scholarly journals A tale of two populations: surviving and destroyed dwarf galaxies and the build-up of the Milky Way’s stellar halo

2020 ◽  
Vol 497 (4) ◽  
pp. 4459-4471 ◽  
Author(s):  
Azadeh Fattahi ◽  
Alis J Deason ◽  
Carlos S Frenk ◽  
Christine M Simpson ◽  
Facundo A Gómez ◽  
...  

ABSTRACT We use magnetohydrodynamical simulations of Milky Way-mass haloes from the Auriga project to investigate the properties of surviving and destroyed dwarf galaxies that are accreted by these haloes over cosmic time. We show that the combined luminosity function of surviving and destroyed dwarfs at infall is similar in the various Auriga haloes, and is dominated by the destroyed dwarfs. There is, however, a strong dependence on infall time: destroyed dwarfs typically have early infall times of less than 6 Gyr (since the big bang), whereas the majority of dwarfs accreted after 10 Gyr have survived to the present day. Because of their late infall, the surviving satellites have higher metallicities at infall than their destroyed counterparts of similar mass at infall; the difference is even more pronounced for the present-day metallicities of satellites, many of which continue to form stars after infall, in particular for $M_{\rm star}\gt 10^7 \, {\rm M}_\odot$. In agreement with previous work, we find that a small number of relatively massive destroyed dwarf galaxies dominate the mass of stellar haloes. However, there is a significant radial dependence: while 90 per cent of the mass in the inner regions (${\lt}20\,$ kpc) is contributed, on average, by only three massive progenitors, the outer regions (${\gt}100\,$ kpc) typically have ∼8 main progenitors of relatively lower mass. Finally, we show that a few massive progenitors dominate the metallicity distribution of accreted stars, even at the metal-poor end. Contrary to common assumptions in the literature, stars from dwarf galaxies of mass $M_{\rm star}\lt 10^7 \, {\rm M}_\odot$ make up less than 10 per cent of the accreted, metal poor stars ([Fe/H] ${\lt}-3$) in the inner $50\,$ kpc.

2021 ◽  
Vol 4 (1) ◽  

Recent observations show that there are many more and much older black holes than previously known. What is particularly puzzling is that supermassive black holes containing more than a billion solar masses already existed in the very early universe. To date, there is no conclusive explanation for how such gravity monsters could have been created in such a short time after the Big Bang. The "Cosmic Time Hypothesis (CTH)" offers a solution to this problem [1]. According to this hypothesis, the early universe had much more time at its disposal than according to the "present-time scale" and the material-condensing forces were much stronger than now. Therefore, objects with extremely large masses could form in a very short "today-time".


Recent observations show that there are many more and much older black holes than previously known. What is particularly puzzling is that supermassive black holes containing more than a billion solar masses already existed in the very early universe. To date, there is no conclusive explanation for how such gravity monsters could have been created in such a short time after the Big Bang. The “Cosmic Time Hypothesis (CTH)” offers a solution to this problem [1]. According to this hypothesis, the early universe had much more time at its disposal than according to the “present-time scale” and the material-condensing forces were much stronger than now. Therefore, objects with extremely large masses could form in a very short “todaytime”.


2009 ◽  
Vol 5 (S265) ◽  
pp. 237-240
Author(s):  
Anna Frebel ◽  
Joshua D. Simon ◽  
Evan Kirby ◽  
Marla Geha ◽  
Beth Willman

AbstractWe present Keck/HIRES spectra of six metal-poor stars in two of the ultra-faint dwarf galaxies orbiting the Milky Way, Ursa Major II and Coma Berenices, and a Magellan/MIKE spectrum of a star in the classical dwarf spheroidal galaxy (dSph) Sculptor. Our data include the first high-resolution spectroscopic observations of extremely metal-poor stars ([Fe/H] < −3.0) not belonging to the Milky Way (MW) stellar halo field population. We obtain abundance measurements and upper limits for up to 26 elements between carbon and europium. The stars span a range of −3.8 < [Fe/H] < −2.3, with the ultra-faints having large spreads in Fe. A comparison with MW halo stars of similar metallicity reveals substantial agreement between the abundance patterns of the ultra-faint dwarf galaxies and Sculptor and the MW halo for the light, α and iron-peak elements (C to Zn). This agreement contrasts with the results of earlier studies of more metal-rich stars (−2.5 ≲[Fe/H]≲ −1.0) in more luminous dwarfs, which found significant abundance discrepancies with respect to the MW halo data. The abundances of neutron-capture elements (Sr to Eu) in all three galaxies are extremely low, consistent with the most metal-poor halo stars, but not with the typical halo abundance pattern at [Fe/H]≳ −3.0. Our results are broadly consistent with a galaxy formation model which predicts that massive dwarf galaxies are the source of the metal-rich component ([Fe/H]≳ −2.5) of the MW inner halo, but we propose that dwarf galaxies similar to the dSphs are the primary contributors to the metal-poor end of the metallicity distribution of the MW outer halo.


