111,922 research outputs found
Recommended from our members
GRB Cosmology: Probing The Early Universe
No full textCurrent observations are about to open up a direct observational window into the final frontier of cosmology: the crucial first billion years in cosmic history when the first stars and galaxies formed. Even before the launch of the James Webb Space Telescope, it would be possible to utilize Gamma-Ray Bursts (GRBs) as unique probes of cosmic star formation and the state of the intergalactic medium up to redshifts of the first stars. The ongoing Swift mission might be the first observatory to detect individual Population III stars, provided that massive metal-free stars were able to trigger GRBs. Swift will empirically constrain the redshift at which Population III star formation was terminated, thus providing crucial input to models of cosmic reionization and metal enrichment.Astronom
Formation Of The First Galaxies
No full textThe emergence of the first stars and galaxies ended the cosmic dark ages, thus fundamentally transforming the simple initial state of the universe into one of ever increasing complexity. We will review the basic physics governing the formation of the first galaxies. Their properties sensitively depend on the feedback exerted by the first, Population III, stars, which in turn reflects how massive those stars were. The key goal is to derive their observational signature, to be probed with upcoming next-generation facilities, such as the James Webb Space Telescope.Astronom
Early cosmic star formation in the Milky Way environment
No full textUnderstanding how the first stars formed and gave rise to the first galaxies is one of the major challenges of modern Cosmology. In standard Λ Cold Dark Matter (ΛCDM) cosmological models the formation of dark matter (DM) structures occurs hierarchically from smaller DM halos (the so-called “mini-halos”, with virial temperatures T_vir 104K). Prior to the formation of the stars, the gas in the primordial Universe was made mostly by H and He, with traces of Li and Be. Indeed, heavier elements (the so-called metals) are formed in the interior of stars and released into the surrounding gas through stellar winds and supernova (SN) explosions. Hence, the first stars, referred to as Population III (Pop III) stars, have formed in DM halos out of metal-free gas. Numerical simulations that study the formation of the first stars and look at different physical processes do not agree in the final mass of Pop III stars: while some simulations predict them to be much more massive than present-day stars, from ∼10s to 100s M⊙ (or even ∼1000 M⊙), others show that they can also have subsolar masses. Hence the initial mass function (IMF) of Pop III stars is still largely unknown.
Being likely massive, Pop III stars do not survive until today, however they are the first sources of chemical enrichment, producing metals and dust. Heavy elements provide efficient cooling channels for the formation of subsequent less massive stars, the so called Population II (Pop II) stars, which could survive to present-day. Although it has been demonstrated that the presence of heavy elements can trigger the transition to the first low-mass stars at a given metallicity Z, the role of metal line-cooling and dust cooling in driving this transition is still debated.
The duration of Pop III star formation and their impact on cosmic evolution depend on the complex interplay of different feedback processes, other than the chemical one. The ultraviolet (UV) radiation quenches star formation by photo-dissociating H2 molecules (i.e. the main coolant at high-z) and gradually (re)ionize H in the intergalactic medium (IGM), reducing/suppressing gas infall in mini-halos. When exploding, Pop III stars also eject gas from the halo through SN-driven outflows. Thus the duration of the Pop III epoque is unknown due the complex interplay between these feedback effects.
