Found 51 talks archived in The Galaxy

Thursday May 12, 2022
Dr. Eugene Vasiliev
IoA Cambridges


I discuss the dynamical interactions between the Milky Way and its satellite galaxies, focusing on the closest and most massive satellites - the Large Magellanic Cloud (LMC) and the Sagittarius dwarf galaxy. The former just has had its first close encounter with the Milky Way very recently, and the latter has been orbiting our Galaxy for several Gyr and is tidally disrupting, leaving a prominent tidal stream spanning the entire sky. Thanks to the abundant and precise observational data from the Gaia satellite and various spectroscopic surveys, we now have a very detailed view of the Sagittarius stream and the remnant. It appears that to reproduce its observed properties, one needs to take into account the gravitational effect of the LMC itself and the effect that it produces on the motion of the Milky Way: an intricate dance of three galaxies. The LMC also affects the motion of other streams and satellite galaxies in the outskirts of the Milky Way, and I discuss an approach for compensating these perturbations in the context of dynamical modelling of the Milky Way mass distribution and the analysis of satellite orbits.

Thursday April 21, 2022
Dr. Robert Grand


Recent years have seen impressive development in cosmological simulations for spiral disc galaxies like the Milky Way. I present a suite of high-resolution magneto-hydrodynamic simulations that include many physical processes relevant for galaxy formation, including star formation, stellar evolution and feedback, active galactic nuclei and magnetic fields. I will discuss how these processes affect the formation of galactic discs, and what these simulations can tell us about the formation of the Milky Way, such as the properties of the Galaxy's putative last significant merger and its effect on the formation of the thick disc and stellar halo. 

Thursday January 20, 2022
Dr. David S. Aguado
Universidad de Florencia


One prediction of ΛCDM is the existence of partially phase-mixed substructures from accreted dwarf galaxies in the Milky Way stellar halo. Substructure originating in a single accretion event can be readily identified as a tight cluster of stars in phase space with similar chemical properties. Recently, the discovery of the Gaia Sausage Enceladus (GSE) has revolutionised our understanding of the complex assembly of the Milky Way halo. We present a review of the chemistry that characterises the last major merger that happened to the Milky Way some 9-10 Gy ago.

Thursday November 25, 2021
Dr. Florent Renaud
Lund Observatory


The formation and evolution of galaxies across cosmic time proceeds in different phases, paced by their internal evolution and external factors like gas accretion and mergers. The complex and always changing interplay between these mechanisms drives the assembly of galaxies and the physical conditions for star formation, which leaves observable imprints on the stellar populations. Large astrometric and spectroscopic surveys (e.g. Gaia, APOGEE, GALAH) collect the signatures of these past events in the building history of the Milky Way. However, simulations and models are necessary to decode the data. In this talk, I will present results from a series of hydrodynamical simulations of Milky Way-like galaxies, both in isolation and in cosmological context using the VINTERGATAN simulation. I will show the crucial role of mergers, and of the end of the merger phase, in forming the thick and thin Galactic discs, and making the transition between the two. I will then nuance this conclusion by explaining why the secular consumption of gas enables a similar transition, as well as the emergence of spirals, without any external factors.

Thursday May 20, 2021
Dr. Guillaume Thomas


At present, our understanding of the formation history of the MW is limited due to the complexity of observing the imprints of accretion events and of reproducing them in numerical simulations. Moreover, though being the only galaxy, in which the Galactic potential can be probed in detail, the distribution of mass in the MW, and hence of the dark matter, is poorly constraint, especially at large distances. In addition, the MW is not isolated, and it has recently been suggested that the infall of the LMC can induce a perturbation in the stellar and dark matter distribution of the MW. As a consequence, the details of the formation history of our Galaxy are still unknown, such as the number of accretion events, the mass of the accreted galaxies, and the epoch of these events. Yet this information is crucial to understand our environment and to constrain the theoretical models and simulations that try to reproduce it.

