Using high resolution, fully cosmological smoothed particle hydrodynamical simulations of dwarf galaxies in a Lambda cold dark matter Universe, we show how high redshift gas outflows can modify the ...baryon angular momentum distribution and allow pure disc galaxies to form. We outline how galactic outflows preferentially remove low angular momentum material due a combination of (a) star formation peaking at high redshift in shallow dark matter potentials, an epoch when accreted gas has relatively low angular momentum, (b) the existence of an extended reservoir of high angular momentum gas in the outer disc to provide material for prolonged SF at later times and (c) the tendency for outflows to follow the path of least resistance which is perpendicular to the disc. We also show that outflows are enhanced during mergers, thus expelling much of the gas which has lost its angular momentum during these events, and preventing the formation of 'classical', merger driven bulges in low-mass systems. Stars formed prior to such mergers form a diffuse, extended stellar halo component similar to those detected in nearby dwarfs.
Within a fully cosmological hydrodynamical simulation, we form a galaxy which rotates at 140 km s−1, and it is characterized by two loose spiral arms and a bar, indicative of a Hubble-type SBc/d ...galaxy. We show that our simulated galaxy has no classical bulge, with a pure disc profile at z = 1, well after the major merging activity has ended. A long-lived bar subsequently forms, resulting in the formation of a secularly formed 'pseudo-'bulge, with the final bulge-to-total light ratio of 0.21. We show that the majority of gas which loses angular momentum and falls to the central region of the galaxy during the merging epoch is blown back into the hot halo, with much of it returning later to form stars in the disc. We propose that this mechanism of redistribution of angular momentum via a galactic fountain, when coupled with the results from our previous study which showed why gas outflows are biased to have low angular momentum, can solve the angular momentum/bulgeless disc problem of the cold dark matter paradigm.
The RAdial Velocity Experiment survey, combined with proper motions and distance estimates, can be used to study in detail stellar kinematics in the extended solar neighbourhood (solar suburb). Using ...72 365 red-clump stars, we examine the mean velocity components in 3D between 6 < R < 10 kpc and −2 < Z < 2 kpc, concentrating on north-south differences. Simple parametric fits to the (R, Z) trends for V
φ and the velocity dispersions are presented. We confirm the recently discovered gradient in mean Galactocentric radial velocity, V
R, finding that the gradient is marked below the plane (δ〈V
R〉/δR = −8 km s−1 kpc−1 for Z < 0, vanishing to zero above the plane), with a Z gradient thus also present. The vertical velocity, V
Z
, also shows clear, large-amplitude (|V
Z
| = 17 km s−1) structure, with indications of a rarefaction-compression pattern, suggestive of wave-like behaviour. We perform a rigorous error analysis, tracing sources of both systematic and random errors. We confirm the north-south differences in V
R and V
Z
along the line of sight, with the V
R estimated independent of the proper motions. The complex three-dimensional structure of velocity space presents challenges for future modelling of the Galactic disc, with the Galactic bar, spiral arms and excitation of wave-like structures all probably playing a role.
We use data from the Radial Velocity Experiment (RAVE) and the Tycho-Gaia astrometric solution (TGAS) catalogue to compute the velocity fields yielded by the radial (V-R), azimuthal (V-phi), and ...vertical (V-z) components of associated Galactocentric velocity. We search in particular for variation in all three velocity components with distance above and below the disc midplane, as well as how each component of V-z (line-of-sight and tangential velocity projections) modifies the obtained vertical structure. To study the dependence of velocity on proper motion and distance, we use two main samples: a RAVE sample including proper motions from the Tycho-2, PPMXL, and UCAC4 catalogues, and a RAVE-TGAS sample with inferred distances and proper motions from the TGAS and UCAC5 catalogues. In both samples, we identify asymmetries in V-R and V-z. Below the plane, we find the largest radial gradient to be partial derivative V-R/partial derivative R = -7.01 +/- 0.61 km s(-1) kpc(-1), in agreement with recent studies. Above the plane, we find a similar gradient with partial derivative V-R/partial derivative R = -9.42 +/- 1.77 km s(-1) kpc(-1). By comparing our results with previous studies, we find that the structure in V-z is strongly dependent on the adopted proper motions. Using the Galaxia Milky Way model, we demonstrate that distance uncertainties can create artificial wave-like patterns. In contrast to previous suggestions of a breathing mode seen in RAVE data, our results support a combination of bending and breathing modes, likely generated by a combination of external or internal and external mechanisms.
This paper presents a fast algorithm for restoring video sequences. The proposed algorithm, as opposed to existing methods, does not consider video restoration as a sequence of image restoration ...problems. Rather, it treats a video sequence as a space-time volume and poses a space-time total variation regularization to enhance the smoothness of the solution. The optimization problem is solved by transforming the original unconstrained minimization problem to an equivalent constrained minimization problem. An augmented Lagrangian method is used to handle the constraints, and an alternating direction method is used to iteratively find solutions to the subproblems. The proposed algorithm has a wide range of applications, including video deblurring and denoising, video disparity refinement, and hot-air turbulence effect reduction.
