Like many galaxies of its size, the Milky Way is a disk with prominent spiral arms rooted in a central bar, although our knowledge of its structure and origin is incomplete. Traditional attempts to understand our Galaxy's morphology assume that it has been unperturbed by major external forces. Here we report simulations of the response of the Milky Way to the infall of the Sagittarius dwarf galaxy (Sgr), which results in the formation of spiral arms, influences the central bar and produces a flared outer disk. Two ring-like wrappings emerge towards the Galactic anti-Centre in our model that are reminiscent of the low-latitude arcs observed in the same area of the Milky Way. Previous models have focused on Sgr itself to reproduce the dwarf's orbital history and place associated constraints on the shape of the Milky Way gravitational potential, treating the Sgr impact event as a trivial influence on the Galactic disk. Our results show that the Milky Way's morphology is not purely secular in origin and that low-mass minor mergers predicted to be common throughout the Universe probably have a similarly important role in shaping galactic structure.

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.

We calculate multiwavelength spectral energy distributions (SEDs) from simulations of major galaxy mergers with black hole feedback that produce submillimeter bright galaxies (SMGs), using the self-consistent three-dimensional radiative transfer code RADISHE. These calculations allow us to predict multiwavelength correlations for this important class of galaxies. We review star formation rates, the time evolution of the 850 m fluxes, along with the time evolution of the M BH -M ? relation of the SMGs formed in the mergers. We reproduce correlations for local AGNs observed in Spitzer Space Telescope's IRAC bands, and make definitive predictions for infrared X-ray correlations. Our dynamical approach allows us to directly correlate observed clustering in the data as seen in IRAC color-color plots with the relative amount of time the system spends in a region of color-color space. We also find that this clustering is positively correlated with the stars dominating in their contribution to the total bolometric luminosity. We compare our calculated SEDs to observations of SMGs and find good agreement. We introduce a simple, heuristic classification scheme which we present in terms of the L IR /L X ratios of these galaxies, which may be interpreted as an evolutionary scheme, as these galaxies evolve in L IR /L X while transiting from a X-ray underluminous infrared bright phase (class I, L IR /L X k100), through a quasar phase (class II, L IR /L X $ 25), to a merger remnant (class III, L IR /L X P 10). We find that SMGs are a broader class of systems than starbursts or quasars, traversing the range from class I to class II systems.

We present a new analysis of the observed perturbations of the H I disc of the Milky Way to infer the existence of a dark subhalo that tidally interacted with the Milky Way disc. We examine tidal interactions between perturbing dark subhaloes and the gas disc of the Milky Way using high-resolution Smoothed Particle Hydrodynamics simulations. We compare our results to the observed H I map of the Milky Way to find that the Fourier amplitudes of the planar disturbances are best fit by a perturbing dark subhalo with a mass that is one-hundredth of the Milky Way with a pericentric distance of 5 kpc. This best fit to the Fourier modes occurs about a dynamical time after pericentric approach, when the perturber is 90 kpc from the Galactic Centre. Our analysis here represents a new method to indirectly characterize dark subhaloes from the tidal gravitational imprints they leave on the gaseous discs of galaxies. We also elucidate a fundamental property of parabolic orbits. We show that under certain conditions, one can break the degeneracy between the mass of the perturber and the pericentric distance in the evaluation of the tidal force -to directly determine the mass of the dark perturber that produced the observed disturbances.

We calculate infrared spectral energy distributions (SEDs) from simulations of major galaxy mergers and study the effect of AGN and starburst driven feedback on the evolution of the SED as a function of time. We use a self-consistent three-dimensional radiative equilibrium code to calculate the emergent SEDs and to make images. To facilitate a simple description of our findings, we describe our results in reference to an approximate analytic solution for the far-IR SED. We focus mainly on the luminous infrared galaxy (LIRG) and ultraluminous infrared galaxy (ULIRG) phases of evolution. We contrast the SEDs of simulations performed with AGN feedback to simulations performed with starburst driven wind feedback. We find that the feedback processes critically determine the evolution of the SED. Changing the source of illumination (whether stellar or AGN) has virtually no impact on the reprocessed far-infrared SED. We find that AGN feedback is particularly effective at dispersing gas and rapidly injecting energy into the ISM. The observational signature of such powerful feedback is a warm SED. In general, simulations performed with starburst driven winds have colder spectra and reprocess more of their emission into the infrared, resulting in higher infrared to bolometric luminosities compared to (otherwise equivalent) simulations performed with AGN feedback. We depict our results in IRAS bands, as well as in Spitzer's MIPS bands, and in Herschel's PACS bands.