The thermal and chemical evolution of star-forming clouds is studied for different gas metallicities, Z, using the model of Omukai (2000), 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 − 10 −3 Z ⊙ and density ≈ 10 5 cm −3 . Early on, CMB radiation prevents the gas temperature to fall below T CM B , 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 E ≡ (b − a)/a > E NL ∼ 1, where a (b) is the short (long) core axis length. The fragment mass is given by the thermal Jeans mass at E = E NL . Given these assumptions and the initial (gaussian) distribution of E we compute the fragment mass distribution as a function of metallicity. We find that: (i) For Z = 0, all fragments are very massive, 10 3 M ⊙ , consistently with previous studies; (ii) for Z > 10 −6 Z ⊙ a few clumps go through an additional high density ( 10 10 cm −3 ) fragmentation phase driven by dustcooling, leading to low-mass fragments; (iii) 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; (iv) as a result of the two fragmentation modes, a bimodal mass distribution emerges in 0.01 < Z/Z ⊙ < 0.1. (v) For 0.1Z ⊙ , the two peaks merge into a singly-peaked mass function which might be regarded as the precursor of the ordinary Salpeter-like IMF.

The presence of dust at high redshift requires efficient condensation of grains in supernova (SN) ejecta, in accordance with current theoretical models. Yet observations of the few wellstudied supernovae (SNe) and supernova remnants (SNRs) imply condensation efficiencies which are about two orders of magnitude smaller. Motivated by this tension, we have (i) revisited the model of Todini & Ferrara 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 SNR. 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 interstellar medium, between 2 and 20 per cent 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 (∼0.1 M ) of cold dust (T ∼ 40 K) can be present in SNRs, without being in conflict with the observed infrared emission.

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.

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 À4 Z , when a transition to efficient cooling-driven fragmentation producing d1 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 topheavy 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 e22.1, and for case 2, $3 Â 10 5 , a very negligible fraction of the initial stars produce the metals and the transition redshift occurs at z f e5:4. 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.

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, resulting from the explosions of metal-free progenitors with masses in the range 12 - 30 Msun and 140 - 260 Msun. Using a self-consistent approach, we show that: (i) metals depleted onto dust grains play a fundamental role, enabling fragmentation to solar or sub-solar mass scales already at metallicities Zcr = 10^{-6} Zsun; (ii) even at metallicities as high as 10^{-2} Zsun, metals diffused in the gas-phase lead to fragment mass scales which are ~ 100 Msun; (iii) C atoms are strongly depleted onto amorphous carbon grains and CO molecules so that CII plays a minor role in gas cooling, leaving OI as the main gas-phase cooling agent in low-metallicity clouds. These conclusions hold independently of the assumed supernova progenitors and suggest that the onset of low-mass star formation is conditioned to the presence of dust in the parent clouds.Comment: 9 pages, 5 figures, accepted for publication in MNRA