Periodic Reporting for period 2 - VIA LACTEA (Numerical Simulations of the Milky Way's Accretion History)
Reporting period: 2023-05-01 to 2024-11-30
How did our Galaxy form? What were the major events that shaped its formation and evolution? Can we use the present-day positions, motions, chemical compositions and ages of stars in the Milky Way to trace its past formation history? These are some of the big questions driving the study of our Milky Way, commonly known as Galactic Archaeology as part of the VIA LACTEA project.
In the current cosmological model, galaxies form hierarchically through the condensation of baryons into stars at the center of dark matter halos. A galaxy like our Milky Way is thought to have experienced many mergers in its early formation history when the Universe was only 2-4 billion years old and later evolved more quiescently to the present-day. However, a number of signs seem to suggest that its evolution may not be as quiescent as previously thought and this may also have important implications for how our Sun was even born. Understanding the formation of the Milky Way and its history throughout cosmic time, is ultimately tied to a fundamentally existential question: "Where do we come from?".
In the stellar halo of the Milky Way, Gaia has uncovered signs of a potentially massive ancient galaxy which merged with our own some 10 bilion years ago. Much is still uncertain about the significance of this event. How massive was the galaxy? Are we certain it was just one galaxy? How was our Galaxy affected by it? What were the implications of this event for the formation and later evolution of the disc of the Milky Way? Did it mark a new phase in the formation of the disc? Did the GSE merger give rise to the thick disc or was it born thick to begin with? Did the GSE merger bring the necessary gas to trigger the formation of the chemical bimodality seen in the Milky Way disc?
In the disc, close to our Sun, Gaia has shown clear signs of disequilibrium which have been mapped out to large distances even into the stellar halo. This points out that our Galaxy is not in a steady-state equilibrium as often assumed opening new opportunities in defining the tale of its formation history as well as the distribution and nature of dark matter. Two candidate galaxies have been proposed to be the culprit for all this recent state of Galactic unrest: the Magellanic Clouds and the Sagittarius dwarf galaxy. How did our Galaxy respond and evolve subject to its interaction with these two alien intruders? What other signs have these interactions left in the chemical, age and phase-space structure of the disc and stellar halo?
Together, these questions motivate the objectives of the VIA LACTEA project which aims to gain a better understanding of the role of massive accretion events in shaping the age, chemical and structural/kinematical properties of the Galaxy from very high-redshift mergers (e.g. Gaia-Sausage-Enceladus) to the present-day subject (e.g. Magellanic Clouds and the Sagittarius dwarf galaxy), test whether we can tell the formation tale of the Milky Way Galaxy from observations of its present-day stellar populations and ultimately test how dark matter behaves on Galactic scales.
In the current cosmological model, galaxies form hierarchically through the condensation of baryons into stars at the center of dark matter halos. A galaxy like our Milky Way is thought to have experienced many mergers in its early formation history when the Universe was only 2-4 billion years old and later evolved more quiescently to the present-day. However, a number of signs seem to suggest that its evolution may not be as quiescent as previously thought and this may also have important implications for how our Sun was even born. Understanding the formation of the Milky Way and its history throughout cosmic time, is ultimately tied to a fundamentally existential question: "Where do we come from?".
In the stellar halo of the Milky Way, Gaia has uncovered signs of a potentially massive ancient galaxy which merged with our own some 10 bilion years ago. Much is still uncertain about the significance of this event. How massive was the galaxy? Are we certain it was just one galaxy? How was our Galaxy affected by it? What were the implications of this event for the formation and later evolution of the disc of the Milky Way? Did it mark a new phase in the formation of the disc? Did the GSE merger give rise to the thick disc or was it born thick to begin with? Did the GSE merger bring the necessary gas to trigger the formation of the chemical bimodality seen in the Milky Way disc?
