Shaping Disk Galaxy Stellar Populations via Internal and External Processes

In recent years, effects such as the radial migration of stars in disks have been recognized as important drivers of the properties of stellar populations. Radial migration arises due to perturbative effects of disk structures such as bars and spiral arms, and can deposit stars formed in disks to regions far from their birthplaces. Migrant stars can significantly affect the demographics of their new locales, especially in low-density regions such as in the outer disks. However, in the cosmological environment, other effects such as mergers and filamentary gas accretion also influence the disk formation process. Understanding the relative importance of these processes on the detailed evolution of stellar population signatures is crucial for reconstructing the history of the Milky Way and other nearby galaxies. In the Milky Way disk in particular, the formation of the thickened component has recently attracted much attention due to its potential to serve as a diagnostic of the galaxy's early history. Some recent work suggests, however, that the vertical structure of Milky Way stellar populations is consistent with models that build up the thickened component through migration. I discuss these developments in the context of cosmological galaxy formatio

A Systematic Look at the Effects of Radiative Feedback on Disc Galaxy Formation

Galaxy formation models and simulations rely on various feedback mechanisms to reproduce the observed baryonic scaling relations and galaxy morphologies. Although dwarf galaxy and giant elliptical properties can be explained using feedback from supernova and active galactic nuclei, Milky Way-sized galaxies still represent a challenge to current theories of galaxy formation. In this paper, we explore the possible role of feedback from stellar radiation in regulating the main properties of disk galaxies such as our own Milky Way. We have performed a suite of cosmological simulations of the same $\sim10^{12} {\rm M}_{\odot}$ halo selected based on its rather typical mass accretion history. We have implemented radiative feedback from young stars using a crude model of radiative transfer for ultraviolet (UV) and infrared (IR) radiation. However, the model is realistic enough such that the dust opacity plays a direct role in regulating the efficiency of our feedback mechanism. We have explored various models for the dust opacity, assuming different constant dust temperatures, as well as a varying dust temperature model. We find that while strong radiative feedback appears as a viable mechanism to regulate the stellar mass fraction in massive galaxies, it also prevents the formation of discs with reasonable morphologies. In models with strong stellar radiation feedback, stellar discs are systematically too thick while the gas disc morphology is completely destroyed due to the efficient mixing between the feedback-affected gas and its surroundings. At the resolution of our simulation suite, we find it impossible to preserve spiral disc morphology while at the same time expelling enough baryons to satisfy the abundance matching constraints.Comment: accepted to MNRA

Probing the shape and history of the Milky Way halo with orbital spectral analysis

Accurate phase-space coordinates (three components of position and velocity) of individual stars are rapidly becoming available with current and future resolved star surveys. These data will enable the computation of the full three-dimensional orbits of tens of thousands of stars in the Milky Way’s stellar halo. We demonstrate that the analysis of stellar halo orbits in frequency space can be used to construct a ‘frequency map’ which provides a highly compact, yet intuitively informative way to represent the six-dimensional halo phase-space distribution function. This representation readily reveals the most important major orbit families in the halo, and the relative abundances of the different orbit families, which in turn reflect the shape and orientation of the dark matter halo relative to the disc. We demonstrate the value of frequency space orbit analysis by applying the method to halo orbits in a series of controlled simulations of disc galaxies. We show that the disc influences the shape of the inner halo making it nearly oblate, but the outer halo remains largely unaffected. Since the shape of the halo varies with radius, the frequency map provides a more versatile way to identify major and minor orbit families than traditional orbit classification schemes. Although the shape of the halo varies with radius, frequency maps of local samples of halo orbits confined to the inner halo contain most of the information about the global shape of the halo and its major orbit families. Frequency maps show that adiabatic growth of a disc traps halo orbits in numerous resonant orbit families (i.e. having commensurable frequencies). The locations and strengths of these resonant families are determined by both the global shape of the halo and its stellar distribution function. If a good estimate of the Galactic potential in the inner halo (within ∼ 50 kpc) is available, the appearance of strong, stable resonances in frequency maps of halo orbits will allow us to determine the degree of resonant trapping induced by the disc potential. We show that if the Galactic potential is not known exactly, a measure of the diffusion rate of a large sample of ∼ 104 halo orbits can help distinguish between the true potential and an incorrect potential. The orbital spectral analysis methods described in this paper provide a strong complementarity to existing methods for constraining the potential of the Milky Way halo and its stellar distribution function

