Counts of high-redshift GRBs as probes of primordial non-Gaussianities

Umberto Maio (1), Ruben Salvaterra (2), Lauro Moscardini (3,4,5) and Benedetta Ciardi (6)

We propose to use high-redshift long γ-ray bursts (GRBs) as cosmological tools to constrain the amount of primordial non-Gaussianity in the density field. By using numerical, N-body, hydrodynamic, chemistry simulations of different cosmological volumes with various Gaussian and non-Gaussian models, we self-consistently relate the cosmic star formation rate density to the corresponding GRB rate. Assuming that GRBs are fair tracers of cosmic star formation, we find that positive local non-Gaussianities, described in terms of the non-linear parameter, ƒ_NL, might boost significantly the GRB rate at high redshift, z ≫ 6. Deviations with respect to the Gaussian case account for a few orders of magnitude if ƒ_NL ∼ 1000, one order of magnitude for ƒ_NL ∼ 100 and a factor of ∼2 for ƒ_NL ∼ 50. These differences are found only at large redshift, while at later times the rates tend to converge. Furthermore, a comparison between our predictions and the observed GRB data at z > 6 allows us to exclude large negative ƒ_NL, consistently with previous works. Future detections of any long GRB at extremely high redshift (z ∼ 15–20) could favour non-Gaussian scenarios with positive ƒ_NL. More stringent constraints require much larger high-z GRB complete samples, currently not available in the literature. By distinguishing the contributions to the GRB rate from the metal-poor Population III regime, and the metal-enriched Population II–I regime, we conclude that the latter is a more solid tracer of the underlying matter distribution, while the former is strongly dominated by feedback mechanisms from the first, massive, short-lived stars, rather than by possible non-Gaussian fluctuations. This holds quite independently of the assumed Population III initial mass function.

Read the article published in "Monthly Notices of the Royal Astronomical Society" (V. 426 (3), pp 2078-2088)

1. Max-Planck-Institut für extraterrestrische Physik, München, Germany
2. INAF/IASF — Italian National Astrophysical Institute, Milano, Italy
3. Dipartimento di Astronomia, Università di Bologna, Bologna, Italy
4. INAF, Osservatorio Astronomico di Bologna, Bologna, Italy
5. INFN, Sezione di Bologna, Bologna, Italy
6. Max-Planck-Institut für Astrophysik, München, Germany   

Look below the presentation "The early bursty Universe - High-redshift GRB hosts as cosmological tool" at 2nd Extreme Universe Laboratory Workshop on Gamma-Ray Bursts in Mosca on 7-11 October 2013.