We address the origin of the robust bimodality observed in galaxy properties about a characteristic stellar mass ∼3 x 10 10 M ⊙ . Less massive galaxies tend to be ungrouped blue star forming discs, while more massive galaxies are typically grouped red old-star spheroids. Colour-magnitude data show a gap between the red and blue sequences, extremely red luminous galaxies already at z ∼ 1, a truncation of today's blue sequence above L*, and massive starbursts at z ∼ 2-4. We propose that these features are driven by the thermal properties of the inflowing gas and their interplay with the clustering and feedback processes, all functions of the dark matter halo mass and associated with a similar characteristic scale. In haloes below a critical shock-heating mass M shock ≤ 10 12 M ⊙ , discs are built by cold streams, not heated by a virial shock, yielding efficient early star formation. It is regulated by supernova feedback into a long sequence of bursts in blue galaxies constrained to a 'fundamental line'. Cold streams penetrating through hot media in M ≥ M shock haloes preferentially at z ≥ 2 lead to massive starbursts in L > L* galaxies. At z M shock haloes hosting groups, the gas is heated by a virial shock, and being dilute it becomes vulnerable to feedback from energetic sources such as active galactic nuclei. This shuts off gas supply and prevents further star formation, leading by passive evolution to 'red-and-dead' massive spheroids starting at z ∼ 1. A minimum in feedback efficiency near M shock explains the observed minimum in M/L and the qualitative features of the star formation history. The cold flows provide a hint for solving the angular momentum problem. When these processes are incorporated in simulations they recover the main bimodality features and solve other open puzzles.

Galaxy bimodality due to cold flows and shock heating

A reexamination is conducted of the formation of dwarf, diffuse, metal-poor galaxies due to supernova-driven winds, in view of data on the systematic properties of dwarfs in the Local Group and Virgo Cluster. The critical condition for global gas loss as a result of the first burst of star formation is that the virial velocity lie below an approximately 100 km/sec critical value. This leads, as observed, to two distinct classes of galaxies, encompassing the diffuse dwarfs, which primarily originate from typical density perturbations, and the normal, brighter galaxies, including compact dwarfs, which can originate only from the highest density peaks. This furnishes a statistical biasing mechanism for the preferential formation of bright galaxies in denser regions, enhancing high surface brightness galaxies' clustering relative to the diffusive dwarfs.

Dwarf galaxies, cold dark matter, and biased galaxy formation

Within the Local Universe galaxies can be studied in great detail star by star, and here we review the results of quantitative studies in nearby dwarf galaxies. The color-magnitude diagram synthesis method is well established as the most accurate way to determine star-formation histories of galaxies back to the earliest times. This approach received a large boost from the exceptional data sets that wide-field CCD imagers on the ground and the Hubble Space Telescope could provide. Spectroscopic studies using large ground-based telescopes such as VLT, Magellan, Keck, and HET have allowed the determination of abundances and kinematics for significant samples of stars in nearby dwarf galaxies. These studies have shown how the properties of stellar populations can vary spatially and temporally. This leads to important constraints to theories of galaxy formation and evolution. The combination of spectroscopy and imaging and what they have taught us about dwarf galaxy formation and evolution is the aim of this r...

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Star-Formation Histories, Abundances, and Kinematics of Dwarf Galaxies in the Local Group

A significant fraction of nearby galaxies show evidence of weak nuclear activity unrelated to normal stellar processes. Recent high-resolution, multiwavelength observations indicate that the bulk of this activity derives from black hole accretion with a wide range of accretion rates. The low accretion rates that typify most low-luminosity active galactic nuclei induce significant modifications to their central engine. The broad-line region and obscuring torus disappear in some of the faintest sources, and the optically thick accretion disk transforms into a three-component structure consisting of an inner radiatively inefficient accretion flow, a truncated outer thin disk, and a jet or outflow. The local census of nuclear activity supports the notion that most, perhaps all, bulges host a central supermassive black hole, although the existence of active nuclei in at least some late-type galaxies suggests that a classical bulge is not a prerequisite to seed a nuclear black hole.

