I: A double Damped Lyman-alpha Absorber towards a z=5 gamma-ray burst
The spectrum of the afterglow of gamma-ray burst 111008A, a cosmic explosion at redshift z=5, reveals not only the host galaxy of this GRB, but also another foreground absorber at z=4.6. This system is, like the host, classified as a Damped Ly-alpha (DLA) system: because of the high neutral hydrogen column density, Lyman-alpha is detected as a very strong line with characteristic damping wings.
Intervening absorbers are very common in GRB afterglow spectra, but having one that is so strong, which forms a high-redshift pair with the host-galaxy DLA, is quite an unique situation. Furthermore, this spectrum is the most detailed we have for a GRB afterglow at this redshift, which allows us tell something about the chemical composition of a galaxy when the universe was only a few gigayears old. The host looks surprisingly like a present-day galaxy. But if iron-group elements are formed in Supernovae type Ia, where do they come from? The progenitors of these phenomena did not yet have enough time to die when the universe was so young.
The foreground z=4.6 appears to be very metal-rich for this redshift (about 20% solar). The broadband SED from the afterglow tells us that there is only little absorption in the line of sight, which must therefore all happen in the foreground object, because of its metals and expected dust content. This leaves essentially no extinction and thus no dust for the host galaxy, which gives interesting constraints to the metals-to-dust ratio as a function of redshift, and to the question how dust is formed in the early universe.
Olga Hartoog
Thursday 13 June 2013, 12:00
Location: A1.04