One of the major challenges of the field is that a tremendous number of multi-aspect (astrometric, photometric and spectroscopic) observations at significant depth is required to study the morphology, the kinematics and the chemistry of the outskirts of our Galaxy, where are located the signatures of these events. Hopefully, the advent of recent and incoming complementary large surveys, such as the European Gaia mission, UNIONS (Ultraviolet Near Infrared Optical Northern Survey), Pristine, Pan-STARRS (PS), WEAVE or LSST (Legacy Survey of Space and Time), is offering a new global point of view on our Galaxy’s halo, allowing us to precisely probe the Galactic potential our the MW, and to retrace itsaccretion history.

In this talk I will present recent works that have been conducted to better catarerized our Galaxy and its history with some of the existing surveys mentioned above. In addition, I will present the major improvement that will bring this new generation of large, multi-aspect surveys, to study both our Galactic history, as well as the fundamental nature of the dark matter.

Tuesday November 3, 2020
Drs. Paola Di Matteo
Observatoire de Paris


Reconstructing the past of the Milky Way depends on the study of its metal-poor stars, which either have been formed in the Galaxy itself in the first billion years, or have been accreted through mergers of satellite galaxies over time. These stars are usually found in what is known as the Milky Way halo, a light — in terms of total mass —  stellar component which is usually made of stars whose kinematics significantly deviates from that of the Galactic disc.
In this talk, I will discuss how it has been possible to use the astrometric and spectroscopic data delivered by Gaia and complementary surveys  to shed light on the past of our Galaxy, through the study of its halo. Besides the discovery of the possible last significant merger experienced by the Milky Way, the use of 6D phase space information and chemical abundances allowed to reconstruct the impact this merger had on the early Milky Way disc, and the time it occurred, as well as to discover that some of the most metal-poor stars in the Galaxy possibly formed in a disc.  This last finding would imply that the dissipative collapse that led to the formation of the old Galactic disc must have been extremely fast.


Thursday October 15, 2020
Dr. Karla Arellano


The emission line spectrum of H II regions provides information about the chemical composition of the present-day interstellar medium. The study as a function of their galactocentric distances helps to constrain chemical evolution models.  In this talk, I present a reanalysis of the abundance gradients of C, N, O, Ne, S, Cl, and Ar for a sample of 33 Galactic H II regions covering a range in Galactocentric Distances from 6-17 kpc. New values of the Galactocentric distances were calculated using Gaia DR2 parallaxes for some objects. We study in detail the different ICF schemes to improve the results of the total abundances in Galactic H II regions. We found that the re-evaluation of the distances using Gaia DR2 parallaxes produces an O gradient that discards a flattening of the gradient in the inner part of the GalaxyThe radial distribution of Ne/O, S/O, Cl/O and Ar/O are almost flat confirming a lockstep evolution of those elements respect to O. Our Galaxy also shows an almost flat N/O gradient respect to other nearby spiral galaxies. We compare our results with those from B type stars and cepheids, young planetary nebulae and those slopes using optical and infrared data for H II regions.

Thursday March 14, 2019
Prof. Jon Holtzman
Fundacion Jesus Serra


The SDSS Apache Point Observatory Galactic Evolution 
Experiment (APOGEE) has
collected high resolution near-IR spectra of several hundred thousand stars
across the Milky Way. I'll describe some observational results about the
spatial variation of chemical abundances as a function of Galactocentric
radius and distance from the midplane, discussing mean abundances, 
distribution function, and the variation of abundance ratios of multiple
elements. Additional information related to stellar ages can be obtained
from [C/N] for red giant stars. Several lines of evidence suggest that 
migration has had a significant impact on the Galactic disk. The 
observed patterns of
abundance ratios may provide observational constraints on 
nucleosynthetic yields.

Wednesday September 12, 2018
Mr. Francisco Surot Madrid


The Milky Way (MW) galaxy is not much different from its faraway cousins. However, our position within the MW allows us to study the properties of its stellar populations with exquisite detail in comparison to extragalactic sources.  The bulge of the MW (i.e. the stellar population within ~3 kpc from the Galactic center) is the most massive stellar component of the MW hosting very old stars (>10 Gyr), therefore the study of its stellar population properties can shed light on the formation and evolution of the MW as a whole, and of other spiral galaxies at large.

