The outer low-α disc of the Milky Way – I: evidence for the first pericentric passage of Sagittarius?

Phase-space data, chemistry, and ages together reveal a complex structure in the outer low-α disc of the Milky Way. The age-vertical velocity dispersion profiles beyond the Solar Neighbourhood show a jump at 6 Gyr for stars beyond the Galactic plane. Stars older than 6 Gyr are significantly hotter than younger stars. The chemistry and age histograms reveal a bump at [Fe/H] = −0.5, [α/Fe] = 0.1, and an age of 7.2 Gyr in the outer disc. Finally, viewing the stars beyond 13.5 kpc in the age-metallicity plane reveals a faint streak just below this bump, towards lower metallicities at the same age. Given the uncertainty in age, we believe these features are linked and suggest a pericentric passage of a massive satellite ∼6 Gyr ago that heated pre-existing stars, and led to a starburst in existing gas. New stars also formed from the metal-poorer infalling gas. The impulse approximation was used to characterize the interaction with a satellite, finding a mass of ∼1011 M⊙, and a pericentric position between 12 and 16 kpc. The evidence points to an interaction with the Sagittarius dwarf galaxy, likely its first pericentric passage

Bars and boxy/peanut bulges in thin and thick discs. II. Can bars form in hot thick discs?

The Milky Way as well as a majority of external galaxies possess a thick disc. However, the dynamical role of the (geometrically) thick disc on the bar formation and evolution is not fully understood. Here, we investigate the effect of thick discs in bar formation and evolution by means of a suite of N-body models of (kinematically cold) thin-(kinematically hot) thick discs. We systematically vary the mass fraction of the thick disc, the thin-to-thick disc scale length ratio as well as thick disc's scale height to examine the bar formation under diverse dynamical scenarios. Bars form almost always in our models, even in presence of a massive thick disc. The part of the bar constituted by the thick disc closely follows the overall growth and temporal evolution of the part of the bar constituted by the thin disc, only the part of the bar in the thick disc is weaker than the part of the bar in the thin disc. The formation of stronger bars is associated with a simultaneous larger loss of angular momentum and a larger radial heating. In addition, we demonstrate a preferential loss of angular momentum and a preferential radial heating of disc stars, along the azimuthal direction within the extent of the bar, in both thin and thick disc stars. For purely thick disc models (without any thin disc), the bar formation critically depends on the disc scale length and scale height. A larger scale length and/or a larger vertical scale height delays the bar formation time and/or suppresses the bar formation almost completely in thick-disc-only models. We find that the Ostriker-Peeble criterion predicts the bar instability scenarios in our models better than the Efstathiou-Lake-Negroponte criterion.Comment: 20 pages, 14 figures, 1 table (including appendix), accepted for publication in A&

Did the Gaia Enceladus/Sausage merger form the Milky Way's bar?

The Milky Way's last significant merger, the Gaia Enceladus/Sausage (GES), is thought to have taken place between 8-11 Gyr ago. Recent studies in the literature suggest that the bar of the Milky Way is rather old, indicating that it formed at a similar epoch to the GES merger. We investigate the possible link between these events using one of the Auriga cosmological simulations which has salient features in common with the Milky Way, including a last significant merger with kinematic signatures resembling that of the GES. In this simulation, the GES-like merger event triggers tidal forces on the disc, gas inflows and a burst of star formation, with the formation of a bar occuring within 1 Gyr of the first pericentre. To highlight the effects of the merger, we rerun the simulation from z=4 with the progenitors of the GES-like galaxy removed well before the merger time. The consequence is a delay in bar formation by around 2 Gyr, and this new bar forms without any significant external perturbers. We conclude that this Milky Way-like simulation shows a route to the real Milky Way's bar forming around the epoch of the GES merger due to tidal forces on its first pericentre. We explore all Auriga galaxies with GES-like merger events, and find that those with stellar mass ratios below 10% form bars within 1 Gyr of the merger, while bar formation is delayed in the more massive merger scenarios. These include the 4 oldest bars in the simulation suite. Lastly, we note some later morphological differences between the disc of the original simulation and our rerun, in particular that the latter does not grow radially for the final 7 Gyr. Our study suggests that the GES may therefore be responsible for the formation of the Milky Way's bar, as well as for the build-up of its extended disc.Comment: 14 pages, 9 figures, submitted to MNRA