Up to now no metal-free star has been detected. Yet a promising way to investigate the properties of Pop III stars is by studying the ancient most metal-poor stars observed in our own Milky Way (MW) and its environment. Measuring the chemical abundances in their photosphere and assuming that they are the same of their natal cloud, polluted by Pop III stars, allows to constrain the nucleosynthetic products of Pop III stars. In addition, the number distribution of stars as function of their [Fe/H], the so-called Metallicity Distribution Function (MDF), provides constraints on the physical processes regulating star formation at high-z. Large surveys provide the Galactic halo MDF and chemical abundances of most metal-poor stars, showing an increasing [C/Fe] with decreasing [Fe/H]. These observations could give important information on the nature of the first stars, on the physical processes driving the transition to the first low-mass stars, and on feedback effects regulating star formation at high-z. To unveil the potential of these observations, adequate theoretical models must be adopted to connect high-z star formation with the MW local relics. Here we use two complementary tools: (i) GAMETE, a code which follows star formation in a large sample of semi-analytical DM merger trees (plausible MW formation histories, which is unknown) thus allowing to rapidly explore different model parameters and to statistically quantify the errors induced by different histories, although no spatial information is provided;
(ii) GAMESH, a pipeline where GAMETE is applied to a DM merger tree from an N-body simulation, and combined with the radiative transfer code CRASH. This approach cannot account for different MW formation histories, but allows to follow in detail the build-up of the MW accounting for the spatial distribution of the star forming progenitors and for the ionizing radiation they produce locally.
The outline of the Thesis is the following: In Chapter 1 we describe the theory of formation of DM halos where first stars form. In particular we focus on the physics leading to the formation of Pop III stars and to the transition to low-mass stars, describing the main feedback processes which could affect the star formation at high-z. We also present available observations for ancient metal-poor stars in the MW Galactic halo. In Chapter 2 we investigate whether current observations of the Galactic halo MDF can provide constraints on the physics of the Pop III/II transition and some indications on the mass of Pop III stars. To this end we use GAMETE to follow the chemical enrichment (metals and dust) across the MW formation. We explore different mass ranges and chemical yields of Pop III stars and compare simulated and observed MDF. This part of the work has been published in de Bennassuti, Schneider, Valiante & Salvadori (2014), MNRAS, 445, 3039. In Chapter 3, we investigate the role of star formation in mini-halos and its effect in shaping the Galactic halo MDF, providing robust data-driven constraints on the PopIII IMF. To this aim we use an improved version of GAMETE, to self-consistently describe the physical processes regulating star-formation in mini-halos: the poor sampling of the Pop III IMF and the effect of UV radiation. We study the effect of this new physics and of the IMF of Pop III stars on the MDF and on the properties of C-enhanced and C-normal stars. This part of the work is published in de Bennassuti, Salvadori, Schneider, Valiante (2017), MNRAS, 465, 926. In Chapter 4 we study the interplay between different feedback processes along the MW formation. To this aim, we apply GAMESH to a low-resolution N-body simulation and we account for the radiative transfer of ionizing photons to follow the inhomogeneous reionization and heating of the IGM. This part of the work has been published in Graziani, Salvadori, Schneider, Kawata, de Bennassuti, Maselli (2015), MNRAS, 449, 3137. In Chapter 5, we study the history of the dark and luminous MW progenitors and their role in shaping the properties of the MW. This is done by applying GAMESH to a higher resolution simulation which allows a more detailed investigation of the MW properties, also providing a larger statistics of mini-halos and satellite galaxies. This part of the work will be published in a forthcoming paper. Finally, in Chapter 6 we present the main conclusions of the work
The Fragmentation of Pre-enriched Primordial Objects
