ALBERT

Description: CR7 is the brightest z  = 6.6 Ly α emitter (LAE) known to date, and spectroscopic follow-up by Sobral et al. suggests that CR7 might host Population (Pop) III stars. We examine this interpretation using cosmological hydrodynamical simulations. Several simulated galaxies show the same ‘Pop III wave’ pattern observed in CR7. However, to reproduce the extreme CR7 Ly α/He ii 1640 line luminosities ( $L_{\rm \alpha /He\,\small {II}}$ ) a top-heavy initial mass function and a massive ( 10 7 M ) Pop III burst with age 2 Myr are required. Assuming that the observed properties of Ly α and He ii emission are typical for Pop III, we predict that in the COSMOS/UDS/SA22 fields, 14 out of the 30 LAEs at z  = 6.6 with L α  〉 10 43.3 erg s –1 should also host Pop III stars producing an observable $L_{\rm He\,\small {II}}\gtrsim 10^{42.7}\,{\rm erg}\,{\rm s}^{-1}$ . As an alternate explanation, we explore the possibility that CR7 is instead powered by accretion on to a direct collapse black hole. Our model predicts L α , $L_{\rm He\,\small {II}}$ , and X-ray luminosities that are in agreement with the observations. In any case, the observed properties of CR7 indicate that this galaxy is most likely powered by sources formed from pristine gas. We propose that further X-ray observations can distinguish between the two above scenarios.

Description: Here we introduce gamesh , a novel pipeline that implements self-consistent radiative and chemical feedback in a computational model of galaxy formation. By combining the cosmological chemical-evolution model gamete with the radiative transfer code crash , gamesh can post-process realistic outputs of a N -body simulation describing the red-shift evolution of the forming galaxy. After introducing the gamesh implementation and its features, we apply the code to a low-resolution N -body simulation of the formation of the Milky Way and we investigate the combined effects of self-consistent radiative and chemical feedback. Many physical properties, which can be directly compared with observations in the Galaxy and its surrounding satellites, are predicted by the code along with the merger-tree assembly. The resulting red-shift evolution for the Local Group of star-formation rates, reionization and metal enrichment along with the predicted metallicity distribution function of halo stars are critically compared with observations. We discuss the merits and limitations of the first release of gamesh , which also opens new directions to a full implementation of feedback processes in galaxy-formation models by combining semi-analytic and numerical methods.

Description: We investigate the frequency and origin of carbon-enhanced metal-poor (CEMP) stars in Local Group dwarf galaxies by means of a statistical, data-calibrated cosmological model for the hierarchical build-up of the Milky Way and its dwarf satellites. The model self-consistently explains the variation with dwarf galaxy luminosity of the observed: (i) frequency and [Fe/H] range of CEMP stars; (ii) metallicity distribution functions; (iii) star formation histories. We show that if primordial faint supernovae dominated the early metal-enrichment, then CEMP-no stars enriched by the first stellar generations should be present in all dwarf galaxies, with similar number of stars and CEMP fractions at [Fe/H] 〈 –4. We demonstrate that the probability to observe a star that is carbon-enhanced within a given [Fe/H] range strongly depends on the luminosity of the dwarf galaxy and, on average, it is an order of magnitude lower in ‘classical’ Sculptor-like dwarf spheroidal (dSph) galaxies ( P  ≤ 0.02) than in the least luminous ultra-faint dwarfs ( P   0.1). In addition, we explain why it may be easier to find CEMP-no stars at [Fe/H]  –2 in classical dSph galaxies than in ultra-faint dwarfs. These are consequences of the dramatic variation in the fraction of stars at [Fe/H] 〈 –3 with galaxy luminosity: ≥40 per cent for galaxies with L  〈 10 5 L , and ≤0.2 per cent for L  〉 10 7 L . We present model predictions for the low-Fe tail and CEMP fraction of stars in dwarf galaxies, with particular emphasis on the Sculptor dSph, that can be used to shed light on the properties of the first stars.

Description: We estimate the potential contribution of M  〈 10 9 M dwarf galaxies to the reionization and early metal enrichment of the Milky Way environment, or circum-Galactic medium. Our approach is to use the observed properties of ancient stars (12 Gyr old) measured in nearby dwarf galaxies to characterize the star formation at high z . We use a merger-tree model for the build-up of the Milky Way, which self-consistently accounts for feedback processes, and which is calibrated to match the present-day properties of the Galaxy and its dwarf satellites. We show that the high- z analogues of nearby dwarf galaxies can produce the bulk of ionizing radiation (〉80 per cent) required to reionize the Milky Way environment. Our fiducial model shows that the gaseous environment can be 50 per cent reionized at z 8 by galaxies with 10 7 M  ≤  M  〈 10 8 M . At later times, radiative feedback stops the star formation in these small systems, and reionization is completed by more massive dwarf galaxies by z rei = 6.4 ± 0.5. The metals ejected by supernova-driven outflows from M  〈 10 9 M dwarf galaxies almost uniformly fill the Milky Way environment by z 5, enriching it to Z 2 10 –2 Z . At z 2, these early metals are still found to represent the 50 per cent of the total mass of heavy elements in the circum-Galactic medium.

