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  • Articles  (9)
  • Monthly Notices of the Royal Astronomical Society  (8)
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  • Articles  (9)
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  • 1
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    Spiral arm kinematics for Milky Way stellar populations (2016)
    Oxford University Press
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    Publication Date: 2016-07-15
    Description: We present a new theoretical population synthesis model (the Galaxy model) to examine and deal with large amounts of data from surveys of the Milky Way and to decipher the present and past structure and history of our own Galaxy. We assume the Galaxy to consist of a superposition of many composite stellar populations belonging to the thin and thick discs, the stellar halo and the bulge, and to be surrounded by a single dark matter halo component. A global model for the Milky Way's gravitational potential is built up self-consistently with the density profiles from the Poisson equation. In turn, these density profiles are used to generate synthetic probability distribution functions (PDFs) for the distribution of stars in colour–magnitude diagrams (CMDs). Finally, the gravitational potential is used to constrain the stellar kinematics by means of the moment method on a (perturbed)-distribution function. Spiral arms perturb the axisymmetric disc distribution functions in the linear response framework of density-wave theory where we present an analytical formula of the so-called ‘reduction factor’ using hypergeometric functions. Finally, we consider an analytical non-axisymmetric model of extinction and an algorithm based on the concept of probability distribution function to handle CMDs with a large number of stars. A genetic algorithm is presented to investigate both the photometric and kinematic parameter space. This galaxy model represents the natural framework to reconstruct the structure of the Milky Way from the heterogeneous data set of surveys such as Gaia -ESO, SEGUE, APOGEE2, RAVE and the Gaia mission.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 2
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    The stellar kinematics of corotating spiral arms in Gaia mock observations (2015)
    Oxford University Press
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    Publication Date: 2015-05-02
    Description: We have observed an N -body/smoothed particle hydrodynamics simulation of a Milky Way-like barred spiral galaxy. We present a simple method that samples N -body model particles into mock Gaia stellar observations and takes into account stellar populations, dust extinction and Gaia ’ s science performance estimates. We examine the kinematics of stars with V  ≤ 16 mag around a nearby spiral arm at a similar position to the Perseus arm at three lines of sight in the disc plane; ( l , b ) = (90, 0), (120, 0) and (150, 0) deg. We find that the structure of the peculiar kinematics around the corotating spiral arm, which is found in Kawata et al. ( 2014b ), is still visible in the observational data expected to be produced by Gaia , despite the dust extinction and expected observational errors of Gaia . These observable kinematic signatures will enable testing whether the Perseus arm of the Milky Way is similar to the corotating spiral arms commonly seen in N -body simulations.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 3
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    Gas and stellar motions and observational signatures of corotating spiral arms (2014)
    Oxford University Press
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    Publication Date: 2014-08-07
    Description: We have observed a snapshot of our N -body/smoothed particle hydrodynamics simulation of a Milky Way-sized barred spiral galaxy in a similar way to how we can observe the Milky Way. The simulated galaxy shows a corotating spiral arm, i.e. the spiral arm rotates with the same speed as the circular speed. We observed the rotation and radial velocities of the gas and stars as a function of the distance from our assumed location of the observer at the three lines of sight on the disc plane, ( l , b ) = (90, 0), (120, 0) and (150, 0) deg. We find that the stars tend to rotate slower (faster) behind (at the front of) the spiral arm and move outwards (inwards), because of the radial migration. However, because of their epicycle motion, we see a variation of rotation and radial velocities around the spiral arm. On the other hand, the cold gas component shows a clearer trend of rotating slower (faster) and moving outwards (inwards) behind (at the front of) the spiral arm, because of the radial migration. We have compared the results with the velocity of the maser sources from Reid et al., and find that the observational data show a similar trend in the rotation velocity around the expected position of the spiral arm at l  = 120 deg. We also compared the distribution of the radial velocity from the local standard of the rest, V LSR , with the Apache Point Observatory Galactic Evolution Experiment (APOGEE) data at l  = 90 deg as an example.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
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  • 4
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    Disc galaxy modelling with a particle-by-particle made-to-measure method (2013)
    Oxford University Press
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    Publication Date: 2013-03-16
    Description: We have developed the initial version of a new particle-by-particle adaptation of the made-to-measure (M2M) method, aiming to model the Galactic disc from upcoming Galactic stellar survey data. In our new particle-by-particle M2M, the observables of the target system are compared with those of the model galaxy at the position of the target stars (i.e. particles). The weights of the model particles are changed to reproduce the observables of the target system, and the gravitational potential is automatically adjusted by the changing weights of the particles. This paper demonstrates, as the initial work, that the particle-by-particle M2M can recreate a target disc system created by an N -body simulation in a known dark matter potential, with no error in the observables. The radial profiles of the surface density, velocity dispersion in the radial and perpendicular directions, and the rotational velocity of the target disc are all well reproduced from the initial disc model, whose scalelength is different from that of the target disc. We also demonstrate that our M2M can be applied to an incomplete data set and recreate the target disc reasonably well when the observables are restricted to a part of the disc. We discuss our calibration of the model parameters and the importance of regularization.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 5
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    Impacts of a flaring star-forming disc and stellar radial mixing on the vertical metallicity gradient (2016)
    Oxford University Press
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    Publication Date: 2016-10-21
    Description: Using idealized N -body simulations of a Milky Way-sized disc galaxy, we qualitatively study how the metallicity distributions of the thin disc star particles are modified by the formation of the bar and spiral arm structures. The thin disc in our numerical experiments initially has a tight negative radial metallicity gradient and a constant vertical scaleheight. We show that the radial mixing of stars drives a positive vertical metallicity gradient in the thin disc. On the other hand, if the initial thin disc is flared, with vertical scaleheight increasing with galactocentric radius, the metal-poor stars, originally in the outer disc, become dominant in regions above the disc plane at every radii. This process can drive a negative vertical metallicity gradient, which is consistent with the current observed trend. This model mimics a scenario where the star-forming thin disc was flared in the outer region at earlier epochs. Our numerical experiment with an initial flared disc predicts that the negative vertical metallicity gradient of the mono-age relatively young thin disc population should be steeper in the inner disc, and the radial metallicity gradient of the mono-age population should be shallower at greater heights above the disc plane. We also predict that the metallicity distribution function of mono-age young thin disc populations above the disc plane would be more positively skewed in the inner disc compared to the outer disc.
