Simulations of the formation of late-type spiral galaxies in a cold dark matter (ΛCDM) universe have traditionally failed to yield realistic candidates. Here we report a new cosmological N-body/smooth particle hydrodynamic simulation of extreme dynamic range in which a close analog of a Milky Way disk galaxy arises naturally. Named "Eris," the simulation follows the assembly of a galaxy halo of mass M vir = 7.9 × 10 11 M with a total of N = 18.6 million particles (gas + dark matter + stars) within the final virial radius, and a force resolution of 120 pc. It includes radiative cooling, heating from a cosmic UV field and supernova explosions (blastwave feedback), a star formation recipe based on a high gas density threshold (n SF = 5 atoms cm −3 rather than the canonical n SF = 0.1 atoms cm −3 ), and neglects any feedback from an active galactic nucleus. Artificial images are generated to correctly compare simulations with observations. At the present epoch, the simulated galaxy has an extended rotationally supported disk with a radial scale length R d = 2.5 kpc, a gently falling rotation curve with circular velocity at 2.2 disk scale lengths of V 2.2 = 214 km s −1 , an i-band bulge-to-disk ratio B/D = 0.35, and a baryonic mass fraction within the virial radius that is 30% below the cosmic value. The disk is thin, has a typical H I-to-stellar mass ratio, is forming stars in the region of the Σ SFR -Σ H i plane occupied by spiral galaxies, and falls on the photometric Tully-Fisher and the stellar-mass-halo-virial-mass relations. Hot (T > 3 × 10 5 K) X-ray luminous halo gas makes up only 26% of the universal baryon fraction and follows a "flattened" density profile ∝ r −1.13 out to r = 100 kpc. Eris appears then to be the first cosmological hydrodynamic simulation in which the galaxy structural properties, the mass budget in the various components, and the scaling relations between mass and luminosity are all consistent with a host of observational constraints. A twin simulation with a low star formation density threshold results in a galaxy with a more massive bulge and a much steeper rotation curve, as in previously published work. A high star formation threshold appears therefore key in obtaining realistic late-type galaxies, as it enables the development of an inhomogeneous interstellar medium where star formation and heating by supernovae occur in a clustered fashion. The resulting outflows at high redshifts reduce the baryonic content of galaxies and preferentially remove low-angular-momentum gas, decreasing the mass of the bulge component. Simulations of even higher resolution that follow the assembly of galaxies with different merger histories shall be used to verify our results.

We analyze the present-day structure and assembly history of a high resolution hydrodynamic simulation of the formation of a Milky Way (MW)-like disk galaxy, from the "Eris" simulation suite, dissecting it into cohorts of stars formed at different epochs of cosmic history. At z = 0, stars with t form < 2 Gyr mainly occupy the stellar spheroid, with the oldest (earliest forming) stars having more centrally concentrated profiles. The younger age cohorts populate disks of progressively longer radial scale length and shorter vertical scale height. At a given radius, the vertical density profiles and velocity dispersions of stars vary smoothly as a function of age, and the superposition of old, vertically-extended and young, vertically-compact cohorts gives rise to a double-exponential profile like that observed in the MW. Turning to formation history, we find that the trends of spatial structure and kinematics with stellar age are largely imprinted at birth, or immediately thereafter. Stars that form during the active merger phase at z > 3 are quickly scattered into rounded, kinematically hot configurations. The oldest disk cohorts form in structures that are radially compact and relatively thick, while subsequent cohorts form in progressively larger, thinner, colder configurations from gas with increasing levels of rotational support. The disk thus forms "inside-out" in a radial sense and "upside-down" in a vertical sense. Secular heating and radial migration influence the final state of each age cohort, but the changes they produce are small compared to the trends established at formation. The predicted correlations of stellar age with spatial and kinematic structure are in good qualitative agreement with the correlations observed for mono-abundance stellar populations in the MW.

