Raffaella Schneider

First Stars, Very Massive Black Holes, and Metals

Recent studies suggest that the initial mass function (IMF) of the first stars (Population III) is likely to have been extremely top-heavy, unlike what is observed at present. We propose a scenario to generate fragmentation to lower masses once the first massive stars have formed and derive constraints on the primordial IMF. We estimate the mass fraction of pair-unstable supernovae (SN??), shown to be the dominant sources of the first heavy elements. These metals enrich the surrounding gas up to ?10-4 Z?, when a transition to efficient cooling-driven fragmentation producing 1 M? clumps occurs. We argue that the remaining fraction of the first stars ends up in ?100 M? VMBHs (very massive black holes). If we further assume that all these VMBHs are likely to end up in the centers of galactic nuclei constituting the observed supermassive black holes (SMBHs), then ?6% of the first stars contributed to the initial metal enrichment and the IMF remained top-heavy down to a redshift z ? 18.5%. Interestingly, this is the epoch at which the cool metals detected in the Ly? forest at z ? 3 must have been ejected from galaxies. At the other extreme, if none of these VMBHs has as yet ended up in SMBHs, we expect them to be either (1) en route toward galactic nuclei, thereby accounting for the X-ray-bright off-center sources detected locally by ROSAT, or (2) the dark matter candidate composing the entire baryonic halos of galaxies. For case 1 we expect all but a negligible fraction of the primordial stars to produce metals, causing the transition at the maximum possible redshift of 22.1, and for case 2, ~3 ? 105, a very negligible fraction of the initial stars produce the metals and the transition redshift occurs at zf 5.4. In this paper, we present a framework (albeit one that is not stringently constrained at present) that relates the first episode of star formation to the fate of their remnants at late times. Clearly, further progress in understanding the formation and fragmentation of Population III stars within the cosmological context will provide tighter constraints in the future. We conclude with a discussion of several hitherto unexplored implications of a high-mass-dominated star formation mode in the early universe.

Thermal and Fragmentation Properties of Star-forming Clouds in Low-Metallicity Environments

The thermal and chemical evolution of star-forming clouds is studied for different gas metallicities, Z, using the model of Omukai, updated to include deuterium chemistry and the effects of cosmic microwave background (CMB) radiation. HD-line cooling dominates the thermal balance of clouds when Z ~ 10-5 to 10-3 Z☉ and density ≈105 cm-3. Early on, CMB radiation prevents the gas temperature from falling below TCMB, although this hardly alters the cloud thermal evolution in low-metallicity gas. From the derived temperature evolution, we assess cloud/core fragmentation as a function of metallicity from linear perturbation theory, which requires that the core elongation ≡ (b - a)/a > NL ~ 1, where a (b) is the short (long) core axis length. The fragment mass is given by the thermal Jeans mass at = NL. Given these assumptions and the initial (Gaussian) distribution of , we compute the fragment mass distribution as a function of metallicity. We find that (1) for Z = 0, all fragments are very massive, 103 M☉, consistent with previous studies; (2) for Z > 10-6 Z☉ a few clumps go through an additional high-density (1010 cm-3) fragmentation phase driven by dust cooling, leading to low-mass fragments; (3) the mass fraction in low-mass fragments is initially very small, but at Z ~ 10-5 Z☉ it becomes dominant and continues to grow as Z is increased; (4) as a result of the two fragmentation modes, a bimodal mass distribution emerges in 0.01 < Z/Z☉ < 0.1; and (5) for 0.1 Z☉, the two peaks merge into a single-peaked mass function, which might be regarded as the precursor of the ordinary Salpeter-like initial mass function.

LSD: Lyman-break galaxies Stellar populations and Dynamics – I. Mass, metallicity and gas at z∼ 3.1

We present the first results of a project, Lyman-break galaxies Stellar populations and Dynamics (LSD), aimed at obtaining spatially resolved, near-infrared (IR) spectroscopy of a complete sample of Lyman-break galaxies at z ∼ 3. Deep observations with adaptive optics resulted in the detection of the main optical lines, such as [O II] λ3727, Hβ and [O III] λ5007, which are used to study sizes, star formation rates (SFRs), morphologies, gas-phase metallicities, gas fractions and effective yields. Optical, near-IR and Spitzer/Infrared Array Camera photometry are used to measure stellar mass. We obtain that morphologies are usually complex, with the presence of several peaks of emissions and companions that are not detected in broad-band images. Typical metallicities are 10–50 per cent solar, with a strong evolution of the mass– metallicity relation from lower redshifts. Stellar masses, gas fraction and evolutionary stages vary significantly among the galaxies, with less massive galaxies showing larger fractions of gas. In contrast with observations in the local universe, effective yields decrease with stellar mass and reach solar values at the low-mass end of the sample. This effect can be reproduced by gas infall with rates of the order of the SFRs. Outflows are present but are not needed to explain the mass–metallicity relation. We conclude that a large fraction of these galaxies is actively creating stars after major episodes of gas infall or merging.