Abstract
Using the Radial Velocity Experiment (RAVE) survey, we recently brought to light a gradient in the mean galactocentric radial velocity of stars in the extended solar neighbourhood. This ...gradient likely originates from non-axisymmetric perturbations of the potential, among which a perturbation by spiral arms is a possible explanation. Here, we apply the traditional density wave theory and analytically model the radial component of the two-dimensional velocity field. Provided that the radial velocity gradient is caused by relatively long-lived spiral arms that can affect stars substantially above the plane, this analytic model provides new independent estimates for the parameters of the Milky Way spiral structure. Our analysis favours a two-armed perturbation with the Sun close to the inner ultra-harmonic 4:1 resonance, with a pattern speed and a small amplitude per cent of the background potential (14 per cent of the background density). This model can serve as a basis for numerical simulations in three dimensions, additionally including a possible influence of the Galactic bar and/or other non-axisymmetric modes.
We report new constraints on the local escape speed of our Galaxy. Our analysis is based on a sample of high-velocity stars from the RAVE survey and two previously published data sets. We use ...cosmological simulations of disc galaxy formation to motivate our assumptions on the shape of the velocity distribution, allowing for a significantly more precise measurement of the escape velocity compared to previous studies. We find that the escape velocity lies within the range 498 < vesc < 608 km s−1 (90 per cent confidence), with a median likelihood of 544 km s−1. The fact that v2esc is significantly greater than 2v2circ (where vcirc= 220 km s−1 is the local circular velocity) implies that there must be a significant amount of mass exterior to the solar circle, that is, this convincingly demonstrates the presence of a dark halo in the Galaxy. We use our constraints on vesc to determine the mass of the Milky Way halo for three halo profiles. For example, an adiabatically contracted NFW halo model results in a virial mass of 1.42+1.14−0.54× 1012 M⊙ and virial radius of (90 per cent confidence). For this model the circular velocity at the virial radius is 142+31−21 km s−1. Although our halo masses are model dependent, we find that they are in good agreement with each other.
Aims. We examine the role of energy feedback in shaping the distribution of metals within cosmological hydrodynamical simulations of L∗ disc galaxies. While negative abundance gradients today provide ...a boundary condition for galaxy evolution models, in support of inside-out disc growth, empirical evidence as to whether abundance gradients steepen or flatten with time remains highly contradictory. Methods. We made use of a suite of L∗ discs, realised with and without “enhanced” feedback. All the simulations were produced using the smoothed particle hydrodynamics code Gasoline, and their in situ gas-phase metallicity gradients traced from redshift z ~ 2 to the present-day. Present-day age-metallicity relations and metallicity distribution functions were derived for each system. Results. The “enhanced” feedback models, which have been shown to be in agreement with a broad range of empirical scaling relations, distribute energy and re-cycled ISM material over large scales and predict the existence of relatively “flat” and temporally invariant abundance gradients. Enhanced feedback schemes reduce significantly the scatter in the local stellar age-metallicity relation and, especially, the O/Fe−Fe/H relation. The local O/Fe distribution functions for our L∗ discs show clear bimodality, with peaks at O/Fe = −0.05 and + 0.05 (for stars with Fe/H > −1), consistent with our earlier work on dwarf discs. Conclusions. Our results with “enhanced” feedback are inconsistent with our earlier generation of simulations realised with “conservative” feedback. We conclude that spatially-resolved metallicity distributions, particularly at high-redshift, offer a unique and under-utilised constraint on the uncertain nature of stellar feedback processes.
We present accurate models of the gravitational potential produced by a radially exponential disc mass distribution. The models are produced by combining three separate Miyamoto–Nagai discs. Such ...models have been used previously to model the disc of the Milky Way, but here we extend this framework to allow its application to discs of any mass, scalelength, and a wide range of thickness from infinitely thin to near spherical (ellipticities from 0 to 0.9). The models have the advantage of simplicity of implementation, and we expect faster run speeds over a double exponential disc treatment. The potentials are fully analytical, and differentiable at all points. The mass distribution of our models deviates from the radial mass distribution of a pure exponential disc by <0.4 per cent out to 4 disc scalelengths, and <1.9 per cent out to 10 disc scalelengths. We tabulate fitting parameters which facilitate construction of exponential discs for any scalelength, and a wide range of disc thickness (a user-friendly, web-based interface is also available). Our recipe is well suited for numerical modelling of the tidal effects of a giant disc galaxy on star clusters or dwarf galaxies. We consider three worked examples; the Milky Way thin and thick disc, and a discy dwarf galaxy.
We analyse a high-resolution, fully cosmological, hydrodynamical disc galaxy simulation, to study the source of the double-exponential light profiles seen in many stellar discs, and the effects of ...stellar radial migration upon the spatiotemporal evolution of both the disc age and metallicity distributions. We find a ‘break’ in the pure exponential stellar surface brightness profile, and trace its origin to a sharp decrease in the star formation per unit surface area, itself produced by a decrease in the gas volume density due to a warping of the gas disc. Star formation in the disc continues well beyond the break. We find that the break is more pronounced in bluer wavebands. By contrast, we find little or no break in the mass density profile. This is, in part, due to the net radial migration of stars towards the external parts of the disc. Beyond the break radius, we find that ∼60 per cent of the resident stars migrated from the inner disc, while ∼25 per cent formed in situ. Our simulated galaxy also has a minimum in the age profile at the break radius but, in disagreement with some previous studies, migration is not the main mechanism producing this shape. In our simulation, the disc metallicity gradient flattens with time, consistent with an ‘inside-out’ formation scenario. We do not find any difference in the intensity or the position of the break with inclination, suggesting that perhaps the differences found in empirical studies are driven by dust extinction.
PDF
