We use Gaia data-release 2 (DR2) magnitudes, colours, and parallaxes for stars with G<12 to explore a parameter space with 15 dimensions that simultaneously includes the initial mass function (IMF) and a non-parametric star formation history (SFH) for the Galactic disc. This inference is performed by combining the Besançon Galaxy Model fast approximate simulations (BGM FASt) and an approximate Bayesian computation algorithm. We find in Gaia DR2 data an imprint of a star formation burst 2-3 Gyr ago in the Galactic thin disc domain, and a present star formation rate (SFR) of ≈ 1M /yr. Our results show a decreasing trend of the SFR from 9-10 Gyr to 6-7 Gyr ago. This is consistent with the cosmological star formation quenching observed at redshifts z < 1.8. This decreasing trend is followed by a SFR enhancement starting at ∼ 5Gyr ago and continuing until ∼ 1Gyr ago which is detected with high statistical significance by discarding the null hypothesis of an exponential SFH with a p-value=0.002. We estimate, from our best fit model, that about 50% of the mass used to generate stars, along the thin disc life, was expended in the period from 5 to 1 Gyr ago. The timescale and the amount of stellar mass generated during the SFR enhancement event lead us to hypothesise that its origin, currently under investigation, is not intrinsic to the disc. Thus, an external perturbation is needed for its explanation. Additionally, for the thin disc we find a slope of the IMF of α 3 ≈ 2 for masses M > 1.53M and α 2 ≈ 1.3 for the mass range between 0.5 and 1.53 M . This is the first time that we consider a non-parametric SFH for the thin disc in the Besançon Galaxy Model. This new step, together with the capabilities of the Gaia DR2 parallaxes to break degeneracies between different stellar populations, allow us to better constrain the SFH and the IMF.

Using an isolated Milky Way-mass galaxy simulation, we compare results from 9 state-of-the-art gravitohydrodynamics codes widely used in the numerical community. We utilize the infrastructure we have built for the AGORA High-resolution Galaxy Simulations Comparison Project. This includes the common disk initial conditions, common physics models (e.g., radiative cooling and UV background by the standardized package GRACKLE) and common analysis toolkit yt, all of which are publicly available. Subgrid physics models such as Jeans pressure floor, star formation, supernova feedback energy, and metal production are carefully constrained across code platforms. With numerical accuracy that resolves the disk scale height, we find that the codes overall agree well with one another in many dimensions including: gas and stellar surface densities, rotation curves, velocity dispersions, density and temperature distribution functions, disk vertical heights, stellar clumps, star formation rates, and Kennicutt-Schmidt relations. Quantities such as velocity dispersions are very robust (agreement within a few tens of percent at all radii) while measures like newly-2 J. KIM ET AL. FOR THE AGORA COLLABORATION formed stellar clump mass functions show more significant variation (difference by up to a factor of ∼3). Systematic differences exist, for example, between mesh-based and particle-based codes in the low density region, and between more diffusive and less diffusive schemes in the high density tail of the density distribution. Yet intrinsic code differences are generally small compared to the variations in numerical implementations of the common subgrid physics such as supernova feedback. Our experiment reassures that, if adequately designed in accordance with our proposed common parameters, results of a modern high-resolution galaxy formation simulation are more sensitive to input physics than to intrinsic differences in numerical schemes.

High resolution N-body simulations using different codes and initial condition techniques reveal two different behaviours for the rotation frequency of transient spiral arms like structures. Whereas unbarred disks present spiral arms nearly corotating with disk particles, strong barred models (bulged or bulge-less) quickly develop a bar-spiral structure dominant in density, with a pattern speed almost constant in radius. As the bar strength decreases the arm departs from bar rigid rotation and behaves similar to the unbarred case. In strong barred models we detect in the frequency space other subdominant and slower modes at large radii, in agreement with previous studies, however we also detect them in the configuration space. We propose that the distinctive behaviour of the dominant spiral modes can be exploited in order to constraint the nature of Galactic spiral arms by the astrometric survey GAIA and by 2-D spectroscopic surveys like CALIFA and MANGA in external galaxies.

We introduce a new set of simulations of Milky Way-sized galaxies using the AMR code ART + hydrodynamics in a ΛCDM cosmogony. The simulation series is named GARROTXA and follow the formation of a halo/galaxy from z = 60 to z = 0. The final virial mass of the system is ∼7.4×10 11 M . Our results are as follows: (a) contrary to many previous studies, the circular velocity curve shows no central peak and overall agrees with recent MW observations. (b) Other quantities, such as M * (6×10 10 M ) and R d (2.56 kpc), fall well inside the observational MW range. (c) We measure the disk-to-total ratio kinematically and find that D/T=0.42. (d) The cold gas fraction and star formation rate (SFR) at z=0, on the other hand, fall short from the values estimated for the Milky Way. As a first scientific exploitation of the simulation series, we study the spatial distribution of the hot X-ray luminous gas. We have found that most of this X-ray emitting gas is in a halo-like distribution accounting for an important fraction but not all of the missing baryons. An important amount of hot gas is also present in filaments. In all our models there is not a massive disk-like hot gas distribution dominating the column density. Our analysis of hot gas mock observations reveals that the homogeneity assumption leads to an overestimation of the total mass by factors 3 to 5 or to an underestimation by factors 0.7−0.1, depending on the used observational method. Finally, we confirm a clear correlation between the total hot gas mass and the dark matter halo mass of galactic systems.

We introduce a new set of eight Milky Way-sized cosmological simulations performed using the AMR code ART + Hydrodynamics in a ΛCDM cosmology. The set of zoom-in simulations covers present-day virial masses that range from 8.3 × 10 11 M ⊙ to 1.56 × 10 12 M ⊙ and is carried out with our simple but effective deterministic star formation (SF) and "explosive" stellar feedback prescriptions. The work is focused on showing the goodness of the simulated set of "field" Milky Way-sized galaxies. To this end, we compare some of the predicted physical quantities with the corresponding observed ones. Our results are as follows. (a) In agreement with some previous works, we found circular velocity curves that are flat or slightly peaked. (b) All simulated galaxies with a significant disk component are consistent with the observed Tully-Fisher, radius-mass, and cold gas-stellar mass correlations of latetype galaxies. (c) The disk-dominated galaxies have stellar specific angular momenta in agreement with those of late-type galaxies, with values around 10 3 km/s/kpc. (d) The SF rates at z = 0 of all runs but one are comparable to those estimated for the star-forming galaxies. (e) The two most spheroid-dominated galaxies formed in halos with late active merger histories and late bursts of SF, but the other run that ends also as dominated by an spheroid, never had major mergers. (f) The simulated galaxies lie in the semi-empirical stellar-to-halo mass correlation of local central galaxies, and those that end up as disk dominated, evolve mostly along the low-mass branch of this correlation. Moreover, the baryonic and stellar mass growth histories of these galaxies are proportional to their halo mass growth histories since the last 6.5-10 Gyr. (g) Within the virial radii of the simulations, ≈ 25 − 50% of the baryons are missed; the amount of gas in the halo is similar to the one in stars in the galaxy, and most of this gas is in the warm-hot phase. (h) The z ∼ 0 vertical gas velocity dispersion profiles, σ z (r), are nearly flat and can be mostly explained by the kinetic energy injected by stars. The average values of σ z increase at higher redshifts, following roughly the shape of the SF history.