B. Thomsen

On the Lyalpha emission from gamma-ray burst host galaxies: Evidence for low metallicities

We report on the results of a search for Ly emission from the host galaxy of the z= 2:140 GRB 011211 and other galaxies in its surrounding field. We detect Ly emission from the host as well as from six other galaxies in the field. The restframe equivalent width of the Ly line from the GRB 011211 host is about 21 A. This is the fifth detection of Ly emission out of five possible detections from GRB host galaxies, strongly indicating that GRB hosts, at least at high redshifts, are Ly emitters. This is intriguing as only 25% of the Lyman-Break selected galaxies at similar redshifts have Ly emission lines with restframe equivalent width larger than 20 A. Possible explanations are i) a preference for GRB progenitors to be metal-poor as expected in the collapsar model, ii) an optical afterglow selection bias against dusty hosts, and iii) a higher fraction of Ly emitters at the faint end of the luminosity function for high- z galaxies. Of these, the current evidence seems to favour i).

GRB 050509B: Constraints on Short Gamma-Ray Burst Models*

We have obtained deep optical images with the Very Large Telescope at ESO of the first well-localized short-duration gamma-ray burst, GRB 050509b. We observed in the V and R bands at epochs starting at {approx}2 days after the GRB trigger and lasting up to three weeks. We detect no variable objects inside the small Swift/XRT X-ray error circle down to 5{sigma} limiting magnitudes of V = 26.5 and R = 25.2. The X-ray error circle includes a giant elliptical galaxy at z = 0.225, which has been proposed as the likely host of this GRB. Our limits indicate that if the GRB originated at z = 0.225, any supernova-like event accompanying the GRB would have to be over 100 times fainter than normal Type Ia SNe or Type Ic hypernovae, 5 times fainter than the faintest known Ia or Ic SNe, and fainter than the faintest known Type II SNe. Moreover, we use the optical limits to constrain the energetics of the GRB outflow, and conclude that there was very little radioactive material produced during the GRB explosion. These limits strongly constrain progenitor models for this short GRB.We have obtained deep optical images with the Very Large Telescope at ESO of the first well-localized short-duration gamma-ray burst, GRB 050509B. From V and R imaging, initiated ~2 days after the GRB trigger and lasting up to three weeks, we detect no variable object inside the small Swift XRT X-ray error circle down to 2 σ limits of V = 26.5 and R = 25.1. The X-ray error circle includes a giant elliptical galaxy at z = 0.225, which has been proposed as the likely host of this GRB. Our limits indicate that if the GRB originated at z = 0.225, any supernova-like event accompanying the GRB would have to be over 100 times fainter than normal Type Ia SNe or Type Ic hypernovae, 5 times fainter than the faintest known Ia or Ic SNe, and fainter than the faintest known Type II SNe. Moreover, we use the optical limits to constrain the energetics of the GRB outflow. Simple models indicate that unless the intrinsic energy in the outflow from GRB 050509B was 1051 ergs, there was very little radioactive material with efficient decay timescales for generating a large luminosity. These limits strongly constrain progenitor models for this short GRB.

Ly+ and ultraviolet emission from high-redshift gamma-ray burst hosts: to what extent do gamma-ray bursts trace star formation?

We report the result of a search for Lyemission from the host galaxies of the gamma- ray bursts (GRBs) 030226 (z = 1.986), 021004 (z = 2.335) and 020124 (z = 3.198). We find that the host galaxy of GRB021004 is an extended (around 8kpc) strong Ly� emitter with a restframe equivalent width (EW) of 68 +1211 u and a star-formation rate of 10.6±2.0M⊙ yr −1 . We do not detect the hosts of GRB030226 and GRB020124, but the upper limits on their Lyfluxes do not rule out large restframe EWs. In the fields of GRB021004 and GRB030226 we find seven and five other galaxies, respectively, with excess emission in the narrow-band filter. These galaxies are candidate Lyemitting galaxies in the environment of the host galaxies. We have also compiled a list of all z & 2 GRB hosts, and demonstrate that a scenario where they trace star formation in an unbiased way is compatible with current observational constraints. Fitting the z = 3 luminosity function (LF) under this assumption, results in a characteristic luminosity of R ∗ = 24.6 and a faint end slope of � = 1.55, consistent with the LF measured for Lyman-break galaxies.

The extended lyman-α emission surrounding the z=3.04 radio-quiet QSO1205-30 : Primordial infalling gas illuminated by the quasar?

We present spectroscopic observations obtained with the FORS1 instrument on the ESO VLT under good seeing conditions of the radio-quiet

The optical afterglow and host galaxy of GRB 000926

z_{\rm em}=3.04

The supernova 2003lw associated with X-ray flash 031203

quasar Q1205-30 and its associated extended Ly α emission. The extended Ly α emission was originally found in a deep narrow band image targeting a

The afterglow and the host galaxy of GRB 011211

z_{\rm abs} \approx z_{\rm em}

The Building the Bridge survey for z = 3 Ly

Lyman-limit system in the spectrum of the QSO. Using spectral point-spread function fitting to subtract the QSO spectrum, we clearly detect the extended Ly α emission as well as two foreground galaxies at small impact parameters (

\alpha

2.12\,\pm\, 0.04

emitting galaxies - II. Completion of the survey

and