The effect of stellar feedback on a Milky Way-like galaxy and its gaseous halo

We present the study of a set of N-body+smoothed particle hydrodynamics simulations of a Milky Way-like system produced by the radiative cooling of hot gas embedded in a dark matter halo. The galaxy and its gaseous halo evolve for 10 Gyr in isolation, which allows us to study how internal processes affect the evolution of the system. We show how the morphology, the kinematics and the evolution of the galaxy are affected by the input supernova feedback energy ESN, and we compare its properties with those of the Milky Way. Different values of ESN do not significantly affect the star formation history of the system, but the disc of cold gas gets thicker and more turbulent as feedback increases. Our main result is that, for the highest value of ESN considered, the galaxy shows a prominent layer of extraplanar cold (log (T/K) < 4.3) gas extended up to a few kiloparsec above the disc at column densities of 1019 cm-2. The kinematics of this material is in agreement with that inferred for the H I haloes of our Galaxy and NGC 891, although its mass is lower. Also, the location, the kinematics and the typical column densities of the hot (5.3 < log (T/K) < 5.7) gas are in good agreement with those determined from the O VI absorption systems in the halo of the Milky Way and external galaxies. In contrast with the observations, however, gas at log (T/K) < 5.3 is lacking in the circumgalactic region of our systems

Probing the shape and history of the Milky Way halo with orbital spectral analysis

Accurate phase-space coordinates (three components of position and velocity) of individual stars are rapidly becoming available with current and future resolved star surveys. These data will enable the computation of the full three-dimensional orbits of tens of thousands of stars in the Milky Way's stellar halo. We demonstrate that the analysis of stellar halo orbits in frequency space can be used to construct a ‘frequency map' which provides a highly compact, yet intuitively informative way to represent the six-dimensional halo phase-space distribution function. This representation readily reveals the most important major orbit families in the halo, and the relative abundances of the different orbit families, which in turn reflect the shape and orientation of the dark matter halo relative to the disc. We demonstrate the value of frequency space orbit analysis by applying the method to halo orbits in a series of controlled simulations of disc galaxies. We show that the disc influences the shape of the inner halo making it nearly oblate, but the outer halo remains largely unaffected. Since the shape of the halo varies with radius, the frequency map provides a more versatile way to identify major and minor orbit families than traditional orbit classification schemes. Although the shape of the halo varies with radius, frequency maps of local samples of halo orbits confined to the inner halo contain most of the information about the global shape of the halo and its major orbit families. Frequency maps show that adiabatic growth of a disc traps halo orbits in numerous resonant orbit families (i.e. having commensurable frequencies). The locations and strengths of these resonant families are determined by both the global shape of the halo and its stellar distribution function. If a good estimate of the Galactic potential in the inner halo (within ∼ 50 kpc) is available, the appearance of strong, stable resonances in frequency maps of halo orbits will allow us to determine the degree of resonant trapping induced by the disc potential. We show that if the Galactic potential is not known exactly, a measure of the diffusion rate of a large sample of ∼ 104 halo orbits can help distinguish between the true potential and an incorrect potential. The orbital spectral analysis methods described in this paper provide a strong complementarity to existing methods for constraining the potential of the Milky Way halo and its stellar distribution functio

