Resumen:

In the context of hierarchical assembly of galaxies in a $\Lambda$ cold dark matter cosmology, the stellar halos of massive, Milky Way-like, systems are expected to be entirely comprised of debris from tidally-disrupted dwarf galaxies. The intermediate stage of this process is characterized by the appearance of stellar streams, spatially elongated structures produced by accreted debris that remains kinematically cohesive. Therefore, in principle, it should be possible to isolate which stars in the Milky Way’s halo belong to these substructures and, hence, reconstruct the chemical-evolution of their original parent low-mass galaxies, providing a glimpse into their early formation and evolution at high redshift using observations of nearby stars. Here, I will discuss what can be learned from the elemental-abundance patterns of these ancient now-destroyed dwarf galaxies. Because their stars are much closer, hence brighter, than surviving present-day satellites of the Milky Way, this approach constitutes an alternative to investigate the early formation history of these small galaxies. I will go through several results using spectroscopic data for the so-called Helmi streams, and Sagittarius stream, and Wukong stream, which are all halo stellar streams of dwarf-galaxy origin. For example, (i) we found that high-metallicity stars in the Helmi streams have low $\alpha$-element abundances in comparison to the overall halo, (ii) the Sagittarius stream contains a population of carbon-enhanced metal-poor stars similar to the Milky Way, and (iii) the Wukong progenitor was massive enough to higher star formation efficiency than nearby dwarf spheroidal galaxies such as Sculptor and Fornax. Although these chemical-abundance studies of disrupted dwarf galaxies are still incipient, they present the opportunity to bridge the gap between low- and high-redshift observations.

Resumen:

Astrophysical ices play a crucial role in the chemical evolution of space environments, as they undergo ionizing radiation-induced reactions and desorption processes. Here, we present our pioneering computational methodology, PROCODA, aimed at elucidating the intricate chemical evolution of experimentally investigated ices under photolysis/radiolysis processes until chemical equilibrium phase (CE) is attained. The code solves a system of coupled differential equations, effectively traces the evolution of molecular abundances over time during the radiation processing of ices. Our investigation primarily focuses on pure H2O, CO, and CO2 ices subjected to different types of ionizing radiation, such as cosmic rays, UV and energetic electrons [1,2,3]. For CO and CO2 ices, we consider 11 distinct chemical species within the ice, encompassing 4 observed species (CO2, CO, O3, and CO3) and 7 non-observed or unknown species (O, O2, C, C2, C2O, C2O2, and C2O3). Additionally, we account for 100 reaction routes, including direct dissociation reactions, bimolecular reactions, and radiation-induced desorption processes. For H2O ices, our analysis involves 9 different chemical species, including 5 observed species (H2, H2O, O2, H2O2, and virtually zero O3) and 61 coupled reactions. The best-fit models obtained through PROCODA provide not only the rate coefficients for the considered equations but also yield invaluable insights into desorption parameters and the characterization of the CE phase. The determined values from our comprehensive study hold significant potential for incorporation into future astrochemical models, enabling the mapping of chemical evolution within astrophysical environments under the influence of ionizing radiation. We gratefully acknowledge the generous financial support of the Brazilian agencies FAPESP, CAPES, and CNPq, which has made this research possible. References: [1] Pilling et. al. 2022, ApJ, 925, 147 (CO2) [2] Pilling et. al. 2023, ApJ, 952, 17 (CO) [3] Pilling et. al. 2023, MNRAS, 523, 2858 (H2O)

Resumen:

Sgr A*, located only 8 kpc away, allows us to study in detail the accretion process on to a super-massive black hole. Only in our Galactic centre we can directly observe the source of the material feeding the accretion, which in this case corresponds to $\sim 25$ young, massive stars, with powerful stellar winds. We will describe our hydrodynamical models of the gas surrounding Sgr A*, originating from the observed stars, with known orbits and stellar wind properties. Our simulations show that these winds and their collisions can naturally account for the properties of the hot plasma detected in X-ray observations. Moreover, the complex interaction of the winds with the medium produces cold clumps and streams, and perhaps even a disc around the black hole, as inferred from ALMA observations. Finally, the stellar orbits and the infall of streams make the accretion vary on time-scales of decades to millennia, potentially explaining the observed X-ray echoes related to past activity from Sgr A*.

