Galaxie naine | EPFL Graph Search

Une galaxie naine est une petite galaxie composée de 100 millions à quelques milliards d'étoiles, un nombre relativement faible par rapport aux 200 à 400 milliards d'étoiles estimés de la Voie lactée. Ces galaxies gravitent souvent autour de galaxies plus importantes comme la Voie lactée, la galaxie d'Andromède ou la galaxie du Triangle. La Voie lactée possède plus d'une trentaine de galaxies naines connues qui gravitent autour d'elle. La plus grande d'entre elles est le Grand Nuage de Magellan, d'un diamètre d'environ 15000 années-lumière. Les deux plus petites, la galaxie naine de la Carène et la galaxie du Lion II ont un diamètre d'environ 1600 années-lumière. La galaxie d'Andromède possède aussi ses galaxies naines satellites. Ces deux grandes galaxies et leurs galaxies naines satellites forment le Groupe local. Les galaxies naines présentent plusieurs morphologies différentes :

galaxie naine elliptique et le sous-type galaxie naine sphéroïdale ;
galaxie naine irrégulière

The last stages of star formation in the dwarf spheroidal galaxy Sextans

Romaine Theler

This thesis is anchored in the research of better understanding of the chemical evolution of galaxies. Many dwarf spheroidal galaxies (dSph) are orbiting the Milky Way displaying various star formation histories. The formation of these dwarf galaxies and the drivers of their evolution are still not well understood. We studied in detail one of them, Sextans, a low mass dSph composed of an old population. 90% of Sextans stars were formed 11-14 Gyr ago (Lee et al. 2009). Battaglia et al. (2011) show the presence of a radial metallicity gradient in Sextans considering observation based on intermediate resolution spectroscopy until the tidal radius. For a projected radius R < 0.8 deg stars cover the whole range of [Fe/H], while for R > 0.8 deg all stars are metal-poor [Fe/H] < -2.2 dex. Due to its distance and its low surface brightness high resolution spectra are required to obtain accurate abundances. Up to recently only 14 Sextans stars have been observed in high resolution spectroscopy (Shetrone et al. 2001; Aoki et al. 2009; Tafelmeyer et al. 2010; Honda et al. 2011) but in majority they were focused only on extremely metal-poor stars. In order to understand the chemical evolution of Sextans we needed to study a statistically significant sample of stars covering a larger metallicity range. This has been done in this work based on the largest high resolution spectroscopic sample of 87 red giant branch stars ever obtained in such low mass dwarf spheroidal galaxies. Our sample of stars located in the center of Sextans displays a large metallicity distribution ranging from -1.0 dex to -3.5 dex. We derived abundances of 10 elements : 3 alpha-elements (Mg, Ca and Ti), 5 iron-peak elements (Sc, Cr,Mn, Co and Ni) and 2 neutron-capture elements (Ba and Eu). Combining the results we had the first picture of the chemical evolution of Sextans. From [alpha/Fe] distribution reflecting the ratio between supernovae [SNe II/SNe Ia], we found that the domination of SNeIa in chemical evolution occurred around [Fe/H] = -2.0 dex. A lower knee than for Fornax and Sculptor suggests that star formation was less efficient in Sextans in agreement with their respective estimated mass. The absence of intrinsic scatter in [alpha/Fe] showed that in the center of Sextans the interstellar medium has been homogeneously enriched through star formation. Some hypothesis like tidal interactions with the Milky Way could explain this abundance homogeneity in the center of the galaxy and the metallicity gradient toward the outskirts. Abundances in iron-peak elements and neutron-capture elements give new observational constraints for a better understanding of the various nucleosynthesis sources of these elements.

EPFL2015

The first generations of stars in the Local Group

Romain Ely Roland Lucchesi

The different projects of this thesis contribute to improving our understanding of the early chemical evolution of the galaxies.The standard cosmological model states an hierarchical formation of the galaxies, the dwarf galaxies being the first stellar systems to form in the early Universe, merging to build larger galaxies over time. Observing the oldest stars in different environments can therefore answer many questions for the models of galaxy formation. Do the old stellar populations of the dwarf galaxies share the same chemical abundance patterns as the Milky Way halo ?These oldest and extremely metal-poor stars (EMP, [Fe/H]

EPFL2022

Constraining the primordial magnetic field with dwarf galaxy simulations

Pascale Jablonka, Yves Revaz, Mahsa Sanati

Using a set of cosmological hydro-dynamical simulations, we constrained the properties of primordial magnetic fields by studying their impact on the formation and evolution of dwarf galaxies. We performed a large set of simulations (8 dark matter only and 72 chemo-hydrodynamical) including primordial magnetic fields through the extra density fluctuations they induce at small length scales (k >= 10 h Mpc(-1)) in the matter power spectrum. Our sample of dwarfs includes nine systems selected out of the initial (3.4 Mpc h(-1))(3) parent box, resimulated from z=200 to z=0 using a zoom-in technique and including the physics of baryons. We explored a wide variety of primordial magnetic fields with strength B-lambda ranging from 0.05 to 0.50 nG and magnetic energy spectrum slopes n(B) from -2.9 to -2.1. Strong magnetic fields characterized by a high amplitude (B-lambda=0.50, 0.20 nG with n(B)=-2.9) or by a steep initial power spectrum slope (n(B)=-2.1, -2.4, with B-lambda=0.05 nG) induce perturbations on mass scales from 10(7) to 10(9) M-circle dot. In this context emerging galaxies see their star formation rates strongly boosted. They become more luminous and metal rich than their counterparts without primordial magnetic fields. Such strong fields are ruled out by their inability to reproduce the observed scaling relations of dwarf galaxies. They predict that dwarf galaxies are at the origin of an unrealistically early reionization of the Universe and that they also overproduce luminous satellites in the Local Group. Weaker magnetic fields impacting the primordial density field at corresponding masses less than or similar to 10(6) M-circle dot, produce a large number of mini dark matter halos orbiting the dwarfs, however out of reach for current lensing observations. This study allows us, for the first time, to constrain the properties of primordial magnetic fields based on realistic cosmological simulations of dwarf galaxies.

EDP SCIENCES S A2020