The structure of the outer parts of galactic disks and the nature of their stellar populations are fundamental to our understanding of the formation and evolution of spiral galaxies. Ages and metallicity distributions of stars in the outermost regions of spiral disks provide important clues on how and when the disks are assembled. In our earlier work we trace the extended stellar disk of NGC 300 out to a radius of at least 10 disk scale lengths, with no sign of truncation. We now revisit the outer disk of NGC 300 in order to derive the metallicity distribution of the faint stellar population in its outskirts. We find that predominantly old stellar population in the outer disk exhibits a negative abundance gradient – as predicted by the chemical evolution models – out to about 10 kpc, followed by the metallicity plateau in the outermost disk.

Global star formation is the key to understanding galaxy disk formation. This in turn depends on gravitational instability of disks and continuing gas accretion as well as minor merging. A key component is feedback from supernovae. Primary observational constraints on disk galaxy formation and evolution include the Schmidt-Kennicutt law, the Tully-Fisher relation and the galaxy luminosity function. I will review how theory confronts phenomenology, and discuss future prospects for refining our understanding of disk formation.

Our knowledge on the age structure, the chemical evolution, and the kinematics of the Galactic disk has grown substantially during the last years. Recent results on the properties of the stellar populations in the Galactic disk are summarized, and ongoing and future surveys and facilities are discussed. A short overview of recent mass estimates for the Milky Way is presented, and a brief summary of some of the key properties of the Galactic companions is given. The coming decade promises major breakthroughs in understanding our Milky Way, its disk, and the role of its satellites.

The Sloan Extension for Galactic Exploration and Understanding (SEGUE) has now been completed. This is one of three surveys that were executed as part of the first extension of the Sloan Digital Sky Survey (SDSS-II), which consist of LEGACY, SUPERNOVA SURVEY, and SEGUE. The SEGUE program has obtained over 3600 square degrees of ugriz imaging of the sky outside the original SDSS-I footprint. The regions of sky targeted for SEGUE imaging were primarily at lower Galactic latitudes (|b| < 35°), in order to better sample the disk/halo interface of the Milky Way. SEGUE also obtained medium-resolution (R = 2000) spectroscopy, over the wavelength range 3800-9200 Å, for over 200,000 stars in 200 selected areas over the sky available from Apache Point, New Mexico. We discuss the determination of stellar atmospheric parameters (Teff, log g, and [Fe/H]) for these stars, and highlight several of the scientific results obtained to date. The proposed second extension of SDSS, known as SDSS-III, will include SEGUE-2, a program to roughly double the numbers of stars with available spectroscopy, as well as APOGEE, a program to obtain high-resolution (R = 20000) near-IR spectroscopy for over 100,000 stars in the disk, bulge and halo populations of the Galaxy. Other massive spectroscopic surveys of interest to Galactic science are also briefly discussed.

We present an update of the Bologna Open Clusters Chemical Evolution project (BOCCE, in short). We are conducting a photometric and spectroscopic survey of Open Clusters, to be used as tracers of the Galactic disk properties and evolution. We obtain the clusters parameters (age, distance, metallicity, and detailed abundances) in a precise and homogeneous way. We have collected data for about 40 Open Clusters and have fully analyzed the photometric data for about one half and the spectra for one quarter of them. We present here results based on these works and indicate what will come next.

We study the effects of Supernova (SN) feedback on the formation of galaxies using hydrodynamical simulations in a ΛCDM cosmology. We use an extended version of the code GADGET-2 which includes chemical enrichment and energy feedback by Type II and Type Ia SN, metal-dependent cooling and a multiphase model for the gas component. We focus on the effects of SN feedback on the star formation process, galaxy morphology, evolution of the specific angular momentum and chemical properties. We find that SN feedback plays a fundamental role in galaxy evolution, producing a self-regulated cycle for star formation, preventing the early consumption of gas and allowing disks to form at late times. The SN feedback model is able to reproduce the expected dependence on virial mass, with less massive systems being more strongly affected.

In this talk I revisit the problem of gas accretion onto minihalos after reionization. I show that primordial minihalos with vcir < 20 km s−1 stop accreting gas after reionization, as is usually assumed, but in virtue of their increasing concentration and the decreasing temperature of the intergalactic medium as redshift decreases, they have a late phase (at redshift z<2) of gas accretion and possibly star formation. As a result we expect that pre-reionization fossils have a more complex star formation history than previously envisioned. A signature of this model is a bimodal star formation history. The dwarf spheroidal galaxy Leo T, that inspired the present work, fits with this scenario. Another prediction of the model is the existence of a population of gas rich minihalos that never formed stars. A subset of compact high-velocity clouds may be identified as such objects but the bulk of them may still be undiscovered.

