2025.11.25, IAU-H1, Francois Hammer talk

The epoch of accretion of Milky Way dwarfs from Gaia DR3 IAU H1 – The Local universe Seminar - 25 November 2025 Presented by François Hammer LIRA From Gaia DR2 to DR3 Li, Hammer, Babusiaux et al. (2021): Battaglia, Taibi, Thomas et al. (2022): 46 dwarf galaxies full account of Gaia systematics 66 dwarf galaxies with reliable measurements Bayesian method Two papers on dwarf orbits based on Gaia DR3: •The number of stars that have combined radial velocities & proper motions (3D): increases from 7 224 631 to 33 812 183 •Proper motion uncertainty is divided by ~ 2 •Parallax uncertainty is divided by ~ 1.3 è Binding energy and angular momentum of dwarf orbits with very good accuracy Content 1Infall time versus binding energy: the MW accretion history 2Gaia predicts a recent infall (< 3 Gyr ago) for most MW dSph galaxies: it is confirmed by the presence of young stars 3Consequences on dSph formation and on their dark matter content 4Conclusions François Hammer 1Infall time versus binding energy: the accretion history of the MW François Hammer The halo accretion history Rocha et al. 2012 Prediction of the hierarchical scenario : •Galaxy mass grows with time, satellites arrived the first are the most bound, while the newcomers are the least bound (Gott, 1975, “onion model”) •Proven by all cosmological simulations, e.g., Rocha et al. 2012, Boylan-Kolchin et al. 2013 Consistent with the prediction of the hierarchical scenario : •Galaxy mass grows with time, satellites arrived the first are the most bound, while the newcomers are the least bound (Gott, 1975, “onion model”) •Proven by cosmological simulations, e.g., Rocha et al. 2012, Boylan-Kolchin et al. 2013, also because the Milky Way has a relatively quiet merger history (when compared to e.g., M31, Cen A). Ebinding = - Eorbital= -(K + F) 4000 2000 0 -2000 -4000 Lz(km s-1 kpc) 0.5 1.5 1.0 2.0 2.5 Binding Energy (105km2s-2) GSE Disk Sgr Bulge The Milky way halo accretion history Adapted from Callingham et al. (2022) and Deason & Belokurov (2025) 12.5 Gyr 9 Gyr 6 Gyr 4000 2000 0 -2000 -4000 Lz(km s-1 kpc) 0.5 1.5 1.0 2.0 2.5 Binding Energy (105km2s-2) GSE Disk Sgr Bulge The Milky way halo accretion history Points from stars Error bars from associated GCs 12.5 Gyr 9 Gyr 6 Gyr Ruiz-Lara et al. 2022 4000 2000 0 -2000 -4000 Lz(km s-1 kpc) 0.5 1.5 1.0 2.0 2.5 Binding Energy (105km2s-2) GSE Disk Sgr Bulge Most dSph galaxies are newcomers into the MW halo Points from stars Error bars from associated GCs 12.5 Gyr 9 Gyr 6 Gyr < 3 Gyr Binding energy based from accurate 3D velocities provided by Gaia DR3 Hammer et al. 2021, 2023 Can we use the relation between infall time and binding energy to predict dwarf infall epoch? D’Souza & Bell (2022) studied the ELVIS simulation suite (from 1 to 3 1012 Msun ). They confirm the strong relationship between Ebinding and Taccr for 45 of the 48 studied main halo histories. iDouglas Recent star formation in all MW dSphs Y. Yang , E.Caffau, P. Bonifacio, F. Hammer, H. Wang, G. Mamon 2024, A&A 691, 363 After PM-filter Before Proper-Motion -filtering All Gaia sources in r< 1.1 degree of Sculptor After Proper-Motion -filtering Model: 86% of > 10 Gyr stars + 14% at < 2 Gyr Fornax 2.1 degrees ✔ UMi Draco Sextans CarinaSculptor ✔✔✔✔ ✔ Gaia star Counts Images Star formation < 2 Gyr ago in all dSphs! 3DSph late arrivals: consequences on their (trans)formation and on their dark matter content François Hammer •Simulation performed with GIZMO (Hopkins 2014), ram pressure is due to the same Galactic coronae that fit well the Magellanic stream formation (see Wang et al. 2019); Ram pressure helps to capture the Sculptor progenitor. •Gas fully lost near pericenter passage: this shaking process leads to large velocity dispersions (Wang et al. 2024). As newcomers into the halo, dSph gas-rich progenitors are loosing their gas and become out of equilibrium Dwarf motion Wang et al. 2024 Here, large slos values are caused by turbulence, shocks, tides and gas losses: the Sculptor progenitor is transformed in less than a single orbit! dark matter free Sculptor Dwarf gas (cyan dots) is compressed by the Galactic corona inducing star formation and supernovas, which explosions expell the gas to dwarf outskirts Galactic corona density increases during the fall of the dwarf, and