Examinando por Autor "Tissera, Patricia"
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Ítem Baryon effects on void statistics in the EAGLE simulation(Oxford University Press, 2017-10) Paillas, Enrique; Lagos, Claudia D.P.; Padilla, Nelson; Tissera, Patricia; Helly, John; Schaller, MatthieuCosmic voids are promising tools for cosmological tests due to their sensitivity to dark energy, modified gravity and alternative cosmological scenarios. Most previous studies in the literature of void properties use cosmological N-body simulations of dark matter (DM) particles that ignore the potential effect of baryonic physics. Using a spherical underdensity finder, we analyse voids using the mass field and subhalo tracers in the Evolution and Assembly of Galaxies and their Environment (EAGLE) simulations, which follow the evolution of galaxies in a Λ cold dark matter universe with state-of-the-art subgrid models for baryonic processes in a (100 cMpc)3 volume. We study the effect of baryons on void statistics by comparing results with DM-only simulations that use the same initial conditions as EAGLE. When identifying voids in the mass field, we find that a DM-only simulation produces 24 per cent more voids than a hydrodynamical one due to the action of galaxy feedback polluting void regions with hot gas, specially for small voids with rvoid ≤ 10 Mpc. We find that the way in which galaxy tracers are selected has a strong impact on the inferred void properties. Voids identified using galaxies selected by their stellar mass are larger and have cuspier density profiles than those identified by galaxies selected by their total mass. Overall, baryons have minimal effects on void statistics, as void properties are well captured by DM-only simulations, but it is important to account for howgalaxies populateDMhaloes to estimate the observational effect of different cosmological models on the statistics of voids. © 2017 The Authors.Ítem Evidence for the Third Stellar Population in the Milky Way's Disk(Institute of Physics Publishing, 2019-12) Carollo, Daniela; Chiba, Masashi; Ishigaki, Miho; Freeman, Ken; Beers, Timothy C.; Lee, Young Sun; Tissera, Patricia; Battistini, Chiara; Primas, FrancescaThe Milky Way is a unique laboratory in which stellar properties can be measured and analyzed in detail. In particular, stars in the older populations encode information on the mechanisms that led to the formation of our Galaxy. In this article, we analyze the kinematics, spatial distribution, and chemistry of a large number of stars in the solar neighborhood, where all of the main Galactic components are well represented. We find that the thick disk comprises two distinct and overlapping stellar populations with different kinematic properties and chemical compositions. The metal-weak thick disk (MWTD) contains two-times less metal content than the canonical thick disk, and exhibits enrichment of light elements typical of the oldest stellar populations of the Galaxy. The rotational velocity of the MWTD around the Galactic center is ∼150 km s-1, corresponding to a rotational lag of 30 km s-1 relative to the canonical thick disk (∼180 km s-1), with a velocity dispersion of 60 km s-1. This stellar population likely originated from the merger of a dwarf galaxy during the early phases of our Galaxy's assembly, or it is a precursor disk, formed in the inner Galaxy and brought into the solar neighborhood by bar instability or spiral-arm formation mechanisms. © 2019. The American Astronomical Society. All rights reserved..Ítem Galactic archaeology in the 21st century : unveiling accretion in the MW disc(Universidad Andrés Bello, 2022) Tronrud, Thorold; Tissera, Patricia; Gómez, Facundo A.; Gómez, Matías; Bignone, Lucas; Facultad de Ciencias ExactasThe evolution of large galaxies such as the Milky Way (MW) in Λ Cold Dark Matter (ΛCDM) cosmology is driven by interactions with other galaxies. These interactions affect the structure of the galaxy in long-lasting ways, not only forming the stellar halo, and modifying the size and characteristics of the stellar disc, but also contributing gas that fuels star formation and depositing stars directly into the galaxy. This stellar debris, formed outside the galaxy in which they now reside (a.k.a. ex-situ), retain the chemical fingerprint of the environment in which they formed, leaving them distinct from the stars that were formed within the primary galaxy (a.k.a. in-situ). If these ex-situ stars are located beyond the disc, in the stellar halo, the distributions they occupy in space — based on the kinematics of their progenitor object — may be readily apparent as streams or great circles surrounding the primary galaxy. A population of ex-situ stars are also expected to reside in the stellar disc, even at high circularities. These stars have been found in simulations to be deposited by relatively few massive mergers, but finding them poses a challenge in this dense region. I propose a method, based on training and utilizing neural networks, to classify disc stars as in- or ex-situ based on their chemical parameters. The use of this method is motivated by my research into the effects mergers and accretion have on the galactic stellar disc of a suite of simulated MW-like galaxies, in which I demonstrate that accretion events and accreted stars cause observable changes to calculated metallicity gradients, and can cause disagreements in correlations between observed parameters based on the ages of the stars being used. Galactic discs form inside-out, starting small and slowly expanding with new star formation at the outer edges. This imparts a natural negative age gradient. Stars that have been deposited onto the disc will not necessarily follow this age gradient, which is one of the primary drivers of the stellar disc’s metallicity gradient. These ex-situ stars drive several age-specific correlations between metallicity gradient and 𝑅 −1 eff and 𝜆 ★ gal that vanish entirely once these contaminants are removed. A chemistry-based method for flagging potentially-accreted stars will allow researchers to remove the bulk of these from their samples, which will allow them to examine the evolution of the galactic disc more finely. Furthermore, these flagged, ex-situ stars can be grouped by their characteristics, for attempts to discern early progenitors of the primary galaxy with significantly less noise from stars formed in-situ. I demonstrate that the Galactic Archaeology Neural Network (GANN) recovers usable fractions of nearly every contributor to the suite of simulated stellar discs. Thus, a catalogue of stars flagged by GANN will contain most of the merger information that is present in the stellar disc of a galaxy. Future applications of this method could aid in the discovery of many ancient remnants, and broaden our understanding of how our galaxy formed.Ítem Gradientes de metalicidad de poblaciones estelares(Universidad Andrés Bello, 2019) Catalán Véliz, Tracy; Tissera, Patricia; Facultad de Ciencias Exactas; Departamento de Ciencias FísicasAnalizamos la evolución de los gradientes de metalicidad de poblaciones estelares con distintas edades del survey MaNGA, en un rango de masa estelar entre 108:5M y 1012:5M con z < 0:04. La evolución de los gradientes de metalicidad sera analizado según masa estelar, morfología, tasa de formación estelar, radio efectivo y redshift de cada galaxia. Para determinar la morfología de las galaxias utilizamos el índice de Sersic, donde las galaxias con n < 2 corresponderán en su mayoría a galaxias dominadas por espirales, y las galaxias n > 2 están dominadas por galaxias elípticas. Analizaremos, además, los gradientes de metalicidad en galaxias masivas con masas estelares de aproximadamente 1011M .Ítem Mild evolution of the stellar metallicity gradients of disc galaxies(EDP Sciences, 2017-08) Tissera, Patricia; MacHado, Rubens E. G.; Vilchez, José M.; Pedrosa, Susana E.; Sanchez-Blazquez, Patricia; Varela, SilvioContext. The metallicity gradients of the stellar populations in disc galaxies and their evolution store relevant information on the disc formation history and on those processes which could mix stars a posteriori, such as migration, bars and/or galaxy-galaxy interactions. Aims. We aim to investigate the evolution of the metallicity gradients of the whole stellar populations in disc components of simulated galaxies in a cosmological context. Methods. We analyse simulated disc galaxies selected from a cosmological hydrodynamical simulation that includes chemical evolution and a physically motivated supernova feedback capable of driving mass-loaded galactic winds. Results. We detect a mild evolution with redshift in the metallicity slopes of-0.02 ± 0.01 dex kpc-1 from z ∼ 1. If the metallicity profiles are normalised by the effective radius of the stellar disc, the slopes show no clear evolution for z< 1, with a median value of approximately-0.23 dex reff-1. As a function of stellar mass, we find that metallicity gradients steepen for stellar masses smaller than ∼1010.3M· while the trend reverses for higher stellar masses, in the redshift range z = [0,1]. Galaxies with small stellar masses have discs with larger reff and flatter metallicity gradients than expected. We detect migration albeit weaker than in previous works. Conclusions. Our stellar discs show a mild evolution of the stellar metallicity slopes up to z ∼ 1, which is well-matched by the evolution calculated archeologically from the abundance distributions of mono-age stellar populations at z ∼ 0. The dispersion in the relations allows for stronger individual evolutions. Overall, supernova feedback could explain the trends but an impact of migration can not be totally discarded. Galaxy-galaxy interactions or small satellite accretions can also contribute to modify the metallicity profiles in the outer parts. Disentangling the effects of these processes for individual galaxies is still a challenge in a cosmological context. © 2017 ESO.Ítem The age–chemical abundance structure of the Galaxy I: evidence for a late-accretion event in the outer disc at z ∼ 0.6(Oxford University Press, 2020-05) Lian, Jianhui; Thomas, Daniel; Claudia, Maraston; Zamora, Olga; Tayar, Jamie; Pan, Kaike; Tissera, Patricia; Fernandez-Trincado, Jose G.; Garcia-Hernandez, D. A.We investigate the age-chemical abundance structure of the outer Galactic disc at a galactocentric distance of r > 10 kpc as recently revealed by the SDSS/APOGEE survey. Two sequences are present in the [α/Fe]-[Fe/H] plane with systematically different stellar ages. Surprisingly, the young sequence is less metal rich, suggesting a recent dilution process by additional gas accretion. As the stars with the lowest iron abundance in the younger sequence also show an enhancement in α-element abundance, the gas accretion event must have involved a burst of star formation. In order to explain these observations, we construct a chemical evolution model. In this model, we include a relatively short episode of gas accretion at late times on top of an underlying secular accretion over long time-scales. Our model is successful at reproducing the observed distribution of stars in the three-dimensional space of [α/Fe]-[Fe/H]-age in the outer disc. We find that a late-time accretion with a delay of 8.2 Gyr and a time-scale of 0.7 Gyr best fits the observed data, in particular the presence of the young, metal-poor sequence. Our best-fitting model further implies that the amount of accreted gas in the late-time accretion event needs to be about three times the local gas reservoir in the outer disc at the time of accretion in order to sufficiently dilute the metal abundance. Given this large fraction, we interpret the late-time accretion event as a minor merger presumably with a gas-rich dwarf galaxy with a mass M∗ < 109 M☉ and a gas fraction of ∼75 per cent. © 2020 The Author(s)