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.

Context. The ages of individual Red Giant Branch stars can range from 1 Gyr old to the age of the Universe, and it is believed that the abundances of most chemical elements in their photospheres remain unchanged with time (those that are not affected by the first dredge-up). This means that they trace the interstellar medium in the galaxy at the time the star formed, and hence the chemical enrichment history of the galaxy. Aims. Colour-Magnitude Diagram analysis has shown the Carina dwarf spheroidal to have had an unusually episodic star formation history and this is expected to be reflected in the abundances of different chemical elements. Methods. We use the VLT-FLAMES multi-fibre spectrograph in high-resolution mode (R≈20000) to measure the abundances of several chemical elements, including Fe, Mg, Ca and Ba, in a sample of 35 individual Red Giant Branch stars in the Carina dwarf spheroidal galaxy. We also combine these abundances with photometry to derive age estimates for these stars. This allows us to determine which of two distinct star formation episodes the stars in our sample belong to, and thus to define the relationship between star formation and chemical enrichment during these two episodes.

We investigate the nature of the newly discovered Ultra Faint dwarf spheroidal galaxies (UF dSphs) in a general cosmological context simultaneously accounting for various ``classical`` dSphs and Milky Way properties including their Metallicity Distribution Function (MDF). To this aim we extend the merger tree approach previously developed to include the presence of star-forming minihaloes, and an heuristic prescription for radiative feedback. The model successfully reproduces both the observed [Fe/H]-Luminosity relation and the mean MDF of UFs. In this picture UFs are the oldest, most dark matter-dominated (M/L > 100) dSphs with a total mass M= 10^{7-8}Msun; they are leftovers of H_2-cooling minihaloes formed at z > 8.5, i.e. before reionization. Their MDF is broader (because of a more prolonged SF) and shifted towards lower [Fe/H] (as a result of a lower gas metallicity at the time of formation) than that of classical dSphs. These systems are very ineffectively star-forming, turning into stars by z=0 only <3% of the potentially available baryons. We provide a useful fit for the star formation efficiency of dSphs.Comment: 5 pages, 4 figures, accepted for publication in MNRAS http://saveitalianbrains.wordpress.co

We study the stellar population history and chemical evolution of the Milky Way (MW) in a hierarchical cold dark matter model for structure formation. Using a Monte Carlo method based on the semi-analytical extended Press & Schechter formalism, we develop a new code GALAXY MERGER TREE AND EVOLUTION (GAMETE) to reconstruct the merger tree of the Galaxy and follow the evolution of gas and stars along the hierarchical tree. Our approach allows us to compare the observational properties of the MW with model results, exploring different properties of primordial stars, such as their initial mass function and the critical metallicity for low-mass star formation, Z cr . In particular, by matching our predictions to the metallicity distribution function (MDF) of metal-poor stars in the Galactic halo we find that: (i) a strong supernova (SN) feedback is required to reproduce the observed properties of the MW; (ii) stars with [Fe/H] < −2.5 form in haloes accreting Galactic medium (GM) enriched by earlier SN explosions; (iii) the fiducial model (Z cr = 10 −4 Z , m PopIII = 200 M ) provides an overall good fit to the MDF, but cannot account for the two hyper-metal-poor (HMP) stars with [Fe/H] < −5; the latter can be accommodated if Z cr 10 −6 Z but such model overpopulates the 'metallicity desert', that is, the range −5.3 < [Fe/H] < −4 in which no stars have been detected; (iv) the current non-detection of metal-free stars robustly constrains either Z cr > 0 or the masses of the first stars m PopIII > 0.9 M ; (v) the statistical impact of truly secondgeneration stars, that is, stars forming out of gas polluted only by metal-free stars, is negligible in current samples; and (vi) independent of Z cr , 60 per cent of metals in the GM are ejected through winds by haloes with masses M < 6 × 10 9 M , thus showing that low-mass haloes are the dominant population contributing to cosmic metal enrichment. We discuss the limitations of our study and comparison with previous work.

We study the formation and evolution of a sample of Lyman Break Galaxies in the Epoch of Reionisation by using high-resolution (∼ 10 pc), cosmological zoom-in simulations part of the serra suite. In serra, we follow the interstellar medium (ISM) thermo-chemical non-equilibrium evolution, and perform on-the-fly radiative transfer of the interstellar radiation field (ISRF). The simulation outputs are post-processed to compute the emission of far infrared lines ([C II], [N II], and [O III]). At z = 8, the most massive galaxy, "Freesia", has an age t 409 Myr, stellar mass M 4.2 × 10 9 M , and a star formation rate SFR 11.5 M yr −1 , due to a recent burst. Freesia has two stellar components (A and B) separated by 2.5 kpc; other 11 galaxies are found within 56.9 ± 21.6 kpc. The mean ISRF in the Habing band is G = 7.9 G 0 and is spatially uniform; in contrast, the ionisation parameter is U = 2 +20 −2 × 10 −3 , and has a patchy distribution peaked at the location of star-forming sites. The resulting ionising escape fraction from Freesia is f esc 2%. While [C II] emission is extended (radius 1.54 kpc), [O III] is concentrated in Freesia-A (0.85 kpc), where the ratio Σ [OIII] /Σ [CII] 10. As many high-z galaxies, Freesia lies below the local [C II]-SFR relation. We show that this is the general consequence of a starburst phase (pushing the galaxy above the Kennicutt-Schmidt relation) which disrupts/photodissociates the emitting molecular clouds around star-forming sites. Metallicity has a sub-dominant impact on the amplitude of [C II]-SFR deviations.

