Rosa Valiante

Stellar sources of dust in the high-redshift Universe

With the aim of investigating whether stellar sources can account for the ≥10 8 Mdust masses inferred from mm/sub-mm observations of samples of 5 <z< 6.4 quasars, we develop a chemical evolution model which follows the evolution of metals and dust on the stellar characteristic lifetimes, taking into account dust destruction mechanisms. Using a grid of stellar dust yields as a function of the initial mass and metallicity over the range 1-40 M � and 0-1 Z� , we show that the role of asymptotic giant branch (AGB) stars in cosmic dust evolution at high redshift might have been overlooked. In particular, we find that (i) for a stellar population forming according to a present-day Larson initial mass function (IMF) with mch = 0.35 M� , the characteristic time-scale at which AGB stars dominate dust production ranges between 150 and 500 Myr, depending both on the assumed star formation history and on the initial stellar metallicity; (ii) this result is only moderately dependent on the adopted stellar lifetimes, but it is significantly affected by variations of the IMF: for a mch = 5M � , dust from AGB starts to dominate only on time-scales larger than 1 Gyr and SNe are found to dominate dust evolution when mch ≥10 M� . We apply the chemical evolution model with dust to the host galaxy of the most distant quasar at z = 6.4, SDSS J1148+5251. Given the current uncertainties on the star formation history of the host galaxy, we have considered two models: (i) the star formation history obtained in a numerical simulation by Li et al. which predicts that a large stellar bulge is already formed at z = 6.4, and (ii) a constant star formation rate of 1000 Myr −1 , as suggested by the observations if most of the far-infrared luminosity is due to young stars. The total mass of dust predicted at z = 6.4 by the first model is 2 × 10 8 M� , within the range of values inferred by observations, with a substantial contribution (∼80 per cent) of AGB dust. When a constant star formation rate is adopted, the contribution of AGB dust decreases to ∼50 per cent but the total mass of dust formed is a factor of 2 smaller. Both models predict a rapid enrichment of the interstellar medium with metals and a relatively mild evolution of the carbon abundance, in agreement with observational constraints. This supports the idea that stellar sources can account for the dust observed but show that the contribution of AGB stars to dust production cannot be neglected, even at the most extreme redshifts currently accessible to observations.

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

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 10(13)-M-circle dot 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 >Z(cr) = 10(-6)-10(-4) Z(circle dot) 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 m(ch) = 0.35 M-circle dot, a final stellar mass of (1-5) x 10(11) M-circle dot 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 m(ch) = 5M(circle dot). 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) x 10(8) M-circle dot 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 first low-mass stars: critical metallicity or dust-to-gas ratio?

We explore the minimal conditions which enable the formation of metal-enriched solar and sub-solar mass stars. We find that in the absence of dust grains, gas fragmentation occurs at densities nH ~ [10^4-10^5]cm^{-3} when the metallicity exceeds Z ~ 10^{-4} Zsun. The resulting fragmentation masses are > 10 Msun. The inclusion of Fe and Si cooling does not affect the thermal evolution as this is dominated by molecular cooling even for metallicities as large as Z = 10^{-2} Zsun. The presence of dust is the key driver for the formation of low-mass stars. We focus on three representative core-collapse supernova (SN) progenitors, and consider the effects of reverse shocks of increasing strength: these reduce the depletion factors, fdep = Mdust/(Mdust+Mmet), alter the shape of the grain size distribution function and modify the relative abundances of grain species and of metal species in the gas phase. We find that the lowest metallicity at which fragmentation occurs is Z=10^{-6} Zsun for gas pre-enriched by the explosion of a 20 Msun primordial SN (fdep > 0.22) and/or by a 35 Msun, Z=10^{-4} Zsun SN (fdep > 0.26); it is ~ 1 dex larger, when the gas is pre-enriched by a Z = 10^{-4} Zsun, 20 Msun SN (fdep > 0.04). Cloud fragmentation depends on the depletion factor and it is suppressed when the reverse shock leads to a too large destruction of dust grains. These features are all consistent with the existence of a minimum dust-to-gas ratio, Dcr, above which fragmentation is activated. We derive a simple analytic expression for Dcr which, for grain composition and properties explored in the present study, reads Dcr = [2.6 - 6.3] x 10^{-9}. When the dust-to-gas ratio of star forming clouds exceeds this value, the fragmentation masses range between 0.01 Msun and 1 Msun, thus enabling the formation of the first low-mass stars.