Description: We investigate the metallicity distribution function (MDF) in the Galactic halo and the relative fraction of carbon-normal and carbon-rich stars. To this aim, we use an improved version of the semi-analytical code GAlaxy MErger Tree and Evolution ( gamete ), that reconstructs the hierarchical merger tree of the Milky Way (MW), following the star formation history and the metal and dust evolution in individual progenitors. The predicted scaling relations between the dust, metal and gas masses for MW progenitors show a good agreement with observational data of local galaxies and of gamma-ray burst (GRB) host galaxies at 0.1 〈  z  〈 6.3. Comparing the simulated and the observed MDF, we find that in order to predict the formation of hyper-iron-poor stars at [Fe/H] 〈 –4, faint supernova (SN) explosions have to dominate the metal yields produced by Population III (Pop III) stars, disfavouring a Pop III initial mass function that extends to stellar masses 〉140 M , into the Pair-Instability SN progenitor mass range. The relative contribution of C-normal and C-enhanced stars to the MDF and its dependence on [Fe/H] points to a scenario where the Pop III/II transition is driven by dust cooling, and the first low-mass stars form when the dust-to-gas ratio in their parent clouds exceeds a critical value of ${\cal D}_{\rm crit} = 4.4 \times 10^{-9}$ . Other transition criteria do not predict any C-normal stars below [Fe/H] 〈 –4, at odds with observations.

Description: 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) x 10 11 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.

Description: 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 ( ≤ 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. At $z$ = 6, a galactic mass–metallicity relation is established. At $z$ = 4, galaxies with a stellar mass M *  ~= 10 8.5 M show $\log ({\rm O}/H)+12=8.19$, consistent with observations. The total amount of heavy elements rises from $\Omega ^{\rm SFH}_{Z}=1.52\times 10^{-6}$ 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 density–metallicity ($\Delta {\rm -}Z$) 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 ($T\, \mu ^{-1}〉10^{4.5}\,{\rm K}$) state. Due to these physical conditions, ${{\rm C}\,{\small {iv}}}$ 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.

Description: We study the impact of star-forming minihaloes, and the initial mass function (IMF) of Population III (Pop III) stars, on the Galactic halo metallicity distribution function (MDF) and on the properties of C-enhanced and C-normal stars at [Fe/H] 〈 –3. For our investigation we use a data-constrained merger tree model for the Milky Way formation, which has been improved to self-consistently describe the physical processes regulating star formation in minihaloes, including the poor sampling of the Pop III IMF. We find that only when star-forming minihaloes are included the low-Fe tail of the MDF is correctly reproduced, showing a plateau that is built up by C-enhanced metal-poor stars imprinted by primordial faint supernovae. The incomplete sampling of the Pop III IMF in inefficiently star-forming minihaloes (〈10 –3 M  yr –1 ) strongly limits the formation of pair-instability supernovae (PISNe), with progenitor masses m PopIII  = [140–260] M , even when a flat Pop III IMF is assumed. Second-generation stars formed in environments polluted at 〉50 per cent level by PISNe are thus extremely rare, corresponding to 0.25 per cent of the total stellar population at [Fe/H] 〈 –2, which is consistent with recent observations. The low-Fe tail of the MDF strongly depends on the Pop III IMF shape and mass range. Given the current statistics, we find that a flat Pop III IMF model with m PopIII  = [10–300] M is disfavoured by observations. We present testable predictions for Pop III stars extending down to lower masses, with m PopIII  = [0.1–300] M .

Description: We present zoom-in, adaptive mesh refinement, high-resolution (~=30 pc) simulations of high-redshift ( z ~= 6) galaxies with the aim of characterizing their internal properties and interstellar medium. Among other features, we adopt a star formation model based on a physically sound molecular hydrogen prescription, and introduce a novel scheme for supernova feedback, stellar winds and dust-mediated radiation pressure. In the zoom-in simulation, the target halo hosts ‘Dahlia’, a galaxy with a stellar mass M * = 1.6 x 10 10 M , representative of a typical z  ~ 6 Lyman-break galaxy. Dahlia has a total H 2 mass of 10 8.5 M that is mainly concentrated in a disc-like structure of effective radius ~=0.6 kpc and scale height ~=200 pc. Frequent mergers drive fresh gas towards the centre of the disc, sustaining a star formation rate per unit area of ~=15 M yr –1 kpc –2 . The disc is composed of dense ( n 25 cm –3 ), metal-rich ( Z ~= 0.5 Z ) gas that is pressure supported by radiation. We compute the 158 μm [C ii ] emission arising from Dahlia, and find that ~=95 per cent of the total [C ii ] luminosity ( $L_{\rm [C\,\small {II}]}\simeq 10^{7.5}\,{\rm L}_{{\odot }}$ ) arises from the H 2 disc. Although 30 per cent of the C ii mass is transported out of the disc by outflows, such gas negligibly contributes to [C ii ] emission, due to its low density ( n 10 cm –3 ) and metallicity ( Z 10 –1 Z ). Dahlia is underluminous with respect to the local [C ii ]–SFR relation; however, its luminosity is consistent with upper limits derived for most z  ~ 6 galaxies.