Gravitational stability and dynamical overheating of galactic stellar disks (original) (raw)
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Stellar velocity dispersion and mass estimation for galactic disks
Astronomy Letters, 2004
Available velocity dispersion estimates for the old stellar population of galactic disks at galactocentric distances r 2L (where L is the photometric radial scale length of the disk) are used to determine the threshold local surface density of disks that are stable against gravitational perturbations. The mass of the disk M d calculated under the assumption of its marginal stability is compared with the total mass M t and luminosity L B of the galaxy within r = 4L. We corroborate the conclusion that a substantial fraction of the mass in galaxies is probably located in their dark halos. The ratio of the radial velocity dispersion to the circular velocity increases along the sequence of galactic color indices and decreases from the early to late morphological types. For most of the galaxies with large color indices (B − V ) 0 > 0.75, which mainly belong to the S0 type, the velocity dispersion exceeds significantly the threshold value required for the disk to be stable. The reverse situation is true for spiral galaxies: the ratios M d /L B for these agree well with those expected for evolving stellar systems with the observed color indices. This suggests that the disks of spiral galaxies underwent no significant dynamical heating after they reached a quasi-equilibrium stable state. c 2004 MAIK "Nauka/Interperiodica".
The Formation of Large Galactic Disks: Revival or Survival?
Using the deepest and the most complete set of observations of distant galaxies, we investigate how extended disks could have formed. Observations include spatially-resolved kinematics, detailed morphologies and photometry from UV to mid-IR. Six billion years ago, half of the present-day spiral progenitors had anomalous kinematics and morphologies, as well as relatively high gas fractions. We argue that gas-rich major mergers, i.e., fusions between gas-rich disk galaxies of similar mass, can be the likeliest driver for such strong peculiarities. This suggests a new channel of disk formation, e.g. many disks could be reformed after gas-rich mergers. This is found to be in perfect agreement with predictions from the state-of-the-art LCDM semi-empirical models: due to our sensitivity in detecting mergers at all phases, from pairs to relaxed post-mergers, we find a more accurate merger rate. The scenario can be finally confronted to properties of nearby galaxies, including M31 and galax...
Formation and evolution of disk galaxies
2003
Global star formation is the key to understanding galaxy disk formation. This in turn depends on gravitational instability of disks and continuing gas accretion as well as minor merging. A key component is feedback from supernovae. Primary observational constraints on disk galaxy formation and evolution include the Schmidt-Kennicutt law, the Tully-Fisher relation and the galaxy luminosity function. I will review how theory confronts phenomenology, and discuss future prospects for refining our understanding of disk formation.
Disk galaxy formation and evolution: models up to intermediate redshifts
Arxiv preprint astro-ph/9810293, 1998
Making use of a seminumerical method we develop a scenario of disk galaxy formation and evolution in the framework of inflationary cold dark matter (CDM) cosmologies. Within the virializing dark matter halos, disks in centrifugal equilibrium are built-up and their galactic evolution is followed through an approach which considers the gravitational interactions among the galaxy components, the turbulence and energy balance of the ISM, the star formation (SF) process due to disk gravitational instabilities, the stellar evolution and the secular formation of a bulge. We find that the main properties and correlations of disk galaxies are determined by the mass, the hierarchical mass aggregation history and the primordial angular momentum. The models follow the same trends across the Hubble sequence than the observed galaxies. The predicted TF relation is in good agreement with the observations except for the standart CDM. While the slope of this relation remains almost constant up to intermediate redshifts, its zero-point decreases in the H-band and slightly increases in the B-band. A maximum in the SF rate for most of the models is attained at z ∼ 1.5 − 2.5.
The thick disc of the Galaxy: Sequel of a merging event
Astronomy and Astrophysics
Accurate characterization of thick disc properties from recent kinematic and photometric surveys provides converging evidences that this intermediate population is a sequel of the violent heating of early disc populations by a merging satellite galaxy.
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.
