We use the new ultra-deep, near-infrared imaging of the Hubble Ultra-Deep Field (HUDF) provided by our UDF12 Hubble Space Telescope (HST) WFC3/IR campaign to explore the rest-frame ultraviolet (UV) properties of galaxies at redshifts z > 6.5. We present the first unbiased measurement of the average UV power-law index, β , (f λ ∝ λ β ) for faint galaxies at z ≃ 7, the first meaningful measurements of β at z ≃ 8, and tentative estimates for a new sample of galaxies at z ≃ 9. Utilising galaxy selection in the new F140W (J 140 ) imaging to minimize colour bias, and applying both colour and power-law estimators of β, we find β = −2.1 ± 0.2 at z ≃ 7 for galaxies with M UV ≃ −18. This means that the faintest galaxies uncovered at this epoch have, on average, UV colours no more extreme than those displayed by the bluest star-forming galaxies at low redshift. At z ≃ 8 we find a similar value, β = −1.9 ± 0.3. At z ≃ 9, we find β = −1.8 ± 0.6, essentially unchanged from z ≃ 6 − 7 (albeit highly uncertain). Finally, we show that there is as yet no evidence for a significant intrinsic scatter in β within our new, robust z ≃ 7 galaxy sample. Our results are most easily explained by a population of steadily star-forming galaxies with either ≃ solar metallicity and zero dust, or moderately sub-solar (≃ 10 − 20%) metallicity with modest dust obscuration (A V ≃ 0.1 − 0.2). This latter interpretation is consistent with the predictions of a state-of-the-art galaxy-formation simulation, which also suggests that a significant population of very-low metallicity, dust-free galaxies with β ≃ −2.5 may not emerge until M UV > −16, a regime likely to remain inaccessible until the James Webb Space Telescope.

Galaxy formation is at the heart of our understanding of cosmic evolution. Although there is a consensus that galaxies emerged from the expanding matter background by gravitational instability of primordial fluctuations, a number of additional physical processes must be understood and implemented in theoretical models before these can be reliably used to interpret observations.In parallel, the astonishing recent progresses made in detecting galaxies that formed only a few hundreds of million years after the Big Bang is pushing the quest for more sophisticated and detailed studies of early structures. In this review, we combine the information gleaned from different theoretical models/studies to build a coherent picture of the Universe in its early stages which includes the physics of galaxy formation along with the impact that early structures had on large-scale processes as cosmic reionization and metal enrichment of the intergalactic medium.

We present a theoretical model embedding the essential physics of early galaxy formation (z 5 − 12) based on the single premise that any galaxy can form stars with a maximal limiting efficiency that provides enough energy to expel all the remaining gas, quenching further star formation. This simple idea is implemented into a merger-tree based semi-analytical model that utilises two mass and redshiftindependent parameters to capture the key physics of supernova feedback in ejecting gas from low-mass halos, and tracks the resulting impact on the subsequent growth of more massive systems via halo mergers and gas accretion. Our model shows that: (i) the smallest halos (halo mass M h 10 10 M ) build up their gas mass by accretion from the intergalactic medium; (ii) the bulk of the gas powering star formation in larger halos (M h 10 11.5 M ) is brought in by merging progenitors; (iii) the faint-end UV luminosity function slope evolves according to α = −1.75 log z − 0.52. In addition, (iv) the stellar mass-to-light ratio is well fit by the functional form log M * = −0.38M U V − 0.13 z + 2.4, which we use to build the evolving stellar mass function to compare to observations. We end with a census of the cosmic stellar mass density (SMD) across galaxies with UV magnitudes over the range −23 M U V −11 spanning redshifts 5 < z < 12: (v) while currently detected LBGs contain ≈ 50% (10%) of the total SMD at z = 5 (8), the JWST will detect up to 25% of the SMD at z 9.5.

We interpret recent ALMA observations of z > 6 normal star forming galaxies by means of a semi-numerical method, which couples the output of a cosmological hydrodynamical simulation with a chemical evolution model which accounts for the contribution to dust enrichment from supernovae, asymptotic giant branch stars and grain growth in the interstellar medium. We find that while stellar sources dominate the dust mass of small galaxies, the higher level of metal enrichment experienced by galaxies with M star > 10 9 M ⊙ allows efficient grain growth, which provides the dominant contribution to the dust mass. Even assuming maximally efficient supernova dust production, the observed dust mass of the z = 7.5 galaxy A1689-zD1 requires very efficient grain growth. This, in turn, implies that in this galaxy the average density of the cold and dense gas, where grain growth occurs, is comparable to that inferred from observations of QSO host galaxies at similar redshifts. Although plausible, the upper limits on the dust continuum emission of galaxies at 6.5 < z < 7.5 show that these conditions must not apply to the bulk of the high redshift galaxy population.

