Time Evolution of the Reverse Shock in SN 1006

The Schweizer-Middleditch star, located behind the SN 1006 remnant and near its center in projection, provides the opportunity to study cold, expanding ejecta within the SN 1006 shell through UV absorption. Especially notable is an extremely sharp red edge to the Si II 1260 Angstrom feature, which stems from the fastest moving ejecta on the far side of the SN 1006 shell--material that is just encountering the reverse shock. Comparing HST far-UV spectra obtained with COS in 2010 and with STIS in 1999, we have measured the change in this feature over the intervening 10.5-year baseline. We find that the sharp red edge of the Si II feature has shifted blueward by 0.19 +/- 0.05 Angstroms, which means that the material hitting the reverse shock in 2010 was moving slower by 44 +/- 11 km/s than the material that was hitting it in 1999, a change corresponding to - 4.2 +/- 1.0 km/s/yr. This is the first observational confirmation of a long-predicted dynamic effect for a reverse shock: that the shock will work its way inward through expanding supernova ejecta and encounter ever slower material as it proceeds. We also find that the column density of shocked Si II (material that has passed through the reverse shock) has decreased by 7 +/- 2% over the ten-year period. The decrease could indicate that in this direction the reverse shock has been ploughing through a dense clump of Si,leading to pressure and density transients.

Intergalactic baryon-rich regions at high redshift

Using a high resolution cosmological simulation of reionization we have examined the differing structures formed by gas and dark matter at a redshift of 5.1. Baryon-rich regions form a small number of filaments, which connect the largest galaxies in the simulation. More detailed examination of the ten largest galaxies reveals long, slender gaseous filaments about 5 proper kpc in width radiating from the galaxy centers. Extending out from each filament are a few smooth, thin, nearly planar gaseous sheets. By contrast, the dark matter concentrates into quasi-spherical bodies. The results have implications for our understanding of structure formation in the early universe and of the Lyman alpha forest.