Journal Article

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

Umberto Maio, Ruben Salvaterra, Lauro Moscardini and Benedetta Ciardi

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 426, issue 3, pages 2078-2088
Published in print November 2012 | ISSN: 0035-8711
Published online November 2012 | e-ISSN: 1365-2966 | DOI:
Counts of high-redshift GRBs as probes of primordial non-Gaussianities

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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, ƒNL, 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 ƒNL ∼ 1000, one order of magnitude for ƒNL ∼ 100 and a factor of ∼2 for ƒNL ∼ 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 ƒNL, consistently with previous works. Future detections of any long GRB at extremely high redshift (z ∼ 15–20) could favour non-Gaussian scenarios with positive ƒNL. 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.

Keywords: cosmology: theory; dark ages, reionization, first stars; early Universe; large-scale structure of Universe; gamma-rays: general; gamma-rays: stars

Journal Article.  8305 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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