hard sphere, while in the other it accretes onto a central sink. We find that in both cases, radiative cooling by H-2 affects the collapse dynamics of the gas only after it has already virialized and become part of the bound object. Therefore, radiative cooling is unlikely to have triggered the initial collapse of perturbations at redshifts z>10. Nevertheless, baryonic objects with masses well below the linear theory Jeans mass (less than or similar to 10(3) M.) form at high redshifts because of shell crossing by the dark matter. Such objects could be the progenitors of a primordial population of high-mass stars in the intergalactic medium. [less ▲] Detailed reference viewed: 5 (2 ULg) |
||
