GRB-DLAs and the Cosmic Metallicity

In a recent paper we explored the possibility of  using long Gamma-ray Bursts to trace the metallicity of their host galaxies up to redshift z~6. We collected 55 GRB afterglow spectra, which shows strong Lyman-alpha line, such to classify these absorbers as Damped Lyman-alpha systems (DLA). Similarly to Quasars-DLA (QSO-DLAs) several weak absorption transitions at the same redshift of these Ly-α lines are identified enabling a metallicity estimates from these systems. The advantage of GRB-DLAs reside on the fact that the DLA is within the GRB host galaxies and so provide direct information of the metal content within the star-forming galaxy. Such systems can easily be identified and studied a posteriori, something that is almost impossible for QSO-DLA because the absorbers is located along the line of sight of the QSO and it is subjected to impact parameter effect.

(see Figure 1). It is evident that GRB-DLAs at z>3 explode in environment enriched of metals, while at z<3 GRB-DLAs show the same metallicity distribution compared to QSO-DLAs. This is probably due to the fact that in the high-z Universe GRBs trace better star-forming environment, where metals were already present due to earlier supernovae, while at later times, the metal where better mixed with the environments.

Figure 1: Metallicity trend derived from a sample of GRB-DLAs (red) and QSO-DLAs (grey).

Triangles represents lower limits derived mostly by low-resolution spectra (R<2400). The dashed lines represent a linear fit of the 2 samples obtained with survival analysis technique. The GRB-DLAs seem to be enriched in metals compared with the QSO-DLAs, in particular at z>4.

The puzzle of MgII absorbers

Spectra of Gamma-ray Burst Afterglows provide the distance of their Host Galaxy, insights about its metallicity, and dust content. Often, more than a single set of absorption lines at a similar redshift, but lower than the main GRB host redshift, are identified. We call these systems “intervening” systems, because due to foreground objects (usually galaxies) not related with the GRB (similarly this happens along quasars lines of sight).

An important set of absorption features is the Magnesium doublet at rest frame 2796 and 2803 Angstroms. The presence of a strong absorbers, defined as those absorbers with rest-frame equivalent width of the blue component larger than 1 Angstrom, usually indicate a galaxy with high neutral hydrogen content (at least along the GRB line of sight) called Damped Lyman Alpha system (DLA).

A unexpected discovery was made by Prochter et al. in 2006: they found that in small sample of GRBs, the number of strong absorbers along their same redshift path was 4 times larger than along a sample of QSOs, tracing the same redshift path.

In our work, in 2013, with Prof. J. X. Prochaska, we demonstrated that the incidence was likely a statistical fluke, mostly due to the only few lines of sight used in the original work.

Cucchiara et al. 2013 : we collected the largest sample of optical/NIR GRB afterglow spectra (see also here). We determine the redshift pathlength of such sample, increasing of a factor almost of 4 from the original work. We also included spectra with high and low resolution.

Our conclusion, similar to others papers using different approaches, suggests that the incidence of strong (EW(2796) > 1 Angstrom) MgII absorbers along GRBs is indeed similar (within the statistical error) of the one along Quasars.