4,219 research outputs found
The age structure of stellar populations in the solar vicinity. Clues of a two-phase formation history of the Milky Way disk
We analyze high quality abundances data of solar neighborhood stars and show
that there are two distinct regimes of [alpha/Fe] versus age which we identify
as the epochs of the thick and thin disk formation. A tight correlation between
metallicity and [alpha/Fe] versus age is clearly identifiable on thick disk
stars, implying that this population formed from a well mixed ISM, over a time
scale of 4-5 Gyr. Thick disk stars vertical velocity dispersion correlate with
age, with the youngest objects having as small scale heights as those of thin
disk stars. A natural consequence of these two results is that a vertical
metallicity gradient is expected in this population. We suggest that the thick
disk set the initial conditions for the formation of the inner thin disk. This
provides also an explanation of the apparent coincidence between the step in
metallicity at 7-10 kpc in the thin disk and the confinment of the thick disk
at about R<10 kpc. We suggest that the outer thin disk developped outside the
influence of the thick disk, but also that the high alpha-enrichment of the
outer regions may originate from a primordial pollution by the gas expelled
from the thick disk. Local metal-poor thin disk stars, whose properties are
best explained by an origin in the outer disk, are shown to be as old as the
youngest thick disk (9-10 Gyr), implying that the outer thin disk started to
form while the thick disk formation was still on-going in the inner Galaxy. We
point out that, given the tight age-abundance relations in the thick disk, an
inside-out process would give rise to a radial gradient in abundances in this
population which is not observed. Finally, we argue that the data discussed
here leave little room for radial migration, either to have contaminated the
solar vicinity, or, to have redistributed stars in significant proportion
across the solar annulus.Comment: Accepted in A&A, Revised version with new figures and extended
discussio
The Genus 0 Gromov-Witten Invariants of Projective Complete Intersections
We describe the structure of mirror formulas for genus 0 Gromov-Witten
invariants of projective complete intersections with any number of marked
points and provide an explicit algorithm for obtaining the relevant structure
coefficients. The structural description alone suffices for some qualitative
applications, such as vanishing results and the bounds on the growth of these
invariants predicted by R. Pandharipande.Comment: two conjectures added; typos corrected 61 pages, 3 figures, 4 table
A new look at the kinematics of the bulge from an N-body model
(Abridged) By using an N-body simulation of a bulge that was formed via a bar
instability mechanism, we analyse the imprints of the initial (i.e. before bar
formation) location of stars on the bulge kinematics, in particular on the
heliocentric radial velocity distribution of bulge stars. Four different
latitudes were considered: , , , and
, along the bulge minor axis as well as outside it, at
and . The bulge X-shaped structure comprises
stars that formed in the disk at different locations. Stars formed in the outer
disk, beyond the end of the bar, which are part of the boxy peanut-bulge
structure may show peaks in the velocity distributions at positive and negative
heliocentric radial velocities with high absolute values that can be larger
than 100 , depending on the observed direction. In some
cases the structure of the velocity field is more complex and several peaks are
observed. Stars formed in the inner disk, the most numerous, contribute
predominantly to the X-shaped structure and present different kinematic
characteristics. Our results may enable us to interpret the cold high-velocity
peak observed in the APOGEE commissioning data, as well as the excess of
high-velocity stars in the near and far arms of the X-shaped structure at
= and =. When compared with real data, the kinematic
picture becomes more complex due to the possible presence in the observed
samples of classical bulge and/or thick disk stars. Overall, our results point
to the existence of complex patterns and structures in the bulge velocity
fields, which are generated by the bar. This suggests that caution should be
used when interpreting the bulge kinematics: the presence of substructures,
peaks and clumps in the velocity fields is not necessarily a sign of past
accretion events.Comment: 21 pages, 18 figures. Accepted for publication in A&
Bars & boxy/peanut bulges in thin & thick discs: I. Morphology and line-of-sight velocities of a fiducial model
We explore trends in the morphology and line-of-sight (los) velocity of
stellar populations in the inner regions of disc galaxies, using N-body
simulations with both a thin (kinematically cold) and a thick (kinematically
hot) disc which form a bar and boxy/peanut (b/p) bulge. The bar in the thin
disc component is 50\% stronger than the thick disc bar and is more
elongated, with an axis ratio almost half that of the thick disc bar. The thin
disc b/p bulge has a pronounced X-shape, while the thick disc b/p is weaker
with a rather boxy shape. This leads to the signature of the b/p bulge in the
thick disc to be weaker and further away from the plane than in the thin disc.
Regarding the kinematics, we find that the los velocity of thick disc stars in
the outer parts of the b/p bulge can be \emph{larger} than that of thin disc
stars, by up to 40\% and 20\% for side-on and Milky Way-like orientations of
the bar respectively. This is due to the different orbits followed by thin and
thick disc stars in the bar-b/p region, which are affected by the fact that: i)
thin disc stars are trapped more efficiently in the bar - b/p instability and
thus lose more angular momentum than their thick disc counterparts and ii)
thick disc stars have large radial excursions and therefore stars from large
radii with high angular momenta can be found in the bar region. We also find
that the difference between the los velocities of the thin and thick disc in
the b/p bulge () correlates with the initial difference between
the radial velocity dispersions of the two discs () . We
therefore conclude that stars in the bar - b/p bulge will have considerably
different morphologies and kinematics depending on the kinematic properties of
the disc population they originate from.Comment: Accepted for publication in A&A. 15 pages (2 page appendix). 16
figure
When the Milky Way turned off the lights: APOGEE provides evidence of star formation quenching in our Galaxy
Quenching, the cessation of star formation, is one of the most significant
events in the life cycle of galaxies. We show here the first evidence that the
Milky Way experienced a generalised quenching of its star formation at the end
of its thick disk formation 9 Gyr ago. Elemental abundances of stars
studied as part of the APOGEE survey reveal indeed that in less than 2
Gyr the star formation rate in our Galaxy dropped by an order-of-magnitude.
