Genesis and evolution of dust in galaxies in the early Universe

II. Rapid dust evolution in quasars at z ≳ 6

Dark Cosmology Centre, University of Copenhagen, Niels Bohr Institute, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
e-mail: christa@dark-cosmology.dk

Received: 18 August 2010
Accepted: 7 January 2011

Abstract

Aims. We intend to assess the most plausible scenarios for generating large amounts of dust in high-z quasars (QSOs) on the basis of observationally derived physical properties of QSOs at z ≳ 6.

Methods. We use a chemical evolution model to compute the temporal progression of quantities such as the amount of dust and gas, stellar masses, star formation rates (SFRs) and the metallicity for various combinations of the initial mass function (IMF), the mass of the galaxy, dust production efficiencies, and the degree of dust destruction in the ISM. We investigate the influence of the SFR on the evolution of these quantities, and determine the earliest epochs at which agreement with observations can be achieved. We apply the obtained results to individual QSOs at z ≳ 6.

Results. We find that large quantities of dust can be generated rapidly as early as 30 Myr after the onset of the starburst when the SFR of the starburst is ≳10³ M_⊙ yr^-1. The amount of dust and several other physical quantities of individual QSOs at z ≳ 6 are satisfactorily reproduced by models at epochs 30, 70, 100, and 170 Myr for galaxies with initial gas masses of 1–3 × 10¹¹ M_⊙. The best agreement with observations is obtained with top-heavy IMFs. A sizable dust contribution from supernovae (SNe) is however required, while at these epochs dust production by asymptotic giant branch (AGB) stars is negligible. Moderate dust destruction in the ISM can be accommodated.