2020 ◽  
Vol 1668 (1) ◽  
pp. 012028
Author(s):  
Viviana Mossa

Abstract The Big Bang Nucleosynthesis (BBN) describes the production of light nuclides occurred during the first minutes of cosmic time. It started with the accumulation of deuterium, whose primordial abundance is sensitive to the universal baryon density and to the amount of relativistic particles. Currently the main source of uncertainty to an accurate theoretical deuterium abundance evaluation is due to the poor knowledge of the D(p, γ)3He cross section at BBN energies. The present work wants to describe one of the two experimental approaches proposed by the LUNA collaboration, whose goal is to measure with unprecedented precision, the reaction cross section in the energy range 30 < Ecm[keV] < 300.


2021 ◽  
Vol 4 (3) ◽  

The Cosmic Time Hypothesis (CTH) presented in this paper is a purely axiomatic theory. In contrast to today's standard model of cosmology, the ɅCDM model, it does not contain empirical parameters such as the cosmological constant Ʌ, nor does it contain sub-theories such as the inflation theory. The CTH was developed solely on the basis of the general theory of relativity (GRT), aiming for the greatest possible simplicity. The simplest cosmological model permitted by ART is the Einstein-de Sitter model. It is the basis for solving some of the fundamental problems of cosmology that concern us today. First of all, the most important results of the CTH: It solves one of the biggest problems of cosmology the problem of the cosmological constant (Ʌ)-by removing the relation between and the vacuum energy density ɛv (Λ=0, ɛv > 0). According to the CTH, the vacuum energy density ɛv is not negative and constant, as previously assumed, but positive and time-dependent (ɛv ̴ t -2). ɛv is part of the total energy density (Ɛ) of the universe and is contained in the energy-momentum tensor of Einstein's field equations. Cosmology is thus freed from unnecessary ballast, i.e. a free parameter (= natural constant) is omitted (Ʌ = 0). Conclusion: There is no "dark energy"! According to the CTH, the numerical value of the vacuum energy density v is smaller by a factor of ≈10-122 than the value calculated from quantum field theory and is thus consistent with observation. The measurement data obtained from observations of SNla supernovae, which suggest a currently accelerated expansion of the universe, result - if interpreted from the point of view of the CTH - in a decelerated expansion, as required by the Einstein-de Sitter universe. Dark matter could also possibly not exist, because the KZH demands that the "gravitational constant" is time-dependent and becomes larger the further the observed objects are spatially and thus also temporally distant from us. Gravitationally bound local systems, e.g. Earth - Moon or Sun - Earth, expand according to the same law as the universe. This explains why Hubble's law also applies within very small groups of galaxies, as observations show. The CTH requires that the strongest force (strong nuclear force) and the weakest (gravitational force) at Planck time (tp ≈10-43 seconds after the "big bang") when all forces of nature are supposed to have been united in a single super force, were of equal magnitude and had the same range. According to the KZH, the product of the strength and range of the gravitational force is constant, i.e. independent of time, and is identical to the product of the strength and range of the strong nuclear force. At Planck time, the universe had the size of an elementary particle (Rp = rE ≈10-15 m). This value also corresponds to the range of the strong nuclear force (Yukawa radius) and the Planck length at Planck time. The CTH provides a possible explanation for Mach's first and second principles. It solves some old problems of the big bang theory in a simple and natural way. The problem of the horizon, flatness, galaxy formation and the age of the world. The inflation theory thus becomes superfluous. • The CTH provides the theoretical basis for the theory of Earth expansion • In Cosmic Time, there was no Big Bang. The universe is infinitely old. • Unlike other cosmological models, the CTH does not require defined "initial conditions" because there was no beginning. • The CTH explains why the cosmic expansion is permanently in an unstable state of equilibrium, which is necessary for a long-term flat (Euclidean), evolutionarily developing universe.


