Stefania Salvadori

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 ACS LCID Project: On the Origin of Dwarf Galaxy Types—A Manifestation of the Halo Assembly Bias?

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 only being 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 color-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 versus 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 primarily reflects 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. Based on observations made with the NASA/ESA HST, which is operated by the AURA, under NASA contract NAS5-26555. Observations associated with programs #8706, #10505, and #10590.

Life and times of dwarf spheroidal galaxies

We propose a cosmological scenario for the formation and evolution of dwarf spheroidal galaxies (dSphs), satellites of the Milky Way (MW). An improved version of the semi-analytical code GAMETE (GAlaxy Merger Tree and Evolution) is used to follow the dSphs evolution simultaneously with the MW formation, matching the observed properties of both. In this scenario dSph galaxies represent fossil objects virializing at z = 7.2 ± 0.7 (i.e. in the prereionization era z > zrei = 6) in the MW environment which at that epoch has already been pre-enriched up to [Fe/H] ∼− 3; their dynamical masses are in the narrow range M = (1.6 ± 0.7) × 10 8 M� , although a larger spread might be introduced by a more refined treatment of reionization. Mechanical feedback effects are dramatic in such low-mass objects, causing the complete blow-away of the gas ∼100 Myr after the formation epoch: 99 per cent of the present-day stellar mass, M∗ = (3 ± 0.7) × 10 6 M� , forms during this evolutionary phase, i.e. their age is >13 Gyr. Later on, star formation is re-ignited by returned gas from evolved stars and a second blow-away occurs. The cycle continues for about 1 Gyr during which star formation is intermittent. At z = 0 the dSph gas content is Mg = (2.68 ± 0.97) × 10 4 M� . Our results match several observed properties of Sculptor, used as a template of dSphs: (i) the metallicity distribution function; (ii) the colour‐magnitude diagram; (iii) the decrement of the stellar [O/Fe] abundance ratio for [Fe/H] > −1.5; (iv) the dark matter content and the light-to-mass ratio; (v) the H I gas mass content.

Simulating cosmic metal enrichment by the first galaxies

We study cosmic metal enrichment via adaptive mesh refinement hydrodynamical simulations in a (10 Mpc h-1)3 volume following the Population III (PopIII)-PopII transition and for different PopIII initial mass function (IMFs). We have analysed the joint evolution of metal enrichment on galactic and intergalactic scales at z = 6 and z = 4. Galaxies account for ≲9 per cent of the baryonic mass; the remaining gas resides in the diffuse phases: (a) voids, i.e. regions with extremely low density (Δ ≤ 1), (b) the true intergalactic medium (IGM, 1 10^{4.5} K state. Due to these physical conditions, C {IV} absorption line experiments can probe only ≃2 per cent of the total carbon present in the IGM/CGM; however, metal absorption line spectra are very effective tools to study reionization. Finally, the PopIII star formation history is almost insensitive to the chosen PopIII IMF. PopIII stars are preferentially formed in truly pristine (Z = 0) gas pockets, well outside polluted regions created by previous star formation episodes.

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.

The first carbon-enhanced metal-poor star found in the Sculptor dwarf spheroidal

The origin of carbon-enhanced metal-poor (CEMP) stars and their possible connection with the chemical elements produced by the first stellar generation is still highly debated. In contrast to the Galactic halo, not many CEMP stars have been found in the dwarf spheroidal galaxies around the Milky Way. Here we present detailed abundances from ESO VLT/UVES high-resolution spectroscopy for ET0097, the first CEMP star found in the Sculptor dwarf spheroidal. This star has

Carbon-enhanced metal-poor stars in dwarf galaxies

\text{[Fe/H]}=-2.03\pm0.10

Mining the Galactic halo for very metal‐poor stars

,

Metals and ionizing photons from dwarf galaxies

\text{[C/Fe]}=0.51\pm0.10

The origin of the far-infrared continuum of z ~ 6 quasars. A radiative transfer model for SDSS J1148+5251

and