142 research outputs found
A statistical method for measuring the Galactic potential and testing gravity with cold tidal streams
We introduce the Minimum Entropy Method, a simple statistical technique for
constraining the Milky Way gravitational potential and simultaneously testing
different gravity theories directly from 6D phase-space surveys and without
adopting dynamical models. We demonstrate that orbital energy distributions
that are separable (i.e. independent of position) have an associated entropy
that increases under wrong assumptions about the gravitational potential and/or
gravity theory. Of known objects, `cold' tidal streams from low-mass
progenitors follow orbital distributions that most nearly satisfy the condition
of separability. Although the orbits of tidally stripped stars are perturbed by
the progenitor's self-gravity, systematic variations of the energy distribution
can be quantified in terms of the cross-entropy of individual tails, giving
further sensitivity to theoretical biases in the host potential. The
feasibility of using the Minimum Entropy Method to test a wide range of gravity
theories is illustrated by evolving restricted N-body models in a Newtonian
potential and examining the changes in entropy introduced by Dirac, MONDian and
f(R) gravity modifications.Comment: Accepted for publication in ApJ. 11 pages 6 figure
Constraining the shape of Milky Way satellites with distance gradients
We combine the Dark Energy Camera Legacy Survey (DECaLS) DR8 photometry with
Gaia photometry to study the 3-D structure of Bootes I, Draco, Ursa Minor,
Sextans and Sculptor dwarf galaxies using blue horizontal branch (BHB) stars as
distance indicators. We construct a new colour-absolute magnitude of BHB stars
that we use to measure the distance gradients within the body of the dwarf
galaxies. We detect a statistically significant non-zero gradient only in
Sextans and Sculptor. Through modeling of the gradient and 2-D density of the
systems by triaxial Plummer models we find that the distance gradients in both
dwarf galaxies are inconsistent with prolate shape, but compatible with oblate
or triaxial shapes. In order to explain the observed gradients, oblate models
of Sextans and Sculptor need to have a significant intrinsic ellipticity larger
than for Sextans and for Sculptor. The flattened oblate shape may
imply a significant anisotropy in velocity distribution in order to be
consistent with the lack of significant velocity gradients in these systems.Comment: 21 pages, 7 figure
Search for globular clusters associated with the Milky Way dwarf galaxies using Gaia DR2
We report the result of searching for globular clusters (GCs) around 55 Milky
Way satellite dwarf galaxies within the distance of 450 kpc from the Galactic
Center except for the Large and Small Magellanic Clouds and the Sagittarius
dwarf. For each dwarf, we analyze the stellar distribution of sources in Gaia
DR2, selected by magnitude, proper motion, and source morphology. Using the
kernel density estimation of stellar number counts, we identify eleven possible
GC candidates. Crossed-matched with existing imaging data, all eleven objects
are known either GCs or galaxies and only Fornax GC 1-6 among them are
associated with the targeted dwarf galaxy. Using simulated GCs, we calculate
the GC detection limit that spans the range from for distant dwarfs to for
nearby systems. Assuming a Gaussian GC luminosity function, we compute that the
completeness of the GC search is above 90 percent for most dwarf galaxies. We
construct the 90 percent credible intervals/upper limits on the GC specific
frequency of the MW dwarf galaxies: for
Fornax, for the dwarfs with , for the dwarfs with , and for
the dwarfs with . Based on , we derive the
probability of galaxies hosting GCs given their luminosity, finding that the
probability of galaxies fainter than to host GCs is lower than
0.1
Identifying RR Lyrae in the ZTF DR3 dataset
We present a RR Lyrae (RRL) catalogue based on the combination of the third
data release of the Zwicky Transient Facility (ZTF DR3) and \textit{Gaia} EDR3.