In the disc, close to our Sun, Gaia has shown clear signs of disequilibrium which have been mapped out to large distances even into the stellar halo. This points out that our Galaxy is not in a steady-state equilibrium as often assumed opening new opportunities in defining the tale of its formation history as well as the distribution and nature of dark matter. Two candidate galaxies have been proposed to be the culprit for all this recent state of Galactic unrest: the Magellanic Clouds and the Sagittarius dwarf galaxy. How did our Galaxy respond and evolve subject to its interaction with these two alien intruders? What other signs have these interactions left in the chemical, age and phase-space structure of the disc and stellar halo?
Together, these questions motivate the objectives of the VIA LACTEA project which aims to gain a better understanding of the role of massive accretion events in shaping the age, chemical and structural/kinematical properties of the Galaxy from very high-redshift mergers (e.g. Gaia-Sausage-Enceladus) to the present-day subject (e.g. Magellanic Clouds and the Sagittarius dwarf galaxy), test whether we can tell the formation tale of the Milky Way Galaxy from observations of its present-day stellar populations and ultimately test how dark matter behaves on Galactic scales.
We developed hydrodynamical simulations of the interaction of the Gaia-Sausage-Enceladus (GSE) merger with the Milky Way (MW) galaxy to study its dispersal in the stellar halo of the Milky Way as part of a larger programme aimed to study impact of the GSE on the formation and evolution of the MW (GASTRO, led by ERC postdoc Dr. Amarante). Prior experiments have only considered purely N-body experiments, however we showed that the various prior passages induce star formation in the GSE progenitor and its dispersal in near integrals of motion space (Energy and Angular momentum) produces complex knots in chemodynamical space. These end up appearing as distinct clusters in this space which naturally get misinterpreted as debris from smaller satellites. Since the discovery of the GSE debris, many groups have actively been searching for clustering of stars in chemodynamical spaces to identify smaller mass objects that may have been accreted in the Milky Way, however based on the interpretation of our models many of these results would need to be re-assessed with further scrutiny.
We have also prepared and been running genetically modified cosmological simulations of MW-like galaxies containing GSE-like features (led by ERC postdoc Dr. Orkney) of study 1. In parallel we have further analysed the properties of MW-like galaxies containing GSE-like debris finding a wide range of scenarios in which such a feature can be reproduced raising doubts on the significance of this galaxy for the formation history of the Milky Way. We have also identified accretion histories reminiscent of two massive objects which may be similar to the hypothesised and highly-debated Kraken galaxy. Our results seem to suggest that such a merger may be extremely challenging to identify chemically because at high redshifts it becomes difficult to chemically disentangle the proto-Milky Way from another merging satellite galaxy with a mass ratio of 1:3. However, we make the falsifiable prediction that the effects of a double merger could have important impacts on the star formation history of the Galaxy.
As part of Study 2, we ran new idealised simulations of the Milky Way interacting with Sgr and the LMC in preparation for the larger cosmological runs these galaxies with the Milky Way. On this front, from the observational side we have conducted a systematic study of the stellar halo using Blue Horizontal Branch stars using the largest/deepest publicly available photometric catalog from the DECaLS survey characterising the structure of the stellar halo out to very large distances (out to 120 kpc). Through this analysis of the outer halo, we were able to make the first quantitative measurements of the transient DM halo wake excited by the infall of the Large Magellanic Cloud as traced by BHBs in terms by characterising its length and width. Such a measurement should provide important constraints on the mass of the LMC and the structure of our own Galactic dark matter halo. We have also identified new disequilirbium features in the Milky Way in the outer disc of the Galaxy. Using Gaia proper motions we produced the first resolved image of outer disc of the Milky Way in the midplane where typically this region used to be highly inaccessible due to the intervening dust.