No full textRecent theoretical investigations have suggested that the formation of the very first stars, forming out of metal-free gas, was fundamentally different from the present-day case. The question then arises which effect was responsible for this transition in the star formation properties. In this paper, we study the effect of metallicity on the evolution of the gas in a collapsing dark matter mini-halo. We model such a system as an isolated 3σ peak of mass 2 × 106M⊙ that collapses at zcoll ≃ 30, using smoothed particle hydrodynamics. The gas has a supposed level of pre-enrichment of either Z = 10−4 Z⊙ or 10−3 Z⊙. We assume that H2 has been radiatively destroyed by the presence of a soft UV background. Metals therefore provide the only viable cooling at temperatures below 104 K. We find that the evolution proceeds very differently for the two cases. The gas in the lower metallicity simulation fails to undergo continued collapse and fragmentation, whereas the gas in the higher metallicity case dissipatively settles into the centre of the dark matter halo. The central gas, characterized by densities nH ≳ 104 cm−3, and a temperature, T ≃ 90 K, that closely follows that of the cosmic microwave background, is gravitationally unstable and undergoes vigorous fragmentation. We discuss the physical reason for the existence of a critical metallicity, Zcrit ∼ 5 × 10 −4 Z⊙, and its possible dependence on redshift. Compared with the pure H/He case, the fragmentation of the Z = −3 Z⊙ gas leads to a larger relative number of low-mass clumps
Recommended from our members
Assembly Of The First Dwarf Galaxies
No full textUnderstanding the formation and evolution of the first stars and galaxies is crucial to understanding reionization, a key epoch in the history of the Universe. Detailed theoretical studies of the galaxies before and during reionization are now particularly urgent because of the wealth of observational data that will soon be provided by the next generation of telescopes, such as JWST, ALMA, LOFAR, MWA, and others. We simulate the formation of the first galaxies using cosmological smoothed particle hydrodynamics simulations. Zooming in on individual galaxies, we explore how various physical processes affect their assembly and further evolution. A highlight of our study will be the simulation of the radiation-hydrodynamics of galaxy assembly, which we will perform using our multi-frequency radiative transfer method TRAPHIC. Feedback from radiation has long been suspected to play a decisive role in galaxy formation and we will investigate its implications for observable properties of the first galaxies.Astronom
The first stars: formation of binaries and small multiples
No full textWe investigate the formation of metal-free, Population III (Pop III), stars within a minihalo at z similar or equal to 20, starting from cosmological initial conditions. We follow the collapsing gas in the center of the minihalo up to number densities of 10(12) cm(-3). We then study the protostellar accretion onto the initial hydrostatic core, which we represent as a growing sink particle. We continue our simulation for 5000 yr after the first sink particle has formed. During this time, a disk-like configuration is assembled around the first protostar. At the end of the simulation, a small multiple system has formed within this disk, dominated by a binary with masses similar to 40M(circle dot) and similar to 10M(circle dot). If Pop Ill stars were to form typically in binaries or small multiples, the standard model of primordial star formation, where single, isolated stars are predicted to form in minihalos, would have to be modified.Astronom
The first galaxies: chemical enrichment, mixing, and star formation
No full textUsing three-dimensional cosmological simulations, we study the assembly process of one of the first galaxies, with a total mass of similar to 10(8) M(circle dot), collapsing at z similar or equal to 10. Our main goal is to trace the transport of the heavy chemical elements produced and dispersed by a pair-instability supernova exploding in one of the minihalo progenitors. To this extent, we incorporate an efficient algorithm into our smoothed particle hydrodynamics code that approximately models turbulent mixing as a diffusion process. We study this mixing with and without the radiative feedback from Population III (Pop III) stars that subsequently form in neighboring minihalos. Our simulations allow us to constrain the initial conditions for second-generation star formation, within the first galaxy itself, and inside of minihalos that virialize after the supernova explosion. We find that most minihalos remain unscathed by ionizing radiation or the supernova remnant, while some are substantially photoheated and enriched to supercritical levels, likely resulting in the formation of low-mass Pop III or even Population II (Pop II) stars. At the center of the newly formed galaxy, similar to 10(5) M(circle dot) of cold, dense gas uniformly enriched to similar to 10(-3) Z(circle dot) is in a Stateof collapse, suggesting that a cluster of Pop II stars will form. The first galaxies, as may be detected by the James Webb Space Telescope, would therefore already contain stellar populations familiar from lower redshifts.NSF AST-0708795NASA NNX08AL43GGerman Bundesministerium fur Bildung und Forschung 05A09VHADeutsche Forschungsgemeinschaft (DFG) KL 1358/1, KL 1358/4, KL 1359/5, KL 1358/10, KL 1358/11German Excellence InitiativeLandesstiftung Baden-Wurttemberg P-LS-SPII/18Astronom