Dust formation and survival in supernova ejecta

The presence of dust at high redshift requires efficient condensation of grains in SN ejecta, in accordance with current theoretical models. Yet, observations of the few well studied SNe and SN remnants imply condensation efficiencies which are about two orders of magnitude smaller. Motivated by this tension, we have (i) revisited the model of Todini & Ferrara (2001) for dust formation in the ejecta of core collapse SNe and (ii) followed, for the first time, the evolution of newly condensed grains from the time of formation to their survival - through the passage of the reverse shock - in the SN remnant. We find that 0.1 - 0.6 M⊙ of dust form in the ejecta of 12 - 40 M⊙ stellar progenitors. Depending on the density of the surrounding ISM, between 2-20% of the initial dust mass survives the passage of the reverse shock, on time-scales of about 4 8×10 4 yr from the stellar explosion. Sputtering by the hot gas induces a shift of the dust size distribution towards smaller grains. The resulting dust extinction curve shows a good agreement with that derived by observations of a reddened QSO at z = 6.2. Stochastic heating of small grains leads to a wide distribution of dust temperatures. This supports the idea that large amounts (� 0.1M⊙) of cold dust (T � 40K) can be present in SN remnants, without being in conflict with the observed IR emission.

Fragmentation of star-forming clouds enriched with the first dust

The thermal and fragmentation properties of star forming clouds have important consequences on the corresponding characteristic stellar mass. The initial composition of the gas within these clouds is a record of the nucleosynthetic products of previous stellar generations. In this paper, we present a model for the evolution of star forming clouds enriched by metals and dust from the first supernovae (SNe), resulting from the explosions of metal-free progenitors with masses in the range 12-30 M ⊙ and 140-260 M ⊙ . Using a self-consistent approach, we show that: (i) metals depleted on to dust grains play a fundamental role, enabling fragmentation to solar or subsolar mass scales already at metallicities Z cr = 10 -6 Z ⊙ ; (ii) even at metallicities as high as 10 -2 Z ⊙ , metals diffused in the gas phase lead to fragment mass scales which are? 100 M ⊙ ; (iii) C atoms are strongly depleted on to amorphous carbon grains and CO molecules so that C II plays a minor role in gas cooling, leaving O I as the main gas-phase cooling agent in low-metallicity clouds. These conclusions hold independently of the assumed SN progenitors and suggest that the onset of low-mass star formation is conditioned to the presence of dust in the parent clouds.

Population III stars: hidden or disappeared?

ABSTRACT A Pop III/Pop II transition from massive to normal stars is predicted to occur whenthe metallicity of the star forming gas crosses the critical range Z cr = 10 −5±1 Z ⊙ . Toinvestigate the cosmic implications of such process we use numerical simulations whichfollow the evolution, metal enrichment and energy deposition of both Pop III and PopII stars. We find that: (i) due to inefficient heavy element transport by outflows andslow ”genetic” transmission during hierarchical growth, large fluctuations around theaverage metallicity arise; as a result Pop III star formation continues down to z = 2.5,but at a low peak rate of 10 −5 M ⊙ yr 1 Mpc −3 occurring at z ≈6 (about 10 −4 of thePop II one); (ii) Pop III star formation proceeds in a ”inside-out” mode in whichformation sites are progressively confined at the periphery of collapsed structures,where the low gas density and correspondingly long free-fall timescales result in a veryinefficient astration. These conclusions strongly encourage deep searches for pristinestar formation sites at moderate (2 < z < 5) redshifts where metal free stars are likelyto be hidden.Key words: galaxies: formation - cosmology: theory - cosmology: observations -intergalactic medium