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Nuclear Activity in Nearby Galaxies

We present Keck DEIMOS spectroscopy of stars in eight of the newly discovered ultra-faint dwarf galaxies around the Milky Way. We measure the velocity dispersions of Canes Venatici I, Canes Venatici II, Coma Berenices, Hercules, Leo IV, Leo T, Ursa Major I, and Ursa Major II from the velocities of 18-214 stars in each galaxy and find dispersions ranging from 3.3 to 7.6 km s^(-1). The six galaxies with absolute magnitudes M_V < -4 are highly dark matter dominated, with mass-to-light ratios approaching 1000 M_☉/L_(☉,V). For the fainter galaxies we find tentative evidence for tidal disruption. The measured velocity dispersions of the ultra-faint dwarfs are correlated with their luminosities, indicating that a minimum mass for luminous galactic systems may not yet have been reached. We also measure the metallicities of the observed stars and find that the new dwarfs have mean metallicities of [Fe/H] = -2.0 to -2.3; these galaxies represent some of the most metal-poor stellar systems known. The six brightest of the ultra-faint dwarfs extend the luminosity-metallicity relationship followed by more luminous dwarfs by a factor of ~30 in luminosity. We detect metallicity spreads of up to 0.5 dex in several objects, suggesting multiple star formation epochs. UMa II and Com, despite their exceptionally low luminosities, have higher metallicities that suggest they may once have been much more massive. Having established the masses of the ultra-faint dwarfs, we re-examine the missing satellite problem. After correcting for the sky coverage of the Sloan Digital Sky Survey, we find that the ultra-faint dwarfs substantially alleviate the discrepancy between the predicted and observed numbers of satellites around the Milky Way, but there are still a factor of ~4 too few dwarf galaxies over a significant range of masses. We show that if galaxy formation in low-mass dark matter halos is strongly suppressed after reionization, the simulated circular velocity function of CDM subhalos can be brought into approximate agreement with the observed circular velocity function of Milky Way satellite galaxies.

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The Kinematics of the Ultra-faint Milky Way Satellites: Solving the Missing Satellite Problem

We study the formation and dynamical evolution of clusters with multiple stellar generations. Observational studies have found that some globular clusters host a population of second generation (SG) stars which show chemical anomalies and must have formed from gas containing matter processed in the envelopes of first generation (FG) cluster stars. We study the SG formation process by means of 1D hydrodynamical simulations, starting from a FG already in place and assuming that the SG is formed by the gas ejected by the Asymptotic Giant Branch (AGB) stars. This gas collects in a cooling flow into the cluster core, where it forms SG stars. The SG subsystem emerging from this process is initially strongly concentrated in the cluster innermost regions and its structural properties are largely independent of the FG initial properties. We also present the results of a model in which pristine gas contributes to the SG formation. In this model a very helium-rich SG population and one with a moderate helium enrichment form; the resulting SG bimodal helium distribution resembles that observed for SG stars in NGC 2808. By means of N-body simulations, we then study the two-population cluster dynamical evolution and mass loss. In our simulations, a large fraction of FG stars are lost early in the cluster evolution due to the expansion and stripping of the cluster outer layers resulting from early mass loss associated with FG SN ejecta. The SG population, initially concentrated in the innermost cluster regions, is largely unscathed by this early mass loss, and this early evolution leads to values of the number ratio of SG to FG stars consistent with observations. We also demonstrate possible evolutionary routes leading to the loss of most of the FG population, leaving an SG-dominated cluster. As the cluster evolves and the two populations mix, the local ratio of SG to FG stars, initially a decreasing function of radius, tends to a constant value in the inner parts of the cluster. Until mixing is complete, the radial profile of this number ratio is characterized by a flat inner part and a declining portion in the outer cluster regions.

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Formation and dynamical evolution of multiple stellar generations in globular clusters

The novel class of 1D hydrodynamical evolutionary sequences for gas flows in elliptical galaxies presented employs model galaxies whose virialized density distributions are constrained to lie on their fundamental plane. The models evolve through as many as three consecutive (wind, outflow, inflow) evolutionary stages. These evolutionary sequences indicate that the X-ray-faint galaxies are still in their wind phase, while the bulk of ellipticals are in the outflow phase, and a few of the brightest galaxies may already have undergone transition to the inflow regime. The effects of interactions with the intracluster medium, ram pressure, encounters, and merging events are qualitatively discussed. 114 refs.