Observations of distant quasars indicate that supermassive black holes of billions of solar masses already existed less than a billion years after the Big Bang. Models in which the 'seeds' of such black holes form by the collapse of primordial metal-free stars cannot explain the rapid appearance of these supermassive black holes because gas accretion is not sufficiently efficient. Alternatively, these black holes may form by direct collapse of gas within isolated protogalaxies, but current models require idealized conditions, such as metal-free gas, to prevent cooling and star formation from consuming the gas reservoir. Here we report simulations showing that mergers between massive protogalaxies naturally produce the conditions for direct collapse into a supermassive black hole with no need to suppress cooling and star formation. Merger-driven gas inflows give rise to an unstable, massive nuclear gas disk of a few billion solar masses, which funnels more than 10(8) solar masses of gas to a sub-parsec-scale gas cloud in only 100,000 years. The cloud undergoes gravitational collapse, which eventually leads to the formation of a massive black hole. The black hole can subsequently grow to a billion solar masses on timescales of about 10(8) years by accreting gas from the surrounding disk.

The tidal stirring model posits the formation of dwarf spheroidal galaxies (dSphs) via the tidal interactions between late-type, rotationally-supported dwarfs and Milky Way-sized host galaxies. Using a comprehensive set of collisionless N-body simulations, we investigate the efficiency of the tidal stirring mechanism for the origin of dSphs. In particular, we examine the degree to which the tidal field of the primary galaxy affects the sizes, masses, shapes, and kinematics of the disky dwarfs for a range of dwarf orbital and structural parameters. Our study is the first to employ self-consistent, equilibrium models for the progenitor dwarf galaxies constructed from a composite distribution function and consisting of exponential stellar disks embedded in massive, cosmologically-motivated dark matter halos. Exploring a wide variety of dwarf orbital configurations and initial structures, we demonstrate that in the majority of cases the disky dwarfs experience significant mass loss and their stellar distributions undergo a dramatic morphological, as well as dynamical, transformation. Specifically, the stellar components evolve from disks to bars and finally to pressure-supported, spheroidal systems with kinematic and structural properties akin to those of the classic dSphs in the Local Group (LG) and similar environments. The self-consistency of the adopted dwarf models is crucial for confirming this complex transformation process via tidally-induced dynamical instabilities and impulsive tidal heating of the stellar distribution. Our results suggest that such tidal transformations should be common occurrences within the currently favored cosmological paradigm and highlight the key factor responsible for an effective metamorphosis to be the strength of the tidal shocks at the pericenters of the orbit. We also demonstrate that the combination of short orbital times and small pericentric distances, characteristic of dwarfs being accreted by their hosts at high redshift, induces the strongest and most complete transformations. Our models also indicate that the efficiency of the transformation via tidal stirring is affected significantly by the structure of the progenitor disky dwarfs. While the mass-to-light ratios, M/L, of the dwarf galaxies typically decrease monotonically with time as the extended dark matter halos are efficiently tidally stripped, we identify a few cases where this trend is reversed later in the evolution when stellar mass loss becomes more effective. We also find that the dwarf remnants satisfy the relation V max = √ 3 σ * , where σ * is the one-dimensional, central stellar velocity dispersion and V max is the maximum halo circular velocity, which has intriguing implications for the missing satellites problem. Assuming that the distant dSphs in the LG, such as Leo I, Tucana, and Cetus are the products of tidal stirring, our findings suggest that these galaxies should have only been partially stirred by the tidal field of their hosts. We thus predict that these remote dwarfs should exhibit higher val...

Supermassive black holes (SMBHs) have been detected in the centers of most nearby massive galaxies. Galaxies today are not only the products of billions of years of galaxy mergers, but also billions of years of SMBH activity as active galactic nuclei (AGNs) that is connected to galaxy mergers. In this context, detection of AGN pairs should be relatively common. Observationally, however, dual AGNs are scant, being just a few percent of all AGNs. In this Letter, we investigate the triggering of AGN activity in merging galaxies via a suite of highresolution hydrodynamical simulations. We follow the dynamics and accretion onto the SMBHs as they move from separations of tens of kiloparsecs to tens of parsecs. Our resolution, cooling, and star formation implementation produce an inhomogeneous, multi-phase interstellar medium, allowing us to accurately trace star formation and accretion onto the SMBHs. We study the impact of gas content, morphology, and mass ratio, focusing on AGN activity and dynamics across a wide range of relevant conditions. We test when the two AGNs are simultaneously detectable, for how long and at which separations. We find that strong dual AGN activity occurs during the late phases of the mergers, at small separations (<1-10 kpc) below the resolution limit of most surveys. Much of the SMBH accretion is not simultaneous, limiting the dual AGN fraction detectable through imaging and spectroscopy to a few percent, in agreement with observational samples.