THE DETECTABILITY OF THE FIRST STARS AND THEIR CLUSTER ENRICHMENT SIGNATURES

We conduct a comprehensive investigation of the detectability of the first stars and their enrichment signatures in galaxy clusters. As the initial mass function (IMF) of these Population III stars is unknown and likely to be biased to high masses, we base our study on analytical models that parameterize these uncertainties and allow us to make general statements. We show that the mean metallicity of outflows from Population III objects containing these stars is well above the critical transition metallicity (Zcr ~ 10-4 Z☉) that marks the formation of normal stars. Thus, the fraction of Population III objects formed as a function of redshift is heavily dependent on the distribution of metals and fairly independent of the mean metallicity of the universe, or the precise value of Zcr. Using an analytic model of inhomogeneous structure formation, we study the evolution of Population III objects as a function of the star formation efficiency, IMF, and efficiency of outflow generation. For all models, Population III objects tend to be in the 106.5-107.0 M☉ mass range, just large enough to cool within a Hubble time, but small enough that they are not clustered near areas of previous star formation. Although the mean metallicity exceeds Zcr at z ~ 15 in all models, the peak of Population III star formation occurs at z ~ 10, and such stars continue to form well into the observable range. We discuss the observational properties of these objects, some of which may have already been detected in ongoing surveys of high-redshift Lyα emitters. Finally, we combine our Population III distributions with the yield models of Heger & Woosley to study their impact on the intracluster medium (ICM) in galaxy clusters. We find that Population III stars can contribute no more than 20% of the iron observed in the ICM, but if they form with characteristic masses ~200-260 M☉, their peculiar elemental yields help to reconcile theoretical models with the observed Fe and Si/Fe abundances. However, these stars tend to overproduce S/Fe and can account only for the O/Fe ratio in the inner regions of poor clusters. Additionally, the associated supernova heating falls far short of the observed level of ~1 keV per ICM gas particle. Thus, the properties of the first objects may be best determined by direct observation.

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.

CAN SUPERMASSIVE BLACK HOLES FORM IN METAL-ENRICHED HIGH-REDSHIFT PROTOGALAXIES?

Primordial gas in protogalactic DM halos with virial temperatures -->Tvir 104 K begins to cool and condense via atomic hydrogen. Provided that this gas is irradiated by a strong UV flux and remains free of H2 and other molecules, it has been proposed that the halo with -->Tvir ~ 104 K may avoid fragmentation and lead to the rapid formation of an SMBH as massive as -->M ? 105?106 M?. This head start would help explain the presence of SMBHs with inferred masses of several times 109 M?, powering the bright quasars discovered in the SDSS at redshift -->z 6. However, high-redshift DM halos with -->Tvir ~ 104 K are likely already enriched with at least trace amounts of metals and dust produced by prior star formation in their progenitors. Here we study the thermal and chemical evolution of low-metallicity gas exposed to extremely strong UV radiation fields. Our results, obtained in one-zone models, suggest that gas fragmentation is inevitable above a critical metallicity, whose value is between -->Zcr ? 3 ? 10?4 Z? (in the absence of dust) and as low as -->Zcr ? 5 ? 10?6 Z? (with a dust-to-gas mass ratio of about -->0.01Z/Z?). We propose that when the metallicity exceeds these critical values, dense clusters of low-mass stars may form at the halo nucleus. Relatively massive stars in such a cluster can then rapidly coalesce into a single more massive object, which may produce an intermediate-mass BH remnant with a mass up to -->M 102?103 M?.

Dust formation in very massive primordial supernovae

At redshift z >∼ 5, Type II supernovae (SNeII) are the only known dust sources with evolutionary time-scales shorter than the Hubble time. We extend the model of dust formation in the ejecta of SNeII by Todini & Ferrara to investigate the same process in pair-instability supernovae (PISNe), which are though to arise from the explosion of the first, metal-free, very massive (140-260 M ○. ) cosmic stars. We find that 15-30 per cent of the PISN progenitor mass is converted into dust, a value > 10 times higher than for SNeII; PISN dust depletion factors (the fraction of produced metals locked into dust grains) range between 0.3 and 0.7. These conclusions depend very weakly on the mass of the PISN stellar progenitor, which in contrast affects considerably the composition and size distribution. For the assumed temperature evolution, grain condensation starts 150-200 d after the explosion; the dominant compounds for all progenitor masses are SiO 2 and Mg 2 SiO 4 while the contribution of amorphous carbon and magnetite grains grows with progenitor mass; typical grain sizes range between 10 -3 and a few times 0.1 μm and are always smaller than 1 μm. We give a brief discussion of the implications of dust formation for the initial mass function evolution of the first stars, cosmic reionization and the intergalactic medium.