Examinando por Autor "Tissera, Patricia B."
Mostrando 1 - 13 de 13
Resultados por página
Opciones de ordenación
Ítem Angular momentum evolution for galaxies in a Λ-CDM scenario(EDP Sciences, 2015-12) Pedrosa, Susana E.; Tissera, Patricia B.Galaxy formation in the current cosmological paradigm is a very complex process in which inflows, outflows, interactions, and mergers are common events. These processes can redistribute the angular momentum content of baryons. Recent observational results suggest that disc formed conserving angular momentum while elliptical galaxies, although they lose angular momentum, determine a correlation between the specific angular momentum of the galaxy and the stellar mass. These observations provide stringent constraints for galaxy formation models in a hierarchical clustering scenario. Aims. We aim to analyse the specific angular momentum content of the disc and bulge components as a function of virial mass, stellar mass, and redshift. We also estimate the size of the simulated galaxies and compare them with observations. Methods. We use cosmological hydrodynamical simulations that include an effective, physically motivated supernova feedback which is able to regulate the star formation in haloes of different masses. We analyse the morphology and formation history of a sample of galaxies in a cosmological simulation by performing a bulge-disc decomposition of the analysed systems and their progenitors. We estimate the angular momentum content of the stellar and gaseous discs, stellar bulges, and total baryons. Results. In agreement with recent observational findings, our simulated galaxies have disc and spheroid components whose specific angular momentum content determine correlations with the stellar and dark matter masses with the same slope, although the spheroidal components are offset by a fixed fraction. The average angular momentum efficiency for the simulated discs is η ~ 1, while for bulges it is η ~ 0.10-0.20. For the simulated sample, the correlations found for the specific angular momentum content as a function of virial mass or stellar mass are found not to evolve significantly with redshift (up to z ~ 2). Both dynamical components seem to move along the correlations as they evolve. The total specific angular momentum of galaxies occupy different positions filling the gap between pure rotational-dominated and dispersion-dominated systems. The scaling relations derived from the simulated galaxies determine a similar relation with the virial radius, which is in agreement with recent observations. © 2015 ESO.Ítem APOGEE Chemical Abundances of the Sagittarius Dwarf Galaxy(Institute of Physics Publishing, 2017-09) Hasselquist, Sten; Shetrone, Matthew; Smith, Verne; Holtzman, Jon; McWilliam, Andrew; Fernández-Trincado J.G.; Beers, Timothy C.; Majewski, Steven R.; Nidever, David L.; Tang, Baitian; Tissera, Patricia B.; Alvar, Emma Fernández; Allende Prieto, Carlos; Almeida, Andres; Anguiano, Borja; Battaglia, Giuseppina; Carigi, Leticia; Delgado Inglada, Gloria; Frinchaboy, Peter; Garcia-Hernández D.A.; Geisler, Doug; Minniti, Dante; Placco, Vinicius M.; Schultheis, Mathias; Sobeck, Jennifer; Villanova, SandroThe Apache Point Observatory Galactic Evolution Experiment provides the opportunity of measuring elemental abundances for C, N, O, Na, Mg, Al, Si, P, K, Ca, V, Cr, Mn, Fe, Co, and Ni in vast numbers of stars. We analyze thechemical-abundance patterns of these elements for 158 red giant stars belonging to the Sagittarius dwarf galaxy (Sgr). This is the largest sample of Sgr stars with detailed chemical abundances, and it is the first time that C, N, P, K, V, Cr, Co, and Ni have been studied at high resolution in this galaxy. We find that the Sgr stars with [Fe/H] ≈ -0.8 are deficient in all elemental abundance ratios (expressed as [X/Fe]) relative to the Milky Way, suggesting that the Sgr stars observed today were formed from gas that was less enriched by Type II SNe than stars formed in the Milky Way. By examining the relative deficiencies of the hydrostatic (O, Na, Mg, and Al) and explosive (Si, P, K, and Mn) elements, our analysis supports the argument that previous generations of Sgr stars were formed with a top-light initial mass function, one lacking the most massive stars that would normally pollute the interstellar medium with the hydrostatic elements. We use a simple chemical-evolution model, flexCE, to further support our claim and conclude that recent stellar generations of Fornax and the Large Magellanic Cloud could also have formed according to a top-light initial mass function. © 2017. The American Astronomical Society. All rights reserved..