Stellar and Gaseous Nuclear Disks Observed in Nearby (U)LIRGs

We present near-infrared integral field spectroscopy of the central kiloparsec of 17 nearby luminous and ultra-luminous infrared galaxies undergoing major mergers. These observations were taken with OSIRIS assisted by the Keck I and II Adaptive Optics systems, providing spatial resolutions of a few tens of parsecs. The resulting kinematic maps reveal gas disks in at least 16 out of 19 nuclei and stellar disks in 11 out of 11 nuclei observed in these galaxy merger systems. In our late-stages mergers, these disks are young (stellar ages $<30$ Myr) and likely formed as gas disks which became unstable to star formation during the merger. On average, these disks have effective radii of a few hundred parsecs, masses between $10^{8}$ and $10^{10} M_{Sun}$, and $v/\sigma$ between 1 and 5. These disks are similar to those created in high-resolution hydrodynamical simulations of gas-rich galaxy mergers, and favor short coalescence times for binary black holes. The few galaxies in our sample in earlier stages of mergers have disks which are larger ($r_{eff}\sim200-1800$ pc) and likely are remnants of the galactic disks that have not yet been completely disrupted by the merger.Comment: accepted for publication in Ap

Orbital decay of supermassive black hole binaries in clumpy multiphase merger remnants

We simulate an equal-mass merger of two Milky Way-size galaxy discs with moderate gas fractions at parsec-scale resolution including a new model for radiative cooling and heating in a multiphase medium, as well as star formation and feedback from supernovae. The two discs initially have a 2.6×106 M⊙ supermassive black hole (SMBH) embedded in their centres. As the merger completes and the two galactic cores merge, the SMBHs form a pair with a separation of a few hundred pc that gradually decays. Due to the stochastic nature of the system immediately following the merger, the orbital plane of the binary is significantly perturbed. Furthermore, owing to the strong starburst the gas from the central region is completely evacuated, requiring ∼10Myr for a nuclear disc to rebuild. Most importantly, the clumpy nature of the interstellar medium has a major impact on the dynamical evolution of the SMBH pair, which undergo gravitational encounters with massive gas clouds and stochastic torquing by both clouds and spiral modes in the disc. These effects combine to greatly delay the decay of the two SMBHs to separations of a few parsecs by nearly two orders of magnitude, ∼108yr, compared to previous work. In mergers of more gas-rich, clumpier galaxies at high redshift stochastic torques will be even more pronounced and potentially lead to stronger modulation of the orbital decay. This suggests that SMBH pairs at separations of several tens of parsecs should be relatively common at any redshif

The origin of type I profiles in cluster lenticulars: an interplay between ram pressure stripping and tidally induced spiral migration

Using N-body + smooth particle hydrodynamics simulations of galaxies falling into a cluster, we study the evolution of their radial density profiles. When evolved in isolation, galaxies develop a type II (down-bending) profile. In the cluster, the evolution of the profile depends on the minimum cluster-centric radius the galaxy reaches, which controls the degree of ram pressure stripping. If the galaxy falls to ∼50 per cent of the virial radius, then the profile remains type II, but if the galaxy reaches down to ∼20 per cent of the virial radius, the break weakens and the profile becomes more type I like. The velocity dispersions are only slightly increased in the cluster simulations compared with the isolated galaxy; random motion therefore cannot be responsible for redistributing material sufficiently to cause the change in the profile type. Instead, we find that the joint action of radial migration driven by tidally induced spirals and the outside-in quenching of star formation due to ram pressure stripping alters the density profile. As a result, this model predicts a flattening of the age profiles amongst cluster lenticulars with type I profiles, which can be observationally tested

The formation of stellar nuclear discs in bar-induced gas inflows

The role of gas in the mass assembly at the nuclei of galaxies is still subject to some uncertainty. Stellar nuclear discs bridge the gap between the large-scale galaxy and the central massive objects that reside there. Using a high-resolution simulation of a galaxy forming out of gas cooling and settling into a disc, we study the formation and properties of nuclear discs. Gas, driven to the centre by a bar, settles into a rotating star-forming nuclear disc (ND). This ND is thinner, younger, kinematically cooler and more metal rich than the surrounding bar. The ND is elliptical and orthogonal to the bar. The complex kinematics in the region of the ND are a result of the superposition of older stars streaming along the bar and younger stars circulating within the ND. The signature of the ND is therefore subtle in the kinematics. Instead the ND stands out clearly in metallicity and age maps. We compare the model to the density and kinematics of real galaxies with NDs finding qualitative similarities. Our results suggest that gas dissipation is very important for forming nuclear structure