We have also prepared and been running genetically modified cosmological simulations of MW-like galaxies containing GSE-like features (led by ERC postdoc Dr. Orkney) of study 1. In parallel we have further analysed the properties of MW-like galaxies containing GSE-like debris finding a wide range of scenarios in which such a feature can be reproduced raising doubts on the significance of this galaxy for the formation history of the Milky Way. We have also identified accretion histories reminiscent of two massive objects which may be similar to the hypothesised and highly-debated Kraken galaxy. Our results seem to suggest that such a merger may be extremely challenging to identify chemically because at high redshifts it becomes difficult to chemically disentangle the proto-Milky Way from another merging satellite galaxy with a mass ratio of 1:3. However, we make the falsifiable prediction that the effects of a double merger could have important impacts on the star formation history of the Galaxy.
As part of Study 2, we ran new idealised simulations of the Milky Way interacting with Sgr and the LMC in preparation for the larger cosmological runs these galaxies with the Milky Way. On this front, from the observational side we have conducted a systematic study of the stellar halo using Blue Horizontal Branch stars using the largest/deepest publicly available photometric catalog from the DECaLS survey characterising the structure of the stellar halo out to very large distances (out to 120 kpc). Through this analysis of the outer halo, we were able to make the first quantitative measurements of the transient DM halo wake excited by the infall of the Large Magellanic Cloud as traced by BHBs in terms by characterising its length and width. Such a measurement should provide important constraints on the mass of the LMC and the structure of our own Galactic dark matter halo. We have also identified new disequilirbium features in the Milky Way in the outer disc of the Galaxy. Using Gaia proper motions we produced the first resolved image of outer disc of the Milky Way in the midplane where typically this region used to be highly inaccessible due to the intervening dust.
As part of study 1, we have broken a number of pre-conceived ideas about the Milky Way's formation history. For example, we have shown that nothing so far in the observational datasets points to the fact that the debris from Gaia-Sausage-Enceladus galaxy came from one massive LMC-like galaxy 10 Gyrs ago. This is already a sign that the very notion that this galaxy debris had a major impact on our Galaxy is slowly starting to be put into question. We expect new results to come in due the course this year which will wrap up our goals in Study as part of our genetically modified cosmological simulations of Milky Way-like galaxies with GSE-like mergers to understand their impacts on the later formation and evolution of the Galaxy.
As part of study 2, we are currently developing new state-of-the-art simulations of the interaction of Sgr and the LMC in a full cosmological context. We have also developed new and tested methods to infer the distribution of dark matter in the Galaxy which we set some of the strongest constraints on exotic dark matter models on the market. Before the project began all previous methods assumed equilibrium, this method was the first one to go beyond the state-of-the art. Furthermore, the identification of more stream-like structures in the outer disc with Gaia has already set a new leap for Galactic Structure thanks to the Gaia data. Prior to the beginning of the project, we did not know what the disc of the Galaxy looked like in the midplane due to the intervening dust. I expect that the new cosmological runs we are developing for study 2 will not only help us understand the impact of satellites like the Sagittarius dwarf galaxy or the LMC on the structure and kinematics of the Galaxy but also bring new falsifiable predictions on what is driving the disequilibrium we see in our Galaxy which will be tested with observational datasets such as Gaia DR4, DECaLS, SDSS-V, LSST and EUCLID into the future.
As part of study 2, we are currently developing new state-of-the-art simulations of the interaction of Sgr and the LMC in a full cosmological context. We have also developed new and tested methods to infer the distribution of dark matter in the Galaxy which we set some of the strongest constraints on exotic dark matter models on the market. Before the project began all previous methods assumed equilibrium, this method was the first one to go beyond the state-of-the art. Furthermore, the identification of more stream-like structures in the outer disc with Gaia has already set a new leap for Galactic Structure thanks to the Gaia data. Prior to the beginning of the project, we did not know what the disc of the Galaxy looked like in the midplane due to the intervening dust. I expect that the new cosmological runs we are developing for study 2 will not only help us understand the impact of satellites like the Sagittarius dwarf galaxy or the LMC on the structure and kinematics of the Galaxy but also bring new falsifiable predictions on what is driving the disequilibrium we see in our Galaxy which will be tested with observational datasets such as Gaia DR4, DECaLS, SDSS-V, LSST and EUCLID into the future.