Unexpected metallicity in very distant galaxies

First detected during the 1960s and officially confirmed in the 1973, gamma ray bursts (GRBs) are the brightest explosions in the universe.  There are two types of GRBs: long burst (longer than two seconds) are associated with supernova explosions, while short bursts are thought to originate from the merger of two compact objects such as neutron stars.

Two instruments on board the NASA Fermi Gamma-ray Space Telescope are used to detect and study GRBs: the Gamma-ray Burst Monitor (GBM) and the Large Area Telescope (LAT). Once a GRB is detected and its position determined, requests for follow-up  observations of the visible and infrared afterglow are sent out to ground-based observers via the groups such as the Gamma-ray Coordinates Network (GCN). These follow-up observations might be conducted by some of the world’s largest telescopes, including the  ESO Very Large Telescope (VLT).

In a paper submitted to MNRAS, Sandra Savaglio and collaborators analysed the VLT spectrum of the GRB 090323 afterglow. The VLT observations show that the emission from GRB 090323 passed through its host galaxy and another galaxy nearby, which are about 12 billion lightyears from Earth. Surprisingly, these galaxies are rich in heavy  elements. Astronomers expect  very distant galaxies to have low metallicities because there has not been enough time for stars to produce the heavy element.

Schematic of the GRB 090323 (left) whose light passed through its own host galaxy and a nearby galaxy in the arly Universe. The resulting spectrum indicated that the two galaxies are rich in heavy elements. ESO/L. Calçada

Savaglio et al. conclude that these galaxies must be forming new stars at a tremendous rate to explain the observed GRB spectra.  The discovery of these metal rich galaxies at high-redshift (z > 3) questions our  understanding of the cosmic chemical evolution.

For more information, see

• Super-solar Metal Abundances in Two Galaxies at z ~ 3.57 revealed by the GRB 090323 Afterglow Spectrum, Savaglio et al. (2011), MNRAS, in press (arXiv:1110.4642)
• VLT Observations of Gamma-ray Burst Reveal Surprising Ingredients of Early Galaxies, ESO press release

[Posting by Fracnesco Pigantale, Catarina Ubach, Glen Mackie and Sarah Maddison]