We examine the pairing process of supermassive black holes (SMBHs) down to scales of 20-100 pc using a set of N-body/SPH simulations of binary mergers of disk galaxies with mass ratios of 1:4 and 1:10. Our numerical experiments are designed to represent merger events occurring at various cosmic epochs. The initial conditions of the encounters are consistent with the ΛCDM paradigm of structure formation, and the simulations include the effects of radiative cooling, star formation (SF), and supernovae feedback. We find that the pairing of SMBHs depends sensitively on the amount of baryonic mass preserved in the center of the companion galaxies during the last phases of the merger. In particular, due to the combination of gasdynamics and SF, we find that a pair of SMBHs can form efficiently in 1:10 minor mergers, provided that galaxies are relatively gas-rich (gas fractions of 30% of the disk mass) and that the mergers occur at relatively high redshift (z ∼ 3), when dynamical friction timescales are shorter. Since 1:10 mergers are most common events during the assembly of galaxies, and mergers are more frequent at high redshift when galaxies are also more gas-rich, our results have positive implications for future gravitational wave experiments such as the Laser Interferometer Space Antenna.

We perform a suite of high-resolution smoothed particle hydrodynamics simulations to investigate the orbital decay and mass evolution of massive black hole (MBH) pairs down to scales of ∼ 30 pc during minor mergers of disk galaxies. Our simulation set includes star formation and accretion onto the MBHs, as well as feedback from both processes. We consider 1:10 merger events starting at z ∼ 3, with MBH masses in the sensitivity window of the Laser Interferometer Space Antenna, and we follow the coupling between the merger dynamics and the evolution of the MBH mass ratio until the satellite galaxy is tidally disrupted. While the more massive MBH accretes in most cases as if the galaxy were in isolation, the satellite MBH may undergo distinct episodes of enhanced accretion, owing to strong tidal torques acting on its host galaxy and to orbital circularization inside the disk of the primary galaxy. As a consequence, the initial 1:10 mass ratio of the MBHs changes by the time the satellite is disrupted. Depending on the initial fraction of cold gas in the galactic disks and the geometry of the encounter, the mass ratios of the MBH pairs at the time of satellite disruption can stay unchanged or become as large as 1:2. Remarkably, the efficiency of MBH orbital decay correlates with the final mass ratio of the pair itself: MBH pairs that increase significantly their mass ratio are also expected to inspiral more promptly down to nuclear-scale separations. These findings indicate that the mass ratios of MBH pairs in galactic nuclei do not necessarily trace the mass ratios of their merging host galaxies, but are determined by the complex interplay between gas accretion and merger dynamics.

The asymmetric emission of gravitational waves produced during the coalescence of a massive black hole (MBH) binary imparts a velocity "kick" to the system that can displace the hole from the center of its host. Here we study the trajectories and observability of MBHs recoiling in three (one major, two minor) gas-rich galaxy merger remnants that were previously simulated at high resolution, and in which the pairing of the MBHs had been shown to be successful. We run new simulations of MBHs recoiling in the major merger remnant with Mach numbers in the range 1 ≤ M ≤ 6, and use simulation data to construct a semi-analytical model for the orbital evolution of MBHs in gas-rich systems. We show that: 1) in major merger remnants the energy deposited by the moving hole into the rotationally supported, turbulent medium makes a negligible contribution to the thermodynamics of the gas. This contribution becomes significant in minor merger remnants, potentially allowing for an electromagnetic signature of MBH recoil; 2) in major merger remnants, the combination of both deeper central potential well and drag from high-density gas confines even MBHs with kick velocities as high as 1, 200 km s −1 within 1 kpc from the host's center; 3) kinematically offset nuclei may be observable for timescales of a few Myr in major merger remnants in the case of recoil velocities in the range 700 − 1, 000 km s −1 ; 4) in minor mergers remnants the effect of gas drag is weaker, and MBHs with recoil speeds in the range 300 − 600 km s −1 will wander through the host halo for longer timescales. When accounting for the probability distribution of kick velocities, however, we find that the likelihood of observing recoiling MBHs in gas-rich galaxy mergers is very low, typically below 10 −5 − 10 −6 .