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    Electronic ISSN: 1365-2966
    Topics: Physics
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  • 6
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    M2M modelling of the Galactic disc via PRIMAL: fitting to Gaia error added data (2014)
    Oxford University Press
    Details
    Publication Date: 2014-07-30
    Description: We have adapted our made-to-measure (M2M) algorithm primal to use mock Milky Way like data constructed from an N -body barred galaxy with a boxy bulge in a known dark matter potential. We use M0 giant stars as tracers, with the expected error of the ESA (European Space Agency) space astrometry mission Gaia . We demonstrate the process of constructing mock Gaia data from an N -body model, including the conversion of a galactocentric Cartesian coordinate N -body model into equatorial coordinates and how to add error to it for a single stellar type. We then describe the modifications made to primal to work with observational error. This paper demonstrates that primal can recover the radial profiles of the surface density, radial velocity dispersion, vertical velocity dispersion and mean rotational velocity of the target disc, along with the pattern speed of the bar, to a reasonable degree of accuracy despite the lack of accurate target data. We also construct mock data which take into account dust extinction and show that primal recovers the structure and kinematics of the disc reasonably well. In other words, the expected accuracy of the Gaia data is good enough for primal to recover these global properties of the disc, at least in a simplified condition, as used in this paper.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
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  • 7
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    Investigating bar structure of disc galaxies via PRIMAL: a particle-by-particle M2M algorithm (2013)
    Oxford University Press
    Details
    Publication Date: 2013-06-18
    Description: We have modified our particle-by-particle adaptation of the made-to-measure (M2M) method, with the aim of modelling the Galactic disc from upcoming Galactic stellar survey data. In our new particle-by-particle M2M algorithm, primal , the observables of the target system are compared with those of the model galaxy at the position of the target stars, i.e. particles. The mass of the model particles are adjusted to reproduce the observables of the target system, and the gravitational potential is automatically adjusted by the changing mass of the particles. This paper builds upon our previous work, introducing likelihood-based velocity constraints in primal . In this paper, we apply primal to barred disc galaxies created by an N -body simulation in a known dark matter potential, with no error in the observables. This paper demonstrates that primal can recover the radial profiles of the surface density, velocity dispersion in the radial and perpendicular directions, and the rotational velocity of the target discs, along with the apparent bar structure and pattern speed of the bar, especially when the reference frame is adjusted so that the bar angle of the target galaxy is aligned to that of the model galaxy at every timestep.
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    Electronic ISSN: 1365-2966
    Topics: Physics
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  • 8
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    Spiral- and bar-driven peculiar velocities in Milky Way-sized galaxy simulations (2015)
    Oxford University Press
    Details
    Publication Date: 2015-08-28
    Description: We investigate the kinematic signatures induced by spiral and bar structure in a set of simulations of Milky Way-sized spiral disc galaxies. The set includes test particle simulations that follow a quasi-stationary density wave-like scenario with rigidly rotating spiral arms, and N -body simulations that host a bar and transient, corotating spiral arms. From a location similar to that of the Sun, we calculate the radial, tangential and line-of-sight peculiar velocity fields of a patch of the disc and quantify the fluctuations by computing the power spectrum from a two-dimensional Fourier transform. We find that the peculiar velocity power spectrum of the simulation with a bar and transient, corotating spiral arms fits very well to that of APOGEE red clump star data, while the quasi-stationary density wave spiral model without a bar does not. We determine that the power spectrum is sensitive to the number of spiral arms, spiral arm pitch angle and position with respect to the spiral arm. However, it is necessary to go beyond the line-of-sight velocity field in order to distinguish fully between the various spiral models with this method. We compute the power spectrum for different regions of the spiral discs, and discuss the application of this analysis technique to external galaxies.
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    Electronic ISSN: 1365-2966
    Topics: Physics
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  • 9
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    Spiral arm kinematics for Milky Way stellar populations (2016)
    Oxford University Press
    Details
    Publication Date: 2016-06-20
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press on behalf of The Royal Astronomical Society.
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