Assembly Of The First Disk Galaxies Under Radiative Feedback From Pop III Stars
No full textWe investigate how radiative feedback from the first stars affects the assembly of the first dwarf galaxies. We perform cosmological zoomed smoothed particle hydrodynamics simulations of a galaxy assembling inside a halo reaching a virial mass similar to 10(9) M-circle dot at z = 10. The simulations follow the non-equilibrium chemistry and cooling of primordial gas and the subsequent conversion of the cool dense gas into massive metal-free stars. To quantify the radiative feedback, we compare a simulation in which stars emit both molecular hydrogen dissociating and hydrogen ionizing radiation with a simulation in which stars do not emit radiation but remain dark. Photodissociation and photoionization exert a strong negative feedback on the assembly of the galaxy inside the minihalo progenitor, impeding gas condensation and suppressing star formation. The radiative feedback on the gas implies a suppression of the central dark matter densities in the minihalo by factors of up to a few, which is a significant deviation from the singular isothermal density profile characterizing the dark matter distribution in the absence of radiative feedback. The properties of the galaxy become insensitive to the inclusion of radiation once the minihalo turns into an atomic cooler. The formation of a rotationally supported extended disk inside the atomically cooling galaxy therefore is a robust outcome of our simulations. Our simulations make predictions for observations with the upcoming James Webb Space Telescope.Astronom
The first galaxies: assembly under radiative feedback from the first stars
No full textWe investigate how radiative feedback from the first stars affects the assembly of the first dwarf galaxies. To this end, we perform cosmological zoomed smoothed particle hydrodynamics simulations of a dwarf galaxy assembling inside a halo reaching a virial mass similar to 10(9) M-circle dot at z = 10. The simulations follow the non-equilibrium chemistry and cooling of primordial gas and the subsequent conversion of the cool dense gas into massive metal-free stars. To quantify the radiative feedback, we compare a simulation in which stars emit both molecular hydrogen dissociating and hydrogen/helium ionizing radiation with a simulation in which stars emit only molecular hydrogen dissociating radiation, and further with a simulation in which stars remain dark. Photodissociation and photoionization exert a strong negative feedback on the assembly of the galaxy inside the main minihalo progenitor. Gas condensation is strongly impeded, and star formation is strongly suppressed in comparison with the simulation in which stars remain dark. The feedback on the gas from either dissociating or ionizing radiation implies a suppression of the central dark matter densities in the minihalo progenitor by factors of up to a few, which is a significant deviation from the singular isothermal density profile characterizing the dark matter distribution inside the virial radius in the absence of radiative feedback. The evolution of gas densities, star formation rates, and the distribution of dark matter becomes insensitive to the inclusion of dissociating radiation in the late stages of the minihalo assembly, and it becomes insensitive to the inclusion of ionizing radiation once the minihalo turns into an atomically cooling galaxy. The formation of a rotationally supported extended disk inside the dwarf galaxy is a robust outcome of our simulations not affected by the inclusion of radiation. Low-mass galaxies in the neighborhood of the dwarf galaxy show a large scatter in the baryon fraction which is driven by radiative feedback from sources both internal and external to these galaxies. Our estimates of the observability of the first galaxies show that dwarf galaxies such as simulated here will be among the faintest galaxies the upcoming James Webb Space Telescope will detect. Our conclusions regarding the structure and observability of the first galaxies are subject to our neglect of feedback from supernovae and chemical enrichment as well as to statistical uncertainties implied by the limited number of galaxies in our simulations.NASA through Astrophysics Theory and Fundamental Physics Program NNX09AJ33GNSF AST-1009928European Union 301096-proFeSsORAstronom
author-bios-SRD-19-0063.R1 – Supplemental material for The Network Structure of Police Misconduct
No full textSupplemental material, author-bios-SRD-19-0063.R1 for The Network Structure of Police Misconduct by George Wood, Daria Roithmayr and Andrew V. Papachristos in Socius</p
- …
