360 research outputs found
Signatures of radial migration in barred galaxies: Azimuthal variations in the metallicity distribution of old stars
By means of N-body simulations, we show that radial migration in galaxy
disks, induced by bar and spiral arms, leads to significant azimuthal
variations in the metallicity distribution of old stars at a given distance
from the galaxy center. Metals do not show an axisymmetric distribution during
phases of strong migration. Azimuthal variations are visible during the whole
phase of strong bar phase, and tend to disappear as the effect of radial
migration diminishes, together with a reduction in the bar strength. These
results suggest that the presence of inhomogeneities in the metallicity
distribution of old stars in a galaxy disk can be a probe of ongoing strong
migration. Such signatures may be detected in the Milky Way by Gaia (and
complementary spectroscopic data), as well as in external galaxies, by IFU
surveys like CALIFA and ATLAS3D. Mixing - defined as the tendency toward a
homogeneous, azimuthally symmetric, stellar distribution in the disk - and
migration turns out to be two distinct processes, the effects of mixing
starting to be visible when strong migration is over.Comment: 8 pages, 10 figures, accepted for publication on Astronomy and
Astrophysic
Autocorrelation of Random Matrix Polynomials
We calculate the autocorrelation functions (or shifted moments) of the
characteristic polynomials of matrices drawn uniformly with respect to Haar
measure from the groups U(N), O(2N) and USp(2N). In each case the result can be
expressed in three equivalent forms: as a determinant sum (and hence in terms
of symmetric polynomials), as a combinatorial sum, and as a multiple contour
integral. These formulae are analogous to those previously obtained for the
Gaussian ensembles of Random Matrix Theory, but in this case are identities for
any size of matrix, rather than large-matrix asymptotic approximations. They
also mirror exactly autocorrelation formulae conjectured to hold for
L-functions in a companion paper. This then provides further evidence in
support of the connection between Random Matrix Theory and the theory of
L-functions
The history of stellar metallicity in a simulated disc galaxy
We explore the chemical distribution of stars in a simulated galaxy. Using simulations of the same initial conditions but with two different feedback schemes (McMaster Unbiased Galaxy Simulations – MUGS – and Making Galaxies in a Cosmological Context – MaGICC), we examine the features of the age–metallicity relation (AMR), and the three-dimensional age– [Fe/H]–[O/Fe] distribution, both for the galaxy as a whole and decomposed into disc, bulge, halo and satellites. The MUGS simulation, which uses traditional supernova feedback, is replete with chemical substructure. This substructure is absent from the MaGICC simulation, which includes early feedback from stellar winds, a modified initial mass function and more efficient feedback. The reduced amount of substructure is due to the almost complete lack of satellites in MaGICC. We identify a significant separation between the bulge and disc AMRs, where the bulge is considerably more metal-rich with a smaller spread in metallicity at any given time than the disc. Our results suggest, however, that identifying the substructure in observations will require exquisite age resolution, of the order of 0.25 Gyr. Certain satellites show exotic features in the AMR, even forming a ‘sawtooth’ shape of increasing metallicity followed by sharp declines which correspond to pericentric passages. This fact, along with the large spread in stellar age at a given metallicity, compromises the use of metallicity as an age indicator, although alpha abundance provides a more robust clock at early times. This may also impact algorithms that are used to reconstruct star formation histories from resolved stellar populations, which frequently assume a monotonically increasing AMR
Boundary conditions associated with the Painlev\'e III' and V evaluations of some random matrix averages
In a previous work a random matrix average for the Laguerre unitary ensemble,
generalising the generating function for the probability that an interval at the hard edge contains eigenvalues, was evaluated in terms of
a Painlev\'e V transcendent in -form. However the boundary conditions
for the corresponding differential equation were not specified for the full
parameter space. Here this task is accomplished in general, and the obtained
functional form is compared against the most general small behaviour of
the Painlev\'e V equation in -form known from the work of Jimbo. An
analogous study is carried out for the the hard edge scaling limit of the
random matrix average, which we have previously evaluated in terms of a
Painlev\'e \IIId transcendent in -form. An application of the latter
result is given to the rapid evaluation of a Hankel determinant appearing in a
recent work of Conrey, Rubinstein and Snaith relating to the derivative of the
Riemann zeta function
On the spacing distribution of the Riemann zeros: corrections to the asymptotic result
It has been conjectured that the statistical properties of zeros of the
Riemann zeta function near z = 1/2 + \ui E tend, as , to the
distribution of eigenvalues of large random matrices from the Unitary Ensemble.