Ítem Chronography of the milky way's halo system with field blue horizontal-branch stars(Institute of Physics Publishing, 2015-11) Santucci, Rafael M.; Beers, Timothy C.; Placco, Vinicius M.; Carollo, Daniela; Rossi, Silvia; Lee, Young Sun; Denissenkov, Pavel; Tumlinson, Jason; Tissera, Patricia B.In a pioneering effort, Preston et al. reported that the colors of blue horizontal-branch (BHB) stars in the halo of the Galaxy shift with distance, from regions near the Galactic center to about 12 kpc away, and interpreted this as a correlated variation in the ages of halo stars, from older to younger, spanning a range of a few Gyrs. We have applied this approach to a sample of some 4700 spectroscopically confirmed BHB stars selected from the Sloan Digital Sky Survey to produce the first "chronographic map" of the halo of the Galaxy. We demonstrate that the mean de-reddened g - r color, increases outward in the Galaxy from -0.22 to -0.08 (over a color window spanning [-0.3: 0.0]) from regions close to the Galactic center to ∼40 kpc, independent of the metallicity of the stars. Models of the expected shift in the color of the field BHB stars based on modern stellar evolutionary codes confirm that this color gradient can be associated with an age difference of roughly 2-2.5 Gyr, with the oldest stars concentrated in the central ∼15 kpc of the Galaxy. Within this central region, the age difference spans a mean color range of about 0.05 mag (∼0.8 Gyr). Furthermore, we show that chronographic maps can be used to identify individual substructures, such as the Sagittarius Stream, and overdensities in the direction of Virgo and Monoceros, based on the observed contrast in their mean BHB colors with respect to the foreground/background field population. © 2015. The American Astronomical Society. All rights reserved..Ítem Machine learning for galactic archaeology: a chemistry-based neural network method for identification of accreted disc stars(Oxford University Press, 2022-09) Tronrud, Thorold; Tissera, Patricia B.; Gómez, Facundo A.; Grand, Robert J. J.; Pakmor, Ruediger; Marinacci, Federico; Simpson, Christine M.We develop a method ('Galactic Archaeology Neural Network', gann) based on neural network models (NNMs) to identify accreted stars in galactic discs by only their chemical fingerprint and age, using a suite of simulated galaxies from the Auriga Project. We train the network on the target galaxy's own local environment defined by the stellar halo and the surviving satellites. We demonstrate that this approach allows the detection of accreted stars that are spatially mixed into the disc. Two performance measures are defined - recovery fraction of accreted stars, frecov and the probability that a star with a positive (accreted) classification is a true-positive result, P(TP). As the NNM output is akin to an assigned probability (Pa), we are able to determine positivity based on flexible threshold values that can be adjusted easily to refine the selection of presumed-accreted stars. We find that gann identifies accreted disc stars within simulated galaxies, with high frecov and/or high P(TP). We also find that stars in Gaia-Enceladus-Sausage (GES) mass systems are over 50 per cent recovered by our NNMs in the majority (18/24) of cases. Additionally, nearly every individual source of accreted stars is detected at 10 per cent or more of its peak stellar mass in the disc. We also demonstrate that a conglomerated NNM, trained on the halo and satellite stars from all of the Auriga galaxies provides the most consistent results, and could prove to be an intriguing future approach as our observational capabilities expand. © 2022 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society.Ítem The abundances and properties of Dual AGN and their host galaxies in the EAGLE simulations(Monthly Notices of the Royal Astronomical Society, 2019-02-21) Rosas-Guevara, Yetli M.; Bower, Richard G.; McAlpine, Stuart; Bonoli, Silvia; Tissera, Patricia B.We look into the abundance of Dual AGN (active galactic nucleus) in the largest hydrodynamical simulation from the Evolution and Assembly of GaLaxies and their Environment (EAGLE) project. We define a Dual AGN as two active black holes (BHs) with a separation below 30 kpc. We find that only 1 per cent of AGN with LHX ≥ 1042 erg s-1 are part of a Dual AGN system at z = 0.8-1. During the evolution of a typical binary BH system, the rapid variability of the hard X-ray luminosity on Myr time-scales severely limits the detectability of Dual AGN. To quantify this effect, we calculate a probability of detection, ton/t30, where t30 is the time in which the two BHs are separated at distances below 30 kpc and ton, the time that both AGNs are visible (e.g. when both AGNs have LHX ≥ 1042 erg s-1) in this period. We find that the average fraction of visible Dual systems is 3 per cent. The visible Dual AGN distribution as a function of BH separation presents a pronounced peak at ∼20 kpc that can be understood as a result of the rapid orbital decay of the host galaxies after their first encounter. We also find that 75 per cent of the host galaxies have recently undergone or are undergoing a merger with stellar mass ratio ≥0.1. Finally, we find that the fraction of visible Dual AGN increases with redshift as found in observations.