We use a multi-step classification pipeline relying on the Fourier
decomposition fitting to the multi-band ZTF light curves and random forest
classification. The resulting catalogue contains 71,755 RRLs with period and
light curve parameter measurements and has completeness of 0.92 and purity of
0.92 with respect to the SOS \textit{Gaia} DR2 RRLs. The catalogue covers the
Northern sky with declination , its completeness is for heliocentric distance ~kpc, and the most distant RRL at
132~kpc. Compared with several other RRL catalogues covering the Northern sky,
our catalogue has more RRLs around the Galactic halo and is more complete at
low Galactic latitude areas. Analysing the spatial distribution of RRL in the
catalogue reveals the previously known major over-densities of the Galactic
halo, such as the Virgo over-density and the Hercules-Aquila Cloud, with some
evidence of an association between the two. We also analyse the Oosterhoff
fraction differences throughout the halo, comparing it with the density
distribution, finding increasing Oosterhoff I fraction at the elliptical radii
between 16 and 32 kpc and some evidence of different Oosterhoff fractions
across various halo substructures
Common origin for Hercules-Aquila and Virgo Clouds in Gaia DR2
We use a sample of ~350 RR Lyrae stars with radial velocities and Gaia DR2
proper motions to study orbital properties of the Hercules-Aquila Cloud (HAC)
and Virgo Over-density (VOD). We demonstrate that both structures are dominated
by stars on highly eccentric orbits, with peri-centres around ~1 kpc and
apo-centres between 15 and 25 kpc from the Galactic centre. Given that the
stars in the HAC and the VOD occupy very similar regions in the space spanned
by integrals of motion, we conclude that these diffuse debris clouds are part
of the same accretion event. More precisely, these inner halo sub-structures
likely represent two complementary not-fully-mixed portions of an ancient
massive merger, also known as the "sausage" event.Comment: Submitted to MNRAS. Comments welcome
Kinematics beats dust: unveiling nested substructure in the perturbed outer disc of the Milky Way
We use eDR3 data and legacy spectroscopic surveys to map the Milky Way
disc substructure towards the Galactic Anticenter at heliocentric distances
. We report the discovery of multiple previously undetected
new filaments embedded in the outer disc in highly extincted regions. Stars in
these over-densities have distance gradients expected for disc material and
move on disc-like orbits with , showing
small spreads in energy. Such a morphology argues against a quiescently growing
Galactic thin disc. Some of these structures are interpreted as excited outer
disc material, kicked up by satellite impacts and currently undergoing
phase-mixing ("feathers"). Due to the long timescale in the outer disc regions,
these structures can stay coherent in configuration space over several Gyrs. We
nevertheless note that some of these structures could also be folds in the
perturbed disc seen in projection from the Sun's location. A full 6D
phase-space characterization and age dating of these structure should help
distinguish between the two possible morphologies.Comment: 5 pages, 4 figures, submitted to MNRA
188,000 Candidate Very Metal-poor Stars in Gaia DR3 XP Spectra
Very metal-poor stars ([Fe/H] < -2) in the Milky Way are fossil records of
early chemical evolution and the assembly and structure of the Galaxy. However,
they are rare and hard to find. Gaia DR3 has provided over 200 million
low-resolution (R = 50) XP spectra, which provides an opportunity to greatly
increase the number of candidate metal-poor stars. In this work, we utilise the
XGBoost classification algorithm to identify about 188,000 very metal-poor star
candidates. Compared to past work, we increase the candidate metal-poor sample
by about an order of magnitude, with comparable or better purity than past
studies. Firstly, we develop three classifiers for bright stars (BP < 16). They
are classifier-T (for Turn-off stars), classifier-GC (for Giant stars with high
completeness), and classifier-GP (for Giant stars with high purity) with
expected purity of 47%/47%/74% and completeness of 40%/94%/65% respectively.
These three classifiers obtained a total of 11,000/116,000/45,000 bright
metal-poor candidates. We apply model-T and model-GP on faint stars (BP > 16)
and obtain 13,000/48,500 additional metal-poor candidates with purity 40%/50%,
respectively. We make our metal-poor star catalogs publicly available, for
further exploration of the metal-poor Milky Way.Comment: Catalogs to be made public available after revie
Calibrating Long Period Variables as Standard Candles with Machine Learning
Variable stars with well-calibrated period-luminosity relationships provide
accurate distance measurements to nearby galaxies and are therefore a vital
tool for cosmology and astrophysics. While these measurements typically rely on
samples of Cepheid and RR-Lyrae stars, abundant populations of luminous
variable stars with longer periods of days remain largely unused.
We apply machine learning to derive a mapping between lightcurve features of
these variable stars and their magnitude to extend the traditional
period-luminosity (PL) relation commonly used for Cepheid samples. Using
photometric data for long period variable stars in the Large Magellanic cloud
(LMC), we demonstrate that our predictions produce residual errors comparable
to those obtained on the corresponding Cepheid population. We show that our
model generalizes well to other samples by performing a blind test on
photometric data from the Small Magellanic Cloud (SMC). Our predictions on the
SMC again show small residual errors and biases, comparable to results that
employ PL relations fitted on Cepheid samples. The residual biases are
complementary between the long period variable and Cepheid fits, which provides
exciting prospects to better control sources of systematic error in
cosmological distance measurements. We finally show that the proposed
methodology can be used to optimize samples of variable stars as standard
candles independent of any prior variable star classification.Comment: 14 pages, 10 figures, 1 table, updated to match the version accepted
by the MNRA
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