   So far, there is a general consensus on the global kinematic, chemical and structural properties of the bulge populations, however the age, or rather, the distribution of the ages of the stars in the bulge is yet to be completely understood.
   We aimed at addressing the questions 'How old is the bulge?' and 'Is there a spatial age gradient in the bulge?' through the determination of the stellar ages in the different fields sparsely distributed within a region of 300 deg² centered on the bulge.
  We use images from the VISTA Variables in the Vía Láctea (VVV) survey, based in near infrared passbands, to extract accurate magnitude and color of half a billion stars in the bulge area using point spread function fitting.
 The newly derived photometric catalogs, used in addition to probe the extinction towards the bulge, will be made publicly available to the entire community.
The contribution of the intervening disk population along the bulge lines of sight has been detected and removed by using a statistical approach in order to obtain a final stars sample that is representative of the bulge population only.
The determination of the stellar ages in different fields is provided through the comparison between the observations and synthetic stellar population models, which have been carefully tailored to account for the observational effects (i.e. distance dispersion, differential reddening, photometric completeness,  photometric and systematic uncertainties).
The simulations leading to the construction of synthetic populations have been carried out by using two different methods: i) a model that uses a spectroscopically derived metallicity distribution functions as prior, leaving the age as the only free parameter; ii) a genetic algorithm that finds the best solution within all possible combinations of age and metallicity (i.e. uniform prior in age and metallicity using IAC-POP/Minniac suite).
  We ultimately find that the bulge itself appears to be on average old (>9.5 Gyr) throughout its extension (|l| < 10° and -10° < b < +5°), with a mild gradient of about 0.16 Gyr/deg towards the Galactic center.

Tuesday March 6, 2018
Dr. Giuseppina Battaglia


In a framework where galaxies form hierarchically, extended stellar haloes are predicted to be an ubiquitous feature around Milky Way-like galaxies and to consist mainly of the shredded stellar component of smaller galactic systems. The type of accreted stellar systems are expected to vary according to the specific accretion and merging history of a given galaxy, and so is the fraction of stars formed in situ versus accreted. Analysis of the chemical properties of Milky Way halo stars out to large Galactocentric radii can provide important insights into the properties of the environment in which the stars that contributed to the build-up of different regions of the Milky Way stellar halo formed. In this talk I will first give an overview of some of the main properties of the Milky Way stellar halo based on literature studies. I will then present results concerning the chemical properties of the outer regions of the Milky Way stellar halo, based on the elemental abundances of halo stars with large present-day Galactocentric distances, >15 kpc. The data-set we acquired consists of high resolution HET/HRS, Magellan/MIKE and VLT/UVES spectra for 28 red giant branch stars covering a wide metallicity range, -3.1 ≲ [Fe/H] ≲-0.6. We show that the ratio of α-elements over Fe as a function of [Fe/H] for our sample of outer halo stars is not dissimilar from the pattern shown by MW halo stars from solar neighborhood samples. On the other hand, significant differences appear at [Fe/H] ≳-1.5 when considering chemical abundance ratios such as [Ba/Fe], [Na/Fe], [Ni/Fe], [Eu/Fe], [Ba/Y]. Qualitatively, this type of chemical abundance trends are observed in massive dwarf galaxies, such as Sagittarius and the Large Magellanic Cloud. This appears to suggest a larger contribution in the outer halo of stars formed in an environment with high initial star formation rate and already polluted by asymptotic giant branch stars with respect to inner halo samples. 

Upcoming talks

  • TODAY: Sistema de control de EST
    Jorge Quintero Nehrkorn
    Friday October 7, 2022 - 10:00 GMT+1  (Aula)
  • TBD
    Dr. Roger Hoyland
    Friday October 14, 2022 - 10:00 GMT+1  (Aula)

More upcoming talks

Recent Colloquia

Recent Talks