Recent observations have gathered a considerable sample of high redshift galaxy candidates and determined the evolution of their luminosity function (LF). To interpret these findings, we use cosmological SPH simulations including, in addition to standard physical processes, a detailed treatment of the Pop III-Pop II transition in early objects. The simulated high-z galaxies match remarkably well the amplitude and slope of the observed LF in the redshift range 59.4) start of their star formation activity; the specific star formation rate is almost independent of galactic stellar mass. These objects are enriched rapidly with metals and galaxies identified by HST/WFC3 (M_UV < -18) show metallicities ~0.1 Zsun even at z=7-8. Most of the simulated galaxies at z~7 (noticeably the smallest ones) are virtually dust-free, and none of them has an extinction larger than E(B-V) = 0.01. The bulk (50%) of the ionizing photons is produced by objects populating the faint-end of the LF (M_UV < -16), which JWST will resolve up to z=7.3. PopIII stars continue to form essentially at all redshifts; however, at z=6 (z=10) the contribution of Pop III stars to the total galactic luminosity is always less than 5% for M_UV < -17 (M_UV < -16). The typical high-z galaxies closely resemble the GRB host galaxy population observed at lower redshifts, strongly encouraging the use of GRBs to detect the first galaxies.Comment: 14 pages, 10 figures, MNRAS in pres

We present a simple, redshift-independent analytic model that explains the local Fundamental Metallicity Relation (FMR), taking into account the physical processes of star formation, inflow of metal-poor intergalactic medium (IGM) gas, and the outflow of metal rich interstellar medium (ISM) gas. We show that the physics of the FMR can be summarised as follows: for massive galaxies with stellar mass M * 10 11 M ⊙ , ISM metal enrichment due to star formation is compensated by inflow of metal poor IGM gas, leading to a constant value of the gas metallicity with star formation rate (SFR); outflows are rendered negligible as a result of the large potential wells of these galaxies. On the other hand, as a result of their smaller SFR, less massive galaxies produce less heavy elements that are also more efficiently ejected due to their shallow potential wells; as a result, for a given M * , the gas metallicity decreases with SFR. For such galaxies, the outflow efficiency determines both the slope, and the knee of the metallicity-SFR relation. Without changing any parameters, this simple model is also successfully matched to the gas fraction -gas metallicity relation observed for a sample of about 260 nearby galaxies.

With the aim of understanding the coevolution of star formation rate (SFR), stellar mass (M * ), and oxygen abundance (O/H) in galaxies up to redshift z 3.7, we have compiled the largest available dataset for studying Metallicity Evolution and Galaxy Assembly (MEGA); it comprises ∼1000 galaxies with a common O/H calibration and spans almost two orders of magnitude in metallicity, a factor of ∼ 10 6 in SFR, and a factor of ∼ 10 5 in stellar mass. From a Principal Component Analysis, we find that the 3-dimensional parameter space reduces to a Fundamental Plane of Metallicity (FPZ) given by 12 + log(O/H) = −0.14 log (SFR) + 0.37 log(M * ) + 4.82. The mean O/H FPZ residuals are small (0.16 dex) and consistent with trends found in smaller galaxy samples with more limited ranges in M * , SFR, and O/H. Importantly, the FPZ is found to be approximately redshift-invariant within the uncertainties. In a companion paper, these results are interpreted with an updated version of the model presented by Dayal et al. (2013).

We extend a previous study of Lyman alpha emitters (LAEs) based on hydrodynamical cosmological simulations, by including two physical processes important for LAEs: (i) Lyα and continuum luminosities produced by cooling of collisionally excited H i in the galaxy and (ii) dust formation and evolution; we follow these processes on a galaxy‐by‐galaxy basis. H i cooling on average contributes 16–18 per cent of the Lyα radiation produced by stars, but this value can be much higher in low‐mass LAEs and further increased if the H i is clumpy. The continuum luminosity is instead almost completely dominated by stellar sources. The dust content of galaxies scales with their stellar mass, Mdust∝M0.7*, and stellar metallicity, Z*, such that Mdust∝Z1.7*. As a result, the massive galaxies have Lyα escape fraction as low as fα= 0.1, with a LAE‐averaged value decreasing with redshift: 〈fα〉= (0.33, 0.23) at z= (5.7, 6.6). The ultraviolet (UV) continuum escape fraction shows the opposite trend with z, possibly resulting from clumpiness evolution. The model successfully reproduces the observed Lyα and UV luminosity functions at different redshifts and the Lyα equivalent width scatter to a large degree, although the observed distribution appears to be more more extended than the predicted one. We discuss possible reasons for such tension.