Because of the tight correlation between age and alpha abundance, this event
reflects in the dearth of stars along the inner disk sequence in the
[Fe/H]-[/Fe] plane. Before this phase, which lasted about 1.5 Gyr, the
Milky Way was actively forming stars. Afterwards, the star formation resumed at
a much lower level to form the thin disk. These events are very well matched by
the latest observation of MW-type progenitors at high redshifts. In late type
galaxies, quenching is believed to be related to a long and secular exhaustion
of gas. In our Galaxy, it occurred on a much shorter time scale, while the
chemical continuity before and after the quenching indicates that it was not
due to the exhaustion of the gas. While quenching is generally associated with
spheroids, our results show that it also occurs in galaxies like the Milky Way,
possibly when they are undergoing a morphological transition from thick to thin
disks. Given the demographics of late type galaxies in the local universe, in
which classical bulges are rare, we suggest further that this may hold true
generally in galaxies with mass lower than or approximately , where
quenching could be directly a consequence of thick disk formation. We emphasize
that the quenching phase in the Milky Way could be contemporaneous with, and
related to, the formation of the bar. We sketch a scenario on how a strong bar
may inhibit star formation.Comment: 17 pages, 8 figures. Published versio
Testing Feedback-Modified Dark Matter Haloes with Galaxy Rotation Curves: Estimation of Halo Parameters and Consistency with CDM
Cosmological -body simulations predict dark matter (DM) haloes with steep
central cusps (e.g. NFW, Navarro et al. 1996). This contradicts observations of
gas kinematics in low-mass galaxies that imply the existence of shallow DM
cores. Baryonic processes such as adiabatic contraction and gas outflows can,
in principle, alter the initial DM density profile, yet their relative
contributions to the halo transformation remain uncertain. Recent high
resolution, cosmological hydrodynamic simulations (Di Cintio et al. 2014, DC14)
predict that inner density profiles depend systematically on the ratio of
stellar to DM mass (M/M). Using a Markov Chain Monte Carlo
approach, we test the NFW and the M/M-dependent DC14 halo
models against a sample of 147 galaxy rotation curves from the new {\it
Spitzer} Photometry and Accurate Rotation Curves (SPARC) data set. These
galaxies all have extended H{\small I} rotation curves from radio
interferometry as well as accurate stellar mass density profiles from
near-infrared photometry. The DC14 halo profile provides markedly better fits
to the data compared to the NFW profile. Unlike NFW, the DC14 halo parameters
found in our rotation curve fits naturally fall within two standard deviations
of the mass-concentration relation predicted by CDM and the stellar
mass-halo mass relation inferred from abundance matching with few outliers.
Halo profiles modified by baryonic processes are therefore more consistent with
expectations from cold dark matter (CDM) cosmology and
provide better fits to galaxy rotation curves across a wide range of galaxy
properties than do halo models that neglect baryonic physics. Our results offer
a solution to the decade long cusp-core discrepancy.Comment: 23 Pages, 18 Figures, MNRAS Accepte
A scalable PC-based parallel computer for lattice QCD
A PC-based parallel computer for medium/large scale lattice QCD simulations
is suggested. The Eotvos Univ., Inst. Theor. Phys. cluster consists of 137
Intel P4-1.7GHz nodes. Gigabit Ethernet cards are used for nearest neighbor
communication in a two-dimensional mesh. The sustained performance for
dynamical staggered(wilson) quarks on large lattices is around 70(110) GFlops.
The exceptional price/performance ratio is below $1/Mflop.Comment: 3 pages, 2 figures, Lattice2002(machines
On the WDVV Equation and M-Theory
A wide class of Seiberg-Witten models constructed by M-theory techniques and
described by non-hyperelliptic Riemann surfaces are shown to possess an
associative algebra of holomorphic differentials. This is a first step towards
proving that also these models satisfy the Witten-Dijkgraaf-Verlinde-Verlinde
equation. In this way, similar results known for simpler Seiberg-Witten models
(described by hyperelliptic Riemann surfaces and constructed without recourse
to M-theory) are extended to certain non-hyperelliptic cases constructed in
M-theory. Our analysis reveals a connection between the algebra of holomorphic
differentials on the Riemann surface and the configuration of M-theory branes
of the corresponding Seiberg-Witten model.Comment: 30 pages, Latex, some corrections made, refs adde
Tsallis' entropy maximization procedure revisited
The proper way of averaging is an important question with regards to Tsallis'
Thermostatistics. Three different procedures have been thus far employed in the
pertinent literature. The third one, i.e., the Tsallis-Mendes-Plastino (TMP)
normalization procedure, exhibits clear advantages with respect to earlier
ones. In this work, we advance a distinct (from the TMP-one) way of handling
the Lagrange multipliers involved in the extremization process that leads to
Tsallis' statistical operator. It is seen that the new approach considerably
simplifies the pertinent analysis without losing the beautiful properties of
the Tsallis-Mendes-Plastino formalism.Comment: 17 pages, no figure
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