Resumen:

Current knowledge about the structure and kinematics of the old component of the Galactic bulge still lacks of consensus. RR Lyrae (RRL) stars trace one of the oldest populations of the bulge, meaning that characterising them could give clues about the Galaxy formation. We study the kinematics of bulge RRLs to define this old population as new in the context of the bulge components. We used APOGEE-2S spectra combined with OGLE-IV light curves for more than 4000 RRLs in the bulge region to derive their systemic radial velocities. Furthermore, we matched the data with the VVV survey to obtain near-IR photometry and proper motions. Then, an orbital analysis was performed using the 6D RRL motion to find which fraction is confined in the bulge. We obtain a rotation curve for bulge RRLs, which show behaviour similar to the RC metal-poor population rather than the metal-rich one. As in previous studies, the RRL rotation is slow, and the velocity dispersion is constant along longitude. From this RRL sample, we find a considerable contamination by halo interlopers. We remark that appropriate orbital analysis is crucial to decontaminate the sample and find the bona fide bulge RRLs.

Resumen:

The exploration of the outer halo of the Milky Way, through wide-sky photometric and spectroscopic surveys, has unequivocally confirmed that the our Galaxy have formed as a result of the continuous merger of protogalactic fragment, aligning with predictions from cosmological Lambda-Cold Dark Matter simulations. Here, we will talk about the hypervelocity stars, considered potential remnants of past accretion events, with velocities exceeding the local escape speed. Alternatively, hypervelocity stars may arise from binary system disruptions during close encounters with the supermassive black hole at the center of the Milky Way. Thanks to the invaluable data provided by Gaia, we have successfully identified stars exhibiting a high likelihood of belonging to this intriguing stellar family. Follow-up VLT spectroscopy has allowed us to estimate their total velocities, which, in certain cases, reach astonishing values of up to 1200 km/s. By reconstructing their past orbits, we have uncovered evidence suggesting that some of these stars might have originated within a now-disrupted dwarf galaxy or experienced another violent event far from the Galactic center.

Resumen:

Studying dwarf galaxies and their interactions with the Milky Way (MW) is crucial for reconstructing the MW formation history, it also helps us improve constraints on cosmological models. Furthermore, the field of galactic dynamics emphasizes the importance of understanding the stability of stellar systems. In such context, the identification of parameters that influence the resilience of stellar systems against disruption by MW tidal forces plays a crutial role in grasping the evolution of the Galaxy and its constituents. This work aims to investigate the key properties of dwarf spheroidal galaxies (dSph) and their orbits around the MW, determining how susceptible their core structures are to disruption. To achieve this, we use semi-analytical $N$-body simulations: embedding an $N$-body system, representing a dSph, in a MW analytical axisymmetric potential. By studying the fate of different systems, we aim to identify combinations of structural and orbital parameters that either preserve or dismantle their cores. Preliminary findings suggest that the initial concentration and the orbit's inclination with respect to the galactic plane are significant factors in this problem. More specifically, when considering Plummer spheres with a total mass $M_{\rm total} = 10^{10} \ \rm{M_{\odot}}$, initial scale lengths $a_1 = 0.5 \ \rm{kpc}$ (less concentrated) and $a_2 = 0.3 \ \rm{kpc}$ (more concentrated), both consisting of $N = 10^4$ particles, and placed on two distinct orbits ($xy$-plane and $yz$-plane), the polar orbit and the initially less concentrated cases show the most persistent cores. Further investigations into the structures of the parameter and phase spaces are expected to deepen our understanding of the related phenomena. Additionally, exploring more extreme orbits may lead to insights on the disruption of globular clusters, and incorporating the time dependent potential of the bar could enrich the analysis.

