The origin of the dust in high-redshift quasars: the case of SDSS J1148+5251

Abstract

We present a semi‐analytical model for the formation and evolution of a high‐redshift quasar (QSO). We reconstruct a set of hierarchical merger histories of a 1013‐M⊙ dark matter halo and model the evolution of the corresponding galaxy and of its central supermassive black hole. The code gamete/QSOdust consistently follows (i) the black hole assembly via both coalescence with other black holes and gas accretion; (ii) the build‐up and star formation history of the quasar host galaxy, driven by binary mergers and mass accretion; (iii) the evolution of gas, stars and metals in the interstellar medium (ISM), accounting for mass exchanges with the external medium (infall and outflow processes); (iv) the dust formation in supernova (SN) ejecta and in the stellar atmosphere of asymptotic giant branch (AGB) stars, dust destruction by interstellar shocks and grain growth in molecular clouds; and (v) the active galactic nucleus feedback which powers a galactic‐scale wind, self‐regulating the black hole growth and eventually halting star formation. We use this model to study the case of SDSS J1148+5251 observed at redshift 6.4. We explore different star formation histories for the QSO host galaxy and find that Population III stars give a negligible contribution to the final metal and dust masses due to rapid enrichment of the ISM to metallicities >Zcr= 10−6–10−4 Z⊙ in progenitor galaxies at redshifts >10. If Population II/I stars form with a standard initial mass function (IMF) and with a characteristic stellar mass of mch= 0.35 M⊙, a final stellar mass of (1–5) × 1011 M⊙ is required to reproduce the observed dust mass and gas metallicity of SDSS J1148+5251. This is a factor of 3–10 higher than the stellar mass inferred from observations and would shift the QSO closer or on to the stellar bulge–black hole relation observed in the local Universe; alternatively, the observed chemical properties can be reconciled with the inferred stellar mass, assuming that Population II/I stars form according to a top‐heavy IMF with mch= 5 M⊙. We find that SNe dominate the early dust enrichment and that, depending on the shape of the star formation history and on the stellar IMF, AGB stars contribute at z < 8–10. Yet, a dust mass of (2–6) × 108 M⊙ estimated for SDSS J1148+5251 cannot be reproduced considering only stellar sources, and the final dust mass is dominated by grain growth in molecular clouds. This conclusion is independent of the stellar IMF and star formation history.

The dependency of AGN infrared colour-selection on source luminosity and obscuration

“…But the rest-frame UV selection in these works, on the other hand, may be inducing a selection bias that misses a dust-rich population. For instance, gravitational lensing has recently revealed two dusty sources at z ∼ 5.7 (Vieira et al 2013), and large amounts of dust have indeed been found at z > 6, even though only in a QSO host-galaxy so far (Bertoldi et al 2003;Gall et al 2011;Valiante et al 2011). …”

Two-size approximation: a simple way of treating the evolution of grain size distribution in galaxies

“…This process is simply referred to as accretion in this paper. Including accretion into dust evolution models is motivated not only to explain the dust abundance in nearby galaxies (Hirashita 1999;Inoue 2011;Zhukovska & Henning 2013;de Bennassuti et al 2014;Schneider et al 2014), but also to explain the observations of huge amounts of dust (∼ 10 8 M⊙) in high-z quasars and starbursts (Michałowski et al 2010;Mattsson 2011;Valiante et al 2011;Kuo & Hirashita 2012;Rowlands et al 2014;Nozawa et al 2015). Grain growth by accretion occurs efficiently only after the ISM is significantly enriched with dust and metals, since the grain growth rate is proportional to the collision rate between these two components.…”

“…Among them, Dayal, Hirashita, & Ferrara (2010) included dust production and destruction in a cosmological simulation, predicting the statistical properties of dust emission luminosities in a large sample of highredshift galaxies. More elaborate dust evolution models that include two major dust formation mechanisms (dust formation in stellar ejecta and dust growth by accretion in the ISM) are also incorporated in semi-analytic (Valiante et al 2011;de Bennassuti et al 2014) or N -body (Bekki 2013;Yozin & Bekki 2014) frameworks.…”

Planetary nebulae in the Small Magellanic Cloud

“…As a consequence, the role played by AGB stars in several astrophysical contexts is still highly debated. On the other hand, understanding the AGB impact is crucial for several astrophysical fields, such as the determination of the masses of galaxies at high redshifts (Maraston et al 2006), the formation and chemical evolution of galaxies (Romano et al 2010;Santini et al 2014), the dust content of high-redshift quasars (Valiante et al 2011), and the formation of multiple populations of stars in globular clusters (Ventura et al 2001).…”