At finite numerical results show that the nearest-neighbour spacing
distribution presents deviations with respect to the conjectured asymptotic
form. We give here arguments indicating that to leading order these deviations
are the same as those of unitary random matrices of finite dimension , where is a well
defined constant.Comment: 9 pages, 3 figure
The Halo Shape and Evolution of Polar Disc Galaxies
We examine the properties and evolution of a simulated polar disc galaxy.
This galaxy is comprised of two orthogonal discs, one of which contains old
stars (old stellar disc), and the other, containing both younger stars and the
cold gas (polar disc) of the galaxy. By exploring the shape of the inner region
of the dark matter halo, we are able to confirm that the halo shape is a oblate
ellipsoid flattened in the direction of the polar disc. We also note that there
is a twist in the shape profile, where the innermost 3 kpc of the halo flattens
in the direction perpendicular to the old disc, and then aligns with the polar
disc out until the virial radius. This result is then compared to the halo
shape inferred from the circular velocities of the two discs. We also use the
temporal information of the simulation to track the system's evolution, and
identify the processes which give rise to this unusual galaxy type. We confirm
the proposal that the polar disc galaxy is the result of the last major merger,
where the angular moment of the interaction is orthogonal to the angle of the
infalling gas. This merger is followed by the resumption of coherent gas
infall. We emphasise that the disc is rapidly restored after the major merger
and that after this event the galaxy begins to tilt. A significant proportion
of the infalling gas comes from filaments. This infalling gas from the filament
gives the gas its angular momentum, and, in the case of the polar disc galaxy,
the direction of the gas filament does not change before or after the last
major merger.Comment: Accepted for publication in MNRAS; 14 pages; 14 figure
A Random Matrix Model for Elliptic Curve L-Functions of Finite Conductor
We propose a random matrix model for families of elliptic curve L-functions
of finite conductor. A repulsion of the critical zeros of these L-functions
away from the center of the critical strip was observed numerically by S. J.
Miller in 2006; such behaviour deviates qualitatively from the conjectural
limiting distribution of the zeros (for large conductors this distribution is
expected to approach the one-level density of eigenvalues of orthogonal
matrices after appropriate rescaling).Our purpose here is to provide a random
matrix model for Miller's surprising discovery. We consider the family of even
quadratic twists of a given elliptic curve. The main ingredient in our model is
a calculation of the eigenvalue distribution of random orthogonal matrices
whose characteristic polynomials are larger than some given value at the
symmetry point in the spectra. We call this sub-ensemble of SO(2N) the excised
orthogonal ensemble. The sieving-off of matrices with small values of the
characteristic polynomial is akin to the discretization of the central values
of L-functions implied by the formula of Waldspurger and Kohnen-Zagier.The
cut-off scale appropriate to modeling elliptic curve L-functions is
exponentially small relative to the matrix size N. The one-level density of the
excised ensemble can be expressed in terms of that of the well-known Jacobi
ensemble, enabling the former to be explicitly calculated. It exhibits an
exponentially small (on the scale of the mean spacing) hard gap determined by
the cut-off value, followed by soft repulsion on a much larger scale. Neither
of these features is present in the one-level density of SO(2N). When N tends
to infinity we recover the limiting orthogonal behaviour. Our results agree
qualitatively with Miller's discrepancy. Choosing the cut-off appropriately
gives a model in good quantitative agreement with the number-theoretical data.Comment: 38 pages, version 2 (added some plots
Gaia Data Release 3: G_RVS photometry from the RVS spectra
Gaia Data Release 3 (DR3) contains the first release of magnitudes estimated
from the integration of Radial Velocity Spectrometer (RVS) spectra for a sample
of about 32.2 million stars brighter than G_RVS~14 mag (or G~15 mag). In this
paper, we describe the data used and the approach adopted to derive and
validate the G_RVS magnitudes published in DR3. We also provide estimates of
the G_RVS passband and associated G_RVS zero-point. We derived G_RVS photometry
from the integration of RVS spectra over the wavelength range from 846 to 870
nm. We processed these spectra following a procedure similar to that used for
DR2, but incorporating several improvements that allow a better estimation of
G_RVS. These improvements pertain to the stray-light background estimation, the
line spread function calibration, and the detection of spectra contaminated by
nearby relatively bright sources. We calibrated the G_RVS zero-point every 30
hours based on the reference magnitudes of constant stars from the Hipparcos
catalogue, and used them to transform the integrated flux of the cleaned and
calibrated spectra into epoch magnitudes. The G_RVS magnitude of a star
published in DR3 is the median of the epoch magnitudes for that star. We
estimated the G_RVS passband by comparing the RVS spectra of 108 bright stars
with their flux-calibrated spectra from external spectrophotometric libraries.
