254 research outputs found
Impacts of Radial Mixing on the Galactic Thick and Thin Disks
Using N-body simulations of the Galactic disks, we qualitatively study how
the metallicity distributions of the thick and thin disk stars are modified by
radial mixing induced by the bar and spiral arms. We show that radial mixing
drives a positive vertical metallicity gradient in the mono-age disk population
whose initial scale-height is constant and initial radial metallicity gradient
is tight and negative. On the other hand, if the initial disk is flaring, with
scale-height increasing with galactocentric radius, radial mixing leads to a
negative vertical metallicity gradient, which is consistent with the current
observed trend. We also discuss impacts of radial mixing on the metallicity
distribution of the thick disk stars. By matching the metallicity distribution
of N-body models to the SDSS/APOGEE data, we argue that the progenitor of the
Milky Way's thick disk should not have a steep negative metallicity gradient.Comment: 4 pages, 1 figure, to appear in the proceedings of the IAU Symposium
334 "Rediscovering our Galaxy", Potsdam, 10-14 July 2017, eds. C. Chiappini,
I. Minchev, E. Starkenburg, M. Valentin
Galaxy Formation from a Low-Spin Density Perturbation in a CDM Universe
In order to understand the formation process of elliptical galaxies which are
not rotationally supported, we have carried out numerical simulations of the
galaxy formation from the density perturbation with a rotation corresponding to
a small spin parameter. The three-dimensional TREE N-Body/SPH simulation code
used in this paper includes the dark matter and gas dynamics, radiative
cooling, star formation, supernova feedback, and metal enrichment. The initial
condition is a slowly rotating, top-hat over-dense sphere on which the
perturbations expected in a cold dark matter (CDM) universe are superposed. By
means of the stellar population synthesis, we calculated the surface brightness
profile, the metallicity distribution, and the photometric properties of the
end-product, and found that these properties quantitatively agree with the
observed properties of bright elliptical galaxies. Thus, we conclude that, in a
CDM universe, the proto-galaxy which has a spin-parameter as small as 0.02
evolves into an elliptical galaxy.Comment: 12 pages, 9 PS figures, 1 GIF figure, accepted for publication in
PASJ; PS version available at
http://www.astr.tohoku.ac.jp/~kawata/research/papers.htm
M2M modelling of the Galactic disc via PRIMAL: Fitting to Gaia Error Added Data
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 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.Comment: 16 pages, 10 figures, submitted to MNRAS 17th Dec 2013, accepted 30th
June 201
Effects of SNe II and SNe Ia Feedback on the Chemo-Dynamical Evolution of Elliptical Galaxies
We numerically investigate the dynamical and chemical processes of the
formation of elliptical galaxies in a cold dark matter (CDM) universe, in order
to understand the origin of the mass-dependence of the photometric properties
of elliptical galaxies. Our three-dimensional TREE N-body/SPH numerical
simulations of elliptical galaxy formation take into account both Type II (SNe
II) and Type Ia (SNe Ia) supernovae (SNe) and follow the time evolution of the
abundances of several chemical elements (C, O, Ne, Mg, Si, and Fe). Moreover we
compare different strengths of SNe feedback.In combination with stellar
population synthesis, we derive the photometric properties of simulation
end-products, including the magnitude, color, half-light radius, and abundance
ratios, and compare them with the observed scaling relations directly and
quantitatively. We find that the extremely strong influence of SNe is required
to reproduce the observed color-magnitude relation (CMR), where we assume each
SN yields energy of 4x10^51 ergs and that 90% of this energy is ejected as
kinetic feedback. The feedback affects the evolution of lower mass systems more
strongly and induces the galactic wind by which a larger fraction of gas is
blown out in a lower mass system. Finally higher mass systems become more metal
rich and have redder colors than lower mass systems. We emphasize based on our
simulation results that the galactic wind is triggered mainly by SNe Ia rather
than SNe II. In addition we examined the Kormendy relation, which prescribes
the size of elliptical galaxies, and the [Mg/Fe]--magnitude relation, which
provides a strong constraint on the star formation history.Comment: 16 pages, 12 figures, accepted for publication in ApJ. Mpeg movies
are available at
http://astronomy.swin.edu.au/staff/dkawata/research/movies.htm
Fourteen candidate RR Lyrae star streams in the inner Galaxy
We apply the GC3 stream-finding method to RR Lyrae stars (RRLS) in the
Catalina survey. We find two RRLS stream candidates at confidence
and another 12 at confidence over the Galactocentric distance
range . Of these, only two are associated with known
globular clusters (NGC 1261 and Arp2). The remainder are candidate `orphan'
streams, consistent with the idea that globular cluster streams are most
visible close to dissolution. Our detections are likely a lower bound on the
total number of dissolving globulars in the inner galaxy, since many globulars
have few RRLS while only the brightest streams are visible over the Galactic
RRLS background, particularly given the current lack of kinematical
information. We make all of our candidate streams publicly available and
provide a new GALSTREAMS Python library for the footprints of all known streams
and overdensities in the Milky Way.Comment: 18 pages, 4 figures. Accepted for publication at MNRAS. GALSTREAMS
Milky Way Streams Footprint Library are available at
https://github.com/cmateu/galstreams . All RRL data and code used in the
paper are available at
https://cmateu.github.io/Cecilia_Mateu_WebPage/CatalinaGC3_Streams.htm
Stellar dynamics around transient co-rotating spiral arms
Spiral density wave theory attempts to describe the spiral pattern in spiral
galaxies in terms of a long-lived wave structure with a constant pattern speed
in order to avoid the winding dilemma. The pattern is consequently a rigidly
rotating, long-lived feature. We run an N-body/SPH simulation of a Milky
Way-sized barred disk, and find that the spiral arms are transient features
whose pattern speeds decrease with radius, in such a way that the pattern speed
is almost equal to the rotation curve of the galaxy. We trace particle motion
around the spiral arms. We show that particles from behind and in front of the
spiral arm are drawn towards and join the arm. Particles move along the arm in
the radial direction and we find a clear trend that they migrate toward the
outer (inner) radii on the trailing (leading) side of the arm. Our simulations
demonstrate that at all radii where there is a co-rotating spiral arm the
particles continue to be accelerated (decelerated) by the spiral arm for long
periods, which leads to strong migration.Comment: 2 pages, 2 figures, to appear in the proceedings of "Assembling the
puzzle of the Milky Way", Le Grand-Bornand, 17-22 April, 2011, eds. C. Reyle,
A. Robin, M. Schulthei
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