The Effect of Different Type Ia Supernova Progenitors on Galactic Chemical Evolution

Aims. Our aim is to show how different hypotheses about type Ia supernova progenitors can affect Galactic chemical evolution. Supernovae Ia are believed to be the main producers of Fe and the timescale with which Fe is restored into the interstellar medium depends on the assumed supernova progenitor model. This is a way of selecting the most appropriate progenitor model for supernovae Ia, a still debated issue. Methods. We include different type Ia SN progenitor models, identified by their distribution of time delays, in a very detailed chemical evolution model for the Milky Way which follows the evolution of several chemical species. We test the single degenerate and the double degenerate models for supernova Ia progenitors, as well as other more empirical models based on differences in the time delay distributions. Results. We find that assuming the single degenerate or the double degenerate scenario produces negligible differences in the predicted [O/Fe] vs. [Fe/H] relation. On the other hand, assuming a percentage of prompt (exploding in the first 100 Myr) type Ia supernovae of 50%, or that the maximum type Ia rate is reached after 3–4 Gyr from the beginning of star formation, as suggested by several authors, produces more noticeable effects on the [O/Fe] trend. However, given the spread still existing in the observational data, no model can be firmly excluded on the basis of only the [O/Fe] ratios. On the other hand, when the predictions of the different models are compared with the G-dwarf metallicity distribution, the scenarios with very few prompt type Ia supernovae can be excluded. Conclusions. Models including the single degenerate or double degenerate scenario with a percentage of 10–13% of prompt type Ia supernovae produce results in very good agreement with the observations. A fraction of prompt type Ia supernovae larger than 30% worsens the agreement with observations and the same occurs if no prompt type Ia supernovae are allowed. In particular, two empirical models for the type Ia SN progenitors can be excluded: the one without prompt type Ia supernovae and the one assuming a delay time distribution that is ∝t −0.5 . We conclude that the typical timescale for the Fe enrichment in the Milky Way is around 1–1.5 Gyr and that type Ia supernovae already should appear during the halo phase.

Dust formation around AGB and SAGB stars: a trend with metallicity?

We calculate the dust formed around asymptotic giant branch (AGB) and super-AGB stars of metallicity Z = 0.008 by following the evolution of models with masses in the range 1 M⊙ ≤ M ≤ 8 M⊙ through the thermal pulses phase, assuming that dust forms via condensation of molecules within a wind expanding isotropically from the stellar surface. We find that, because of the strong hot bottom burning (HBB) experienced, high-mass models produce silicates, whereas lower mass objects are predicted to be surrounded by carbonaceous grains; the transition between the two regimes occurs at a threshold mass of 3.5 M⊙. These findings are consistent with the results presented in a previous investigation, for Z = 0.001. However, in the present higher metallicity case, the production of silicates in the more massive stars continues for the whole AGB phase, because the HBB experienced is softer at Z = 0.008 than at Z = 0.001; thus, the oxygen in the envelope, essential for the formation of water molecules, is never consumed completely. The total amount of dust formed for a given mass experiencing HBB increases with metallicity, because of the higher abundance of silicon, and the softer HBB, both factors favouring a higher rate of silicates production. This behaviour is not found in low-mass stars, because the carbon enrichment of the stellar surface layers, due to repeated third dredge-up episodes, is almost independent of the metallicity. Regarding cosmic dust enrichment by intermediate-mass stars, we find that the cosmic yield at Z = 0.008 is a factor of ∼5 larger than at Z = 0.001. In the lower metallicity case carbon dust dominates after ∼300 Myr, but at Z = 0.008 the dust mass is dominated by silicates at all times, with a prompt enrichment occurring after ∼40 Myr, associated with the evolution of stars with masses M ∼ 7.5–8 M⊙. These conclusions are partly dependent on the assumptions concerning the two important macrophysics inputs needed to describe the AGB phase, and still unknown from first principles: the treatment of convection, which determines the extent of the HBB experienced and of the third dredge-up following each thermal pulse, and mass-loss, essential in fixing the time-scale on which the stellar envelope is lost from the star.

Dust from asymptotic giant branch stars: relevant factors and modelling uncertainties

The dust formation process in the winds of Asymptotic Giant Branch stars is discussed, based on full evolutionary models of stars with mass in the range 1M⊙ 6M6 8M⊙, and metallicities 0.001 < Z < 0.008. Dust grains are assumed to form in an isotropically expanding wind, by growth of pre–existing seed nuclei. Convection, for what concerns the treatment of convective borders and the efficiency of the schematization adopted, turns out to be the physical ingredient used to calculate the evolutionary sequences with the highest impact on the results obtained. Low–mass stars with M6 3M⊙ produce carbon type dust with also traces of silicon carbide. The mass of solid carbon formed, fairly independently of metallicity, ranges from a few 10 −4 M⊙, for stars of initial mass 1 1.5M⊙, to � 10 −2 M⊙ for M� 2 2.5M⊙; the size of dust particles is in the range 0.1µm6 aC 6 0.2µm. On the contrary, the production of silicon carbide (SiC) depends on metallicity. For 10 −3 6 Z 6 8×10 −3 the size of SiC grains varies in the range 0.05µm < aSiC < 0.1µm, while the mass of SiC formed is 10 −5 M⊙ < MSiC < 10 −3 M⊙. Models of higher mass experience Hot Bottom Burning, which prevents the formation of carbon stars, and favours the formation of silicates and corundum. In this case the results scale with metallicity, owing to the larger silicon and aluminium contained in higher–Z models. At Z=8×10 −3 we find that the most massive stars produce dust masses md � 0.01M⊙, whereas models of smaller mass produce a dust mass ten times smaller. The main component of dust are silicates, although corundum is also formed, in not negligible quantities (� 10 20%).