The formation of disks in massive spiral galaxies
2010
The flatness of the rotation curve inside spiral galaxies is interpreted as the imprint of a halo of invisible matter. Using the deepest observations of distant galaxies, we have investigated how large disks could have been formed. Observations include spatially resolved kinematics, detailed morphologies and photometry from UV to mid-IR. Six Gigayears ago, half of the present-day spirals had anomalous kinematics and morphologies that considerably affect the scatter of the Tully Fisher relation. All anomalous galaxies can be modelled through gas-rich, major mergers that lead to a rebuilt of a new disk. The spiral-rebuilding scenario is proposed as a new channel to form large disks in present-day spirals and it accounts for all the observed evolutions since the last 6 Giga-years. A large fraction of the star formation is linked to merging events during their whole durations. PACS: 98.35.Ac, 98.35.Gi, 98.52.Cf, 98.52.Nr, 98.52.Sw, 98.56, 98.62.Ai, 98.62.Dm, 98.62.Gq, 98.62.Hr , 98.62.Lv
Monthly Notices of the Royal Astronomical Society, 2014
Over the past 10 Gyr, star-forming galaxies have changed dramatically, from clumpy and gas rich, to rather quiescent stellar-dominated disks with specific star formation rates lower by factors of a few tens. We present a general theoretical model for how this transition occurs, and what physical processes drive it, making use of 1D axisymmetric thin disk simulations with an improved version of the Gravitational Instability-Dominated Galaxy Evolution Tool (GIDGET) code. We show that at every radius galaxies tend to be in a slowly evolving equilibrium state wherein new accretion is balanced by star formation, galactic winds, and radial transport of gas through the disk by gravitational instability (GI) -driven torques. The gas surface density profile is determined by which of these terms are in balance at a given radius, -direct accretion is balanced by star formation and galactic winds near galactic centers, and by transport at larger radii. We predict that galaxies undergo a smooth transition from a violent disk instability phase to secular evolution. This model provides a natural explanation for the high velocity dispersions and large clumps in z ∼ 2 galaxies, the growth and subsequent quenching of bulges, and features of the neutral gas profiles of local spiral galaxies.
Early-type disk galaxies: Structure and kinematics
Astronomy Reports, 2008
Spectroscopic observations of three lenticular (S0) galaxies (NGC 1167, NGC 4150, and NGC 6340) and one SBa galaxy (NGC 2273) have been taken with the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences aimed to study the structure and kinematic properties of early-type disk galaxies. The radial profiles of the stellar radial velocities and the velocity dispersion are measured. N -body simulations are used to construct dynamical models of galaxies containing a stellar disk, bulge, and halo. The masses of individual components are estimated for maximum-mass disk models. A comparison of models with estimated rotational velocities and the stellar velocity dispersion suggests that the stellar disks in lenticular galaxies are "overheated"; i.e., there is a significant excess velocity dispersion over the minimum level required to maintain the stability of the disk. This supports the hypothesis that the stellar disks of S0 galaxies were subject to strong gravitational perturbations. The relative thickness of the stellar disks in the S0 galaxies we consider substantially exceed the typical disk thickness of spiral galaxies.
The Thick Disks of Spiral Galaxies as Relics from Gas-Rich, Turbulent, Clumpy Disks at High Redshift
The Astrophysical Journal, 2009
The formation of thick stellar disks in spiral galaxies is studied. Simulations of gas-rich young galaxies show formation of internal clumps by gravitational instabilities, clump coalescence into a bulge, and disk thickening by strong stellar scattering. The bulge and thick disks of modern galaxies may form this way. Simulations of minor mergers make thick disks too, but there is an important difference. Thick disks made by internal processes have a constant scale height with galactocentric radius, but thick disks made by mergers flare. The difference arises because in the first case, perpendicular forcing and disk-gravity resistance are both proportional to the disk column density, so the resulting scale height is independent of this density. In the case of mergers, perpendicular forcing is independent of the column density and the low density regions get thicker; the resulting flaring is inconsistent with observations. Late-stage gas accretion and thin disk growth are shown to preserve the constant scale heights of thick disks formed by internal evolution. These results reinforce the idea that disk galaxies accrete most of their mass smoothly and acquire their structure by internal processes, in particular through turbulent and clumpy phases at high redshift.