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.

Mergers and tidal interactions between massive galaxies and their dwarf satellites are a fundamental prediction of the Lambda-Cold Dark Matter cosmology. These events are thought to provide important observational diagnostics of nonlinear structure formation. Stellar streams in the Milky Way and Andromeda are spectacular evidence for ongoing satellite disruption. However, constructing a statistically meaningful sample of tidal streams beyond the Local Group has proven a daunting observational challenge, and the full potential for deepening our understanding of galaxy assembly using stellar streams has yet to be realised. Here we introduce the Stellar Stream Legacy Survey, a systematic imaging survey of tidal features associated with dwarf galaxy accretion around a sample of ∼ 3100 nearby galaxies within 𝑧 ∼ 0.02, including about 940 Milky Way analogues. Our survey exploits public deep imaging data from the DESI Legacy Imaging Surveys, which reach surface brightness as faint as ∼ 29 mag arcsec −2 in the 𝑟 band. As a proof of concept of our survey, we report the detection and broad-band photometry of 24 new stellar streams in the local Universe. We discuss how these observations can yield new constraints on galaxy formation theory through comparison to mock observations from cosmological galaxy simulations. These tests will probe the present-day mass assembly rate of galaxies, the stellar populations and orbits of satellites, the growth of stellar halos and the resilience of stellar disks to satellite bombardment.

We study the components of cool and warm/hot gas in the circumgalactic medium (CGM) of simulated galaxies and address the relative production of OVI by photoionization versus collisional ionization, as a function of halo mass, redshift, and distance from the galaxy halo center. This is done utilizing two different suites of zoom-in hydro-cosmological simulations, VELA (6 halos; z > 1) and NIHAO (18 halos; to z = 0), which provide a broad theoretical basis because they use different codes and physical recipes for star formation and feedback. In all halos studied in this work, we find that collisional ionization by thermal electrons dominates at high redshift, while photoionization of cool or warm gas by the metagalactic radiation takes over near z ∼ 2. In halos of ∼ 10 12 M and above, collisions become important again at z < 0.5, while photoionization remains significant down to z = 0 for less massive halos. In halos with M v > 3 × 10 11 M , at z ∼ 0 most of the photoionized OVI is in a warm, not cool, gas phase (T 3 × 10 5 K). We also find that collisions are dominant in the central regions of halos, while photoionization is more significant at the outskirts, around R v , even in massive halos. This too may be explained by the presence of warm gas or, in lower mass halos, by cool gas inflows.

We analyse the distribution and origin of O vi in the Circumgalactic Medium (CGM) of dark-matter haloes of ∼1012 M⊙ at z ∼ 1 in the VELA cosmological zoom-in simulations. We find that the O vi in the inflowing cold streams is primarily photoionized, while in the bulk volume it is primarily collisionally ionized. The photoionized component dominates the observed column density at large impact parameters (≳0.3Rvir), while the collisionally ionized component dominates closer in. We find that most of the collisional O vi, by mass, resides in the relatively thin boundaries of the photoionized streams. Thus, we predict that a reason previous work has found the ionization mechanism of O vi so difficult to determine is because the distinction between the two methods coincides with the distinction between two significant phases of the CGM. We discuss how the results are in agreement with analytic predictions of stream and boundary properties, and their compatibility with observations. This allows us to predict the profiles of O vi and other ions in future CGM observations and provides a toy model for interpreting them.