it becomes very effective to remove progressively the dwarf gas and to free the stars (orange dots) Dwarf motion to the Galaxy •If rotationally supported dSph progenitors were dark matter dominated, this would prevent them to be transformed into dispersion supported systems; •This is because dark matter shields stellar content and preserve their initial dynamics. As newcomers into the halo, if dark matter dominated, dSph gas-rich progenitors are also loosing their gas Wang et al. 2024 Here, large slos values are caused by turbulence, shocks, tides and gas losses: the Sculptor progenitor is transformed in less than a single orbit! Sculptor dominated by dark matter Conclusions François Hammer •Dwarf average binding energy is 5 (3) times smaller than that of GSE (Sgr), respectively, suggesting their infall ≤ 3 Gyr ago, during which they perform less than a single orbit; •Their progenitors are gas-rich, rotation-supported: to transform them in short times into dispersion dominated dSph requires them to be out of equilibrium, and this could explain their high slos values; •This however requires a small dark matter content in the progenitors (e.g., WLM type, Kolhe et al. 2026, A&A submitted) because massive DM halo would prevent such a fast transformation. Conclusions François Hammer •Dwarf average binding energy is 5 (3) times smaller than that of GSE (Sgr), respectively, suggesting their infall ≤ 3 Gyr ago, during which they perform less than a single orbit; •Their progenitors are gas-rich, rotation-supported: to transform them in short times into dispersion dominated dSph requires them to be out of equilibrium, and this could explain their high slos values; •This however requires a small dark matter content in the progenitors (e.g., WLM type, Kolhe et al. 2026, A&A submitted) because massive DM halo would prevent such a fast transformation. •Recent arrival of dSphs confirmed by the discovery of young (CHEB) stars by Yang et al. 2024; •Expansion of the stellar content after gas exhaustion well explain the presence of stars very far from the dwarf centers (Sestito+2023, Waller+2023, Tau+2024, see also works of Anirudth Chiti, Jaclyn Jensen) Conclusions François Hammer •Dwarf average binding energy is 5 (3) times smaller than that of GSE (Sgr), respectively, suggesting their infall ≤ 3 Gyr ago, i.e., they can’t proceed even a single orbit; •Their progenitors are gas-rich, rotation-supported: to transform them in short times into dispersion dominated dSph requires them to be out of equilibrium, and this could explain their high slos values; •This however requires a small dark matter content in the progenitors (e.g., WLM type, Kolhe et al. 2026, A&A submitted) because massive DM halo would prevent such a fast transformation. •Recent arrival of dSphs confirmed by the discovery of young (CHEB) stars by Yang et al. 2024; •Expansion of the stellar content after gas exhaustion well explain the presence of stars very far from the dwarf centers (Sestito+2023, Waller+2023, Tau+2024, see also works of Anirudth Chiti, Jaclyn Jensen) •Consequently, dwarf orbits cannot be used to estimate the Milky Way mass; •Keep posted for a full hydrodynamical model of the Milky Way formed from GSE merger (Akib et al. in preparation). It also demonstrates that globular clusters keep their energy from their progenitors! François Hammer Akib et al. in preparation An explanation of the internal kinematics of dwarf galaxies without prominent dark matter sub-halos Hammer, Li, Mamon, Pawlowski, Bonifacio, Jiao, Wang, Wang & Yang, 2023, MNRAS 519, 5059 The recent infall of dwarf spheroidal galaxies and their progenitors to be gas-rich dwarf irregular Hammer, Wang, Mamon, Pawlowski, Yang, Jiao, Li, Bonifacio, Caffau & Wang, 2024, MNRAS 527, 2718 Mechanisms to form dwarfs spheroidal from dwarf irregular: the role of ram-pressure and tidal shocks Wang, Hammer, Mamon, Pawloswki, Yang & Wang, 2024, MNRAS 527, 7144 Hydrodynamical simulations to transform dwarf irregular into dwarf spheroidal Yang, Caffau, Bonifacio, Hammer, Wang, Mamon, 2024, A&A 691, 363 An additional proof of a recent infall: detection of young stars in all dwarf spheroidal galaxies Hammer, Jiao, Mamon, et al. 2024, A&A, 692, 1 The Milky Way accretion history compared to cosmological simulations: from bulge formation to dwarf galaxy infall