Heating of galactic discs by infalling satellites (original) (raw)
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The Astrophysical Journal, 2008
We perform a set of fully self-consistent, dissipationless N-body simulations to elucidate the dynamical response of thin galactic disks to bombardment by cold dark matter (CDM) substructure. Our method combines (1) cosmological simulations of the formation of Milky Way (MW)-sized CDM halos to derive the properties of substructure and (2) controlled numerical experiments of consecutive subhalo impacts onto an initially-thin, fully-formed MW type disk galaxy. The present study is the first to account for the evolution of satellite populations over cosmic time in such an investigation of disk structure. In contrast to what can be inferred from statistics of the z = 0 surviving substructure, we find that accretions of massive subhalos onto the central regions of host halos, where the galactic disks reside, since z ∼ 1 should be common. One host halo accretion history is used to initialize the controlled simulations of satellite-disk encounters. The specific merger history involves six dark matter substructures, with initial masses in the range ∼ 20% − 60% of the disk mass and of comparable size to the disk, crossing the central regions of their host host in the past ∼ 8 Gyr. We show that these accretion events severely perturb the thin galactic disk and produce a wealth of distinctive dynamical signatures on its structure and kinematics. These include (1) considerable thickening and heating at all radii, with the disk thickness and velocity ellipsoid nearly doubling at the solar radius; (2) prominent flaring associated with an increase in disk thickness greater than a factor of 4 in the disk outskirts; (3) surface density excesses at large radii, beyond ∼ 5 disk scale lengths, resembling those of observed antitruncated disks; (4) long-lived, lopsidedness at levels similar to those measured in observational samples of disk galaxies; and (5) substantial tilting. The interaction with the most massive subhalo in the simulated accretion history drives the disk response while subsequent bombardment is much less efficient at disturbing the disk. We also explore a variety of disk and satellite properties that influence these responses. We conclude that substructure-disk encounters of the kind expected in the ΛCDM paradigm play a significant role in setting the structure of disk galaxies and driving galaxy evolution.
Simulations of the heating of the Galactic stellar disc
Monthly Notices of the Royal Astronomical Society, 2002
The velocity dispersion of nearby stars in the Galactic disc are well known to increase substantially with age; this is the so-called Age-Velocity relation, and is interpreted as a "heating" of the disc as a function of time. We have studied the heating of the Galactic stellar disc due to giant molecular clouds and halo black holes, via simulations of the orbits of tracer stars embedded in a patch of the local Galactic disc. We examine a range of masses and number densities of the giant molecular cloud and halo black hole perturbers. The heating of the stellar disc in the simulations is fit with a simple power law of the σ ∝ t α where σ is the velocity dispersion of the tracer stars as a function of time, t. We also fit this form to the best determinations of the increase in the velocity dispersion as a function of time as derived from stars in the solar neighbourhood for which ages can be reliably assigned. Observationally, α is found to lie in the range 0.3 to 0.6, i.e. it remains poorly constrained and its determination is probably still dominated by systematic errors. Better constrained observationally is the ratio of the velocity dispersions of the stars in the vertical z and horizontal x (i.e. toward the Galactic center) directions, being σ z /σ x = 0.5 ± 0.1.
Heating of Galaxy Disks by Dark Satellites
Satellites and Tidal …, 2004
Satellites arid Tidal Streams ASP Conference Series, Vol. 3~7, ~OO4 F. Prada, D. MartInez Delgado, and TJ Mahoney Heating of Galaxy Disks by Dark Satellites Andrew J. Benson California Institute of Technology, MC105-24, 1200 E. California Blvd., Pasadena, CA 91125, USA Cedric G. Lacey, Carlos S. Frenk, Shaun Cole, and Canton M. Baugh Institute for Computational Cosmology, University of Durham, Science Laboratories, South Road, Durham DH1 SLE, UK Abstract. We develop an analytic model to calculate the rate at which galaxy disks are ...
