Victor P. Debattista

Constraints from Dynamical Friction on the Dark Matter Content of Barred Galaxies

We show that bars in galaxy models having halos of moderate density and a variety of velocity distributions all experience a strong drag from dynamical friction unless the halo has large angular momentum in the same sense as the disk. The frictional drag decreases the bar pattern speed, driving the corotation point out to distances well in excess of those estimated in barred galaxies. The halo angular momentum required to avoid strong braking is unrealistically large, even when rotation is confined to the inner halo only. We conclude, therefore, that bars are able to maintain their observed high pattern speeds only if the halo has a central density low enough for the disk to provide most of the central attraction in the inner galaxy. We present evidence that this conclusion holds for all bright galaxies.

The Secular Evolution of Disk Structural Parameters

We present a comprehensive series of simulations to study the secular evolution of disk galaxies expected in a ΛCDM universe. Our simulations are organized in a hierarchy of increasing complexity, ranging from rigid-halo collisionless simulations to fully live simulations with gas and star formation. Our goal is to examine which structural properties of disk galaxies may result from secular evolution rather than from hierarchical assembly. In the vertical direction, we find that various mechanisms lead to heating, the strongest of which is the buckling instability of a bar, which leads to peanut-shaped bulges; these can be recognized face-on even in the presence of gas. We find that bars are robust structures that survive buckling and require a large (~20% of the total mass of the disk) central mass concentration to be destroyed. This can occur in dissipative simulations, where bars induce strong gas inflows, but requires that radiative cooling overcome heating. We show how angular momentum redistribution leads to increasing central densities and disk scale lengths and to profile breaks at large radii. The breaks in these simulations are in excellent agreement with observations, even when the evolution is collisionless. Disk scale lengths increase even when the total disk angular momentum is conserved; thus, mapping halo angular momenta to scale lengths is nontrivial. A decomposition of the resulting profile into a bulge+disk gives structural parameters in reasonable agreement with observations although kinematics betrays their bar nature. These findings have important implications for galaxy formation models, which have so far ignored or introduced in a very simplified way the effects of nonaxisymmetric instabilities on the morphological evolution of disk galaxies.

Beyond Inside-Out Growth: Formation and Evolution of Disk Outskirts

We have performed a high mass and force resolution simulation of an idealized galaxy forming from dissipational collapse of gas embedded in a spherical dark matter halo. The simulation includes star formation and effects of stellar feedback. In our simulation a stellar disk forms with a surface density profile consisting of an inner exponential breaking to a steeper outer exponential. The break forms early on and persists throughout the evolution, moving outward as more gas is able to cool and add mass to the disk. The parameters of the break are in excellent agreement with observations. The break corresponds to a rapid drop in the star formation rate associated with a drop in the cooled gas surface density, but the outer exponential is populated by stars that were scattered outward on nearly circular orbits from the inner disk by spiral arms. The resulting profile and its associated break are therefore a consequence of the interplay between a radial star formation cutoff and redistribution of stellar mass by secular processes. A consequence of such evolution is a sharp change in the radial mean stellar age profile at the break radius.

Thin, thick and dark discs in ΛCDM

In acold dark matter (� CDM) cosmology, the Milky Way accretes satellites into the stellar disc. We use cosmological simulations to assess the frequency of near disc plane and higher inclination accretion events, and collisionless simulations of satellite mergers to quantify the final state of the accreted material and the effect on the thin disc. On average, a Milky Way-sized galaxy has three subhaloes with vmax > 80 km s −1 ; seven with vmax > 60 km s −1 and 15 with vmax > 40 km s −1 merge at redshift z 1. Assuming isotropic accretion, a third of these merge at an impact angle θ 20 ◦ are twice as likely as low-inclination ones. These lead to structures that closely resemble the recently discovered inner and outer stellar haloes. They also do more damage to the Milky Way stellar disc creating a more pronounced flare, and warp; both long-lived and consistent with current observations. The most massive mergers (vmax 80 km s −1 ) heat the thin disc enough to produce a thick disc. These heated thin-disc stars are essential for obtaining a thick disc as massive as that seen in the Milky Way; they likely comprise some ∼50-90 per cent of the thick disc stars. The Milky Way thin disc must reform from fresh gas after z = 1. Only one in four of our sample Milky Way haloes experiences mergers massive and late enough to fully destroy the thin disc. We conclude that thick, thin and dark discs occur naturally within aCDM cosmology.

DYNAMICAL FRICTION AND THE DISTRIBUTION OF DARK MATTER IN BARRED GALAXIES

We use fully self-consistent N-body simulations of barred galaxies to show that dynamical friction from a dense dark matter halo dramatically slows the rotation rate of bars. Our result supports previous theoretical predictions for a bar rotating within a massive halo. On the other hand, low-density halos, such as those required for maximum disks, allow the bar to continue to rotate at a high rate. There is somewhat meager observational evidence indicating that bars in real galaxies do rotate rapidly, and we use our result to argue that dark matter halos must have a low central density in all high surface brightness disk galaxies, including the Milky Way. Bars in galaxies that have larger fractions of dark matter should rotate slowly, and we suggest that a promising place to look for such candidate objects is among galaxies of intermediate surface brightness.