Ítem The assembly history of the Galactic inner halo inferred from α-element patterns(Monthly Notices of the Royal Astronomical Society, 2019-02-15) Fernandez-Alvar, Emma; Tissera, Patricia B.; Carigi, Leticia; Schuster, William J.; Beers, Timothy C.; Belokurov, Vasily A.We explore the origin of the observed decline in [O/Fe] (and [Mg/Fe]) with Galactocentric distance for high-metallicity stars ([Fe/H] > −1.1), based on a sample of halo stars selected within the Apache Point Observatory Galactic Evolution Experiment (APOGEE) fourteenth data release (DR14). We also analyse the characteristics of the [α/Fe] distributions in the inner-halo regions inferred from two zoom-in Milky Way mass-sized galaxies that are taken as case studies. One of them qualitatively reproduces the observed trend to have higher fraction of α-rich star for decreasing galactocentric distance; the other exhibits the opposite trend. We find that stars with [Fe/H] > −1.1 located in the range [15–30] kpc are consistent with formation in two starbursts, with maxima separated by about ∼1 Gyr. We explore the contributions of stellar populations with different origin to the [α/Fe] gradients detected in stars with [Fe/H] > −1.1. Our analysis reveals that the simulated halo that best matches the observed chemical trends is characterized by an accretion history involving low-to intermediate-mass satellite galaxies with a short and intense burst of star formation, and contributions from a more massive satellite with dynamical masses about ∼1010 M, distributing low [α/Fe] stars at intermediate radius.Ítem The Auriga stellar haloes: Connecting stellar population properties with accretion and merging history(Monthly Notices of the Royal Astronomical Society, 2019-02-15) Monachesi, Antonela; Gomez, Facundo A.; Grand, Robert J. J.; Simpson, Christine M.; Kauffmann, Guinevere; Bustamante, Sebastian; Marinacci, Federico; Pakmor, Rudiger; Springel, Volker; Frenk, Carlos S.; White, Simon D. M.; Tissera, Patricia B.We examine the stellar haloes of the Auriga simulations, a suite of 30 cosmological magnetohydrodynamical high-resolution simulations of Milky Way-mass galaxies performed with the moving-mesh code AREPO. We study halo global properties and radial profiles out to ∼150 kpc for each individual galaxy. The Auriga haloes are diverse in their masses and density profiles, mean metallicity and metallicity gradients, ages, and shapes, reflecting the stochasticity inherent in their accretion and merger histories. A comparison with observations of nearby late-type galaxies shows very good agreement between most observed and simulated halo properties. However, Auriga haloes are typically too massive. We find a connection between population gradients and mass assembly history: galaxies with few significant progenitors have more massive haloes, possess large negative halo metallicity gradients, and steeper density profiles. The number of accreted galaxies, either disrupted or under disruption, that contribute 90 per cent of the accreted halo mass ranges from 1 to 14, with a median of 6.5, and their stellar masses span over three orders of magnitude. The observed halo mass-metallicity relation is well reproduced by Auriga and is set by the stellar mass and metallicity of the dominant satellite contributors. This relationship is found not only for the accreted component but also for the total (accreted + in situ) stellar halo. Our results highlight the potential of observable halo properties to infer the assembly history of galaxies.Ítem The evolution of the metallicity gradient and the star formation efficiency in disc galaxies(Oxford University Press, 2017-12) Sillero, Emanuel; Tissera, Patricia B.; Lambas, Diego G.; Michel-Dansac, LeoWe study the oxygen abundance profiles of the gas-phase components in hydrodynamical simulations of pre-prepared disc galaxies including major mergers, close encounters and isolated configurations. We analyse the evolution of the slope of oxygen abundance profiles and the specific star formation rate (sSFR) along their evolution. We find that galaxy-galaxy interactions could generate either positive or negative gas-phase oxygen profiles, depending on the state of evolution. Along the interaction, galaxies are found to have metallicity gradients and sSFR consistent with