To improve our understanding of high-z galaxies we study the impact of H 2 chemistry on their evolution, morphology and observed properties. We compare two zoom-in high-resolution (30 pc) simulations of prototypical M ∼ 10 10 M galaxies at z = 6. The first, "Dahlia", adopts an equilibrium model for H 2 formation, while the second, "Althaea", features an improved non-equilibrium chemistry network. The star formation rate (SFR) of the two galaxies is similar (within 50%), and increases with time reaching values close to 100 M yr −1 at z = 6. They both have SFR-stellar mass relation consistent with observations, and a specific SFR of 5 Gyr −1 . The main differences arise in the gas properties. The non-equilibrium chemistry determines the H→ H 2 transition to occur at densities > 300 cm −3 , i.e. about 10 times larger than predicted by the equilibrium model used for Dahlia. As a result, Althaea features a more clumpy and fragmented morphology, in turn making SN feedback more effective. Also, because of the lower density and weaker feedback, Dahlia sits 3σ away from the Schmidt-Kennicutt relation; Althaea, instead nicely agrees with observations. The different gas properties result in widely different observables. Althaea outshines Dahlia by a factor of 7 (15) in [C II] 157.74 µm (H 2 17.03 µm) line emission. Yet, Althaea is under-luminous with respect to the locally observed [C II]-SFR relation. Whether this relation does not apply at high-z or the line luminosity is reduced by CMB and metallicity effects remains as an open question.

We study cosmic metal enrichment via AMR hydrodynamical simulations in a (10 Mpc h −1 ) 3 volume following the Pop III -Pop II transition and for different Pop III IMFs. We have analyzed the joint evolution of metal enrichment on galactic and intergalactic scales at z = 6 and z = 4. Galaxies account for < ∼ 9% of the baryonic mass; the remaining gas resides in the diffuse phases: (a) voids, i.e. regions with extremely low density (∆ 1), (b) the true intergalactic medium (IGM, 1 < ∆ 10) and (c) the circumgalactic medium (CGM, 10 < ∆ 10 2.5 ), the interface between the IGM and galaxies. By z = 6 a galactic mass-metallicity relation is established. At z = 4, galaxies with a stellar mass M 10 8.5 M show log(O/H) + 12 = 8.19, consistent with observations. The total amount of heavy elements rises from Ω SFH Z = 1.52 × 10 −6 at z = 6 to 8.05 × 10 −6 at z = 4. Metals in galaxies make up to 0.89 of such budget at z = 6; this fraction increases to 0.95 at z = 4. At z = 6 (z = 4) the remaining metals are distributed in CGM/IGM/voids with the following mass fractions: 0.06/0.04/0.01 (0.03/0.02/0.01). Analogously to galaxies, at z = 4 a densitymetallicity (∆ − Z) relation is in place for the diffuse phases: the IGM/voids have a spatially uniform metallicity, Z ∼ 10 −3.5 Z ; in the CGM Z steeply rises with density up to 10 −2 Z . In all diffuse phases a considerable fraction of metals is in a warm/hot (T µ −1 > 10 4.5 K) state. Due to these physical conditions, C IV absorption line experiments can probe only 2% of the total carbon present in the IGM/CGM; however, metal absorption line spectra are very effective tools to study reionization. Finally, the Pop III star formation history is almost insensitive to the chosen Pop III IMF. Pop III stars are preferentially formed in truly pristine (Z = 0) gas pockets, well outside polluted regions created by previous star formation episodes.

We discuss how knowledge of the whole evolutionary history of dwarf galaxies, including details on the early star formation events, can provide insight on the origin of the different dwarf galaxy types. We suggest that these types may be imprinted by the early conditions of formation rather than being only the result of a recent morphological transformation driven by environmental effects. We present precise star formation histories of a sample of Local Group dwarf galaxies, derived from colour-magnitude diagrams reaching the oldest main-sequence turnoffs. We argue that these galaxies can be assigned to two basic types: fast dwarfs that started their evolution with a dominant and short star formation event, and slow dwarfs that formed a small fraction of their stars early and have continued forming stars until the present time (or almost). These two different evolutionary paths do not map directly onto the present-day morphology (dwarf spheroidal vs dwarf irregular). Slow and fast dwarfs also differ in their inferred past location relative to the Milky Way and/or M31, which hints that slow dwarfs were generally assembled in lower density environments than fast dwarfs. We propose that the distinction between a fast and slow dwarf galaxy reflects primarily the characteristic density of the environment where they form. At a later stage, interaction with a large host galaxy may play a role in the final gas removal and ultimate termination of star formation.