THE GENESIS OF THE MILKY WAY'S THICK DISK VIA STELLAR MIGRATION

We compare the spatial, kinematic, and metallicity distributions of stars in the Milky Way disk, as observed by the Sloan Digital Sky Survey and Geneva-Copenhagen Survey, to predictions made by N-body simulations that naturally include radial migration as proposed by Sellwood & Binney. In these simulations, stars that migrate radially outward feel a decreased restoring force, consequentially they reach larger heights above the mid-plane. We find that this model is in qualitative agreement with observational data and can explain the disks double-exponential vertical structure and other characteristics as due to internal evolution. In particular, the model reproduces observations of stars in the transition region between exponential components, which do not show a strong correlation between rotational velocity and metallicity. Although such a correlation is present in young stars because of epicyclic motions, radial migration efficiently mixes older stars and weakens the correlation. Classifying stars as members of the thin or thick disk by either velocity or metallicity leads to an apparent separation in the other property, as observed. We find a much stronger separation when using [α/Fe], which is a good proxy for stellar age. The model success is remarkable because the simulation was not tuned to reproduce the Galaxy, hinting that the thick disk may be a ubiquitous Galactic feature generated by stellar migration. Nonetheless, we cannot exclude the possibility that some fraction of the thick disk is a fossil of a more violent history, nor can radial migration explain thick disks in all galaxies, most strikingly those which counterrotate with respect to the thin disk.

Morphological evolution of discs in clusters

The recent discovery of hidden non-axisymmetric and disc-like structures in bright Virgo dwarf elliptical and lenticular galaxies (dE/dSph/dS0) indicates that they may have late-type progenitors. Using N-body simulations we follow the evolution of disc galaxies within a A cold dark matter (ACDM) cluster simulated with 10 7 particles, where the hierarchical growth and galaxy harassment are modelled self-consistently. Most of the galaxies undergo significant morphological transformation, even at the outskirts of the cluster, and move through the Hubble sequence from late-type discs to dwarf spheroidals. None of the discs is completely destroyed, therefore they cannot be the progenitors of ultracompact dwarf (UCD) galaxies. The time evolution of the simulated galaxies is compared with unsharp masked images obtained from Very Large Telescope (VLT) data and the projected kinematics of our models with the latest high-resolution spectroscopic studies from the Keck and Palomar telescopes.

THE CAUSES OF HALO SHAPE CHANGES INDUCED BY COOLING BARYONS : DISKS VERSUS SUBSTRUCTURES

Cold dark matter cosmogony predicts triaxial dark matter halos, whereas observations find quite round halos. This is most likely due to the condensation of baryons leading to rounder halos. We examine the halo phase space distribution basis for such shape changes. Triaxial halos are supported by box orbits, which pass arbitrarily close to the density center. The decrease in triaxiality caused by baryons is thought to be due to the scattering of these orbits. We test this hypothesis with simulations of disks grown inside triaxial halos. After the disks are grown we check whether the phase space structure has changed by evaporating the disks and comparing the initial and final states. While the halos are substantially rounder when the disk is at full mass, their final shape after the disk is evaporated is not much different from the initial. Likewise, the halo becomes (more) radially anisotropic when the disk is grown, but the final anisotropy is consistent with the initial. Only if the baryons are unreasonably compact or massive does the halo change irreversibly. We show that the character of individual orbits is not generally changed by the growing mass. Thus, the central condensation of baryons does not destroy enough box orbits to cause the shape change. Rather, box orbits merely become rounder along with the global potential. However, if angular momentum is transferred to the halo, either via satellites or via bars, a large irreversible change in the halo distribution occurs. The ability of satellites to alter the phase space distribution of the halo is of particular concern to galaxy formation simulations since halo triaxiality can profoundly influence the evolution of disks.

The NGC 404 Nucleus: Star Cluster and Possible Intermediate-mass Black Hole

We examine the nuclear morphology, kinematics, and stellar populations in nearby S0 galaxy NGC 404 using a combination of adaptive optics assisted near-IR integral-field spectroscopy, optical spectroscopy, and Hubble Space Telescope imaging. These observations enable study of the NGC 404 nucleus at a level of detail possible only in the nearest galaxies. The surface brightness profile suggests the presence of three components: a bulge, a nuclear star cluster (NSC), and a central light excess within the cluster at radii < 3 pc. These components have distinct kinematics with modest rotation seen in the NSC and counter-rotation seen in the central excess. Molecular hydrogen emission traces a disk with rotation nearly orthogonal to that of the stars. The stellar populations of the three components are also distinct, with half of the mass of the NSC having ages of ~1 Gyr (perhaps resulting from a galaxy merger), while the bulge is dominated by much older stars. Dynamical modeling of the stellar kinematics gives a total NSC mass of 1.1 × 107 M ☉. Dynamical detection of a possible intermediate-mass black hole (BH) is hindered by uncertainties in the central stellar mass profile. Assuming a constant mass-to-light ratio, the stellar dynamical modeling suggests a BH mass of <1 × 105 M ☉, while the molecular hydrogen gas kinematics are best fitted by a BH with a mass of 4.5+3.5 –2.0 × 105 M ☉. Unresolved and possibly variable dust emission in the near-infrared and active galactic nucleus-like molecular hydrogen emission-line ratios do suggest the presence of an accreting BH in this nearby LINER galaxy.

Radial migration in disc galaxies – I. Transient spiral structure and dynamics

We seek to understand the origin of radial migration in spiral galaxies by analysing in detail the structure and evolution of an idealized, isolated galactic disc. To understand the redistribution of stars, we characterize the time evolution of properties of spirals that spontaneously form in the disc. Our models unambiguously show that in such discs, single spirals are unlikely, but that a number of transient patterns may coexist in the disc. However, we also show that while spirals are transient in amplitude, at any given time the disc favours patterns of certain pattern speeds. Using several runs with different numerical parameters we show that the properties of spirals that occur spontaneously in the disc do not sensitively depend on resolution. The existence of multiple transient patterns has large implications for the orbits of stars in the disc, and we therefore examine the resonant scattering mechanisms that profoundly alter angular momenta of individual stars. We confirm that the corotation scattering mechanism described by Sellwood & Binney is responsible for the largest angular momentum changes in our simulations.