Author(s):  
Mohamed Abdalla Bakry ◽  
Ali Eid ◽  
A. Alkaoud

In this article, we assume that the beginning of the universe was before the Big Bang. In the beginning, all matter in the universe was combined in an infinitesimal spherical shape. This sphere was compressed to an incomprehensible value for a period, and then exploded and expanded time and space. We are referring to the negative time before the Big Bang. The evolution of the universe before the Big Bang, passing through the moment of the explosion to the end of the universe at the Big Rip, has been studied. In this article, we try to answer the questions; did the universe exist before the Big Bang? What is the origin of the universe and how did it arise? What are the stages of the evolution of the universe until the moment of the Big Rip? What is the length of time for the stages of this development?


Author(s):  
J. R. Lucas

This chapter argues for the position of temporal becoming across a wide variety of fields. The chapter's central sections address successively the metaphysics, physics and logic of time. It rebuts McTaggart's argument that temporal becoming involves a contradiction. It admits that special relativity's frame — dependence of simultaneity is inimical to temporal becoming. However it also argues that temporal becoming is rehabilitated both by general relativity's allowance of cosmic time functions and, more fundamentally, by the collapse of the wave-packet in quantum theory. Finally, the discussion considers the logic of time, especially tense logic, and applies this to recent cosmological speculation about the Big Bang and more generally to the idea of the beginning of time.


It is hard to believe today that most scientists in the year 1920 believed that our Galaxy was all there is to the whole Universe—the ‘island Universe’ as it was then called. The difference between our present day view, with all its subtle complexities, and such a primitive notion has been brought about very largely by the accumulation of observational data. Almost immediately following the year 1920 Hubble disposed of the ‘island Universe’ concept and for the past fifty years astronomers have worked on the basis that our Galaxy is but one among thousands of millions strewn more or less uniformly throughout space. Although observation plays the key role in determining which ideas survive and which are rejected, ideas themselves frequently come from theoretical studies. Already in 1922 Friedmann discovered the theoretical models of the Universe which are now often described as the ‘big bang’ cosmologies. In this lecture I shall not be much concerned with these models, for the personal reason that I happen to be not very interested in them. But it is of relevance that I should explain to you why this is so.


2018 ◽  
Vol 615 ◽  
pp. A64 ◽  
Author(s):  
E. Ledinauskas ◽  
K. Zubovas

Context. Irregular dwarf galaxies of the Local Group have very varied properties and star formation histories. Some of them formed the majority of their stars very late compared to others. Extreme examples of this are Leo A and Aquarius, which reached the peak of star formation at z < 1 (more than 6 Gyr after the Big Bang). This fact seemingly challenges the ΛCDM cosmological framework because the dark matter halos of these galaxies on average should assemble the majority of their masses before z ~ 2 (<3 Gyr after the Big Bang). Aims. We investigate whether the delayed star formation histories of some irregular dwarf galaxies might be explained purely by the stochasticity of their mass assembly histories coupled with the effect of cosmic reionization. Methods. We developed a semi-analytic model to follow the accretion of baryonic matter, star formation, and stellar feedback in dark matter halos with present-day virial masses 109 M⊙ < Mdm,0 < 1011 M⊙ and with different stochastic growth histories obtained using the PINOCCHIO code based on Lagrangian perturbation theory. Results. We obtain the distributions of observable parameters and the evolution histories for these galaxies. Accretion of baryonic matter is strongly suppressed after the epoch of reionization in some models, but the galaxies continue to accrete dark matter and eventually reach enough mass for accretion of baryonic matter to begin again. These “reborn” model galaxies show delayed star formation histories that are very similar to those of Leo A and Aquarius. Conclusions. We find that the stochasticity caused by mass assembly histories is enhanced in systems with virial masses ~1010 M⊙ because of their sensitivity to the photoionizing intergalactic radiation field after the epoch of reionization. This results in qualitatively different star formation histories in late- and early-forming galaxies, and it might explain the peculiar star formation histories of irregular dwarf galaxies such as Leo A and Aquarius.


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