Voie lactée

La Voie lactée, aussi nommée la Galaxie (avec une majuscule), est une galaxie spirale barrée qui comprend entre 200 et d'étoiles, et sans doute plus de de planètes. Elle abrite le Système solaire et donc la Terre. Son diamètre est estimé à , voire à ou à , bien que le nombre d'étoiles au-delà de soit très faible. Son cortège de galaxies satellites et elle font partie du Groupe local, lui-même rattaché au superamas de la Vierge appartenant lui-même à Laniakea. Le Système solaire se situe à environ du centre de la Voie lactée, lequel est constitué d'un trou noir supermassif. Observée de la Terre, la Galaxie ressemble à une bande blanchâtre. En effet, le Système solaire est situé sur le bord de sa structure en forme de disque, qui apparaît alors comme une bande, et l'accumulation d'une multitude d'étoiles que l'on ne peut distinguer à l'œil nu lui donne son aspect « lacté », comme l'avaient déjà avancé Démocrite et Anaxagore. Grâce à sa lunette astronomique, Galilée démontre le pre

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PHYS-402: Astrophysics IV : observational cosmology

Cosmology is the study of the structure and evolution of the universe as a whole. This course describes the principal themes of cosmology, as seen from the point of view of observations.

PHYS-209: Astrophysics I: introduction to astrophysics

Ce cours décrit de façon simple les processus physiques qui expliquent l'univers dans lequel nous vivons. En couvrant une large gamme de sujets, le but du cours est aussi de donner un aperçu général des objets astrophysiques qui nous entourent.

PHYS-401: Astrophysics III : stellar and galactic dynamics

The aim of this course is to acquire the basic knowledge on specific dynamical phenomena related to the origin, equilibrium, and evolution of star clusters, galaxies, and galaxy clusters.

Rotation measure and synchrotron emission signatures in simulations of magnetized galactic discs

Yoan Rappaz

We analyse observational signatures of magnetic fields for simulations of a Milky Way-like disc with supernova-driven interstellar turbulence and self-consistent chemical processes. In particular, we post-process two simulations data sets of the SILCC Project for two initial amplitudes of the magnetic field, B-0 = 3 and 6 mu G, to study the evolution of Faraday rotation measures (RM) and synchrotron luminosity. For calculating the RM, three different models of the electron density n(e) are considered. A constant electron density, and two estimations based on the density of ionized species and the fraction of the total gas, respectively. Our results show that the RM profiles are extremely sensitive to the n(e) models, which assesses the importance of accurate electron distribution observations/estimations for the magnetic fields to be probed using Faraday RMs. As a second observable of the magnetic field, we estimate the synchrotron luminosity in the simulations using a semi-analytical cosmic ray model. We find that the synchrotron luminosity decreases over time, which is connected to the decay of magnetic energy in the simulations. The ratios between the magnetic, the cosmic ray, and the thermal energy density indicate that the assumption of equipartition does not hold for most regions of the ISM. In particular, for the ratio of the cosmic ray to the magnetic field energy the assumption of equipartition could lead to a wrong interpretation of the observed synchrotron emission.

OXFORD UNIV PRESS2022

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Tracing the formation of the Milky Way through ultra metal-poor stars

Pascale Jablonka, Nicolas Lawrence Etienne Longeard

We use Gaia DR2 astrometric and photometric data, published radial velocities and MESA models to infer distances, orbits, surface gravities, and effective temperatures for all ultra metal-poor stars ([Fe/H] < -4.0 dex) available in the literature. Assuming that these stars are old (> 11 Gyr) and that they are expected to belong to the Milky Way halo, we find that these 42 stars (18 dwarf stars and 24 giants or sub-giants) are currently within similar to 20 kpc of the Sun and that they map a wide variety of orbits. A large fraction of those stars remains confined to the inner parts of the halo and was likely formed or accreted early on in the history of the Milky Way, while others have larger apocentres (> 30 kpc), hinting at later accretion from dwarf galaxies. Of particular interest, we find evidence that a significant fraction of all known UMP stars (similar to 26 per cent) are on prograde orbits confined within 3 kpc of the Milky Way plane (J(z) < 100 km s(-1) kpc). One intriguing interpretation is that these stars belonged to the massive building block(s) of the proto-Milky Way that formed the backbone of the Milky Way disc. Alternatively, they might have formed in the early disc and have been dynamically heated, or have been brought into the Milky Way by one or more accretion events whose orbit was dragged into the plane by dynamical friction before disruption. The combination of the exquisite Gaia DR2 data and surveys of the very metal-poor sky opens an exciting era in which we can trace the very early formation of the Milky Way.

OXFORD UNIV PRESS2019

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The Pristine Dwarf-Galaxy survey - II. In-depth observational study of the faint Milky Way satellite Sagittarius II

Pascale Jablonka, Nicolas Lawrence Etienne Longeard

We present an extensive study of the Sagittarius II (Sgr II) stellar system using MegaCam g and i photometry, narrow-band, metallicity-sensitive calcium H&K doublet photometry and Keck II/DEIMOS multiobject spectroscopy. We derive and refine the Sgr II structural and stellar properties inferred at the time of its discovery. The colour-magnitude diagram implies Sgr II is old (12.0 +/- 0.5 Gyr) and metal poor. The CaHK photometry confirms the metal-poor nature of the satellite (Fe/H = -2.32 +/- 0.04 dex) and suggests that Sgr II hosts more than one single stellar population (sigma(CaHK)([FeH]) = 0.11(-0.03)(+0.05) dex). Using the Ca infrared triplet measured from our highest signal-to-noise spectra, we confirm the metallicity and dispersion inferred from the Pristine photometric metallicities (Fe/H = -2.23 +/- 0.05 dex, sigma(spectro)([Fe/H]) = 0.10(-0.04)(+0.06) dex). The velocity dispersion of the system is found to be sigma(v) = 2.7(-1.0)(+1.3) km s(-1) after excluding two potential binary stars. Sgr II's metallicity and absolute magnitude (M-V = -5.7 +/- 0.1 mag) place the system on the luminosity-metallicity relation of the Milky Way dwarf galaxies despite its small size. The low but resolved metallicity and velocity dispersions paint the picture of a slightly dark-matter-dominated satellite (M/L = 23.0(-23.0)(+32.8) M-circle dot L-circle dot(-1)). Furthermore, using the Gaia Data Release 2, we constrain the orbit of the satellite and find an apocentre of 118.4(-23.7)(+28.4) kpc and a pericentre of 54.8(-6.1)(+3.3) kpc. The orbit of Sgr II is consistent with the trailing arm of the Sgr stream and indicates that it is possibly a satellite of the Sgr dSph that was tidally stripped from the dwarf's influence.

OXFORD UNIV PRESS2020

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Galaxie

vignette|redresse=1.5|M51, la Galaxie du Tourbillon, un exemple typique de galaxie spirale. Une galaxie est une structure cosmique formée par le rassemblement d'étoiles et de leurs planètes éventuel

Étoile

vignette|Le Soleil, l’étoile la plus proche de la Terre, vu lors d'une éruption en ultraviolets avec de fausses couleurs. Une étoile est un corps céleste plasmatique qui rayonne sa propre lumière par

Système solaire

Le Système solaire (avec majuscule), ou système solaire (sans majuscule), est le système planétaire du Soleil, auquel appartient la Terre. Il est composé de cette étoile et des objets célestes gravi