A dynamo amplifies the magnetic field of a Milky-Way-like galaxy

The magnetic fields of spiral galaxies are so strong that they cannot be primordial. Their typical values are over one billion times higher than any value predicted for the early Universe. Explaining this immense growth and incorporating it in galaxy evolution theories is one of the long-standing challenges in astrophysics. So far, the most successful theory for the sustained growth of the galactic magnetic field is the alpha-omega dynamo. This theory predicts a characteristic dipolar or quadrupolar morphology for the galactic magnetic field, which has been observed in external galaxies. However, so far, there has been no direct demonstration of a mean-field dynamo operating in direct, multi-physics simulations of spiral galaxies. We do so in this work. We employ numerical models of isolated, star-forming spiral galaxies that include a magnetized gaseous disk, a dark matter halo, stars, and stellar feedback. Naturally, the resulting magnetic field has a complex morphology that includes a strong random component. Using a smoothing of the magnetic field on small scales, we are able to separate the mean from the turbulent component and analyze them individually. We find that a mean-field dynamo naturally occurs as a result of the dynamical evolution of the galaxy and amplifies the magnetic field by an order of magnitude over half a Gyr. Despite the highly dynamical nature of these models, the morphology of the mean component of the field is identical to analytical predictions. This result underlines the importance of the mean-field dynamo in galactic evolution. Moreover, by demonstrating the natural growth of the magnetic field in a complex galactic environment, it brings us a step closer to understanding the cosmic origin of magnetic fields.Comment: Accepted for publication in Astronomy & Astrophysic

Insight into the Galactic Bulge Chemodynamical Properties from Gaia DR3

We explore the chemodynamical properties of the Galaxy in the azimuthal velocity $V_\phi$ and metallicity [Fe/H] space using red giant stars from Gaia Data Release 3. The row-normalized $V_\phi$-[Fe/H] maps form a coherent sequence from the bulge to the outer disk, clearly revealing the thin/thick disk and the Splash. The metal-rich stars display bar-like kinematics while the metal-poor stars show dispersion-dominated kinematics. The intermediate-metallicity population ($-1<$[Fe/H]$<-0.4$) can be separated into two populations, one that is bar-like, i.e. dynamically cold ($\sigma_{V_R}\sim80$ $\rm km\ s^{-1}$) and fast rotating ($V_\phi\gtrsim100$ $\rm km\ s^{-1}$), and the Splash, which is dynamically hot ($\sigma_{V_R}\sim110$ $\rm km\ s^{-1}$) and slow rotating ($V_\phi\lesssim100$ $\rm km\ s^{-1}$). We compare the observations in the bulge region with an Auriga simulation where the last major merger event occurred $\sim10$ Gyr ago: only stars born around the time of the merger reveal a Splash-like feature in the $V_\phi$-[Fe/H] space, suggesting that the Splash is likely merger-induced, predominantly made-up of heated disk stars and the starburst associated with the last major merger. Since the Splash formed from the proto-disk, its lower metallicity limit coincides with that of the thick disk. The bar formed later from the dynamically hot disk with [Fe/H] $>-1$ dex, with the Splash not participating in the bar formation and growth. Moreover, with a set of isolated evolving $N$-body disk simulations, we confirm that a non-rotating classical bulge can be spun up by the bar and develop cylindrical rotation, consistent with the observation for the metal-poor stars.Comment: ApJ accepted, 20 pages, 15 figures, comments welcom

Insights into the Galactic Bulge Chemodynamical Properties from Gaia Data Release 3

We explore the chemodynamical properties of the Galaxy in the azimuthal velocity V ϕ and metallicity [Fe/H] space using red giant stars from Gaia Data Release 3. The row-normalized V ϕ –[Fe/H] maps form a coherent sequence from the bulge to the outer disk, clearly revealing the thin/thick disk and the Splash. The metal-rich stars display bar-like kinematics, while the metal-poor stars show dispersion-dominated kinematics. The intermediate-metallicity population (−1 − 1 dex, with the Splash not participating in the bar formation and growth. Moreover, with a set of isolated evolving N-body disk simulations, we confirm that a nonrotating classical bulge can be spun up by the bar and develop cylindrical rotation, consistent with the observations for the metal-poor stars