The dust mass in z > 6 normal star forming galaxies

We interpret recent ALMA observations of z > 6 normal star forming galaxies by means of a semi-numerical method, which couples the output of a cosmological hydrodynamical simulation with a chemical evolution model which accounts for the contribution to dust enrichment from supernovae, asymptotic giant branch stars and grain growth in the interstellar medium. We find that while stellar sources dominate the dust mass of small galaxies, the higher level of metal enrichment experienced by galaxies with Mstar > 10 9 M⊙ allows efficient grain growth, which provides the dominant contribution to the dust mass. Even assuming maximally efficientsupernova dust production, the observed dust mass of the z = 7.5 galaxy A1689-zD1 requires very efficient grain growth. This, in turn, implies that in this galaxy the average density of the cold and dense gas, where grain growth occurs, is comparable to that inferred from observations of QSO host galaxies at similar redshifts. Although plausible, the upper limits on the dust continuum emission of galaxies at 6.5 < z < 7.5 show that these conditions must not apply to the bulk of the high redshift galaxy population.

Dust production rate of asymptotic giant branch stars in the Magellanic Clouds

We compare theoretical dust yields for stars with mass 1M⊙ 6 mstar 6 8M⊙, and metallicities 0.001 6 Z 6 0.008 with observed dust production rates (DPR) by carbonrich and oxygen-rich Asymptotic Giant Branch (C-AGB and O-AGB) stars in the Large and Small Magellanic Clouds (LMC, SMC). The measured DPR of C-AGB in the LMC are reproduced only if the mass loss from AGB stars is very efficient during the carbon-star stage. The same yields over-predict the observed DPR in the SMC, suggesting a stronger metallicity dependence of the mass-loss rates during the carbonstar stage. DPR of O-AGB stars suggest that rapid silicate dust enrichment occurs due to efficient hot-bottom-burning if mstar > 3M⊙ and Z > 0.001. When compared to the most recent observations, our models support a stellar origin for the existing dust mass, if no significant destruction in the ISM occurs, with a contribution from AGB stars of 70% in the LMC and 15% in the SMC.

Scaling relations of metallicity, stellar mass and star formation rate in metal-poor starbursts - I. A Fundamental Plane

Most galaxies follow well-defined scaling relations of metallicity (O/H), star formation rate (SFR), and stellar mass (Mstar). However, low-metallicity starbursts, rare in the Local Universe but more common at high redshift, deviate significantly from these scaling relations. On the “main sequence” of star formation, these galaxies have high SFR for a given Mstar; and on the mass-metallicity relation, they have excess Mstar for their low metallicity. In this paper, we characterize O/H, Mstar, and SFR for these deviant “low-metallicity starbursts”, selected from a sample of �1100 galaxies, spanning almost two orders of magnitude in metal abundance, a factor of � 10 6 in SFR, and of � 10 5 in stellar mass. Our sample includes quiescent star-forming galaxies and blue compact dwarfs at redshift 0, luminous compact galaxies at redshift 0.3, and Lyman Break galaxies at redshifts 1-3.4. Applying a Principal Component Analysis (PCA) to the galaxies in our sample with Mstar6 3 × 10 10 M⊙ gives a Fundamental Plane (FP) of scaling relations; SFR and stellar mass define the plane itself, and O/H its thickness. The dispersion for our sample in the edge-on view of the plane is 0.17dex, independently of redshift and including the metal-poor starbursts. The same FP is followed by 55100 galaxies selected from the Sloan Digital Sky Survey, with a dispersion of 0.06dex. In a companion paper, we develop multi-phase chemical evolution models that successfully predict the observed scaling relations and the FP; the deviations from the main scaling relations are caused by a different (starburst or “active”) mode of star formation. These scaling relations do not truly evolve, but rather are defined by the different galaxy populations dominant at different cosmological epochs.

High-redshift quasars host galaxies: is there a stellar mass crisis?

We investigate the evolutionary properties of a sample of quasars (QSOs) at 5 <z <6.4 using the semi-analytical hierarchical model GAMETE/QSODUST. We find that the observed properties of these QSOs are well reproduced by a common formation scenario in which stars form according to a standard initial mass function, via quiescent star formation and efficient merger-driven bursts, while the central black hole (BH) grows via gas accretion and BH-BH mergers. Eventually, a strong active galactic nuclei-driven wind starts to clear up the interstellar medium of dust and gas, damping the star formation and un-obscuring the line of sight towards the QSO. In this scenario, all the QSOs hosts have final stellar masses in the range (4-6) × 1011 M⊙, a factor of 3-30 larger than the upper limits allowed by the observations. We discuss alternative scenarios to alleviate this apparent tension: the most likely explanation resides in the large uncertainties that still affect dynamical mass measurements in these high-z galaxies. In addition, during the transition between the starburst-dominated and the active QSO phase, we predict that ˜40 per cent of the progenitor galaxies can be classified as Submillimetre Galaxies, although their number rapidly decreases with redshift.