Resumen:

The Milky Way bulge located in the center of the Galaxy is one of the most massive and oldest components. Since it is relatively close to us, and therefore resolved, its observation offers a great opportunity to study the Galaxy formation. However, it is very challenging cause a large amount of extinction and crowding, as well as the superposition of other components of the Galaxy in the same line of sight. Several questions related to the age, metallicity, and distribution of the stellar populations present in the Bulge are still open and their answers could lead us to better understand its formation history. During this talk, I will present two different approaches to solve open discussions involving the innermost 2x2 degrees of the Bulge, one related to how relatively different are the ages of the stellar populations and the other related to the central radial velocity dispersion peak that is poorly mapped. We will present the ongoing combined analysis of photometric data of the HST and mainly spectroscopical data of MUSE to answer these questions.

Resumen:

The Sagittarius dwarf spheroidal galaxy (Sgr) is one of the Milky Way's nearest and most massive satellites. It is being disrupted by the tidal interaction with the Milky Way's gravitational potential, leading to a giant tidal stream that covers a large portion of the sky. As seen from Earth, the core of Sgr lies a few degrees below the Galactic plane, behind the Galactic center; meanwhile, the streams are almost perpendicular to the Galactic plane. Since its discovery, Sgr has been extensively studied; however, understanding the regions at low Galactic latitudes still needs a proper analysis, which has been prevented due to the high crowding and reddening present in this region. Moving into the infrared wavelengths and surveying a broad field with good resolution is a promising alternative for developing this type of analysis. Building on previous research leveraging VISTA capabilities for Sgr dSph analysis, we seek to further our understanding using the improved VVV/VVVX coverage in synergy with the Gaia DR3 survey data. For this purpose, we developed a data-driven approach to perform the decontamination of the region of interest, which allows a careful separation of the Sgr members from field stars. Our results are promising and provide a significant step forward in understanding the intricate characteristics of the Sagittarius dwarf spheroidal galaxy close to the Galactic Bulge. With this work, we aim to provide a better map of the structure of the Sgr dSph and its streams at low Galactic latitudes, completing the view of these structures at higher latitudes.

Resumen:

The moving groups are structures that appears in the velocity distribution of stars in the solar neighborhood in the U-V plane. It is believed that their origins is related to the resonances zones produced by perturbations in the Galaxy's disk. Several studies argue that the central bar is the main mechanism responsible for the formation of moving groups; however, our recent study shows that the spiral structure is also able to produce the observed kinematic distribution of stars due to their dynamics properties in the solar neighborhood, as shown in Michtchenko et al. (2018b). In this context, our work aims to compare the effects of perturbations produced by spiral arms and the bar in the solar neighborhood separately, using theoretical models along with observational data from Gaia. Our model consists of a Hamiltonian that describes the stellar motion in an axisymmetric disk under the influence of perturbations induced by the arms or bars, described through a gravitational potential. For the arms, we used the model with a Gaussian profile from Junqueira et al. (2013), and for the bar, we applied the analytical model from Michtchenko et al. (2018a). We derived the equations of motion for each perturbation and studied their solutions using numerical tools, such as the spectral method and fast Lyapunov indicators, to construct dynamical maps with the aim of characterizing the orbits and identifying zones of stable, resonant, and chaotic motion. By analyzing the dynamical maps in the U-V plane, it was possible to conclude that the model including the spiral arms effects better matched better than those obtained from bars model, having a better compatibility with observational data of stars in the solar neighborhood, indicating that the resonances zones generated by perturbations from the arms may be the mechanisms behind the formation of moving groups.