The G_RVS magnitude provides information that is complementary to that obtained
from the G, G_BP, and G_RP magnitudes, which is useful for constraining stellar
metallicity and interstellar extinction. The median precision of G_RVS
measurements ranges from about 0.006 mag for the brighter stars (i.e. with 3.5
< G_RVS < 6.5 mag) to 0.125 mag at the faint end. The derived G_RVS passband
shows that the effective transmittance of the RVS is approximately 1.23 times
better than the pre-launch estimate.Comment: 16 pages, 18 figures. Accepted for publication in A&
Gaia Data Release 3
CONTEXT:
Gaia Data Release 3 (Gaia DR3) contains the second release of the combined radial velocities. It is based on the spectra collected during the first 34 months of the nominal mission. The longer time baseline and the improvements of the pipeline made it possible to push the processing limit from GRVS = 12 in Gaia DR2 to GRVS = 14 mag.
AIMS:
We describe the new functionalities implemented for Gaia DR3, the quality filters applied during processing and post-processing, and the properties and performance of the published velocities.
METHODS:
For Gaia DR3, several functionalities were upgraded or added to the spectroscopic pipeline. The calibrations were improved in order to better model the temporal evolution of the straylight and of the instrumental point spread function (PSF). The overlapped spectra, which were mostly discarded in Gaia DR2, are now handled by a dedicated module. The hot star template mismatch, which prevented publication of hot stars in Gaia DR2, is largely mitigated now, down to GRVS = 12 mag. The combined radial velocity of stars brighter than or equal to GRVS = 12 mag is calculated in the same way as in Gaia DR2, that is, as the median of the epoch radial velocity time series. The combined radial velocity of the fainter stars is measured from the average of the cross-correlation functions.
RESULTS:
Gaia DR3 contains the combined radial velocities of 33 812 183 stars. With respect to Gaia DR2, the temperature interval has been expanded from Teff ∈ [3600, 6750] K to Teff ∈ [3100, 14 500] K for the bright stars (GRVS ≤ 12 mag) and [3100, 6750] K for the fainter stars. The radial velocities sample a significant part of the Milky Way: they reach a few kiloparsecs beyond the Galactic centre in the disc and up to about 10−15 kpc vertically into the inner halo. The median formal precision of the velocities is 1.3 km s−1 at GRVS = 12 and 6.4 km s−1 at GRVS = 14 mag. The velocity zeropoint exhibits a small systematic trend with magnitude that starts around GRVS = 11 mag and reaches about 400 m s−1 at GRVS = 14 mag. A correction formula is provided that can be applied to the published data. The Gaia DR3 velocity scale agrees satisfactorily with APOGEE, GALAH, GES, and RAVE; the systematic differences mostly remain below a few hundred m s−1. The properties of the radial velocities are also illustrated with specific objects: open clusters, globular clusters, and the Large Magellanic Cloud. For example, the precision of the data allows mapping the line-of-sight rotational velocities of the globular cluster 47 Tuc and of the Large Magellanic Cloud
Ultrasmooth organic–inorganic perovskite thin-film formation and crystallization for efficient planar heterojunction solar cells
To date, there have been a plethora of reports on different means to fabricate organic–inorganic metal halide perovskite thin films; however, the inorganic starting materials have been limited to halide-based anions. Here we study the role of the anions in the perovskite solution and their influence upon perovskite crystal growth, film formation and device performance. We find that by using a non-halide lead source (lead acetate) instead of lead chloride or iodide, the perovskite crystal growth is much faster, which allows us to obtain ultrasmooth and almost pinhole-free perovskite films by a simple one-step solution coating with only a few minutes annealing. This synthesis leads to improved device performance in planar heterojunction architectures and answers a critical question as to the role of the anion and excess organic component during crystallization. Our work paves the way to tune the crystal growth kinetics by simple chemistry
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