We present our recent results on the cosmic evolution of the outskirts of disk galaxies. In particular we focus on disk–like galaxies with stellar disk truncations. Using UDF, GOODS and SDSS data we show how the position of the break (i.e. a direct estimator of the size of the stellar disk) evolves with time since z~1. Our findings agree with an evolution on the radial position of the break by a factor of 1.3 ± 0.1 in the last 8 Gyr for galaxies with similar stellar masses. We also present radial color gradients and how they evolve with time. At all redshifts we find a radial inside-out bluing reaching a minimum at the position of the break radius, this minimum is followed by a reddening outwards. Our results constraint several galaxy disk formation models and favour a scenario where stars are formed inside the break radius and are relocated in the outskirts of galaxies through secular processes.

The first metal enrichment in the universe was made by supernova (SN) explosions of population (Pop) III stars. The history of chemical evolution is recorded in abundance patterns of extremely metal-poor (EMP) stars. We investigate the properties of nucleosynthesis in Pop III SNe by comparing their yields with the abundance patterns of the EMP stars. We focus on (1) jet-induced SNe with various properties of the jets, especially energy deposition rates [Ėdep = (0.3 − 1500) × 1051 ergs s−1], and (2) SNe of stars with various main-sequence masses (Mms = 13 − 50M⊙) and explosion energies [E = (1 − 40) × 1051ergs]. The varieties of Pop III SNe can explain the observations of the EMP stars: (1) higher [C/Fe] for lower [Fe/H] and (2) trends of abundance ratios [X/Fe] against [Fe/H].

We present a new determination of the mass content of the Sculptor dwarf spheroidal galaxy, based on a novel approach which takes into account the two distinct stellar populations present in this galaxy. This method helps to partially break the well-known mass-anisotropy degeneracy present in the modelling of pressure-supported stellar systems.

The chemical enrichment by the first sources of light in the universe ultimately set the stage for the subsequent evolution of the Milky Way system. The oldest and, usually, the most-metal poor stars are our ‘near-field’ link to this ancient epoch as they, apart from tracing the chemical enrichment itself, also indirectly hold information on, e.g., the conditions for star formation and feed-back effects in the early universe. In particular, I will discuss the possible origins of the relatively large number of carbon enhanced metal-poor stars in the Galactic halo. Furthermore, I will argue that the apparent absence of the chemical signature of so-called pair-instability supernovae (PISNe), which are a natural consequence of current theoretical models for primordial star formation at the highest masses, may arise from a subtle observational selection effect. Whereas most surveys traditionally focus on the most metal-poor stars, early PISN enrichment is predicted to ‘overshoot’, reaching enrichment levels of [Ca/H] ~ −2.5 that would be missed by current searches.

The first stars were key drivers of early cosmic evolution. We review the main physical elements of the current consensus view, positing that the first stars were predominantly very massive. We continue with a discussion of important open questions that confront the standard model. Among them are uncertainties in the atomic and molecular physics of the hydrogen and helium gas, the multiplicity of stars that form in minihalos, and the possible existence of two separate modes of metal-free star formation.

We present a study of remarkably luminous and unique dwarf galaxies at redshifts of 0.5 < z < 0.7, selected from the DEEP2 Galaxy Redshift survey by the presence of the temperature sensitive [OIII]λ4363 emission line. Measurements of this important auroral line, as well as other strong oxygen lines, allow us to estimate the integrated oxygen abundances of these galaxies accurately without being subject to the degeneracy inherent in the standard R23 system used by most studies. [O/H] estimates range between 1/5–1/10 of the solar value. Not surprisingly, these systems are exceedingly rare and hence represent a population that is not typically present in local surveys such as SDSS, or smaller volume deep surveys such as GOODS.

Our low-metallicity galaxies exhibit many unprecedented characteristics. With B-band luminosities close to L*, thse dwarfs lie significantly away from the luminosity-metallicity relationships of both local and intermediate redshift star-forming galaxies. Using stellar masses determined from optical and NIR photometry, we show that they also deviate strongly from corresponding mass-metallicity relationships. Their specific star formation rates are high, implying a significant burst of recent star formation. A campaign of high resolution spectroscopic follow-up shows that our galaxies have dynamical properties similar to local HII and compact emission line galaxies, but mass-to-light ratios that are much higher than average star-forming dwarfs.