observations, on average. Strong gas inflows produced during galaxy- galaxy interactions or as a result of strong local instabilities in gas-rich discs are able to produce both a quick dilution of the central gas-phase metallicity and a sudden increase of the sSFR. Our simulations show that, during these events, a correlation between the metallicity gradients and the sSFR can be set up if strong gas inflows are triggered in the central regions in short time-scales. Simulated galaxies without experiencing strong disturbances evolve smoothly without modifying the metallicity gradients. Gas-rich systems show large dispersion along the correlation. The dispersion in the observed relation could be interpreted as produced by the combination of galaxies with different gas-richness and/or experiencing different types of interactions. Hence, our findings suggest that the observed relation might be the smoking gun of galaxies forming in a hierarchical clustering scenario. © 2017 The Authors.Ítem The gas metallicity gradient and the star formation activity of disc galaxies(Oxford University Press, 2016-03) Tissera, Patricia B.; Pedrosa, Susana E.; Sillero, Emanuel; Vilchez, Jose M.We study oxygen abundance profiles of the gaseous disc components in simulated galaxies in a hierarchical universe. We analyse the disc metallicity gradients in relation to the stellar masses and star formation rates of the simulated galaxies. We find a trend that galaxies with low stellar masses have steeper metallicity gradients than galaxies with high stellar masses at z ∼ 0. We also detect that the gas-phase metallicity slopes and the specific star formation rate (sSFR) of our simulated disc galaxies are consistent with recently reported observations at z ∼ 0. Simulated galaxies with high stellar masses reproduce the observed relationship at all analysed red shifts and have an increasing contribution of discs with positive metallicity slopes with increasing red shift. Simulated galaxies with low stellar masses have a larger fraction of negative metallicity gradients with increasing red shift. Simulated galaxies with positive or very negative metallicity slopes exhibit disturbed morphologies and/or have a close neighbour. We analyse the evolution of the slope of the oxygen profile and sSFR for a gas rich galaxy–galaxy encounter, finding that this kind of event could generate either positive or negative gas-phase oxygen profiles depending on their state of evolution. Our results support claims that the determination of reliable metallicity gradients as a function of red shift is a key piece of information in understanding galaxy formation and setting constraints on the subgrid physics.Ítem The origin of accreted stellar halo populations in the milky way using apogee, gaia, and the eagle simulations(Monthly Notices of the Royal Astronomical Society, 2019-01-21) Mackereth, J. Ted; Schiavon, Ricardo P.; Pfeffer, Joel; Hayes, Christian R.; Bovy, Jo; Anguiano, Borja; Prieto, Carlos Allende; Hasselquist, Sten; Holtzman, Jon; Johnson, Jennifer A.; Majewski, Steven R.; O’Connell, Robert; Shetrone, Matthew; Tissera, Patricia B.; Fernandez-Trincado, J. G.Kinematics of halo stars. We show that ∼2/3 of nearby halo stars have high orbital eccentricities (e 0.8), and abundance patterns typical of massive Milky Way dwarf galaxy satellites today, characterized by relatively low [Fe/H], [Mg/Fe], [Al/Fe], and [Ni/Fe]. The trend followed by high-e stars in the [Mg/Fe]-[Fe/H] plane shows a change of slope at [Fe/H] ∼ -1.3, which is also typical of stellar populations from relatively massive dwarf galaxies. Low-e stars exhibit no such change of slope within the observed [Fe/H] range and show slightly higher abundances of Mg, Al, and Ni. Unlike their low-e counterparts, high-e stars show slightly retrograde motion, make higher vertical excursions, and reach larger apocentre radii. By comparing the position in [Mg/Fe]-[Fe/H] space of high-e stars with those of accreted galaxies from the EAGLE suite of cosmological simulations, we constrain the mass of the accreted satellite to be in the range 108.5≲ M ≲ 109M⊙ We show that the median orbital eccentricities of debris are largely unchanged since merger time, implying that this accretion event likely happened at z≲1.5. The exact nature of the low-e population is unclear, but we hypothesize that it is a combination of in situ star formation, high-|z| disc stars, lower mass accretion events, and contamination by the low-e tail of the high-e population. Finally, our results imply that the accretion history of the Milky Way was quite unusual.