The Astrophysical Journal, 2003
We present a detailed analysis of the dynamical properties of a simulated disk galaxy assembled hierarchically in the ΛCDM cosmogony. At z = 0, two distinct dynamical components are easily identified solely on the basis of the orbital parameters of stars in the galaxy: a slowly rotating, centrally concentrated spheroid and a disk-like component largely supported by rotation. These components are also clearly recognized in the surface brightness profile of the galaxy, which can be very well approximated by the superposition of an R 1/4 spheroid and an exponential disk. However, neither does the dynamicallyidentified spheroid follow de Vaucouleurs' law nor is the disk purely exponential, a result which calls for caution when estimating the importance of the disk from traditional photometric decomposition techniques. The disk may be further decomposed into a thin, dynamically cold component with stars on nearly circular orbits and a hotter, thicker component with orbital parameters transitional between the thin disk and the spheroid. Supporting evidence for the presence of distinct thick and thin disk components is found, as in the Milky Way, in the double-exponential vertical structure of the disk and in abrupt changes in the vertical velocity distribution as a function of age. The dynamical origin of these components offers intriguing clues to the assembly of spheroids and disks in the Milky Way and other spirals. The spheroid is old, and has essentially no stars younger than the time elapsed since the last major accretion event; ∼ 8 Gyr ago for the system we consider here. The majority of thin disk stars, on the other hand, form after the merging activity is over, although a significant fraction (∼ 15%) of thin-disk stars are old enough to predate the last major merger event. This unexpected population of old disk stars consists mainly of the tidal debris of satellites whose orbital plane was coincident with the disk and whose orbits were circularized by dynamical friction prior to full disruption. More than half of the stars in the thick disk share this origin, part of a trend that becomes more pronounced with age: nine out of ten stars presently in the old (τ ∼ > 10 Gyr) disk component were actually brought into the disk by satellites. By contrast, only one in two stars belonging to the old spheroid are tidal debris; the rest may be traced to a major merger event that dispersed the luminous progenitor at z ∼ 1.5 and seeded the formation of the spheroid. Our results highlight the role of satellite accretion events in shaping the disk-as well as the spheroidal-component and reveal some of the clues to the assembly process of a galaxy preserved in the detailed dynamics of old stellar populations.
Simulations of Galactic Disks Including a Dark Baryonic Component
2008
The near proportionality between HI and dark matter in outer galactic disks prompted us to run N-body simulations of galactic disks in which the observed gas content is supplemented by a dark gas component representing between zero and five times the visible gas content. While adding baryons in the disk of galaxies may solve some issues, it poses the problem of disk stability. We show that the global stability is ensured if the ISM is multiphased, composed of two partially coupled phases, a visible warm gas phase and a weakly collisionless cold dark phase corresponding to a fraction of the unseen baryons. The phases are subject to stellar and UV background heating and gas cooling, and their transformation into each other is studied as a function of the coupling strength. This new model, which still possesses a dark matter halo, fits the rotation curves as well as the classical CDM halos, but is the only one to explain the existence of an open and contrasting spiral structure, as observed in the outer HI disks
The Astrophysical Journal, 2008
Previous models of galactic disk heating in interactions invoke restrictive assumptions not necessarily valid in modern ΛCDM contexts: that satellites are rigid and orbits are circular, with slow decay over many orbital periods from dynamical friction. This leads to a linear scaling of disk heating with satellite mass: disk heights and velocity dispersions scale ∝ M sat /M disk . In turn, observed disk thicknesses present strong constraints on merger histories: the implication for the Milky Way is that < 5% of its mass could come from mergers since z ∼ 2, in conflict with cosmological predictions. More realistically, satellites merge on nearly radial orbits, and once near the disk, resonant interactions efficiently remove angular momentum while tidal stripping removes mass, leading to rapid merger/destruction in a couple of free-fall plunges. Under these conditions the proper heating efficiency is non-linear in mass ratio, ∝ (M sat /M disk ) 2 . We derive the scaling of disk scale heights and velocity dispersions as a function of mass ratio and disk gas content in this regime, and show that this accurately describes the results of simulations with appropriate "live" halos and disks. Under realistic circumstances, we show that disk heating in minor mergers is suppressed by an order of magnitude relative to the expectations of previous analyses. We show that the Milky Way disk could have experienced ∼ 5 − 10 independent 1:10 mass-ratio mergers since z ∼ 2, in agreement with cosmological models. Because the realistic heating rates are non-linear in mass, the predicted heating is dominated by the more stochastic, rare low mass-ratio mergers, and the existence of populations with little or no thick disk does not require fundamental modifications to the cosmology. This also leads to important differences in the predicted isophotal shapes of bulge-disk systems along the Hubble sequence.