The low metallicities, high specific star formation rates, and small halo masses of our galaxies mark them as lower redshift analogs of Lyman-Break galaxies, which, at z ~ 2 are evolving onto the metallicity sequence that we observe in the galaxy population of today. In this sense, these systems offer fundamental insights into the physical processes and regulatory mechanisms that drive galaxy evolution in that epoch of major star formation and stellar mass assembly.

Galactic disks consist of both stars and gas. The stars gravitationally influence the gas either in disks at large or within spiral arms, leading to the formation of giant clouds and turbulence driving in the gas. In featureless disks as in flocculent galaxies, swing amplification operating in a combined star-gas disk is efficient to form bound condensations and feed a significant level of random gas motions. This occurs when the gaseous Toomre parameter is less than 1.4 for the stellar parameters similar to the solar neighbourhood conditions. In disks with spiral features, on the other hand, spiral-arm spurs and associated giant clouds develop as a consequence of magneto-Jeans instability in which magnetic tension counterbalances the stabilizing Coriolis force. Spiral shocks are inherently unstable when the vertical dimension is taken into account, exhibiting flapping motions of the shock front. This naturally converts the kinetic energy in galaxy rotation into random kinetic energy of the gas. The resulting turbulent motions are supersonic and persist despite strong shock dissipation. Thermal instability occurring in gas flows across spiral arms prompts phases transitions that produce a significant fraction of thermally-unstable, intermediate-temperature gas in the postshock expansion zones.

I will first discuss abundance gradients in the Milky Way and nearby disk galaxies, then the problem of substructures in the Galactic Disk, and finally some new opportunities for investigating substructure in disks.

Encircling the Milky Way at low latitudes, the Low Latitude Stream is a large stellar structure, the origin of which is as yet unknown. As part of the SEGUE survey, several photometric scans have been obtained that cross the Galactic plane, spread over a longitude range of 50° to 203°. These data allow a systematic study of the structure of the Galaxy at low latitudes, where the Low Latitude Stream resides. We apply colour-magnitude diagram fitting techniques to map the stellar (sub)structure in these regions, enabling the detection of overdensities with respect to smooth models. These detections can be used to distinguish between different models of the Low Latitude Stream, and help to shed light on the nature of the system.

We investigate the evolution of dust formed in Population III supernovae (SNe) by considering its transport and processing by sputtering within the SN remnants (SNRs). We find that the fate of dust grains within SNRs heavily depends on their initial radii aini. For Type II SNRs expanding into the ambient medium with density of nH,0 = 1 cm−3, grains of aini < 0.05 μm are detained in the shocked hot gas and are completely destroyed, while grains of aini > 0.2 μm are injected into the surrounding medium without being significantly destroyed. Grains with aini = 0.05–0.2 μm are finally trapped in the dense shell behind the forward shock. We show that the grains piled up in the dense shell enrich the gas up to 10−6–10−4Z⊙, high enough to form low-mass stars with 0.1–1 M⊙. In addition, [Fe/H] in the dense shell ranges from −6 to −4.5, which is in good agreement with the ultra-metal-poor stars with [Fe/H] < −4. We suggest that newly formed dust in a Population III SN can have great impact on the stellar mass and elemental composition of Population II.5 stars formed in the shell of the SNR.

We review the expected properties of Pop III and very metal-poor starbursts and the behaviour the Lyα and He ii λ1640 emission lines, which are most likely the best/easiest signatures to single out such objects. Existing claims of Pop III signatures in distant galaxies are critically examined, and the searches for He ii λ1640 emission at high redshift are summarised. Finally, we briefly summarise ongoing and future deep observations at z > 6 aiming in particular at detecting the sources of cosmic reionisation as well as primeval/Pop III galaxies.

We investigate the size-density relation of H ii regions in blue compact dwarf galaxies (BCDs) by compiling observational data of their size (Di) and electron density (ne). We find that the size-density relation follows a relation with constant column density (ne ∝ Di−1) rather than with constant luminosity (ne ∝ Di−1.5). Such behavior resembles that of Galactic H ii regions, and may imply an underlying “scale-free” connection. Because this size-density relation cannot be explained by static models, we model and examine the evolution of the size-density relation of H ii regions by considering the star formation history and pressure-driven expansion of H ii regions. We find that the size-density relation of the entire BCD sample does not result from an evolutionary sequence of H ii regions but rather reflects a sequence with different initial gas densities (or “hierarchy” of density). We also find that the dust extinction of ionizing photons is significant for the BCD sample, despite their blue optical colors. This means that as long as the emission from H ii regions is used to trace massive star formation, we would miss the star formation activity in dense environments even in low-metallicity galaxies such as BCDs.