Ítem The Origin of the Milky Way's Halo Age Distribution(Institute of Physics Publishing, 2018-05) Carollo, Daniela; Tissera, Patricia B.; Beers, Timothy C.; Gudin, Dmitrii; Gibson, Brad K.; Freeman, Ken C.; Monachesi, AntonelaWe present an analysis of the radial age gradients for the stellar halos of five Milky Way (MW) mass-sized systems simulated as part of the Aquarius Project. The halos show a diversity of age trends, reflecting their different assembly histories. Four of the simulated halos possess clear negative age gradients, ranging from approximately -7 to -19 Myr kpc-1, shallower than those determined by recent observational studies of the Milky Way's stellar halo. However, when restricting the analysis to the accreted component alone, all of the stellar halos exhibit a steeper negative age gradient with values ranging from -8 to -32 Myr kpc-1, closer to those observed in the Galaxy. Two of the accretion-dominated simulated halos show a large concentration of old stars in the center, in agreement with the Ancient Chronographic Sphere reported observationally. The stellar halo that best reproduces the current observed characteristics of the age distributions of the Galaxy is that formed principally by the accretion of small satellite galaxies. Our findings suggest that the hierarchical clustering scenario can reproduce the MW's halo age distribution if the stellar halo was assembled from accretion and the disruption of satellite galaxies with dynamical masses less than ∼109.5 M, and a minimal in situ contribution. © 2018. The American Astronomical Society..Ítem The oxygen abundance gradients in the gas discs of galaxies in the EAGLE simulation(Oxford University Press, 2019) Tissera, Patricia B.; Rosas-Guevara, Yetli; Bower, Richard G.; Crain, Robert A.; Lagos, Claudia del P.; Schaller, Matthieu; Schaye, Joop; Theuns, TomWe use the EAGLE simulations to study the oxygen abundance gradients of gas discs in galaxies within the stellar mass range [109.5, 1010.8] ∼M at z = 0. The estimated median oxygen gradient is -0.011 ± 0.002 dex kpc-1, which is shallower than observed. No clear trend between simulated disc oxygen gradient and galaxy stellar mass is found when all galaxies are considered. However, the oxygen gradient shows a clear correlation with gas disc size so that shallower abundance slopes are found for increasing gas disc sizes. Positive oxygen gradients are detected for ∼40 per cent of the analysed gas discs, with a slight higher frequency in low-mass galaxies. Galaxies that have quiet merger histories show a positive correlation between oxygen gradient and stellar mass, so that more massive galaxies tend to have shallowermetallicity gradients. At high stellarmass, there is a larger fraction of rotationaldominated galaxies in low-density regions. At low stellar mass, non-merger galaxies show a large variety of oxygen gradients and morphologies. The normalization of the disc oxygen gradients in non-merger galaxies by the effective radius removes the trend with stellar mass. Conversely, galaxies that experienced mergers show a weak relation between oxygen gradient and stellar mass. Additionally, the analysed EAGLE discs show no clear dependence of the oxygen gradients on local environment, in agreement with current observational findings.Ítem TYPE Ia SUPERNOVA PROGENITORS and CHEMICAL ENRICHMENT in HYDRODYNAMICAL SIMULATIONS. I. the SINGLE-DEGENERATE SCENARIO(Institute of Physics Publishing, 2015-09) Jiménez, Noelia; Tissera, Patricia B.; Matteucci, FrancescaThe nature of the Type Ia supernova (SN Ia) progenitors remains uncertain. This is a major issue for galaxy evolution models since both chemical and energetic feedback plays a major role in the gas dynamics, star formation, and therefore the overall stellar evolution. The progenitor models for the SNe Ia available in the literature propose different distributions for regulating the explosion times of these events. These functions are known as the delay time distributions (DTDs). This work is the first one in a series of papers aiming at studying five different DTDs for SNe Ia. Here we implement and analyze the single-degenerate (SD) scenario in galaxies dominated by a rapid quenching of the star formation, displaying the majority of the stars concentrated in the bulge component. We find a good fit to both the present observed SN Ia rates in spheroidal-dominated galaxies and the [O/Fe] ratios shown by the bulge of the Milky Way. Additionally, the SD scenario is found to reproduce a correlation between the specific SN Ia rate and the specific star formation rate (sSFR), which closely resembles the observational trend, at variance with previous works. Our results suggest that SN Ia observations in galaxies with very low and very high sSFRs can help to impose more stringent constraints on the DTDs and therefore on SN Ia progenitors. © 2015. The American Astronomical Society. All rights reserved.