Dark matter halo response to the disc growth
Monthly Notices of the Royal Astronomical Society, 2006
We consider the sensitivity of the circular-orbit adiabatic contraction approximation to the baryon condensation rate and the orbital structure of dark matter halos in the ΛCDM paradigm. Using one-dimensional hydrodynamic simulations including the dark matter halo mass accretion history and gas cooling, we demonstrate that the adiabatic approximation is approximately valid even though halos and disks may assemble simultaneously. We further demonstrate the validity of the simple approximation for ΛCDM halos with isotropic velocity distributions using three-dimensional N-body simulations. This result is easily understood: an isotropic velocity distribution in a cuspy halo requires more circular orbits than radial orbits. Conversely, the approximation is poor in the extreme case of a radial orbit halo. It overestimates the response a core dark matter halo, where radial orbit fraction is larger. Because no astronomically relevant models are dominated by low-angular momentum orbits in the vicinity of the disk and the growth time scale is never shorter than a dynamical time, we conclude that the adiabatic contraction approximation is useful in modeling the response of dark matter halos to the growth of a disk.
Constraints of the Clumpiness of Dark Matter Halos through Heating of the Disk Galaxies
The Astrophysical Journal, 2003
Motivated by the presence of numerous dark matter clumps in the Milky Way's halo as expected from the cold dark matter cosmological model, we conduct numerical simulations to examine the heating of the disk. We construct an initial galaxy model in equilibrium, with a stable thin disk. The disk interacts with dark matter clumps for about 5 Gyr. Three physical effects are examined : first the mass spectrum of the dark matter clumps, second the initial thickness of the galactic disk, and third the spatial distribution of the clumps. We find that the massive end of the mass spectrum determines the amount of disk heating. Thicker disks suffer less heating. There is a certain thickness at which the heating owing to the interaction with the clumps becomes saturates. We also find that the heating produced by the model which mimics the distribution found in Standard CDM cosmology is significant and too high to explain the observational constraints. On the other hand, our model that corresponds to the clump distribution in a ΛCDM cosmology produces no significant heating. This result suggests that the ΛCDM cosmology is preferable with respect to the Standard CDM cosmology in explaining the thickness of the Milky Way.
Forming disc galaxies in ?CDM simulations
Monthly Notices of The Royal Astronomical Society, 2007
We used fully cosmological, high-resolution N-body + smooth particle hydrodynamic (SPH) simulations to follow the formation of disc galaxies with rotational velocities between 135 and 270 km s−1 in a Λ cold dark matter (CDM) universe. The simulations include gas cooling, star formation, the effects of a uniform ultraviolet (UV) background and a physically motivated description of feedback from supernovae (SNe). The host dark matter haloes have a spin and last major merger redshift typical of galaxy-sized haloes as measured in recent large-scale N-body simulations. The simulated galaxies form rotationally supported discs with realistic exponential scalelengths and fall on both the I band and baryonic Tully–Fisher relations. An extended stellar disc forms inside the Milky Way (MW)-sized halo immediately after the last major merger. The combination of UV background and SN feedback drastically reduces the number of visible satellites orbiting inside a MW-sized halo, bringing it in fair agreement with observations. Our simulations predict that the average age of a primary galaxy's stellar population decreases with mass, because feedback delays star formation in less massive galaxies. Galaxies have stellar masses and current star formation rates as a function of total mass that are in good agreement with observational data. We discuss how both high mass and force resolution and a realistic description of star formation and feedback are important ingredients to match the observed properties of galaxies.
Interaction between collisionless galactic discs and non-axisymmetric dark matter haloes
Monthly Notices of the Royal Astronomical Society, 2013
Using N -body simulations (N ∼ 10 6 − 10 7 ), we examine how a non-axisymmetric dark halo affects the dynamical evolution of the structure in collisionless (stellar) discs. We demonstrate how the model parameters such as mass of the halo, initial conditions in the disc and the halo axes ratio affect morphology and kinematics of the stellar discs. We show that a non-axisymmetric halo can generate a large-scale spiral density pattern in the embedded stellar disc. The pattern is observed in the disc for many periods of its revolution, even if the disc is gravitationally over-stable. The growth of the spiral arms is not accompanied by significant dynamical heating of the disc, irrelevant to its initial parameters. We also investigate transformation of the dark halo's shape driven by the long-lived spiral pattern in the disc . We show that the analysis of the velocity field in the stellar disc and in the spiral pattern gives us a possibility to figure out the spatial orientation of the triaxial-shaped dark halo and to measure the triaxiality.