Winds, outflows, and inflows in X-ray elliptical galaxies. I

A large number of spectroscopic studies have provided evidence of the presence of multiple populations in globular clusters by revealing patterns in the stellar chemical abundances. This paper is aimed at studying the origin of these abundance patterns. We explore a model in which second generation (SG) stars form out of a mix of pristine gas and ejecta of the first generation of asymptotic giant branch stars. We first study the constraints imposed by the spectroscopic data of SG stars in globular clusters on the chemical properties of the asymptotic and super asymptotic giant branch ejecta. With a simple one-zone chemical model, we then explore the formation of the SG population abundance patterns focussing our attention on the Na-O, Al-Mg anticorrelations and on the helium distribution function. We carry out a survey of models and explore the dependence of the final SG chemical properties on the key parameters affecting the gas dynamics and the SG formation process. Finally, we use our chemical evolution framework to build specific models for NGC 2808 and M4, two Galactic globular clusters which show different patterns in the Na–O and Mg–Al anticorrelation and have different helium distributions. We find that the amount of pristine gas involved in the formation of SG stars is a key parameter to fit the observed O–Na and Mg–Al patterns. The helium distribution function for these models is in general good agreement with the observed one. Our models, by shedding light on the role of different parameters and their interplay in determining the final SG chemical properties, illustrate the basic ingredients, constraints and problems encountered in this self-enrichment scenario which must be addressed by more sophisticated chemical and hydrodynamic simulations.

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Abundance patterns of multiple populations in globular clusters: a chemical evolution model based on yields from AGB ejecta

We introduce the concept of cluster iron mass-to-light ratio (IMLR) and estimate its value for both the intracluster medium and the stars, finding the amount of iron to be nearly the same in the two cluster components. We discuss under which conditions the past supernova activity can account for the observed IMLR and provide evidence that either the past average rate of Type Ia supernovae was at least a factor of ∼10 higher than the present rate in ellipticals, or massive stars in clusters formed with a very flat initial mass function (IMF)

Production and circulation of iron in elliptical galaxies and clusters of galaxies

It is now widely accepted that heating processes play a fundamental role in galaxy clusters, struggling in an intricate but fascinating 'dance' with its antagonist, radiative cooling. Last-generation observations, especially X-ray, are giving us tiny hints about the notes of this endless ballet. Cavities, shocks, turbulence and wide absorption lines indicate that the central active nucleus is injecting a huge amount of energy in the intracluster medium. However, which is the real dominant engine of self-regulated heating? One of the models we propose is massive subrelativistic outflows, probably generated by a wind disc or just the result of the entrainment on kpc scale by the fast radio jet. Using a modified version of the adaptive mesh refinement code FLASH 3.2, we have explored several feedback mechanisms that self-regulate the mechanical power. Two are the best schemes that answer our primary question, that is, quenching cooling flow and at the same time preserving a cool core appearance for a long-term evolution (7 Gyr): one is more explosive (with efficiencies ~ 5 x 10 -3 -10 -2 ), triggered by central cooled gas, and the other is gentler, ignited by hot gas Bondi accretion (with ∈ = 0.1). These three-dimensional simulations show that the total energy injected is not the key aspect, but the results strongly depend on how energy is given to the intracluster medium. We follow the dynamics of the best models (temperature, density, surface brightness maps and profiles) and produce many observable predictions: buoyant bubbles, ripples, turbulence, iron abundance maps and hydrostatic equilibrium deviation. We present an in-depth discussion of the merits and flaws of all our models, with a critical eye towards observational concordance.

Annibale D'Ercole

Papers

Formation and dynamical evolution of multiple stellar generations in globular clusters

Winds, outflows, and inflows in X-ray elliptical galaxies. I

Abundance patterns of multiple populations in globular clusters: a chemical evolution model based on yields from AGB ejecta

Production and circulation of iron in elliptical galaxies and clusters of galaxies

The dance of heating and cooling in galaxy clusters: three-dimensional simulations of self-regulated active galactic nuclei outflows