Resumen:

We propose near-infrared (near-IR) high-resolution spectroscopic follow-up of selected stellar streams to perform a detailed elemental abundance analysis. The stream candidates were identified over the whole sky, and a few selected stars have previously been observed during observations from IGRINS/ Gemini. Using near-IR spectra we aim to perform the first Chemical Tagging of potential stream stars, linking them to their progenitor streams. The new spectral data will allow us to confirm or discard the possible association of these streams with known globular clusters, and provide additional information for a set of chemical species (C, N, O, Mg, Al, Si, Ti, Ni, Cu, Co, Ce, Nd, Yb, among others) accessible from the H-band. We will also examine the presence or absence of the multiple- population phenomenon in the selected streams, and its relation with globular clusters, and strongly constrain the dynamical history for both stars observed with IGRINS and their progenitors.

Resumen:

We derived radial velocities and Ca II Triplet (CaT) metallicity of more than 150 red giants stars in six SMC star clusters and their surrounding fields, with the instrument GMOS on GEMINI South. The mean cluster radial velocity and metallicity were obtained with mean errors of 2.2 km/s and 0.03 dex. We add this information to that available for another 51 clusters and 30 fields with CaT metallicities on the same scale. Using this expanded sample we analyze the chemical properties of the SMC Main Body. We found a high probability that the metallicity distribution of the main body clusters is bimodal with a metalrich and a metalpoor cluster group. Metalrich clusters present a clear agemetallicity relation, while metalpoor clusters present no chemical enrichment throughout the life of the galaxy. We present observational evidence that the chemical enrichment is complex in the SMC main body. Could two cluster groups with potential different origins be coexisting in the main body?

Resumen:

The Magellanic Bridge is a stream of gas and stars connecting the Small and Large Magellanic Clouds, formed by tidal forces as a consequence of a close encounter between both galaxies $\sim$ 200 Myr ago. Recent investigations showed that the Bridge contains two additional branches (Northern and Southern Bridges). The characterization of the stellar populations in the Bridge impose important constraints to the dynamical models that attempt to explain the formation and evolution of the Magellanic System. In this work, currently in progress, we perform a 6D phase$-$space vector, plus age and metallicity, analysis of star clusters in the Southern Bridge. We derived distances, ages, metallicities and velocities from the VISCACHA Survey photometric data and its Gemini/GMOS spectroscopic follow-up, also adopting proper motions from the GAIA Survey. We will present the main results of this investigation.

Resumen:

We present results from a study of six red giant members of the globular cluster (GC) NGC 6558, using high-resolution near-infrared spectra collected with the Apache Point Observatory Galactic Evolution Experiment II survey (APOGEE-2), as part of CAPOS (the bulge Cluster APOgee Survey). We employ the \texttt{BACCHUS} (Brussels Automatic Code for Characterizing High accUracy Spectra) code to provide line-by-line elemental-abundances analysis for Fe-peak (Fe, Ni), $\alpha$-(O, Mg, Si, Ca, Ti), light-(C, N), odd-Z (Al), and $s$-process (Ce) elements. This is the first reliable measure of the CNO abundances for NGC 6558. Our analysis yields a mean metallicity for NGC 6558 of $\langle$[Fe/H]$\rangle=-1.15$, with no evidence for a metallicity spread. We find a solar Ni abundance, $\langle$[Ni/Fe]$\rangle \sim +0.01$, and a moderate enhancement of $\alpha$-elements ranging between $+0.16$ to $< +0.42$, but slightly enhanced of $s$-process $\langle$[Ce/Fe]$\rangle \sim +0.19$. We also found low levels of $\langle$[Al/Fe]$\rangle \sim +0.09$, but with a strong enrichment of nitrogen, [N/Fe]$> +0.99$, well above Galactic levels, which is followed by low levels of carbon abundances, [C/Fe]$< -0.12$. This Nitrogen-Carbon behavior is a typical chemical signature for the presence of multiple stellar populations co-existing in GCs at similar metallicity and this is the first time that it is observed in NGC 6558. We observed a remarkable consistency in the behavior of all the chemical species compared to the other CAPOS bulge GCs with the same metallicity.