Over the past two decades observations and theoretical simulations have established a global frame-work of galaxy formation and evolution in the young Universe (1-3).Galaxies formed as baryonic gas cooled at the centres of collapsing dark matter halos. Mergers of halos and galaxies led to the hierarchical build-up of galaxy mass.It remains unclear, however, over what timescales galaxies were assembled and when and how bulges and disks, the primary components of present day galaxies, were formed. It is also puzzling that the most massive galaxies were more abundant and were forming stars more rapidly at early epochs than expected from models (4-7). A major step forward in understanding these issues requires well resolved physical information on individual galaxies at high redshift. Here we report adaptive optics, spectroscopic observations of a representative luminous star forming galaxy when the Universe was only twenty percent of its current age. The far superior angular resolution of these data compared to our first study (8) reveals the physical and dynamical properties of a high redshift galaxy in unprecedented detail. A large and massive rotating proto-disk is channelling gas toward a growing central stellar bulge hosting an accreting massive black hole. The high gas surface densities, large star formation rate and moderately young stellar ages suggest rapid assembly, fragmentation and conversion to stars of an initially very gas rich protodisk, with no obvious evidence for a major merger. 2Imaging spectroscopy of high redshift galaxies at high angular resolution of well understood rest-frame optical spectral diagnostics is now becoming feasible with advanced instruments on large ground-based telescopes. This promises new empirical information about the crucial epoch of galaxy evolution near cosmological redshift z~2, about 3 billion years after the Big Bang when the Universe was about 20% of its current age. We have recently begun a study of a representative sample of z~2-3 star forming galaxies, selected based on their rest-frame ultra-violet/optical fluxes and colours, with the near-infrared integral field spectrometer SINFONI on the Very Large Telescope of the European Southern Observatory (9, 10). Our first results (8) revealed that fairly large and massive proto-disk galaxies were present already at z~2-3. We did not have sufficient resolution, however, to distinguish unambiguously between a merger and disk interpretation, or to resolve the bulge and disk components. For one of these luminous star forming galaxies, BzK-15504 (z=2.38: 11, 12), the presence of a nearby star and excellent atmospheric conditions now allowed us, for the first time, to take full advantage of the adaptive optics mode of SINFONI. We achieved an angular resolution of ~0.15" (1.2 kpc or 4000 light years), more than three times better than in our previous work.BzK-15504 is a fairly typical representative of rest-frame optically bright, actively star forming galaxies at that redshift (for details see caption of Fig.1 and Suppleme...

We compile a new sample of 115 nearby, non‐Seyfert galaxies spanning a wide range of star formation activities, from starburst to nearly dormant, based on ultraviolet observations with various satellites. We combine these observations with infrared observations to study the relation between ratio of total far‐infrared to ultraviolet luminosity and ultraviolet spectral slope (the IRX–UV relation). We show that, at fixed ultraviolet spectral slope, quiescent star‐forming galaxies in our sample have systematically lower ratio of total far‐infrared to ultraviolet luminosity than starburst galaxies. The strengths of spectral indices sensitive to star formation history, such as the 4000‐Å spectral discontinuity and the Hα emission equivalent width, correlate well with distance from the mean relation for starburst galaxies in the IRX–UV diagram, while there is little or no correlation between the dust‐sensitive Hα/Hβ ratio and this distance. This is strong observational evidence that the star formation history is relevant to the second parameter affecting the IRX–UV relation. We show that these results can be understood in the framework of the simple model of Charlot & Fall for the transfer of starlight through the interstellar medium in galaxies. We confirm that, for starburst galaxies, the tight IRX–UV relation can be understood most simply as a sequence in overall dust content. In addition, we find that the broadening of the relation for quiescent star‐forming galaxies can be understood most simply as a sequence in the ratio of present to past‐averaged star formation rate. We use a library of Monte Carlo realizations of galaxies with different star formation histories and dust contents to quantify the accuracy to which the ultraviolet attenuation AFUV of a galaxy can be estimated from either the ratio of far‐infrared to ultraviolet luminosity or the ultraviolet spectral slope. We provide simple formulae for estimating AFUV as a function of either of these observational quantities and show that the accuracy of these estimates can be significantly improved if some constraints are available on the ratio of present to past‐averaged star formation rate.

Many stellar models present difficulties in reproducing basic observational relations of very low mass stars (VLMS), including the mass-radius relation and the optical colour-magnitudes of cool dwarfs. Here, we improve PARSEC models on these points. We implement the T -τ relations from PHOENIX BT-Settl model atmospheres as the outer boundary conditions in the PARSEC code, finding that this change alone reduces the discrepancy in the mass-radius relation from 8 to 5 per cent. We compare the models with multi-band photometry of clusters Praesepe and M 67, showing that the use of T -τ relations clearly improves the description of the optical colours and magnitudes. But anyway, using both Kurucz and PHOENIX model spectra, model colours are still systematically fainter and bluer than the observations. We then apply a shift to the above T -τ relations, increasing from 0 at T eff = 4730 K to ∼14% at T eff = 3160 K, to reproduce the observed mass-radius radius relation of dwarf stars. Taking this experiment as a calibration of the T -τ relations, we can reproduce the optical and near infrared CMDs of low mass stars in the old metal-poor globular clusters NGC 6397 and 47 Tuc, and in the intermediate-age and young solar-metallicity open clusters M 67 and Praesepe. Thus, we extend PARSEC models using this calibration, providing VLMS models more suitable for the lower main sequence stars over a wide range of metallicities and wavelengths. Both sets of models are available on PARSEC webpage.

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